Draft Risk Management Plan and Rationale for Thidiazuron—1/13/16

Human Health Risks 

o No human health risks were identified, therefore no human health risk mitigation is necessary.

Ecological Risks

Birds/Terrestrial-phase amphibians and reptiles 

o There are no acute or chronic risks to birds (also surrogates for terrestrial-phase amphibians/reptiles). 

o The risk assessment identified a potential pathway of concern for birds drinking water in thidiazuron-treated 

cotton fields on a chronic exposure basis. However, SIP is a conservative screening model and assumes that 

thidiazuron is present in the water source at its solubility limit (31 mg/L), that 100% of drinking water needs are 

obtained from thidiazuron-treated cotton fields, and that the most sensitive bird (20 g) is exposed. If the 

solubility limit is reduced by half (15 mg/L), there is no risk concern for birds. Because even this exposure is 

unlikely, EPA does not have any concern for bird chronic effects from drinking water alone, given the current 

avian toxicity data. 

Terrestrial Plants

o Thidiazuron is expected to adversely affect the survival and growth of terrestrial plants, mainly dicots. The 

highest risk quotient (RQ) is 13 for exposure to a combination of surface water runoff and spray drift. This 

significantly exceeds the plant Level of Concern (LOC) of 1.0.

o The LOC is also exceeded by spray drift exposure alone for the most sensitive dicot at a distance of more than 

1000 feet from the edge of cotton fields.

o For other dicots tested (n=9), only 1 species (other than the most sensitