-Page 2 of 53-

Risk Conclusions

The results of this screening-level risk assessment indicate that the current uses of maleic 

hydrazide are not expected to cause direct adverse effects to Federally listed threatened and 

endangered (hereafter referred to as “listed”) and non-listed freshwater and estuarine/marine fish 

and invertebrates on an acute or chronic exposure basis. Direct adverse effects to listed and nonlisted aquatic plants are also not expected. Acute risk to non-listed mammals is not expected, but 

acute risk to listed mammals is uncertain due due to maximum estimated environmental 

concentrations (EECs) being higher than the nondefinitive endpoint (greater than) values, highest 

concentration tested, in acute toxicity tests with mammals. Chronic risk to mammals is expected

for certain size class/dietary item combinations when application rates are greater than 4.6 lbs 

a.i./A. Acute and chronic risk to listed and non-listed birds is uncertain due to max EECs higher 

than the nondefinitive acute endpoints and a lack of chronic toxicity data with avian species. 

Potential risk to non-listed species of beneficial terrestrial invertebrates is similarly uncertain due 

to nondefinitive ecotoxicity endpoints in acute contact honey bee studies which are lower than 

the EECs. However, risk to listed species of beneficial terrestrial invertebrates is considered 

likely due to toxicity data showing substantial mortality effects to parasitic wasps and predatory 

mites at application rates that are approximately 1/3 the registered maximum application rate. 

Risk to listed and non-listed monocotyledon (monocot) species is expected from the registered 

uses of maleic hydrazide. Risk to listed and non-listed dicotyledon (dicot) species is expected in 

semi-aquatic areas, but may be reduced in dry areas when ground applications are used in place 

of aerial applications. Risk from spray drift alone is not predicted for ground applications but 

may be expected for listed and non-listed monocot species when aerial applications of maleic 

hydrazide are made.

Designations of “uncertain” for taxa in this assessment are a result of incomplete ecotoxicology 

data. Where uncertainty designations are made for a taxon, efforts have been made to 

characterize the risk picture. For instance, multiple lines of evidence support the potential for 

effects to non-target terrestrial invertebrates, whereas for mammals, available lines of evidence 

indicate that the potential for acute risk may be lower.

Table 1 summarizes the representative uses assessed and the taxa that may potentially be directly 

at risk from exposure to maleic hydrazide. Additional description is further summarized in 

Appendix 3.
Page 3 of 53

Table 1: Summary of Risk Determinations for Currently Registered Uses of Maleic Hydrazide1

Use Fish2 Aquatic 

Invertebrates3

Aquatic 

Plants4

Terrestrial Plants Birds5 Mammals Terrestrial 

Invertebrates

AcuteChronicAcuteChronicListedNon-Listed

Monocots Dicots

AcuteChronicAcuteChronicListedNon-ListedListedNonListedListedNonListed

Agricultural 

Fallow/Idleland/

Conservation 

Reserve

X X X X U U Ua X X U

Ornamental 

and/or Shade 

Trees6

X X X X U U Ua X X U

Golf Course 

Turf, 

Nonagricultural 

Rights-of-Way, 

Urban Areas5

X X X Xb U U Ua X X U

Tobacco X X Xc Xc U U Ua X X U

Ornamental 

Woody Shrubs 

and Vines

Xd Xd Xc Xc U U Ua X

 Potato 

(White/Irish)

Xd Xd Xc Xc U U X

Onion Xe Xe Xc Xc Ua U U

Citrus Xe Xe Xf Xf Ua U U

1 A bolded and shaded “X” indicates risk is expected to listed and non-listed species. An italicized “X” indicates that risk is 

expected to listed species only. A “U” indicates that risk is uncertain due to a lack of data. A blank box indicates that no risk is 

expected. See Risk Characterization section for more information.

2 Fish includes both estuarine/marine and freshwater fish and are also considered surrogates for aquatic-phase amphibians. 

3 Aquatic Invertebrates includes both estuarine/marine and freshwater invertebrates.

4 Aquatic Plants includes both vascular and non-vascular plants.

5 Birds are considered surrogates for reptiles and terrestrial-phase amphibians. 

6 Also includes Ornamental Lawns and Turf and Residential Lawns use patterns

7 Also includes Industrial/Construction Areas (Outdoor), Nonagricultural Uncultivated Areas/Soils, Fencerows/Hedgerows, and 

Airports/Landing Fields use patterns.

a Acute risk is not expected for non-listed species and is uncertain for listed species.

 

b Risk to non-listed dicots is only predicted in semi-aquatic areas (regardless of application method)

 

c Risk to both listed and non-listed dicots is only predicted in semi-aquatic areas (regardless of application 

 method)

 

d Risk to non-listed monocots is only predicted in semi-aquatic areas (regardless of application method)

 e Risk to both listed and non-listed monocots is only predicted in semi-aquatic areas (regardless of application 

 method)

 

f Risk to both listed and non-listed dicots is only predicted in semi-aquatic areas, however non-listed dicots in 

 semi-aquatic areas are not considered at risk for scenarios with only ground applications.
Page 4 of 53

Basic Uncertainties 

 No available ecotoxicity or fate data were evaluated by EFED for hydrazine, an impurity

present in low levels in technical grade maleic hydrazide. 

 Several registered product labels do not specify the number of applications per year. This 

assessment assumed that only 1 application at the maximum application rate is made (the 

exception to this is for citrus, where the label does state that 2 applications can be made). 

This may underestimate risk for those use sites that do not state the maximum number of 

applications. 

 No chronic data are available for avian reproduction. 

 Terrestrial animal acute and chronic studies did not test concentrations up to the 

maximum EECs predicted in this assessment, but generally did not have effects at the 

concentrations tested.
-Page 5 of 53-

Environmental Fate and Ecological Risk Assessment for the 

Registration of Maleic Hydrazide

and its Potassium Salt (K-MH)

 

Maleic Hydrazide

1,2-dihydro-3,6-pyridazinedione

CAS Registry Number 123-33-1

PC Code 051501

Maleic Hydrazide Potassium salt

CAS Registry Number 28382-15-2

PC Code 051503

Prepared by:

Ibrahim Abdel-Saheb, Ph.D. Environmental Scientist

Michael Wagman, Biologist

U. S. Environmental Protection Agency

Office of Pesticide Programs

Environmental Fate and Effects Division

Environmental Risk Branch VI

1200 Pennsylvania Ave., NW

Mail Code 7507P

Washington DC 20460

Reviewed by:

Amy Blankinship, Senior Science Advisor, ERB VI

William P. Eckel, Ph.D, Senior Science Advisor, ERB VI

Mark Corbin, Branch Chief, ERB VI

June 18, 2014
Page 6 of 53

Table of Contents

1. Executive Summary.................................................................................................................................................7

2. Problem Formulation ............................................................................................................................................11

2.1. Nature of Regulatory Action .......................................................................................................................11

2.2. Mode of Action..............................................................................................................................................12

2.3. Use Characterization....................................................................................................................................12

2.4. Conceptual Model.........................................................................................................................................15

3. Analysis...................................................................................................................................................................20

3.1. Exposure Characterization ................................................................................................................................20

3.2. Ecological Effects Characterization ..................................................................................................................26

4. Risk Characterization ...........................................................................................................................................32

4.1. Risk Estimation...................................................................................................................................................32

4.2. Risk Description ...................................................................................................................................................38

4.2.1. Risks to Aquatic Organisms ..............................................................................................................................38

4.2.2. Risks to Terrestrial Organisms ..........................................................................................................................39

5. Federally Threatened and Endangered (Listed) Species Concerns ....................................................................43

6. Literature Cited........................................................................................................................................................46

Appendix 1 – Surface Water Concentration Calculator Modleing Summary....................47

Appendix 2 -Environmental Fate Studies Summary ............................................................48

Appendix 3–Summary of Risk Conclusions by Use Site and Taxa......................................50
Page 7 of 53

1. Executive Summary

EFED has completed the Registration Review preliminary ecological risk assessment for maleic 

hydrazide acid (1,2-dihydro-3,6-pyridazinedione CAS Registry Number 123-33-1; PC Code: 

051501), its potassium salt (CAS Registry Number 28382-15-2; PC Code 051503) and their enduse products. Maleic hydrazide is a systemic growth retardant with end use products generally 

formulated using the potassium salt of maleic hydrazide. When sprayed on plants, it is absorbed 

and moves internally where it blocks cell division. It is a plant growth regulator (sprout inhibitor) 

and herbicide whose mode of activity is as a uracil antimetabolite. Maleic hydrazide is registered 

for use on tobacco, potatoes, onions, non-bearing citrus, turf, utility and highway rights-of-way, 

airports, industrial land, lawns, recreational areas, ornamental/shade trees and ornamental plants. 

Most of the maleic hydrazide used in the U.S. is applied to tobacco (80%), followed by potatoes

(18%). Formulations include an emulsifiable concentrate and soluble concentrate/liquid and solid. 

Maleic hydrazide is applied by aircraft or ground spray equipment. Current use practice limitations 

prohibit treating crops within 7 days of harvest, and grazing or feeding forage or hay from treated 

areas to livestock.

Maleic hydrazide is not persistent unders aerobic soil conditions. It degrades to transformation 

products with a half life of <24 hours. It is stable to hydrolysis though it photolyzes in water 

with an observed half-life of >30 days at pH 9. Maleic hydrazide is expected to be mobile, due to 

its low adsorption to soils with KOC values that range from 11.5 – 70 mL/g o.c. Maleic hydrazide 

is moderately persistent under anaerobic soil conditions (half-life 30-60 days). Maleic hydrazide 

did not persist in the available field dissipation studies and degraded with half-lives of 3 days on 

bare ground and 5 days in turf plots, 2-3 days (sandy loam, potatoes), 7 days (loam, tobacco).

The potential for maleic hydrazide to bioaccumulate in fish is anticipated to be very low, as 

indicated by the octanol/water partition coefficient (log Kow= -0.68).

Maleic hydrazide is not likely to volatilize given its vapor pressure (7.5 x 10-8 mm Hg at 25oC and 

Henry’s Law constant = 2.5 x 10-5 m3 atm g/mol at 25

oC). 

No available fate or ecotoxicity data has been evaluated by EFED for hydrazine, an impurity

present at low levels in technical grade maleic hydrazide. High levels of hydrazine as an impurity 

was responsible for carcinogenicity reported in several mammalian toxicity studies.

Maleic hydrazide is practically non-toxic on an acute basis to freshwater and estuarine/marine fish 

and invertebrates. On a chronic exposure basis, maleic hydrazide was not found to have any 

treatment related effects to freshwater and estuarine/marine fish and invertebrates up to the highest 

concentration tested, 9,600 μg a.i./L (for fathead minnow)—99,000 μg a.i./L (for daphnids). 

Maleic hydrazide is practically nontoxic to birds, mammals, and beneficial insects (i.e., honey 

bees) on an acute exposure basis as no effects were recorded in the available studies (i.e., the 

LD50/LC50 effect level was greater than the highest concentration tested). Chronic toxicity testing 

with mammals identified a decrease in body weight gain of multiple generations in a 2-generation 

rat study (MRID 00128720). No avian reproduction data are available to assess chronic effects to 

birds. The 1994 RED (USEPA, 1994) determined that avian reproduction studies were not 
Page 8 of 53

required for maleic hydrazide because “it is not applied more than once prior to or during the 

breeding season and it is practically nontoxic to birds.” However, the 2009 problem formulation 

(USEPA, 2009a), noted that many uses “do not specify on the label the number of applications 

and application intervals,” but nonetheless did not request avian reproduction data. 

Based on the available plant toxicity data, maleic hydrazide exhibited low toxicity to aquatic 

plants, but may exhibit toxicity to some species of terrestrial plants.

No degradates of maleic hydrazide are considered metabolites of concern. The major degradate 

of maleic hydrazide is CO2. Other nonvolatile degradates, maleic acid and maleimide, were 

identified in small amounts (<5% of the applied) which further degrade into CO2.

Potential effects to federally-listed endangered and threatened species (listed species) based on 

LOC exceedances require an in-depth listed species evaluation determining the potential cooccurrence of listed species and use patterns of maleic hydrazide. Identified potential risks to 

listed species are summarized in Table 2.

Table 2. Potential Effects to Federally Listed Taxa Associated with Direct Effects from the 

Registered Uses of Maleic Hydrazide.

Listed Plant Taxon Potential Direct Effects

Terrestrial and semi-aquatic 

plants – monocots and dicots Yes

Aquatic nonvascular plants No

Aquatic vascular plants No

Listed Animal Taxon Potential Direct Effects

Acute Chronic

Terrestrial invertebrates Yes1 Uncertain

Mammals Uncertain2 Yes

Birds Uncertain2 Uncertain3

Reptiles Uncertain2 Uncertain2,3

Amphibians Uncertain

2,4 Uncertain2,3,4

Freshwater fish No No

Freshwater invertebrates No No

Mollusks No No

Estuarine/marine fish No No

Estuarine/marine invertebrates No No

1 Although an LC50 was not attainable in the acute contact toxicity tests with honey bees, the limit dose (36.3 μg a.i./bee) was not high enough 

to preclude risk at the non-listed species LOC (0.4), based on the maximum EECs (35.1 μg/bee) for registered maleic hydrazide uses. 

Further, given that additional toxicity data with predatory mites and parasitic wasps showed lethal effects at concentrations well below the 

maximum application rate, EFED cannot preclude effects to listed beneficial terrestrial invertebrates.

2 Risk to this taxa was considered uncertain due to the use of non-definitive toxicological endpoints that were below maximum EECs. 

Although no effects were observed in acute toxicological studies, the doses tested did not exceed the maximum EECs predicted by the 

maximum application rates.

3 Chronic risk to this taxa is uncertain due to a lack of avian reproduction data

4Risk to these taxa are uncertain for terrestrial-phase amphibians. No direct effects are expected for aquatic phase amphibians.

Based on this screening-level assessment, acute and chronic risk resulting from direct effects is 

not expected for freshwater and estuarine/marine fish (surrogates for aquatic-phase amphibians) 

and invertebrates or aquatic plants from the use of maleic hydrazide on all of its registered use 

sites. Acute and chronic risk resulting from direct effects cannot be precluded for listed birds 

(surrogates for reptiles and terrestrial-phase amphibians), mammals, and terrestrial invertebrates 
Page 9 of 53

from the use of maleic hydrazide on all its registered use sites. Risk can also not be precluded for 

terrestrial plants, which would be expected given maleic hydrazide’s mode of action.

Potential direct effects on listed birds (and reptiles and terrestrial phase amphibians), mammals, 

terrestrial invertebrates and terrestrial plants and non-listed birds, terrestrial invertebrates and 

terrestrial plants from the use of maleic hydrazide may be associated with modification of primary 

constituent elements (PCEs) of designated critical habitat, where such designations have been 

made. However, at this current stage of the Registration Review process, it is premature to make 

effects determinations until further refinements are conducted. In order to make species-specific 

effects determinations, useful refinements include more detailed 1) species-specific ecological and 

biological data; 2) information on use patterns for maleic hydrazide; and 3) sub-county level spatial 

proximity analyses for the co-occurrence of potential areas of effects and listed species/designated 

critical habitats of concern. Examples of such refinements are described below.

EFED is currently developing tools that are expected to further refine the species-specific 

assessment and designed to aid in making effects determinations for individual listed species 

and/or their designated critical habitats (if applicable). Information obtained from the U.S. Fish & 

Wildlife Service (USFWS), the National Marine Fisheries Service (NMFS), and other reliable 

sources is being collated by EFED to address all currently listed species. The information will be 

stored in an Office of Pesticide Programs Pesticide Registration Information System (PRISM) 

knowledgebase, which consists of an information repository for biological and behavioral data 

relevant to specific species and/or their habitats) and a document repository that contains 

supporting documents (e.g., USFWS recovery plans) and electronic information (e.g., GIS data 

files). These data will be relied upon to provide information on habitat, diet, life history variables, 

and temporal and spatial data. For terrestrial taxa specifically, the biological information relevant 

to Risk Quotient (RQ) calculations (e.g., diet and body weight) will be used to parameterize 

exposure estimates to derive species-specific RQs consistent with currently used methods in the 

T-REX and T-HERPS models. 

Refinements may also include more detailed analyses of the registered uses and their use patterns 

that result in level of concern (LOC) exceedances for listed species in the screening-level 

assessment. The analyses may include more information on where, when, and how maleic 

hydrazide is used on crops including: onion, shallot, potato, tobacco and citrus as well as nonagricultural uses including rights-of-way (ROW), ornamental lawns, turf, and golf courses.

Actual usage data (when available) and national land-cover datasets that indicate potential use sites 

(e.g., national land cover dataset (NLCD), crop data layer (CDL)) may be used to support a more 

refined analysis of where maleic hydrazide is reasonably expected to be used. Similarly, 

refinements for the timing of applications and how maleic hydrazide is used may involve an 

analysis of reported usage data (if available) and a more in-depth exploration of agronomic 

practices. 

The U.S. Environmental Protection Agency (EPA), along with the U.S. Department of Agriculture 

(USDA), the U.S. Fish and Wildlife Service (FWS), and the National Marine Fisheries Service 

(NMFS), have announced the availability of a report developed by the National Academy of 

Sciences’ National Research Council outlining recommendations on specific scientific and 
Page 10 of 53

technical issues related to the development of endangered and threatened species risk assessments 

for pesticides that are compliant with the Endangered Species Act (ESA) and the Federal 

Insecticide, Fungicide, and Rodenticide Act (FIFRA). The EPA, USDA, FWS and NMFS asked 

the NRC to consider a range of scientific and technical issues including:

• identifying best available scientific data and information; 

• considering sub-lethal, indirect and cumulative effects; 

• assessing the effects of chemical mixtures and inert ingredients; 

• analyzing the effects of pesticide use through the use of models; 

• incorporating uncertainties into the evaluations; and 

• using geospatial information and datasets that can be employed by the agencies in the 

course of these assessments. 

After reviewing the NRC’s recommendations on these scientific and technical issues, the EPA 

and its federal partners will determine how these recommendations can be implemented and 

incorporated into the scientific approaches for developing listed species risk assessments. It is 

anticipated that any draft guidance and/or science policies developed by EPA and its federal 

partners as a result of the NAS recommendations would be released for public comment prior to 

implementation. 

The refinements based on species-specific data and detailed usage information, when available, 

are expected to help to more accurately identify potential areas of effect and to better inform effects 

and habitat determinations for listed species and any designated critical habitats. For example, if 

maleic hydrazide is used when a particular species of concern is not present (e.g., it is migratory) 

or is not co-located in space, then risk to the species may often be precluded. If LOCs are still 

exceeded for birds (surrogates for terrestrial-phase amphibians and reptiles), mammals, terrestrial 

invertebrates and terrestrial plants after conducting the refined analyses, further analyses of the 

potential spatial/temporal co-occurrence of listed species of concern (and any designated critical 

habitat) may be conducted. The extent of possible refinement in the analyses of spatial/temporal 

co-occurrence will largely depend on the scale and quality of the available sub-county level use 

site (e.g., NLCD, CDL) and species location data.

Uncertainties

Although the majority of product labels explicitly state that the maximum number of applications 

is 1 per year, several registered product labels do not specify the number of applications per year. 

This assessment assumed only 1 application at the maximum rate for each use site (except for 

citrus, for which labels clearly allow 2 applications at the maximum rate). Thus this assessment 

may underestimate exposure to maleic hydrazide for those use sites with labels that do not specify 

the maximum number of applications per year or crop cycle.

No available fate or ecotoxicity data has been evaluated for hydrazine, an impurity present in low 

levels in technical grade maleic hydrazide. This assessment does not quantitatively evaluate risk

posed by the hydrazine impurity. If hydrazine is more persistant in the environment than maleic 

hydrazide, then potential for risk from maleic hydrazide and its contaminat will be underestimated.

If hydrazine is more toxic than maleic hydrazide and is present in concentrations higher than what 

was tested in the maleic hydrazide ecotoxicity studies (<0.12 ppm in the most recently conducted 
Page 11 of 53

toxicity studies), then this assessment may underestimate risk posed by maleic hydrazide due to 

the hydrazine contamination. No avian reproduction data are available, therefore chronic risk to 

birds cannot be precluded in this assessment. Acute and chronic studies with terrestrial animal 

species did not result in any toxic effects, therefore the maximum concentration tested was used 

as the endpoint (i.e. LD50, LC50, or NOAEC) for risk estimation. However, these studies did not 

test concentrations up to the maximum estimated environmental concentrations (EECs) for maleic 

hydrazide uses, therefore actual risk to terrestrial animal species remains uncertain. 

2. Problem Formulation

The purpose of problem formulation is to provide the foundation for the environmental fate and 

ecological risk assessment in support of the Registration Review of the Plant growth regulator, 

herbicide: maleic hydrazide [(1,2-dihydro-3,6-pyridazinedione); CAS#: 123-33-1; PC Code: 

051501], and its potassium salt (CAS Registry Number 28382-15-2; PC Code 051503). The 

problem formulation sets the objectives for the risk assessment, establishes a risk hypothesis and 

conceptual model depicting potential risk, and provides a plan for analyzing the data and 

characterizing the risk associated with the uses of maleic hydrazide (USEPA, 1998). 

2.1. Nature of Regulatory Action

Under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), all pesticides distributed 

or sold in the United States must be registered by the United States Environmental Protection 

Agency (EPA or the Agency). To determine whether a pesticide can be registered, the EPA 

evaluates the product’s safety to non-target species based on a wide range of environmental and 

health effects studies. In 1996, FIFRA was amended by the Food Quality Protection Act (FQPA), 

and EPA was mandated to implement a new program for the periodic review of pesticides, i.e., 

Registration Review1. The registration review program is intended to ensure that as the ability to 

assess risk evolves and as policies and practices change, all registered pesticides continue to meet 

the statutory standard of no unreasonable adverse effects to human health and the environment. 

Changes in science, public policy, and pesticide use practices will occur over time. Through the 

registration review program, the Agency periodically reevaluates pesticides to make sure that as 

changes occur, products in the marketplace can be used safely.

Additionally, potential effects to federally listed endangered and threatened species (hereafter 

referred to as “listed”) and their designated critical habitat are also considered under the 

Endangered Species Act (ESA) in order to ensure that the continued registration of maleic 

hydrazide is not likely to jeopardize the continued existence of listed species or adversely modify 

their critical habitat. In order to meet the requirements of FIFRA and the ESA, this assessment 

follows EPA guidance on conducting ecological risk assessments (USEPA, 1998) and Office of 

Pesticide Program’s Overview Document, which contains guidance for assessing pesticide risks 

to non-listed and listed species (USEPA, 2004).

 

1 http://www.epa.gov/oppsrrd1/registration_review/
Page 12 of 53

2.2. Mode of Action

Maleic hydrazide is a plant growth regulator (sprout inhibitor) and herbicide whose mode of 

activity is as a uracil antimetabolite. It is registered for use on tobacco, potatoes, onions, nonbearing citrus, turf, utility and highway rights-of-way, airports, industrial land, lawns, 

recreational areas, ornamental/shade trees and ornamental plants. When sprayed on plants, it is 

absorbed and moves internally where it blocks cell division.

2.3. Use Characterization

Formulations include an emulsifiable concentrate and soluble concentrate/liquid and solid. 

Maleic hydrazide is applied by aircraft or ground spray equipment. Formulations include an 

emulsifiable concentrate, liquid soluble concentrate and solid soluble concentrate. Maleic 

hydrazide is applied by aircraft or ground spray equipment. Current use practice limitations 

prohibit treating crops within 7 days of harvest, and grazing or feeding forage or hay from 

treated areas to livestock. For many of the use sites, especially the ones with the highest 

application rates (agricultural fallow, idleland, conservation reserve, and ornamental and shade 

trees), labels are lacking information on the maximum number of applications per year and/or the 

maximum application rate per year. This assessment has assumed that the maximum application 

rate is applied once (unless otherwise stated) without any succeeding retreatment. Therefore, 

this assessment may underestimate risk if multiple applications at the maximum application rate 

are applied in a season. Use patterns for Maleic hydrazide based on crop agricultural and non

agricultural uses are summarized in Tables 3-4, below.

Table 3: Use Site and Application Rates for Registered Maleic Hydrazide Uses

Crop/Use Site Maximum 

Application Rate

(lbs a.i./A)

Number of 

Applications/

Year (min 

retreatment 

interval)

Agricultural 

Fallow/Idleland/Conservation 

Reserve

13 NS

Ornamental and/or Shade 

Trees

9.2 NS

Ornamental Lawns and Turf, 

Residential Lawns

8.7 1

Airports/Landing Fields, 

Golf Course Turf, 

Industrial/Construction Areas 

(Outdoor), Nonagricultural 

Rights-ofWay/Fencrows/Hedgerows, 

8 1
Page 13 of 53

Nonagricultural Uncultivated 

Areas/Soils, Urban Areas

Tobacco 6.1 1

Ornamental Woody Shrubs 

and Vines

4.6 NS

Potato (White/Irish) 4.3 1

Onion 2.9 1

Citrus 1.9 2 (42)

Table 4. Maleic hydrazide Uses by Label

Label Formulation % a.i. Crops/Sites No. 

apps/Yr

Method of 

Application

000400-

00084

L 21.7 POTATO, TOBACCO, 

ONION, AGRICULTURAL 

FALLOW/IDLELAND

1 Ground and Air

000400-

00094

L 21.7 ORNAMENTAL LAWNS 

AND TURF, 

AGRICULTURAL 

FALLOW/IDLELAND, GOLF 

COURSE TURF, 

RESIDENTIAL LAWNS

1 Boom sprayer

000400-

00165

SC/S 80 AGRICULTURAL 

FALLOW/IDLELAND, 

AIRPORTS/LANDING 

FIELDS, 

NONAGRICULTURAL 

RIGHTS-OFWAY/FENCEROWS/HEDGER

OWS, ORNAMENTAL 

AND/OR SHADE TREES, 

GOLF COURSE TURF, 

POTATO, TOBACCO, ONION

1 Tractor-mounted sprayer

Ground, Air, Boom sprayer

000400-

00452

L 30.3 POTATO, TOBACCO, ONION 1 Ground, Air, Boom sprayer

000400-

00482

EC 21 TOBACCO 1 Dropline, Tractor-mounted 

sprayer, Jug

003862-

00159

L 21.7 NONAGRICULTURAL 

RIGHTS-OFWAY/FENCEROWS/HEDGER

OWS, ORNAMENTAL 

LAWNS AND TURF, 

AIRPORTS/LANDING 

FIELDS, ORNAMENTAL 

AND/OR SHADE TREES, 

ORNAMENTAL WOODY 

SHRUBS AND VINES

1 Hose-end sprayer

005549-

00089

EC 21.7 TOBACCO 1 Tractor-mounted sprayer, 

power sprayer

019713-

00001

L 30.2 ORNAMENTAL AND/OR 

SHADE TREES, 

AIRPORTS/LANDING 

FIELDS

1 Airblast, boom sprayer
Page 14 of 53

, NONAGRICULTURAL 

RIGHTS-OFWAY/FENCEROWS/HEDGER

OWS, ORNAMENTAL 

LAWNS AND TURF, GOLF 

COURSE TURF, 

RESIDENTIAL LAWNS

019713-

00002

L 30.2 NONAGRICULTURAL 

RIGHTS-OFWAY/FENCEROWS/HEDGER

OWS, ORNAMENTAL 

LAWNS AND TURF, 

RESIDENTIAL LAWNS, 

CITRUS, POTATO, ONION

1 Animal-drawn sprayer, 

aircraft, 

Tractor-mounted sprayer, 

sprayer

019713-

00017

L 27.8 TOBACCO 1 Animal-drawn sprayer, 

Compressed air sprayer, 

Tractor-mounted sprayer

019713-

00020

L 21.6 TOBACCO 1 Compressed air sprayer

Tractor-mounted sprayer

019713-

00105

EC 11.1 TOBACCO Not 

specified

Sprayer, 

Compressed air sprayer

019713-

00293

L 30.2 TOBACCO Not 

specified

Compressed air sprayer, 

Tractor-mounted sprayer

019713-

00294

L 21.6 TOBACCO Not 

specified

Compressed air sprayer, 

Tractor-mounted sprayer

019713-

00361

DF 80 ORNAMENTAL AND/OR 

SHADE TREES, 

ORNAMENTAL WOODY 

SHRUBS AND VINES, 

AIRPORTS/LANDING 

FIELDS, 

NONAGRICULTURAL 

RIGHTS-OFWAY/FENCEROWS/HEDGER

OWS, NONAGRICULTURAL 

UNCULTIVATED 

AREAS/SOILS, GOLF 

COURSE TURF, POTATO, 

TOBACCO, ONION

1 Ground, Air, 

Tractor-mounted sprayer

019713-

00371

SC/S 80 TOBACCO Not 

specified

Tractor-mounted sprayer, 

Compressed air sprayer

019713-

00372

SC/S 80 ORNAMENTAL LAWNS 

AND TURF, RESIDENTIAL 

LAWNS, 

NONAGRICULTURAL 

RIGHTS-OFWAY/FENCEROWS/HEDGER

OWS, CITRUS, POTATO, 

ONION

Not 

specified

Tractor-mounted sprayer, 

Compressed air sprayer

051873-

00002

EC 21.7 TOBACCO Not 

specified

Animal-drawn sprayer

, Tractor-mounted sprayer, 

Compressed air sprayer,

051873-

00006

EC 11.1 TOBACCO Not 

specified

Compressed air sprayer, 

sprayer
Page 15 of 53

051873-

00008

EC 21.7 ORNAMENTAL AND/OR 

SHADE TREES, 

AIRPORTS/LANDING 

FIELDS, GOLF COURSE 

TURF, INDUSTRIAL AREAS 

(OUTDOOR), 

NONAGRICULTURAL 

RIGHTS-OFWAY/FENCEROWS/HEDGER

OWS, ORNAMENTAL 

WOODY SHRUBS AND 

VINES

Not 

specified

Pump-up sprayer, Air carrier 

sprayer, Boom sprayer

051873-

00009

L

30.15

TOBACCO Not 

specified

Animal-drawn sprayer

, Tractor-mounted sprayer, 

Compressed air sprayer

051873-

00017

SC/S 80 TOBACCO Not 

specified

Animal-drawn sprayer

, Tractor-mounted sprayer, 

Compressed air sprayer

A screening level estimate of current usage of maleic hydrazide is presented in Table 5. This 

screening level usage assessment presents usage data on an annual basis and is derived from 

USDA’s National Agricultural Statistics Service with data updated to 2012. 

Table 5. Maleic hydrazide Screening Level Usage Analysis (SLUA) for 2012

Percent Crop Treated

Crop Lbs. A.I. Average Maximum

1 Onions 30,000 10 20

2 Potatoes 200,000 10 15

3 Shallot* <500 N/C N/C

4 Tobacco 600,000 65 80

* California data, only LBS a.i. available. No information available about use of this chemical on this crop 

in other states.

N/C = not calculated 

2.4. Conceptual Model

The potential terrestrial and aquatic exposure pathways and effects of the registered uses of maleic 

hydrazide are shown in Figure 1. The conceptual model used to depict the potential ecological 

risk associated with the registered uses of maleic hydrazide assumes that, as a plant growth 

regulator (sprout inhibitor) and herbicide, maleic hydrazide is capable of adversely affecting 

terrestrial and aquatic animals and plants, provided that environmental concentrations are 

sufficiently elevated as a result of the labeled uses. Solid arrows in Figure 1 depict the most likely 

routes of exposure and effects; dashed lines depict potential routes of exposure that are not 

considered likely for maleic hydrazide.
Page 16 of 53

The hypothesis for the risks of maleic hydrazide to non-target animals focuses on both aquatic and 

terrestrial environments via potential exposure to the parent from direct spray, spray drift, and 

runoff. Leaching is considered a significant route of transport considering maleic hydrazide is 

mobile in water. Dotted exposure routes are not likely (not anticipated to volatize). For terrestrial 

organisms, the major route of exposure is via diet, such as through consumption of plant leaves or 

insects, which contain maleic hydrazide residues as a result of direct application and/or spray drift, 

or root uptake and translocation. For aquatic animal species, the major routes of exposure are 

direct contact via the respiratory surface (gills) or the integument. Aquatic vascular and nonvascular plants may also be exposed via direct uptake and adsorption. Risk to terrestrial plants is 

also considered in this screening-level assessment. In addition, maleic hydrazide has a low 

potential for sorption to sediment; therefore, exposure to organisms that reside primarily in the 

“benthos” is not likely. Estimated exposure concentrations for all organisms are obtained through 

the use of several Agency exposure models.

The following risk hypothesis is presumed for this screening level assessment:

Based on the application methods, mode of action, direct toxicity to non-target aquatic and 

terrestrial species, the registered uses of maleic hydrazide has the potential to reduce survival, 

reproduction, and/or growth in listed and non-listed terrestrial and aquatic organisms.

 
Page 17 of 53
Page 18 of 53

Analysis Plan

Residues of Concern

In October 1976, maleic hydrazide went into Special Review (then called Rebuttable

Presumption Against Registration or RPAR) because it met the risk criteria for oncogenic, 

mutagenic and reproductive effects. Based on other data submitted, EPA determined that the 

oncogenicity and reproductive effects triggers were not supported, and that only weak evidence 

supported the mutagenicity trigger. In concluding the RPAR in June 1982, EPA allowed 

continued use of maleic hydrazide and its potassium salt (K-MH), but established an upper limit 

of 15 ppm for the impurity hydrazine (associated with tumor induction) in technical grade maleic 

hydrazide. At this level, lifetime cancer risks for both dietary and worker exposure were not

considered of concern (USEPA, 2009a). This assessment considers the presence of hydrazine as 

an uncertainty and risk from hydrazine is qualitatively considered in the risk description section 

(Section 4.2.1 and 4.2.2). No degradates were considered residues of concern in this assessment.

Mixtures

Evaluation of pesticide environmental mixtures is beyond the scope of this assessment because of 

myriad factors that cannot be quantified based on the available data. Those factors include 

identification of other possible co-contaminants and their concentrations, differences in the pattern 

and duration of exposure among contaminants, and the differential effects of other 

physical/chemical characteristics of the receiving waters (e.g. organic matter present in sediment 

and suspended water). Evaluation of factors that could influence additivity/synergism is beyond 

the scope of this assessment and the capabilities of the available data to allow for an evaluation. 

However, it is acknowledged that not considering mixtures could over- or under-estimate risks 

depending on the type of interaction and factors discussed above. 

Measures of Exposure

EFED uses standard available models to evaluate potential exposures to aquatic and terrestrial 

organisms as described at http://www.epa.gov/pesticides/science/models_db.htm.

Since the previous exposure assessments were completed (DP 362546; USEPA 2009), the 

following main changes in standard procedures have been made: 

1) Transformation rates for fate studies were recalculated according to the North American Free 

Trade Agreement (NAFTA) Guidance for Evaluating and Calculating Degradation Kinetics 

in Environmental Media (NAFTA, 2011). These updated results will be used in estimating 

model inputs for estimating aquatic exposure. 

The most up to date procedures and models are used in the Registration Review exposure

assessments.
Page 19 of 53

Measures of Effect

Selected toxicity data are used to calculate risk estimates. Any additional information submitted 

by the registrant or found in the open literature are also to be considered. The open literature 

studies are identified using EPA’s ECOTOXicity (ECOTOX)2database, which employs a 

literature search engine for locating chemical toxicity data for aquatic life, terrestrial plants, and 

wildlife. The evaluation of both sources of data can also provide insight into the direct and indirect 

effects of pesticides on biotic communities from loss of species that are sensitive to the chemicals 

and from changes in structure and functional characteristics of the affected communities.

Endangered Species Assessments

Consistent with the Agency’s responsibility under the Endangered Species Act (ESA), the Agency 

evaluates potential risks to listed species from registered uses of pesticides in Registration Review. 

These assessments are conducted in accordance with standard EPA procedures and good scientific 

judgment. The process for evaluating potential risks to listed species is further described in the 

Overview Document at 

http://www.epa.gov/oppfead1/endanger/litstatus/riskasses.htm.

Endocrine Disruptor Screening Program

As required by FIFRA and FFDCA, EPA reviews numerous studies to assess potential adverse 

outcomes from exposure to chemicals. Collectively, these studies include acute, subchronic and 

chronic toxicity, including assessments of carcinogenicity, neurotoxicity, developmental, 

reproductive, and general or systemic toxicity. These studies include endpoints which may be 

susceptible to endocrine influence, including effects on endocrine target organ histopathology, 

organ weights, estrus cyclicity, sexual maturation, fertility, pregnancy rates, reproductive loss, 

and sex ratios in offspring. For ecological hazard assessments, EPA evaluates acute tests and 

chronic studies that assess growth, developmental and reproductive effects in different 

taxonomic groups. As part of the Problem Formulation for Registration Review for maleic 

hydrazide EPA reviewed these data and selected the most sensitive endpoints for relevant risk 

assessment scenarios from the existing hazard database. However, as required by FFDCA 

section 408(p), maleic hydrazide is subject to the endocrine screening part of the Endocrine 

Disruptor Screening Program (EDSP). 

EPA has developed the EDSP to determine whether certain substances (including pesticide 

active and other ingredients) may have an effect in humans or wildlife similar to an effect 

produced by a “naturally occurring estrogen, or other such endocrine effects as the Administrator 

may designate.” The EDSP employs a two-tiered approach to making the statutorily required 

determinations. Tier 1 consists of a battery of 11 screening assays to identify the potential of a 

chemical substance to interact with the estrogen, androgen, or thyroid (E, A, or T) hormonal 

systems. Chemicals that go through Tier 1 screening and are found to have the potential to 

interact with E, A, or T hormonal systems will proceed to the next stage of the EDSP where EPA 

will determine which, if any, of the Tier 2 tests are necessary based on the available data. Tier 2 

 

2 http://cfpub.epa.gov/ecotox/
Page 20 of 53

testing is designed to identify any adverse endocrine-related effects caused by the substance, and 

establish a dose-response relationship between the dose and the E, A, or T effect. 

Under FFDCA section 408(p), the Agency must screen all pesticide chemicals. Between 

October 2009 and February 2010, EPA issued test orders/data call-ins for the first group of 67 

chemicals, which contains 58 pesticide active ingredients and 9 inert ingredients. A second list 

of chemicals identified for EDSP screening was published on June 14, 20133and includes some 

pesticides scheduled for registration review and chemicals found in water. Neither of these lists 

should be construed as a list of known or likely endocrine disruptors. 

For further information on the status of the EDSP, the policies and procedures, the lists of 

chemicals, future lists, the test guidelines and the Tier 1 screening battery, please visit our 

website.4

3. Analysis

3.1. Exposure Characterization

3.1.1. Environmental Fate and Transport Characterization

The major route of dissipation of maleic hydrazide in the environment is by biotic processes. 

Hydrolysis, photodegradation, and volatilization do not appear to play a significant role in the 

dissipation process. The major degradate of maleic hydrazide is CO2. Other nonvolatile 

degradates, maleic acid and maleimide, were identified in small amounts (<5% of the applied) 

which further degrade into CO2.

Maleic hydrazide is fairly non-persistent, with a soil aerobic half-life of <24 hours, and 

moderately persistent under anaerobic soil conditions (half-life 30-60 days). Maleic hydrazide is 

expected to be mobile, due to its low adsorption to soils. Kd values for parent maleic hydrazide 

were 0.14 to 2.61 (MRID# (00151952). Sorption correlates with organic carbon (Koc ranges 

from 11.5 – 70 mL/g); thus and according to the FAO classification, maliec hydrazide is 

considered as mobile in soil (FAO, 2000).

The terrestrial field dissipation half-lives are 3 days on bare ground and 5 days in turf plots, 2-3 

days (sandy loam, potatoes), 7 days (loam, tobacco). 

No available fate data were evaluated for hydrazine, an impurity in technical grade maleic 

hydrazide. Estimated environmental fate values from TOXNET suggest that if released to air, a 

vapor pressure of 14.4 mm Hg at 25 oC indicates that hydrazine will exist solely in the vapor 

phase in the ambient atmosphere. Vapor-phase hydrazine is degraded in the atmosphere by 

reaction with photochemically produced hydroxyl radicals and ozone with estimated half-lives of 

 

3 See http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPPT-2009-0477-0074 for the final second list of 

chemicals.

4 http://www.epa.gov/endo/
Page 21 of 53

about 6 and 9 hours, respectively. Volatilization in water is not expected to be an important 

environmental fate process for either the free base or the protonated species given the estimated 

Henry's law constant of the free base is 6.1X10-7atm-m3/mole and the fact that cations do not 

volatilize. Hydrazine is soluble in water based on an estimated water solubility value of 1X106

mg/L. The estimated half-life of hydrazine in pond water was about 8.3 days and degradation 

occurred through a combination of abiotic and biotic mechanisms.

Surface Water Impact

Although available data indicate that maleic hydrazide degrades rapidly under aerobic conditions 

(observed half-life of <24 hours) and has a half-life of less than two weeks in the field, maleic 

hydrazide residues were mobile to very mobile (up to 100% of the applied radioactivity was

present in the leachate from sandy soil columns). Its persistence under anaerobic conditions 

could pose a risk of contamination of surface waters if maleic hydrazide were washed into 

anaerobic zones by rainfall events that occur soon after application. Because there is a potential 

for surface water runoff, the following label advisory is required for all maleic hydrazide 

products that do not already contain this language (e.g. Reg. Nos. 400-84 and 400-452):

Surface water advisory:

"Under some conditions, maleic hydrazide may have a significant potential for runoff into 

surface water (primarily via dissolution in runoff water), for several days post-application. 

Conditions favoring runoff include poorly draining soils or wet soils with readily visible slopes, 

frequently flooded areas, areas where an intense or sustained rainfall is forecast to occur

within 48 hours, areas overlying extremely shallow ground water, and areas overlying tile 

drainage systems that flow to surface water." 

Table 6 lists the environmental fate properties of maleic hydrazide, along with the major and 

minor degradates detected in the submitted environmental fate and transport studies. 

Table 6. Summary of maleic hydrazide Environmental Fate Properties

Study Value (units) Major Degradates 

Minor Degradates MRID # Study Status and Comments

Hydrolysis 

pH 3: stable

pH 6: stable

pH 9: stable

00143322 Acceptable.

Direct Aqueous 

Photolysis 

(<10% degraded at 28 

days) - pH 5

Stable- pH 7

t 1/2 >30 days at pH 9

42872301

42692101, 

43141001, 

43177301

00141951

Acceptable

Aerobic Soil 

Metabolism (t 1/2 )

<24

hours

Major: CO2

(70.59-72.59% of the 

applied at 90 days 

posttreatment)

Minor: maleic acid

and maleimide, up to 

3.70% and 0.96%

41896201 Acceptable. Up to five

unidentified nonvolatile 

degradates were detected; 

however, none of these 

were present at greater 

than 2.28% of the applied
Page 22 of 53

Study Value (units) Major Degradates 

Minor Degradates MRID # Study Status and Comments

of the applied, 

respectively, each at 

12 hours posttreatment

Anaerobic Soil 

Metabolism

t 1/2 : 30-60 days Major: CO2

(maximum of 68.31-

72.23% of the

applied after 60 days 

of anaerobic 

incubation).

Minor: maleic acid 

and maleimide

(maximums of 2.34% 

and 0.58% of the 

applied, respectively, 

at the start of

anaerobic incubation).

41918201 Acceptable. Minor 

degradates did not persist 

under anaerobic 

conditions

Kd-ads (mL/g) 0.14 to 2.61 00151952 Supplemental. Aged (30-

day) maleic hydrazide 

residues were very mobile 

in sandy loam soil leached 

under fast rain conditions 

(1 inch/hour) with 47.4% 

of the applied

radioactivity detected in 

the leachate. Under slow 

rain conditions

(0.5 inch/day).

Terrestrial Field 

Dissipation (t 1/2 ) 

days

3 (bare ground)

5 (turf plots)

<14 (bare ground)

<27 tobacco plots

42790901 

42744801

42693301

Acceptable. Maleic 

hydrazide was not

detected below the 30-cm 

soil depth at either 

location.

Fish 

Bioaccumulation 

Bioconcentration factor 

BCF (log Kow= -0.6)

00163301 Acceptable

3.1.2. Measures of Aquatic Exposure

Aquatic exposures are quantitatively estimated for all assessed uses using scenarios that 

represent high exposure sites for maleic hydrazide use. 

Aquatic exposure modeling was performed using the tier 1 ecological exposure model Surface 

Water Concentration Calculator (SWCC, 2014), because the toxicity endpoints were all “greater than” 
Page 23 of 53

values in the part-per-million range (Table 12), and the maximum application rate (13 lb/acre) 

was not expected to result in exposures in the ppm range.

Maleic hydrazide environmental fate data used for generating model parameters are listed in 

Table 7. Chemical property input values were chosen in accordance with current input 

parameter guidance (USEPA, 2009b).

Table 7. Summary of SWCC Environmental Fate Data Used for Aquatic Exposure Inputs 

for Maleic hydrazide. 

Fate Property Value (unit) MRID (or source) Comment

Application rate 

(lbai/A) 13 EPA Reg. No 400-94 (1)

No. Applications 1EPA Reg. No 400-94 (1)

Koc 35 (mean of 5 values) 00151952

Surface Water 

Concentration Calculator

User Manual, 2014

Solubility in Water 4,507 mg/L @ 25 oC

Tomlin, C.D.S. (ed.). The 

Pesticide Manual - World 

Compendium, 11 th ed., 

British Crop Protection 

Council, Surrey, England 

1997, p. 757]

Photolysis in Water Stable

42872301

42692101, 43141001, 

43177301

00141951

Aerobic Soil 

Metabolism Half-lives 

3 days

41896201 Surface Water 

Concentration Calculator

User Manual, 2014

Hydrolysis Stable 00143322

Aerobic Aquatic 

Metabolism (water 

column) 

6 days No data

Surface Water 

Concentration Calculator

User Manual, 2014

Chemical Application 

Method Ground (spray) EPA Reg. No 400-94 (1)

Results from the SWCC model are presented in Table 8. Output that generates peak EECs are 

reported in Appendix 1.
Page 24 of 53

Table 8. Tier I Concentrations of Maleic hydrazide in Surface Water.

*: Represents the crop with Maximum application rate (13 lb/acre)

3.1.3. Measures of Terrestrial Exposure

3.1.3.1. Terrestrial Animals

Ingestion of Food Items Based on Foliar Application

Exposure estimates for terrestrial animals assumed to be in the target area or in an area exposed to 

spray drift are derived using the T-REX5 model (version 1.5.2., 6/11/2013). This model 

incorporates the Kenaga nomograph, as modified by Fletcher et al. (1994), which is based on a 

large set of actual field residue data. The upper limit values from the nomograph represent an 

approximation of the highest residue value observed in the data set (Hoerger and Kenaga 1972). 

Consideration is given to different types of feeding strategies for mammals and birds, including 

herbivores, insectivores and granivores. For dose-based exposures, three weight classes of birds 

(20, 100, and 1000 g) and mammals (15, 35, and 1000 g) are considered. Given that no data on 

interception and subsequent dissipation from foliar surfaces are available for maleic hydrazide, a 

default foliar dissipation half-life of 35 days is used based on the work of Willis and McDowell 

(1987). This assessment assumes a maximum application rate of 13 lbs a.i./A and one application 

of maleic hydrazide per year (maximum labeled rate across all active labels) to estimate terrestrial 

exposure for this assessment. The dietary-based EECs (upper bound Kenaga) on a variety of food 

items from the use of maleic hydrazide applied at the maximum label rate is provided in Table 9; 

dose-based EECs are provided in Table 10. 

Table 9. T-REX Calculated Dietary-Based EECs of Maleic Hydrazide on Food Residues.

Food Type Agricultural Fallow/Idleland Use Site

Maximum Dietary-based EECs (mg/kg)

Short Grass 3120

Tall Grass 1430

Broadleaf plants 1755

 

5 USEPA. 2013. Pesticides: Science and Policy. Terrestrial Models, T-REX Version 1.5.2 

http://www.epa.gov/oppefed1/models/terrestrial/

Crop* Peak (μg/L) 21-day average 

(μg/L)

60-day average 

(μg/L)

Agricultural / fallow/ idleland 

conservation reserve 

469 268 125
Page 25 of 53

Fruits/pods/seeds 195

Arthropods 1222

Table 10. T-REX Calculated Maximum Dose-Based EECs of Maleic Hydrazide Uses on 

Terrestrial Food Residues (Agricultural Fallow/Idleland/Conservation Reserve Use Sites)

Food Type

Dose-based EECs

(mg/kg-bw)

(birds)

Dose-based EECs

(mg/kg-bw)

(mammals)

Small

(20 g)

Medium

(100 g)

Large

(1000 g)

Small

(15 g)

Medium

(35 g)

Large

(1000 g)

Short grass 3553.4 2026.3 907.2 2974.7 2055.9 476.7

Tall grass 1628.6 928.7 415.8 1363.4 942.3 218.5

Broadleaf plants 1998.8 1139.8 510.3 1673.3 1156.5 268.1

Fruits/pods 222.1 126.6 56.7 185.9 128.5 29.8

Arthropods 1391.7 793.6 355.3 1165.1 805.2 186.7

Seeds (granivore) 49.4 28.1 12.6 41.3 28.6 6.6

3.1.3.2. Terrestrial Plants

Maleic hydrazide exposure to terrestrial and semi-aquatic plants is estimated using the TerrPlant6

(version 1.2.2) model. The model generates EECs for plants residing near a use area that may be 

exposed via runoff and/or spray drift. The EECs are generated from one application at the 

maximum rate for a particular use and compound-specific solubility information. Only a single 

application is considered because it is assumed that for plants, toxic effects are likely to manifest 

shortly after the initial exposure and that subsequent exposures do not contribute to the response.

Hence, the model estimates EECs based on application rate, the solubility factor, and default 

assumptions of drift. The EECs for terrestrial and semi-aquatic plants for a single application of 

maleic hydrazide at the maximum label rate are presented in Table 11. 

Table 11. Max EECs for Terrestrial and Semi-Aquatic Plants Near Maleic Hydrazide Use 

Areas.

Single Max. Application Rate

(lbs a.i./A, Agricultural 

Fallow/Idleland/Conservation 

Reserve Use Sites)

EECs (lbs a.i./A)

Total Loading to 

Adjacent Dry Areas 

(sheet runoff + drift)

Total Loading to 

Semi-Aquatic Areas 

(channelized runoff 

+ drift)

Drift EEC

Ground 

spray

Aerial 

spray

Ground 

spray

Aerial 

spray

Ground 

spray

Aerial 

spray

13 0.78 1.3 6.63 7.15 0.13 0.65

 

6 USEPA. 2013a. Pesticides: Science and Policy. Terrestrial Models, TerrPlant Version 1.2.2. 

http://www.epa.gov/oppefed1/models/terrestrial/
Page 26 of 53

 3.2. Ecological Effects Characterization

The ecological effects characterization for maleic hydrazide is based on data generated from 

guideline studies submitted by the registrant and from open literature studies in the ECOTOX 

database (http:/www/epa.gov/ecotox). 

A brief description of available aquatic and terrestrial toxicity data used to calculate RQs is 

provided below in Sections 3.2.1 and 3.2.2. As noted in Section 3.2.3, there are currently no 

reported ecological incidents involving maleic hydrazide.

3.2.1. Aquatic Effects Characterization

Aquatic toxicity data for animals and plants are summarized in Table 12. For freshwater and 

estuarine/marine fish and invertebrates (and for aquatic-phase amphibians, for which fish are 

considered surrogates), the compound is considered practically non-toxic. Several of the aquatic 

studies conducted using maleic hydrazide experienced highly acidic conditions in the highest 

treatment concentrations tested and adequately buffered maleic hydrazide standards were not used. 

Where this occurred, it is unclear whether effects observed were due to the inherent toxicity of the 

test chemical or were rather due to the acidity in the test system. However, treatment related effects 

were not seen in these studies except at very high concentrations that are unlikely to be achieved 

under current use patterns. 

The most sensitive acute endpoint for estuarine/marine fish was an LC50 > 104,000 μg a.i./L (the 

highest treatment level tested) for sheepshead minnow in a study using maleic hydrazide’s 

potassium salt where no mortality or sublethal effects were observed at any test concentration. In 

a recently submitted acute study on the fathead minnow (MRID 48876701), the LC50 was also 

considered non-definitive due to the highly acidic concentrations at the higher test concentrations. 

However, no treatment related mortalities or sublethal effects were seen at concentrations up to 

240,000 μg a.i./L in this study. It was unclear whether the mortality and sublethal effects, 

including fish on the bottom of the test vessel and fin deterioration, observed at the higher test 

concentrations were due to the acidic conditions (pH declined to below 5.0 at 460,000 and 930,000 

μg a.i./L) or to the toxicity of the chemical at those concentrations. An acute toxicological study 

with the potassium salt did not indicate any toxicity differentials between the salt and the acid 

(MRIDs 00146142, 00124739) to rainbow trout. On a chronic exposure basis, maleic hydrazide 

was tested at concentrations up to 9,600 μg a.i./L without any treatment related effects to fathead 

minnow. No chronic studies have been conducted with an estuarine/marine fish species and an 

ACR (acute to chronic ratio) approach cannot be used due to nondefinitive endpoints. However, 

considering the lack of acute effects in both freshwater and saltwater fish species (and no other 

indication that estuarine/marine species may be more sensitive compared to freshwater species), 

and the lack of chronic effects observed in the fathead minnow early life stage study, the chronic 

NOAEC established for freshwater fish species (9,600 μg a.i./L) will also be used to assess risk to 

estuarine/marine fish species in this risk assessment.

In an acute freshwater invertebrate study with Daphnia magna (MRID 00124741), the EC50 was 

determined to be 107,500 μg a.i./L, and there was no increase in mortality compared to controls at 
Page 27 of 53

concentration less than 100,000 μg a.i./L. It was unclear whether some of the mortality observed 

at the higher test concentrations (100,000 and 180,000 μg a.i./L) was due to the toxicity of the test 

compound or the acidic conditions present at the highest test concentrations (pH declined to 5.41 

in the highest test concentration). The potassium salt of maleic hydrazide did not appear to have 

any increased toxicity to daphnids with an LC50 greater than 1,000,000 μg a.i./L (MRID 

00146143). On a chronic exposure basis, maleic hydrazide was tested at concentrations up to 

99,000 μg a.i./L without any treatment related effects to daphnids (MRID 48876702). Similarly, 

no mortality or sublethal effects were observed in an acute mysid study (MRID 44411103) using 

the potassium salt, tested at concentrations up to 103,000 μg a.i./L. No mortality was observed in 

an acute eastern oyster study (MRID 44411102), and the IC50 was considered to be > 111,000 μg 

a.i./L however a statistically significant (18%) inhibition on shell growth was observed above the 

73,200 μg a.i./L (LOAEC = 111,000 μg a.i./L) treatment level. No chronic studies have been 

conducted with an estuarine/marine invertebrate and an ACR approach cannot be used due to 

nondefinitive endpoints. However, considering the lack of acute effects in both freshwater and 

saltwater invertebrate studies and the lack of chronic effects in the daphnid life cycle test, the 

chronic NOAEC established for freshwater invertebrate species (99,000 μg a.i./L) will also be used 

to evaluate risk for estuarine/marine species in this risk assessment. 

Aquatic vascular and nonvascular plants were also relatively insensitive to maleic hydrazide. The 

14-day NOAEC and EC50 values for the vascular aquatic plant duckweed (Lemna gibba) were 

23,700 μg a.i./L and 114,000 μg a.i./L, respectively (MRID 43006904) based on reduced front 

count. Of the four nonvascular algal species tested, none had a definitive EC50 value, with 

nondefinite values ranging from > 9,840 to > 102,000 μg a.i./L.. The most sensitive definitive 

NOAEC value for a nonvascular aquatic plant was for blue-green algae (Anabaena flos aquae) 

with a NOAEC of 60,000 μg a.i./L based on reduced cell density at the LOAEC of 95,000 μg 

a.i./L.

Table 12. Summary of Most Sensitive Acute and Chronic Toxicity Data for Aquatic 

Animals Exposed to either Maleic Hydrazide Acid or Potassium Salt. Bolded values 

represent the endpoints used for risk estimation.

Species

TGAI/

TEP 

(% a.i.)

Acute Toxicity Chronic Toxicity

96-hr 

LC50/EC50

(μg a.i./L)

48-hr EC50

(μg a.i./L)

Toxicity 

Classification 

(MRID, study 

classification)

NOAEC/

LOAEC

(μg a.i./L)

Endpoints

(MRID, study 

classification)

Rainbow trout

Oncorhynchus mykiss

TGAI 

%a.i. not 

reported 1,608,000 --

Practically nontoxic

(00124739, 

Acceptable)

No Data N/A

Fathead minnow

Pimephales promelas

TGAI

100% a.i.

Acid > 240,000 - -

Practically nontoxic 

(48876701, 

Supplemental--

Qualitative)

9,600/

> 9,600

None affecected

(48876702, 

Acceptable)

Waterflea

Daphnia magna

TGAI

% a.i. not 

reported -- 107,500

Practically nontoxic 

(00124741, 

Acceptable)

99,000/

 > 99,000

None Affected 

(48877502, 

Acceptable)
Page 28 of 53

Species

TGAI/

TEP 

(% a.i.)

Acute Toxicity Chronic Toxicity

96-hr 

LC50/EC50

(μg a.i./L)

48-hr EC50

(μg a.i./L)

Toxicity 

Classification 

(MRID, study 

classification)

NOAEC/

LOAEC

(μg a.i./L)

Endpoints

(MRID, study 

classification)

Sheepshead minnow

Cyprinodon variegatus

TGAI 

98.98% a.i.

Potassium 

Salt

> 104,000 --

Practically nontoxic

(44411101, 

Supplemental--

Quantitative)

No Data N/A

Mysid shrimp

Americamysis bahia

TGAI 

98.98% a.i. 

Potassium 

Salt

> 103,000 --

Practically nontoxic

(44411103)

No Data N/A

Eastern oyster

Crassostrea virginia

TGAI 

98.98% a.i.

Potassium 

Salt

> 111,000 --

Practically nontoxic 

(44411102)

No Data N/A

Aquatic Plants

Duckweed 

Lemna gibba

TGAI

98.98% a.i.

Potassium 

Salt N/A N/A N/A

14-day EC50

= 114,000

14-d 

NOAEC/

LOAEC = 

23,700/

38,600

Frond numbers 

(43006904, 

Acceptable)

Blue-green algae

Anabaena flos-aquae

TGAI

98.98% a.i.

Potassium 

Salt N/A N/A N/A

96-hr IC50 > 

95,000

NOAEC/

LOAEC =

60,000/

95,000

Cell numbers

(43006901, 

Acceptable)

3.2.2. Terrestrial Effects Characterization

Toxicity values for terrestrial animals and plants are summarized in Tables 13 and 14, 

respectively. Maleic hydrazide is practically non-toxic to mammals, birds (and to reptiles and 

terrestrial-phase amphibians for which birds serve as surrogates), and honey bees (Apis mellifera)

on an acute exposure basis. No sublethal effects or mortalities were observed in either the acute 

bird or mammalian studies. In one avian acute oral study (MRID 00124742), a slight, nonstatistically significant reduction in body weight gain was observed at the 2150 and 4640 mg/kg 

treatment levels. Sub-acute dietary studies with both the bobwhite and mallard (MRIDs 00126033

and 00107417) found no mortality or sublethal effects at the highest treatment level tested (10,000 

mg/kg-diet). The non-definitive highest treatment concentrations from these acute studies will be 

conservatively used as the acute endpoints (LD50/LC50) for risk estimation in this risk assessment.

In a chronic two generation mammalian reproduction study with two litters per generation, the 

potassium salt of maleic hydrazide was incorporated continuously in the feed of male and female 

Sprague-Dawley rats for two successive generations at dietary concentrations of 0,1000, 10,000, 

30,000 and 50,000 ppm. The 50,000 ppm treatment group was discontinued during the study after 
Page 29 of 53

the F1 generation due to decreased body weights noted in both the 30,000 and 50,000 ppm F0 adults 

and F1 pups during lactation. The NOAEL, based on reduced body weights of F0 females at 

30,000 ppm (8.5% inhibition) was considered to be 10,000 ppm. Additional effects observed at 

the 50,000 ppm treatment level included F1 pup weights (21—24% inhibition) and F2 pup weights 

(9-11% inhibition). No reproductive effects were observed in either male or female animals at any 

dietary level.

No chronic avian reproduction data is available. The 1994 RED stated that “Avian reproduction 

studies are not required for maleic hydrazide because it is not applied more than once prior to or 

during the breeding season, and it is practically nontoxic to birds” and the 2009 Problem 

Formulation (USEPA, 2009) did not indicate this as a data gap. Given a lack of acute mortality or 

sublethal effects in all tested terrestrial animals and no terrestrial animal incident data, there is no 

indication of a strong likelihood of chronic effects to avian species at the concentrations of maleic 

hydrazide expected on the field or dietary items. No reproductive effects were observed in the 

mammalian chronic study discussed above. However, due to the lack of chronic avian data and 

the observed effects on body weight in the mammalian chronic reproduction study (discussed 

above), there is uncertainty around the potential for chronic effects to birds and chronic risk to 

avian species cannot be precluded.

In an acute study with honey bees (MRID 00018842) exposed to the MH-30© formulation (% a.i. 

not reported), the LD50 was reported to be > 36.26 μg/bee, with only 3.2% mortality at the limit 

concentration. Two nonguideline studies (MRIDs 44990001 & 44990002) were submitted with 

additional terrestrial invertebrate data. Parasitic wasps (Aphidius rhopalosiphi) and predatory 

mites (Typhlodromus pyri) exposed to the Royal MH-60 SG formulation (60.2% ai maleic 

hydrazide, formulation not registered in the U.S.) applied at 4.45 lbs ai/A experienced 68% 

mortality in wasps after 48 hours and 47% mortality of mites after 24 hours (50% mortality after 

7 days). Additional sublethal effects included reduced coordination in the wasp study. 

Tier II seedling emergence and vegetative vigor testing was conducted with a typical end-use 

product (TEP) containing 30.2% maleic hydrazide potassium salt (23% maleic hydrazide acid 

equivalents). Onion was the most sensitive monocot species in both the seedling emergence and 

vegetative vigor studies with a NOAEC and EC25 of 0.38 and 0.53 lbs ai/A, respectively, for 

seedling emergence and 0.38 and 1.44 lbs ai/A, respectively, for vegetative vigor. Tomato was 

the most sensitive dicot in the seedling emergence test with a NOAEC and EC25 of 0.75 and 1.03, 

respectively. Cabbage was the most sensitive dicot in the vegetative vigor study with a NOAEC 

and EC25 of 0.75 and 1.10, respectively. 
Page 30 of 53

Table 13. Maleic Hydrazide Acid and Potassium Salt Terrestrial Animal Toxicity 

Summary. Bolded values indicate endpoints used in risk estimation.

Parameter Study Type Species Exposure 

Duration

TGAI/

TEP 

(% a.i.)

Toxicity 

Value/End

point 

Affected

Reference

Toxicity 

Classification/

Study 

Classification

1. 

Abundance 

(i.e., survival, 

reproduction, 

and growth) 

of individual 

birds

1a. Acute 

(Dosebased)

Mallard duck

Anas 

platyrhynchos

Single oral 

dose

TGAI

% a.i. not 

reported

Acid

>4,640 

mg/kg bw 

(LD50)/

No effect

00124742

Practically 

non-toxic/

Acceptable

Mallard duck

A. platyrhynchos

Single oral 

dose

TEP

34.5% a.i.

Potassium 

Salt

> 2,250 

mg/kg bw

(LD50)/No 

effect

00146141

Practically 

non-toxic/

Acceptable

Canary

Serinus canaria

Single oral 

dose

TGAI 

100% a.i.

Acid

>2,000

mg/kg bw

(LD50)/No 

effect

48877501

Practically 

non-toxic/

Acceptable

1b. Acute 

(Dietarybased)

Northern 

Bobwhite 

Colinus 

virginianus

8-Day 

dietary

TGAI

% a.i. not 

reported

> 10,000 

mg/kg diet 

(LC50)/No 

effect

00126033

Practically 

non-toxic/

Acceptable

Mallard duck

A. platyrhynchos

8-Day 

dietary

TGAI

% a.i.

Acid

> 10,000 

mg/kg diet 

(LC50)/ No 

effect

00107417

Practically 

non-toxic/

Acceptable

Mallard duck

A. platyrhynchos

8-Day 

dietary

TEP

34.5% a.i.

Potassium 

Salt

>5620

mg/kg diet 

(LC50)/No 

effect

00147000

Practically 

non-toxic/

Acceptable

1c. Chronic 

(Dietarybased) 

Northern 

bobwhite (C. 

virginianus) and 

Mallard Duck (A. 

platyrhynchos)

Avian

reproduction 

study

No Data No Data N/A

2. 

Abundance 

(i.e., survival, 

reproduction, 

and growth) 

of individual 

mammal

2a. Acute Rat (Rattus 

norvegicus)

Single oral 

dose

>5,000

mg/kg bw 

(LD50)

00079657, 

00079658,

41185401, 

00079661,

00079660, and 

41289101

Practically 

non-toxic/

Acceptable
Page 31 of 53

Parameter Study Type Species Exposure 

Duration

TGAI/

TEP 

(% a.i.)

Toxicity 

Value/End

point 

Affected

Reference

Toxicity 

Classification/

Study 

Classification

2b. Chronic Rat (Rattus 

orvegicus)

2-Generation 

reproduction 

study

10,000 

mg/kg-diet

(NOAEL)

LOAEL = 

30,000 

mg/kg-diet 

based on 

decreased

F0 body 

weight

00128720

(Coreminimum)

3. Survival of 

beneficial 

insects 

3a. Acute 

Contact

Honey Bee 

(Apis mellifera)

48 hours

TEP

% a.i. not 

reported

> 36.26

μg/bee

(LD50)/

Mortality 

00018842

Practically 

non-toxic/ 

Acceptable

Table 14. Summary of Tier II Terrestrial Plant Toxicity Data for Maleic Hydrazide 

Potassium Salt

Crop Species EC25 (lbs 

a.i./A)

NOAEC (lbs 

a.i./A)

Most 

sensitive 

parameter

MRID

Seedling Emergence

Monocots

Oat 1.35 < 0.375 Plant Height 41289301

Ryegrass 2.52 1.5 Plant Height 41289301

Corn 3.02 1.5 Plant Height 41289301

Onion 0.53 0.38 Plant Height 41289301

Dicots

Lettuce 2.77 1.5 Plant Height 41289301

Soybean 1.09 0.75 Dry Weight 41289301

Carrot >6.0 6.0 NA 41289301

Tomato 1.03 0.75 Dry Weight 41289301

Cucumber >6.0 6.0 NA 41289301

Cabbage 1.07 0.75 Dry Weight 41289301

Vegetative Vigor

Monocots

Oat 1.95 1.50 Plant Height, 

Dry Weight 41289302

Ryegrass 4.87 3.00 Plant Height 41289302

Corn 4.01 1.50 Plant Height 41289302

Onion 1.44 0.38 Plant Height 41289302

Dicots

Soybean 2.31 0.38 Phytotoxicity 41289302

Carrot 4.73 0.75 Phytotoxicity 41289302

Tomato 4.71 1.50 Plant Height, 

Phytotoxicity 41289302

Cucumber 4.63 1.50 Plant Height, 41289302
Page 32 of 53

Phytotoxicity

Cabbage 1.10 0.75 Dry Weight, 

Phytotoxicity 41289302

Lettuce 2.22 0.75 Plant Height, 

Dry Weight 41289302

3.2.3. Review of Incident Data

A review of the Environmental Incident Information System (EIIS, version 2.1.1) database and the 

Incident Data System (IDS) database, which are maintained by the Agency’s Office of Pesticide 

Programs, and the Avian Monitoring Information System (AIMS), which is maintained by the 

American Bird Conservancy, for ecological incidents involving maleic hydrazide acid and 

potassium salt was completed on March 11, 2014. The results of this review show no reported 

incidents.

However, because of limitations in the incident reporting system, the lack of incident reports 

cannot be construed as the absence of incidents from the registered use of maleic hydrazide. 

Reporting by states is only voluntary, and individuals discovering incidents may not be informed 

on the procedure of reporting these occurrences. Additionally, much of these databases are

generated from registrant-submitted incident reports. In addition, incident reports for non-target 

organisms typically provide information only on mortality events and plant damage incidents. 

Impacts to non-commercial beneficial insects and other invertebrates are not likely to be reported. 

Except for phytotoxic effects in terrestrial plants, sublethal effects for organisms such as reduced 

growth or impaired reproduction are rarely reported. Because of these logistical difficulties, EIIS 

is most likely a minimal representation of all pesticide-related ecological incidents and the absence 

of incident reports cannot be construed as the absence of incidents.

4. Risk Characterization

4.1. Risk Estimation

4.1.1. Aquatic Organisms 

The peak EECs in surface water generated from the SWCC model (using the maximum application 

rate from agricultural fallow/idle land/conservation reserve, the use site with the highest rates) 

represents acute exposure to fish, aquatic invertebrates, and aquatic plants, and the highest 21-day 

and 60-day average EECs represents chronic exposure to aquatic invertebrates and fish, 

respectively. Acute and chronic RQs for freshwater and estuarine/marine animals and aquatic 

vascular and nonvascular plants are summarized in Table 15.
Page 33 of 53

4.1.1.1. Aquatic Animals

Fish

There were no exceedances of any LOC for acute or chronic risk to freshwater or estuarine/marine 

fish. Since no chronic data was available for estuarine/marine fish and no acute mortality or 

sublethal effects were observed in either the acute freshwater or saltwater fish studies and neither 

were effects observed in the chronic freshwater fish early life stage study, the chronic endpoint 

from the fathead minnow ELS study was used to estimate chronic risk to estuarine/marine fish. 

Aquatic invertebrates

There were no exceedances of any LOC for acute or chronic risk to freshwater or estuarine/marine 

invertebrates. Since no chronic data was available for estuarine/marine invertebrates and no acute 

mortality or sublethal effects were observed in either the acute daphnid or mysid studies, the 

chronic endpoint from the daphnid life cycle study was used to estimate chronic risk 

estuarine/marine invertebrates. 

Aquatic Plants

As shown in Table 15, RQs for vascular and nonvascular plants do not exceed any LOCs (1.0) for 

any of the currently registered uses of maleic hydrazide, as evidenced by the RQs derived from 

the most conservative exposure scenario (i.e., Agricultural Fallow/Idleland/Conservation Reserve

at the maximum rate of 13 lbs a.i./A), which do not result in LOC exceedances. 

Table 15. Summary of Acute and Chronic RQs for Freshwater and Estuarine/Marine Fish,

Invertebrates and Aquatic Plants Exposed to Maleic Hydrazide (Maximum EECs based on

the Agricultural Fallow/Idleland/Conservation Reserve Use Pattern)

Species Toxicity 

Endpoint

(ppb)

Maximum

EECs

Maximum 

RQs

Freshwater Fish: 

Fathead Minnow

(Pimmephales promelas)

Acute > 240,000 469 < 0.01

Chronic 9,600 125 0.01

Saltwater Fish: 

Sheepshead minnow 

(Cyprinodon variegatus )

Acute > 104,000 469 < 0.01

Chronic 9,6001 125 0.01

Freshwater Invertebrates:

Waterflea

(Daphnia magna)

Acute 107,500 469 < 0.01

Chronic 99,000 268 < 0.01

Saltwater Invertebrates: 

Mysid shrimp 

(Americamysis bahia)

Acute > 103,000 469 < 0.01

Chronic 99,0002 268 < 0.01

Aquatic Non-Vascular 

Plant

Blue-green algae

(Anabaena flos-aquae)

NonListed> 95,000 469 < 0.01

Listed 60,000 469 < 0.01

Aquatic Vascular Plants:

Duckweed (Lemna gibba)

Non-listed 114,000 469 < 0.01

Listed 23,700 469 0.02
Page 34 of 53

* EECs are estimated surface water concentrations based on SWCC model runs. 

1 No chronic data was available for estuarine/marine fish. Since no acute effects were observed in either the 

acute freshwater or saltwater fish studies, the chronic freshwater fish NOAEC is used in place of an

estuarine/marine fish NOAEC.

2 No chronic data was available for estuarine/marine invertebrates. Since no acute mortality was observed in 

either the acute freshwater or saltwater invertebrate studies, the chronic freshwater NOAEC is used in place 

of an estuarine/marine invertebrate NOAEC.

4.1.2. Terrestrial Organisms

4.1.2.1. Terrestrial Animals

Ingestion of Food Items Based on Foliar Application

Based on the available toxicity data, no acute mortality and/or sublethal effects were observed in 

any of the avian or mammalian studies at the highest levels tested. Only non-definitive (“greater 

than”) acute and sub-acute toxicity endpoints are available for birds and mammals; therefore, the

acute RQs presented in Table 16 are also non-definitive (“less than”) maximum RQs. Further 

discussion of the predicted exposure values relative to the levels at which no mortality occurred is 

provided as part of the risk description in Section 4.2.2. No chronic avian data was available to 

quantitatively evaluate chronic risk to birds, therefore chronic risk to birds is uncertain.

Using the non-definitive acute oral LD50 as a conservative endpoint for birds (the canary LD50 of 

> 2,000 mg/kg-bw and the test organism body weight of 24.5 g) and for the maximum 

application rate across all use sites (13 lbs ai/A for Agricultural Fallow), exceedances of the nonlisted species dose-based LOC (0.5) occur for small birds feeding on short grass, tall grass, 

broadleaf plants and arthropods (RQs range from 0.72 to 1.83), and medium birds feeding on 

short grass (RQ = 0.82). The listed species dose-based LOC (0.1) is exceeded for small birds 

feeding on all dietary items except for granivores (RQs range from 0.11 to 1.83), medium birds 

feeding on all dietary items except for fruits/pods/seeds and granivores (RQs range from 0.32 to 

0.82), and large birds feeding on all dietary items except for fruits/pods/seeds and granivores 

(RQs range from 0.10 to 0.26). Using the non-definitive sub-acute dietary endpoint for birds, the 

listed species dietary-based LOC (0.1) is exceeded for birds consuming all dietary items except 

for fruits/pods/seeds (RQs range from 0.02 to 0.31), but the non-listed species LOC (0.5) was 

only marginally exceeded for birds feeding on short grass (RQ = 0.56).

Using the non-definitive acute endpoints for mammals and for the maximum application rate 

across all use sites (13 lbs ai/A for Agricultural Fallow), there were no exceedances of the nonlisted species LOC. Exceedances of the listed species LOC (0.1) were generally marginal and 

occured for all size classes of mammals feeding on short grass (RQs range from 0.12 to 0.27), 

small and medium mammals feeding on tall grass and broadleaf plants (RQs range from 0.11 to 

0.15), and small mammals feeding on arthropods (RQ = 0.11). The chronic dose-based LOC

(1.0) is exceeded for all size classes of mammals consuming short grass (RQs range from 1.24 to 

2.71, small and medium mammals consuming broadleaf plants and tall grass (RQs range from 

1.06 to 1.52, respectively) and small mammals consuming arthropods (RQ = 1.06) and the 

chronic dietary-based LOC (1.0) was not exceeded for mammals (RQs 0.02—0.31)
Page 35 of 53

Lower application rates associated with other uses of maleic hydrazide result in less uncertainty 

regarding the potential for risk. As the resulting EECs from lower application rates decrease and 

approach the highest doses tested in the non-definitive acute studies, EFED has greater 

confidence that the potential for risk is low. For example, under the ornamental and shade trees 

use pattern (maximum application rate of 9.2 lbs ai/A), there is no longer any expectation for 

acute or chronic risk to large mammals feeding on short grass as the non-definitive endpoints 

exceed the EEC for this use. Under the tobacco use pattern (maximum application rate of 6.1 lbs 

a.i./A) anticipated acute risk becomes very low to birds from dietary exposure to residues on 

broadleaf plants as the non-definitive endpoints exceed the EEC for this use site. Under the 

Ornamental Woody Shrubs use pattern (maximum application rate of 4.6 lbs a.i./A, anticipated 

acute risk becomes very low for large birds or medium mammals and chronic risk would not be 

expected for any mammalian size classes. Under the potato use pattern (maximum application 

rate of 4.3 lbs ai/A), the anticipated risk becomes low for all listed and non-listed mammals. 

Under the onion use pattern (maximum application rate of 2.9 lbs ai/A, the anticipated acute risk 

becomes low for all non-listed avian species. A full comparison of what exceedances occur 

under the different use patterns is described in the beginning of this assessment in Table 1 and in 

Appendix 3.
Page 36 of 53

Table 16. Maximum Acute and Chronic Risk Estimates to Birds and Mammals from the Maximum Application Rates by Use 

Pattern

Use Site (Maximum Application Rate)

Agricultural 

Fallow 

(13 lbs ai/A)

Ornamental 

Shade 

Trees

(9.2 lbs 

ai/A)

Golf Course 

Turf (8 lbs 

ai/A)

Tobacco

(6.1 lbs 

ai/A)

Ornamental 

Woody 

Shrubs (4.6 

lbs ai/A)

Potato

(4.3 lbs 

ai/A)

Onion (2.9 

lbs ai/A)

Acute: Dose-Based

Birds 20 g Short Grass, Tall Grass, Broadleaf 

Plants, Fruits/Pods, Arthropods

0.03—1.83 0.02—1.30 0.02—1.13 0.01—0.86 0.01—0.65 0.01—0.61 0.01—0.41

100 g Short Grass, Tall Grass, Broadleaf 

Plants, Arthropods

0.01—0.82 0.01—0.58 0.01—0.50 0.01—0.38 <0.01—0.29 <0.01—0.27 <0.01—0.18

1000 g Short Grass, Tall Grass, Broadleaf 

Plants, Arthropods

<0.01—0.26 < 0.01—

0.18

<0.01—0.16 <0.01—0.12 <0.10 <0.10 <0.07

Mammals 15 g Short grass, Tall Grass, Broadleaf 

Plants, Arthropods

<0.01—0.27 <0.01—

0.19

<0.01—0.17 <0.01—0.13 <0.01—0.10 <0.10 <0.07

35 g Short Grass, Tall Grass, Broadleaf 

Plants

<0.01—0.23 <0.01—

0.16

<0.01—0.14 <0.01—0.11 <0.08 <0.09 <0.06

1000 g Short Grass <0.01--0.12 <0.01—

0.09

<0.08 <0.06 <0.04 <0.05 <0.04

Acute: 

Dietary-Based

Birds N/A Short Grass, Tall Grass, Broadleaf 

Plants, Arthropods

0.02—0.56 0.01—0.39 0.01—0.34 0.01—0.26 0.01—0.20 0.01—0.18 0.01—0.12

Chronic: Dose-Based

Mammals

15g

Short Grass, Tall Grass, Broadleaf 

Plants, Arthropods 0.04—2.71 0.03—1.92 0.02—1.67 0.02—1.27 <0.97 <0.91 <0.61

35g Short Grass, Tall Grass Broadleaf 

Plants 0.03—2.31 0.02—1.64 0.02—1.42 0.02—1.08 <0.83 <0.77 <0.53

1000g Short Grass 0.02--1.24 0.01—0.88 <0.76 <0.58 <0.45 <0.42 <0.29

1 Food indicates the dietary items for which LOC exceedances occur for the highest use pattern (Agricultural Fallow) only.

2 RQs in bold exceed the non-listed species LOC. RQs in bold italics exceed only the listed species LOC.
Page 37 of 53

4.1.2.3. Terrestrial Invertebrates

On an acute exposure basis, maleic hydrazide is classified as ‘practically nontoxic’ to non-target 

terrestrial insects including honey bees (honey bee acute contact LD50 >36.26 μg ai/bee, with only 

3.2% mortality at the limit dose). If an RQ were calculated for terrestrial invertebrates7,

conservatively assuming the LD50 to be 36.26 μg ai/bee and the acute contact exposure to be the 

application rate (13 lbs a.i./A) * 2.7 μg a.i./bee or 35.1 μg, the resultant RQ would be 0.97 or 

approximately two times the proposed terrestrial invertebrate LOC (0.4). The proposed non-listed 

species LOC would also be exceeded for use patterns with maximum application rates greater than 

6.1 lbs a.i./A (e.g. tobacco with an RQ > 0.45). The ornamental woody shrubs and vines, potato, 

onion and citrus use patterns would not exceed the proposed non-listed species LOC, but would 

exceed the interim listed species LOC of 0.05 (RQs would range from 0.13–0.30). A full 

comparison of what exceedances occur and the resulting risk picture under the different use 

patterns is described at the beginning of this assessment in Table 1 and Appendix 3.

4.1.2.4. Plants

The RQs for terrestrial and semi-aquatic plants for a single application of maleic hydrazide at the 

maximum labeled rate of 13 lbs a.i./A under both ground and aerial application scenarios is 

presented in Table 17. RQs were exceeded for listed and non-listed monocots in both dry and 

semi-aquatic areas under both ground and aerial application scenarios. RQs were also exceeded 

for listed and non-listed monocots due to spray drift for aerial applications only. For dicot plant 

species, RQs were exceeded for listed and non-listed species in semi-aquatic areas from both 

ground and aerial application scenarios, but RQs were only exceeded for listed and non-listed 

species in dry areas under aerial application scenarios and marginally exceeded (RQ = 1.04) for 

listed dicots under ground application scenarios. Spray drift alone did not result in any 

exceedances for dicot species. 

Risk to plant species is may be less for other use patterns. For example, for ornamental and/or 

shade trees (maximum application rate of 9.2 lbs a.i./A applied aerially), non-listed dicots are no 

longer at risk in dry areas (RQ = 0.89) and non-listed monocots no longer exceed the spray drift 

LOC (RQ = 0.87). Under the golf course turf use pattern (maximum application rate of 8.0 lbs 

a.i./A), using ground applications only, non-listed monocots in dry areas no longer exceed the LOC 

(RQ = 0.91). Under the tobacco use pattern (maximum application rate of 6.1 lbs a.i./A), using 

ground applications only, no listed or non-listed monocot or dicot in dry areas would exceed the 

LOC (Max RQ = 0.96) while for aerial application scenarios there would no longer be exceedances 

for listed dicots in dry areas or listed monocots from spray drift alone. For the Ornamental Woody 

Shrubs and vines use pattern (maximum application rate of 4.6 lbs a.i./A) with aerial applications, 

non-listed monocots in dry habitats would no longer be at risk. For aerial applications on onions 

(maximum application rate of 2.9 lbs a.i./A), listed monocots in dry areas would also no longer 

have exceedances. However, even if applied by ground applications only, both listed and nonlisted monocots and dicots in semi-aquatic areas would be considered at risk under the maximum 

 

7 The employed methodology for calculating an RQ for pollinators was first proposed in the 2012 Pollinator Risk 

Assessment Framework SAP White Paper, and subsequently validated by the panel. The White Paper is accessible 

online at regulations.gov, under docket EPA-HQ-OPP-2012-0543
Page 38 of 53

application rate in all use patterns, except for non-listed dicots (RQ of 0.94) under the citrus use 

pattern only. A full comparison of what exceedances occur under the different use patterns is 

described in the executive summary in Table 1 and in Appendix 3.

Table 17. RQ values for plants in dry and semi-aquatic areas exposed to Maleic Hydrazide through 

runoff and/or spray drift from the maximum application rate of 13 lbs a.i./A for Agricultural Fallow.

Plant Type Listed Status

Dry Semi-Aquatic Spray Drift

Ground Aerial Ground Aerial Ground Aerial

Monocot non-listed 1.47 2.45 12.51 13.49 0.25 1.23

Monocot Listed 2.05 3.42 17.45 18.82 0.34 1.71

Dicot non-listed 0.76 1.26 6.44 6.94 0.13 0.63

Dicot listed 1.04 1.73 8.84 9.53 0.17 0.87

*If RQ > 1.0, the LOC is exceeded, resulting in potential for risk to that plant group.

4.2. Risk Description

The results of this screening-level risk assessment indicate that the registered uses of maleic 

hydrazide do not have the potential to cause direct adverse acute or chronic effects to freshwater 

and estuarine/marine fish and invertebrates or adverse effects to aquatic vascular and non-vascular 

plants. Maleic hydrazide is not acutely toxic to avian or mammalian species, however doses tested 

were not high enough to capture toxic effects at predicted maximum EECs. As a result, since 

terrestrial EECs exceed 1/10 and 1⁄2 the tested doses, acute risk to listed and non-listed avian and 

mammalian species cannot be precluded, though the levels of risk estimated may be overly 

conservative. Chronic risk is indicated for mammals, though this varies depending on the size 

class and dietary item consumed. Chronic risk to birds is uncertain due to the lack of avian 

reproduction data. 

Risk to monocots is indicated regardless of habitat or application method, although spray drift 

alone will not result in risk when only ground applications are used. Non-listed dicots are mainly

considered at risk in semi-aquatic habitats and where aerial applications are used in dry areas (there 

is only a marginal exceedance, RQ = 1.04 for listed dicots in dry areas under ground application 

scenarios). 

4.2.1. Risks to Aquatic Organisms

Predicted EEC values are expected to be several orders of magnitude lower than any ecotoxicity 

endpoints for aquatic animals and plants. Although no chronic data are available for a saltwater 

fish species, the freshwater fish NOAEC was used for risk estimation. Estuarine/marine fish 

species would have to be several orders (~2-3 orders) of magnitude more sensitive than freshwater 

fish on a chronic basis in order to exceed the listed species LOC. Therefore, even if 

estuarine/marine fish were to be more sensitive on a chronic basis, there is sufficient distance 

between potential chronic endpoints and EECs to consider risk to this taxa low. 
Page 39 of 53

Since no chronic data was available for estuarine/marine invertebrates and no acute mortality or 

sublethal effects were observed in either the acute daphnid or mysid studies, and there is no other 

indication that estuarine/marine species are more sensitive than freshwater species, the chronic 

endpoint from the daphnid life cycle study was used to estimate chronic risk estuarine/marine 

invertebrates. Sublethal effects (diminished shell growth) was observed in the Eastern oyster acute 

study, therefore it is possible that using the chronic daphnid endpoint underestimates risk to this 

taxa and other bivalves. However, for the chronic LOC to be exceeded, bivalve taxa would need 

to be several orders (~3 orders) of magnitude more sensitive than daphnids or mysids on a chronic 

basis. Given that the sublethal effects observed in the acute oyster study were only observed at 

very high concentrations (NOEC = 73,200 μg a.i/L), chronic risk to this taxa is considered highly 

unlikely given current use patterns. 

No aquatic toxicological data has been evaluated for the impurity hydrazine. Although no 

ecotoxicological information has been evaluated by EFED for hydrazine, the Hazardous 

Substances Data Bank database accessible via Toxnet (http://toxnet.nlm.nih.gov/ does contain 

unreviewed ecotoxicity data. The most sensitive fish and aquatic invertebrate acute endpoints 

recorded in this database are 610 μg a.i./L for the guppy (Lebistes reticulatus) and 160 μg a.i./L 

for the waterflea (D. pulex). The most sensitive aquatic plant endpoint recorded in this database 

was a 96-hr EC50 of 0.02 ug a.i./L for green algae (S. capricornutum). These toxicity values are 

several orders of magnitude (at least 3 orders) more sensitive than for maleic hydrazide. For most 

of the older aquatic maleic hydrazide toxicological data, hydrazine was not analyzed for in the test 

material. Newer aquatic studies with the fathead minnow (MRIDs 48876701 & 48876702) used 

technical maleic hydrazide with analysis showing hydrazine concentrations less than 0.12 ppm. 

EPA had previously established an upper limit of 15 ppm for the impurity hydrazine in technical 

grade maleic hydrazide and technical labels do not indicate the amount of hydrazine present. 

4.2.2. Risks to Terrestrial Organisms

Based on the results of the screening-level assessment, the registered uses of maleic hydrazide 

result in potential for direct acute and chronic effects to certain size class and dietary combinations 

of birds and mammals. However, this conclusion is uncertain due to the use of nondefinitive 

toxicity endpoints that did not test up to the highest concentrations predicted in acute and chronic 

dose- and dietary-based modeling.

In order to gain a better understanding of how the EECs for the maximum maleic hydrazide

application rate relate to the toxicity data currently available for birds, T-REX was used to calculate 

RQs using the conservative assumption that the highest value in the avian acute oral study (i.e., 

acute LD50 = 2,000 mg a.i./kg-bw in the canary study) and the highest value in the avian sub-acute 

dietary study (i.e., acute LC50 = 5,620 mg a.i./kg-diet) represent the avian acute endpoints. The 

resulting acute dose-based and dietary-based acute RQs for all size and dietary classes, based on 

the upper-bound Kenaga values ranged from <0.01 to 1.83. In actuality, these RQs would be lower 

than the estimated values because no mortality or sublethal effects were identified at the 2,000 mg 

a.i./kg-bw acute oral and 5,620 mg a.i./kg-diet subacute dietary exposure levels. For example, if 

the mallard non-definitive acute oral endpoint of 4,600 mg a.i/kg-bw were used (only slight 

decreases in body weight gain were actually seen at this treatment level) instead of the non-
Page 40 of 53

definitive canary endpoint, for the maximum application rate of 13 lbs a.i./A, dose-based RQs 

would range from <0.01 to a maximum of 1.49 and the only exceedances of the non-listed species 

LOC would be for small and medium birds consuming short grass (RQs of 1.49 and 0.67, 

respectively) and small birds consuming tall grass, broadleaf plants and arthropods (RQs of 0.58—

0.84). Similarly, no mortality or sublethal effects were seen in the sub-acute dietary studies on the 

bobwhite and mallard with maleic hydrazide acid at the highest concentrations of 10,000 mg/kgdiet. If this value was used instead of the LC50 of 5,620 mg/kg-diet used in the assessment, than 

the non-listed species dietary LOC would not be exceeded and the listed species LOC would be 

exceeded for short grass, tall grass, broadleaf plants, and arthopods (RQs range from 0.02—0.31). 

Similarly, if the actual sub-acute LC50 value were two times higher than this (i.e. the true LC50 was

20,000 mg ai/kg-diet) than the listed species dietary LOC would only be exceeded for birds feeding 

on short grass (RQ would be 0.16 for this dietary item). Figure 2 shows how upper bound residue 

estimates decline over time compared to the avian dietary-based LOCs. For the most conservative 

dietary item (short grass), residues decline below the listed species LOC over a period of 

approximately 60 days. 

If mean Kenaga EECs (for the maximum application rate of 13 lbs a.i./A) and the original 

nondefinitive toxicity values were used instead of maximum EECs, the non-listed species acute 

dose-based LOC would only be exceeded for small birds feeding on short grass and arthropods 

(RQ = 0.65 and 0.50, respectively) while the listed species LOC would still be exceeded for 

small and medium sized birds feeding on short grass, tall grass, broadleaf plants and arthropods 

(RQs would range from < 0.01 to 0.65). Using mean Kenaga EEC values, the dietary-based 

listed species LOC would only be exceeded for birds consuming short grass, tall grass, broadleaf 

plants and arthopods (RQs would range from 0.02 to 0.24).

No chronic avian reproduction data was submitted. Additionally, effects on body weight were 

seen in multiple generations in the mammalian reproduction test. Therefore, chronic risk cannot 

be precluded for avian species (and to terrestrial-phase amphibians and reptiles for which birds 

serve as surrogates). 
Page 41 of 53

Figure 2: Terrestrial Maleic Hydrazide Residues (Upper Bound Kenaga Estimates) on Different 

Dietary Items over Time. Horizontal lines show the Avian Acute Dietary Listed and Non-Listed 

Species LOCs

0.0

1000.0

2000.0

3000.0

4000.0

5000.0

6000.0

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 Concentration (mg ai/kg dietary item)

Terrestrial Residues (upper bound estimates)

vs

Avian LOCs (as dietary concentrations)

Short Grass Tall Grass Broadleaf plants

Fruits/pods/seeds Arthropods Avian Acute Non-endangered LOC

Avian Chronic LOC Avian Acute Endangered LOC

Time (days)
Page 42 of 53

In order to gain a better understanding of how the EECs for the maximum maleic hydrazide 

application rate relate to the toxicity data currently available for mammals, T-REX was used to 

calculate RQs using the conservative assumption that the highest value in the mammalian acute 

oral study (i.e., acute LD50 = 5,000 mg/kg and the highest value in the mammalian 2-generation 

reproduction study (i.e., NOAEC = 10,000 ppm) represent the mammalian acute and chronic 

endpoints. The resulting dose-based acute RQs for all size and dietary classes, based on the upperbound Kenaga values were all less than 0.27, the resulting chronic dietary-based RQ did not exceed 

any LOCs and the resulting chronic dose-based RQ exceeded the LOC for all size classes of 

mammals consuming short grass, small and medium sized mammals feeding on tall grass and 

broadleaf plants and small sized mammals only feeding on arthopods. The actual LD50 would have 

to be greater than 13,000 (i.e. more than 2x the nondefinitive acute endpoint) in order to completely 

eliminate the potential for acute risk to all mammals. 

The chronic NOAEC endpoint for mammals was based on a statistically significant 8.5% effect 

on body weight at 30,000 ppm in the available rat reproduction study. There is some uncertainty 

if this level of effect would result in decreased biological fitness of listed and non-listed mammals. 

If a NOAEC of 30,000 ppm was used instead (LOAEC = 50,000 ppm in this scenario), than there 

would be no exceedances for chronic dose-based effects to mammals for any size-class or dietary 

combination (the maximum RQ would be 0.90 for small mammals consuming short grass).

If using mean Kenaga EEC values instead of upper bound Kenaga values, there would only be a 

marginal exceedance of the acute listed species LOC for small mammals feeding on short grass 

(RQ = 0.10) and no other acute or chronic LOC exceedances for mammals, although on a chronic 

dose basis, small mammals consuming short grass would only narrowly not exceed the LOC (RQ 

= 0.96).

Only non-definitive acute contact toxicity data is available for honey bees. The proposed LOC 

(0.4) for beneficial terrestrial invertebrates is exceeded by more than a factor of 2 assuming that 

the actual contact LD50 for honey bees was the limit dose (36.26 μg/bee, at which only 3.2% 

mortality actually occurred in the test study). The true LD50 would have to be approximately 90

μg a.i./bee in order not to exceed the proposed LOC, but would have to be at least an order of 

magnitude higher not to exceed the interim listed species LOC (0.05). Hazard to potentially 

sensitive life stages (i.e. egg, larvae and juveniles) may not be evident from the honey bee contact 

toxicity test that used adult worker honey bees. The additional data indicating high levels of 

mortality in predatory mite and parasitic wasp studies at concentrations approximately 1/3 the 

maximum application rate (and only slightly above the maximum application rate in the potato use 

pattern) provide additional lines of evidence that listed species of beneficial terrestrial 

invertebrates may be at risk for all use patterns except potentially for onion and citrus use sites. 

The EECs predicted by EFED’s current screening tool, Terrplant, indicated risk to listed and nonlisted species of monocots and dicots in both semiaquatic and dry habitats when aerial applications 

are made. When only ground applications are permitted, then no risk is predicted for non-listed 

dicots in dry areas and listed species only marginally exceed the LOC (RQ = 1.04) Spray drift 

alone results in potential risk to listed and non-listed monocot species when maleic hydrazide is 
Page 43 of 53

applied aerially, but no risk is indicated for even listed species of either dicots or monocots from 

spray drift caused by ground applications. 

To determine the extent of the area off the field for which RQs exceed the relevant LOC from 

spray drift, EFED uses the refined modeling tool, AgDrift 2.1.1. With standard default 

assumptions, the RQ exceeds the listed species LOC at distances ≥312 feet and ≥158 feet from the 

application site for listed monocots and dicots, respectively when maleic hydrazide is applied 

aerially. For ground applications and standard default assumptions, the RQ from spray drift only 

exceeds the listed species LOC at distances < 30 feet and < 16 feet from the application site for 

listed monocots and dicots, respectively. For non-listed monocots and dicots, when applied 

aerially, the RQ exceeds the non-listed species LOC at distances < 85 feet and < 113 feet, 

respectively while for ground applications, the RQ is only exceeded within 11 feet of the 

application site. Different droplet spectra (i.e. coarser droplet size distributions) would yield less 

spray drift and lower RQs. 

No terrestrial toxicological data has been evaluated for the impurity hydrazine. Although no 

ecotoxicological information has been evaluated by EFED for hydrazine, the Hazardous 

Substances Data Bank database accessible via Toxnet (http://toxnet.nlm.nih.gov/ does contain 

ecotoxicity data that has not been reviewed by EFED. The most sensitive terrestrial animal 

endpoints found in this database for hydrazine are for rabbit (species not listed in the database) 

with an acute oral LD50 of 20 mg/kg and roach (Rutilus rutilus) with a 24-hr LC50 of 0.54 mg/L. 

These toxicity values are several orders of magnitude (at least 2 orders) more sensitive than for 

maleic hydrazide. For most of the older terrestrial maleic hydrazide toxicological data, hydrazine 

was not analyzed for in the test material. A newer terrestrial study with the canary (MRID 

48877501) used technical maleic hydrazide with analysis showing hydrazine concentrations less 

than 0.12 ppm. EPA had previously established an upper limit of 15 ppm for the impurity 

hydrazine in technical grade maleic hydrazide, and technical labels do not indicate the amount of 

hydrazine present. Therefore, the toxicological information from the available maleic hydrazide 

terrestrial studies may underestimate toxicity (and increase the uncertainty in the risk estimates) if 

the actual amount of hydrazine impurity as a result of the maleic hydrazide manufacting process 

results in higher hydrazine concentrations than what was tested in the maleic hydrazide laboratory 

toxicity studies.

5. Federally Threatened and Endangered (Listed) Species Concerns

Section 7 of the Endangered Species Act, 16 U.S.C. Section 1536(a)(2), requires all federal 

agencies to consult with the National Marine Fisheries Service (NMFS) for marine and 

anadromous listed species, and/or the United States Fish and Wildlife Service (USFWS) for listed 

wildlife and freshwater organisms, if they are proposing an "action" that may affect listed species 

or their designated critical habitat. Each federal agency is required under the Act to ensure that 

any action they authorize, fund, or carry out is not likely to jeopardize the continued existence of 

a listed species or result in the destruction or adverse modification of designated critical habitat. 

To jeopardize the continued existence of a listed species means "to engage in an action that 

reasonably would be expected, directly or indirectly, to reduce appreciably the likelihood of both 
Page 44 of 53

the survival and recovery of a listed species in the wild by reducing the reproduction, numbers, or 

distribution of the species" (50 C.F.R. § 402.02).

To facilitate compliance with the requirements of the Endangered Species Act (subsection (a)(2)), 

the Office of Pesticide Programs has established procedures to evaluate whether a proposed 

registration action may directly or indirectly appreciably reduce the likelihood of both the survival 

and recovery of a listed species in the wild by reducing the reproduction, numbers, or distribution 

of any listed species (USEPA, 2004). After the Agency’s screening level risk assessment is 

conducted, if any of the Agency’s listed species’ LOCs are exceeded for either direct or indirect 

effects, an analysis is conducted to determine if any listed or candidate species may co-occur in 

the area of the proposed pesticide use or areas downstream or downwind that could be 

contaminated from drift or runoff/erosion. If listed or candidate species may be present in the 

proposed action area, further biological assessment is undertaken. The extent to which listed 

species may be at risk is considered, which then determines the need for the development of a 

more comprehensive consultation package, as required by the Endangered Species Act.

The federal action addressed herein is the proposed renewed registration of maleic hydrazide on 

agricultural use sites. It is expected that its use could occur nationwide.

5.1. Action Area 

For listed species assessment purposes, the action area is considered to be the area affected directly 

or indirectly by maleic hydrazide use and not merely the immediate area where maleic hydrazide

is applied. At the initial baseline, the risk assessment considers broadly described taxonomic 

groups and conservatively assumes that listed species within those broad groups are co-located 

with the pesticide treatment area. This means that listed terrestrial plants and wildlife are assumed 

to be located on or adjacent to the treated site and listed aquatic organisms are assumed to be 

located in a surface water body adjacent to the treated site. The assessment also assumes that the 

listed species are located within an assumed area, which has the relatively highest potential 

exposure to the pesticide, and that exposures are likely to decrease with distance from the treatment 

area. 

5.2. Taxonomic Groups Potentially at Risk 

If the assumptions associated with the baseline action area result in RQs that are below the listed 

species LOCs, a "no effect" determination conclusion is made with respect to listed species in that 

taxa, and no further refinement of the action area is necessary. Furthermore, RQs below the listed 

species LOCs for a given taxonomic group indicate no concern for indirect effects on listed species 

that depend upon the taxonomic group for which the RQ was calculated. However, in situations 

where the screening assumptions lead to RQs in excess of the listed species LOCs for a given 

taxonomic group, a potential for a "may affect" conclusion exists and may be associated with direct 

effects on listed species belonging to that taxonomic group or may extend to indirect effects upon 

listed species that depend upon that taxonomic group as a resource. In such cases, additional 

information on the biology of listed species, the locations of these species, and the locations of use 

sites are considered to determine the extent to which screening assumptions regarding an action 
Page 45 of 53

area apply to a particular listed organism. These subsequent refinement steps will consider how 

this information would impact the action area for a particular listed organism and potentially 

include areas of exposure that are downwind and downstream of the pesticide use site.

Assessment endpoints, exposure pathways, and the conceptual models addressing maleic 

hydrazide uses, and the associated exposure and effects analyses conducted for the maleic 

hydrazide baseline risk assessment are in Sections 2 to 3. The assessment endpoints used in the 

baseline risk assessment include those defined operationally as reduced survival and reproductive 

impairment for both aquatic and terrestrial animal species and survival, reproduction, and growth 

of aquatic and terrestrial plant species from both direct acute and chronic exposures. These 

assessment endpoints are intended to address the standard set forth in the Endangered Species Act 

requiring federal agencies to ensure that any action they authorize does not appreciably reduce the 

likelihood of both the survival and recovery of a listed species in the wild by reducing the 

reproduction, numbers, or distribution of the species. Risk estimates (RQs) which, integrating 

exposure and effects, are calculated for broad based taxonomic groups in the screening-level risk 

assessment presented in Section 4.

Both acute endangered species and chronic risk LOCs are considered in the baseline risk 

assessment to identify direct and indirect effects to taxa of listed species. This section identifies 

direct effect concerns, by taxa that are triggered by exceeding endangered LOCs in the baseline 

risk assessment, with an evaluation of the potential probability of individual effects for exposures 

that may occur at the established endangered species LOC. Data on exposure and effects collected 

under field and laboratory conditions are evaluated to make determinations on the predictive utility 

of the direct effect screening assessment findings to listed species. Potential for direct effects to 

listed taxa are described in Table 18.

Table 18. Potential Effects to Federally Listed Taxa Associated with Direct Effects from 

the Registered Uses of Maleic Hydrazide.

Listed Plant Taxon Potential Direct Effects

Terrestrial and semi-aquatic 

plants – monocots and dicots Yes

Aquatic nonvascular plants No

Aquatic vascular plants No

Listed Animal Taxon Potential Direct Effects

Acute Chronic

Terrestrial invertebrates Yes1 Uncertain

Mammals Uncertain2 Yes

Birds Uncertain2 Uncertain3

Reptiles Uncertain2 Uncertain2,3

Amphibians Uncertain2,4 Uncertain2,3,4

Freshwater fish No No

Freshwater invertebrates No No

Mollusks No No

Estuarine/marine fish No No

Estuarine/marine invertebrates No No

1 Although an LC50 was not attainable in the acute contact toxicity test with honey bees, the limit dose (36.3 μg a.i./bee) was not high enough 

to preclude risk at the non-listed species LOC (0.4), based on the EECs (35.1 μg/bee) for registered maleic hydrazide uses. Further, given that

additional toxicity data with predatory mites and parasitic wasps showed lethal effects at concentrations well below the maximum application 

rate, EFED cannot preclude effects to this taxon.
Page 46 of 53

2 Risk to this taxa was considered uncertain due to the use of non-definitive toxicological endpoints that were below maximum EECs. 

Although no effects were observed in acute toxicological studies, the doses tested did not exceed the maximum EECs predicted by the 

maximum application rates.

3 Chronic risk to this taxa is uncertain due to a lack of avian reproduction data

4Risk to this taxa is uncertain for terrestrial-phase amphibians. No direct effects are expected for aquatic phase amphibians.

6. Literature Cited 

Ahrens, W.H. Herbicide Handbook of the Weed Science Society of America. 7th ed. 

Champaign, IL: Weed Science Society of America, 1994., p. 180.

Baris, R., M. Barrett, R.F.H. Bohaty, M. Echeverria, P. Villanueva, J. Wolf, and D. Young. 

2012. Guidance for Using PRZM-GW in Drinking Water Exposure Assessments. 

Environmental Fate and Effects Division, Office of Pesticide Programs, Office of 

Chemical Safety and Pollution Prevention, USEPA. Washington, D.C. 

FAO. 2000. Appendix 2. Parameters of pesticides that influence processes in the soil. In FAO 

Information Division Editorial Group (Ed.), Pesticide Disposal Series 8. Assessing Soil 

Contamination. A Reference Manual. Rome: Food & Agriculture Organization of the 

United Nations (FAO). (Accessed March 31, 2014).

NAFTA. 2011. Guidance for Evaluating and Calculating Degradation Kinetics in

Environmental Media. December 2011. NAFTA Technical Working Group on 

Pesticides. Accessed on March 21, 2014 at:

http://www.epa.gov/oppfead1/international/naftatwg/guidance/degradation-kin.pdf

Tomlin, C.D.S. (ed.). The Pesticide Manual - World Compendium, 11 th ed., British Crop 

Protection Council, Surrey, England 1997, p. 757.

USEPA, 1994. Reregistration Eligibility Decision (RED) Maleic Hydrdazide. Office of 

Prevention, Pesticides and Toxic Substances. Washington, D.C. 

USEPA, 2009a. Registration Review - Preliminary Problem Formulation for the Ecological Risk

Assessment of Maleic hydrazide. Office of Pesticide Programs (DP Barcode 362546).

USEPA, 2009b. Input Parameter Guidance. Version 2.1. Environmental Fate and Effects

Division. October 22, 2009

http://www.epa.gov/oppefed1/models/water/input_parameter_guidance.htm
Page 47 of 53

Appendix 1 – Surface Water Concentration Calculator Modeling Summary

Cropped Area Fraction 1.0

Koc (ml/g) 35

Water Half-Life (days) @ 25 °C 6

Benthic Half-Life (days) @ 25 °C 0

Photolysis Half-Life (days) @ 40 °Lat 0

Hydrolysis Half-Life (days) 0

Soil Half-Life (days) @ 22 °C 3

Foliar Half-Life (days) 35

Molecular Wt 112.09

Vapor Pressure (torr) 7.5E-8

Solubility (mg/l) 4507
Page 48 of 53

Appendix 2 -Environmental Fate Studies Summary

835.2120: Hydrolysis (00143322)

[

14C]-4 ,5-Maleic hydrazide (radiochemical purity >99%), at 50 ppm, was stable to hydrolysis 

(<6% degradation) in buffered solutions (pH 3, 6, and 9) maintained for 61 days at 45 0C and for 

an additional 30 days at 80°C.

835.2240: Photodearadation in Water (428723, (4269210)

[1,4-

14C]Maleic hydrazide (6 -hydroxy-2 H-pyridazin-3 -one; radiochemical purity >99%), at 

approximately 0.12 mg/L, degraded slowly (~10% at 28 days) at - pH 5, was stable at pH 7, and 

degraded with an observed half-life of >30 days at pH 9 in sterile aqueous buffer solutions that 

were irradiated for 12 hours/day for 30 days at 23-30°C using a xenon lamp that had an intensity

(daily intensity 24.9-49.3 mwatts/cm2) and emission spectrm: similar to sunlight. At 30 days 

posttreatment, [14~]maleich ydrazide was the only [14~]compound isolated from the pH 5 and 7 

solutions. In the irradiated pH 9 solution, [14~]maleich ydrazide was 0.121-0.124 mg/L 

immediately posttreatment, 0.061-0.091 mg/L at 16 days, 0.084-0.091 mg/L at 24 days, and

0.031-0.078 mg/L at 30 days; two photodegradates were isolated from the 30-day solution, but 

were neither identified nor quantified. [14c]~aleic hydrazide was the only [14C] compound isolated 

from the dark controls. Material balances ranged from 96.4% to 109% of the applied in the 

irradiated solutions and from 97.7% to 114% in dark controls; there was no discernable pattern of 

decline with any solution. The pH of buffer solutions remained constant over the study period.

835.2410: Photodegradation on Soil (00151951)

[

14C]-4 ,5-Maleic hydrazide, potassium salt at 20 ppm on sandy loam soil was stable to 

photodegradation on sandy loam soil irradiated continuously with artificial light (1.4 w/m2) for

17 days (equivalent to 34 days of sunlight).

835.4100: Aerobic Soil Metabolism (4189620)

Maleic hydrazide degraded witlh an observed half-life of approximately 24 hours in loamy sand 

soil that was incubated in the dark at 17-22°C and 75% of 0.33 bar moisture capacity for 90 

days. Two degradates, maleic acid and maleimide, were detected in the soil in small quantities 

(<5%) during the study. At 90 days posttreatment, C02 was the major degradate

(approximately 70% of the applied).

835.4200: Anaerobic Soil Metabolism (41918201)

Maleic hydrazide degraded with an observed half-life of 30- 60 days in anaerobic (flooding plus 

nitrogen atmosphere) loamy sand soil that was incubated in the dark at 18-21°C for 60 days. Two 

nonvolatile degradates, maleic acid and maleimide, were identified in the soil plus floodwater in
Page 49 of 53

small quantities (<5%) at the start of anaerobic incubation, but did not persist. After 60 days of 

anaerobic incubation, (GOz was the major degradate (approximately 70% of the applied).

835.1230; 835.1240: Leaching /Adsorptoion/Desorption (41896202, 00151952)

In the study (41896202), based on column leaching studies, aged (12 hours) maleic hydrazide and 

maleic hydrazide residues were found to be mobile (39.76% of the applied radioactivity was in the 

leachate) in columns of loamy sand soil, and very mobile (100.79% of the applied radioactivity in 

the leachate) in columns of sand soil leached over a period of 7 days.

The study (00151952) using batch equilibrium techniques, unaged maleic hydrazide was very

mobile in a silt loam, a sandy clay loam, a sandy loam, and two sand soils; Freundlich Kads values 

were 0.14- 2.61 and Kdes values were 0.39-14.45. A soil column study of unaged maleic hydrazide 

had high mobility, with all of the radioactivity being eluted in less than 10 inches of water (two 

sand soils).

835.6100: Terrestrial field dissipation studies (42744801 and 42790901)

The study conducted on sandy loam soil located in California (42744801 and 42790901) provides 

acceptable information that maleic hydrazide dissipated with observed half-lives of 3 and 5 days 

in bareground and vegetated (turf) plots, respectively. The study conducted on loam soil located 

in North Carolina (42693301) provides acceptable information that maleic hydrazide dissipated 

with observed halflives of 14 and 27 days in bareground and vegetated (mature tobacco) plots,

respectively. The study conducted on clay loam soil in New Mexico (40034802)

provides supplemental information that maleic hydrazide, potassium salt, dissipated with a halflife of 3-7days; no residues were detected (<0.05 ppm) at depths greater than 12 inches.

850.1730: Fish BCF (0016330)

The 1988 Registration Standard for maleic hydrazide specified that the bioaccumulation in fish 

study was not required because the reviewed octanol/water partition coefficient (Kow) study 

(00163301) gave a value of (log Kow= -0.68), indicating a very low potential for maleic hydrazide 

to bioaccumulate in fish or other aquatic nontarget organisms.

835.4300: Aerobic Aquatic Metabolism studies- Anaerobic Aquatic Metabolism.

No studies were reviewed. No data are required at this time because maleic hydrazide has no 

aquatic or forestry uses.
Page 50 of 53

Appendix 3–Summary of Risk Conclusions by Use Site and Taxa

Use Pattern 

Category1

Aquatic 

Invertebrates

Fish (surrogate 

for aquaticphase 

amphibians)

Birds, (surrogate for 

terrestrial-phase 

amphibians and reptiles)

Mammals Plants Beneficial 

Terrestrial 

Invertebrates

Agricultural 

Fallow/Idleland/

Conservation 

Reserve

Acute risk:

Not expected

Chronic risk: 

Not expected

Acute risk:

Not expected 

Chronic risk: 

Not expected

Acute risk: 

Dietary-risk to listed and 

non-listed birds and dosebased risk to all size classes 

of listed birds and dose-based 

risk to small and medium 

non-listed birds is uncertain 

due to nondefinitive data that 

did not test up to predicted 

EECs.

Chronic risk: 

Uncertian due to lack of 

chronic data

Acute risk: 

Risk to all size classes of 

listed mammals is uncertain 

due to nondefinitive data 

that did not test up to 

predicted EECs. Risk is not 

predicted for non-listed 

mammals

Chronic risk: 

Potential for dose-based risk 

to mammals depending on 

size class and dietary items.

Terrestrial:

Likelihood of risk expected, 

especially for listed and nonlisted monocot species in all 

habitats and from spray drift. 

Risk expected for dicots in 

semi-aquatic habitats (ground 

or air) and dry habitats from 

aerial applications only.

Aquatic:

Not expected

Non-listed species: 

Risk to non-listed 

species is uncertain 

due to nondefinitive data that 

did not test up to 

predicted EECs.

Listed species: 

Likelihood of risk 

expected to listed 

species.

Ornamental 

and/or Shade 

Trees, 

Ornamental 

Lawns and Turf, 

Residential 

Lawns

Acute risk:

Not expected

Chronic risk: 

Not expected

Acute risk:

Not expected 

Chronic risk: 

Not expected

Acute risk: 

Dose-based risk to small and 

medium sized listed and nonlisted birds and large listed 

birds is uncertain due to 

nondefinitive data that did 

not test up to predicted EECs. 

Not expected for large nonlisted birds. Dietary-based 

risk uncertain for listed birds 

only.

Chronic risk: 

Uncertian due to lack of 

chronic data

Acute risk: 

Risk to small and medium 

sized listed mammals is 

uncertain due to 

nondefinitive data that did 

not test up to predicted 

EECs. Not expected for 

large listed mammals or any 

non-listed mammals.

Chronic risk: 

Potential for dose-based risk 

to small and medium sized 

mammals feeding on short 

grass only

Terrestrial:

Likelihood of risk expected, 

especially for listed and nonlisted monocot species in all 

habitats, listed dicots in dry 

(aerial application only) and 

semi-aquatic habitats and nonlisted dicots in semi-aquatic 

habitats only. Risk from spray 

drift is only expected for listed 

monocots under aerial 

applications.

Aquatic:

Not expected

Non-listed species:

Risk to non-listed 

species is uncertain 

due to nondefinitive data that 

did not test up to 

predicted EECs.

Listed species:

Likelihood of risk 

expected to listed 

species
Page 51 of 53

Golf Course 

Turf, Industrial/

Construction 

Areas (Outdoor), 

Nonagricultural 

Rights-ofWay/Fencrows/

Hedgerows, 

Nonagricultural 

Uncultivated 

Areas/Soils, 

Urban Areas, 

Airports/Landing 

Fields

Acute risk:

Not expected

Chronic risk: 

Not expected

Acute risk:

Not expected 

Chronic risk: 

Not expected

Acute risk: 

Dose-based risk to small and 

medium sized listed and nonlisted birds and large listed 

birds is uncertain due to 

nondefinitive data that did 

not test up to predicted EECs. 

Not expected for large nonlisted birds. Dietary-based 

risk uncertain for listed birds 

only.

Chronic risk: 

Uncertian due to lack of 

chronic data

Acute risk: 

Risk to small and medium 

sized listed mammals is 

uncertain due to 

nondefinitive data that did 

not test up to predicted 

EECs. Not expected for 

large listed mammals or any 

non-listed mammals.

Chronic risk: 

Potential for dose-based risk 

to small and medium sized 

mammals feeding on short 

grass only

Terrestrial:

Risk expected for all plants in 

semi-aquatic areas regardless 

of application method, but 

only listed species of

monocots in dry areas when 

ground applications are used 

(under aerial applications in 

dry areas, both listed 

monocots and dicots are 

considered at risk). Risk from 

spray drift is only expected for 

listed monocots under aerial 

applications

Aquatic:

Not expected

Non-listed species:

Risk to non-listed 

species is uncertain 

due to nondefinitive data that 

did not test up to 

predicted EECs.

Listed species:

Likelihood of risk 

expected to listed 

species

Tobacco Acute risk:

Not expected

Chronic risk: 

Not expected

Acute risk:

Not expected 

Chronic risk: 

Not expected

Acute risk: 

Dose-based risk to small 

listed and non-listed birds

and listed medium and large 

sized birds is uncertain due to 

nondefinitive data that did 

not test up to predicted EECs. 

Not expected for medium or 

large non-listed birds. 

Dietary risk uncertain for 

listed birds only.

Chronic risk: 

Uncertian due to lack of 

chronic data

Acute risk:

Risk to small and medium 

sized listed mammals is 

uncertain due to 

nondefinitive data that did 

not test up to predicted 

EECs. Not expected for 

large listed mammals or any 

non-listed mammals.

Chronic risk: 

Potential for dose-based risk 

to small and medium sized 

mammals feeding on short 

grass only

Terrestrial:

Risk expected for all listed and 

non-listed plants in semiaquatic areas, regardless of 

application method. Risk 

expected for listed and nonlisted monocots in dry areas 

only under aerial applications. 

Risk from spray drift alone not 

expected for any plants 

regardless of application 

method.

Aquatic:

Not expected

Non-listed species:

Risk to non-listed 

species is uncertain 

due to nondefinitive data that 

did not test up to 

predicted EECs.

Listed species:

Likelihood of risk 

expected to listed 

species
Page 52 of 53

Ornamental 

Woody Shrubs 

and Vines

Acute risk:

Not expected

Chronic risk: 

Not expected

Acute risk:

Not expected 

Chronic risk: 

Not expected

Acute risk: 

Dose-based risk to small 

listed and non-listed birds

and medium sized listed birds

is uncertain due to 

nondefinitive data that did 

not test up to predicted EECs. 

Not expected for medium or 

large non-listed birds. 

Dietary risk uncertain for 

listed birds only.

Chronic risk: 

Uncertian due to lack of 

chronic data

Acute risk:

Risk to small listed 

mammals is uncertain due to 

nondefinitive data that did 

not test up to predicted 

EECs (marginal 

exceedance). Not expected 

for medium or large listed 

mammals or any non-listed 

mammals.

Chronic risk: 

Not expected 

Terrestrial:

Risk expected for all listed and 

non-listed plants in semiaquatic areas, regardless of 

application method. Risk 

expected for listed monocots 

in dry areas only under aerial 

applications. Risk from spray 

drift alone not expected for 

any plants regardless of 

application method.

Aquatic:

Not expected

Non-listed species:

Not expected

Listed species:

Likelihood of risk 

expected to listed 

species

Potato 

(White/Irish)

Acute risk:

Not expected

Chronic risk: 

Not expected

Acute risk:

Not expected 

Chronic risk: 

Not expected

Acute risk: 

Dose-based risk to small 

listed and non-listed birds 

and medium sized listed birds 

is uncertain due to 

nondefinitive data that did 

not test up to predicted EECs. 

Not expected for medium or 

large non-listed birds. 

Dietary risk uncertain for 

listed birds only.

Chronic risk: 

Uncertian due to lack of 

chronic data

Acute risk:

Not expected

Chronic risk:

Not expected

Terrestrial:

Risk expected for all listed and 

non-listed plants in semiaquatic areas, regardless of 

application method. Risk 

expected for listed monocots 

in dry areas only under aerial 

applications. Risk from spray 

drift alone not expected for 

any plants regardless of 

application method.

Aquatic:

Not expected

Non-listed species:

Not expected

Listed species:

Likelihood of risk 

expected to listed 

species
Page 53 of 53

Onion Acute risk:

Not expected

Chronic risk: 

Not expected

Acute risk:

Not expected 

Chronic risk: 

Not expected

Acute risk:

Dose-based risk to small and 

medium sized listed birds is 

uncertain due to 

nondefinitive data that did 

not test up to predicted EECs. 

Not expected for any nonlisted birds. Dietary risk 

uncertain for listed birds

feeding on short grass only.

Chronic risk: 

Uncertian due to lack of 

chronic data

Acute risk:

Not expected

Chronic risk:

Not expected

Terrestrial:

Risk expected for all listed and 

non-listed plants in semiaquatic areas, regardless of 

application method. No risk 

expected for any plants in dry 

areas or due to spray drift

regardless of application 

method.

Aquatic:

Not expected

Non-listed species:

Not expected

Listed species:

Risk to listed 

species uncertain.

Citrus Acute risk:

Not expected

Chronic risk: 

Not expected

Acute risk:

Not expected 

Chronic risk: 

Not expected

Acute risk:

Dose-based risk to small and 

medium sized listed birds is 

uncertain due to 

nondefinitive data that did 

not test up to predicted EECs. 

Not expected for any nonlisted birds. Dietary risk 

uncertain for listed birds

feeding on short grass only.

Chronic risk: 

Uncertian due to lack of 

chronic data

Acute risk:

Not expected

Chronic risk:

Not expected

Terrestrial:

Risk expected for all listed and 

non-listed plants in semiaquatic areas when aerial 

application is used. Under 

ground applications in semiaquatic areas, risk is not 

expected for non-listed dicots, 

but is expected for listed 

dicots and non-listed and 

listed monocots. No risk 

expected for any plants in dry 

areas or due to spray drift

regardless of application 

method.

Aquatic:

Not expected

Non-listed species:

Not expected

Listed species:

Risk to listed 

species uncertain

1 Use pattern categories are based on similar application rates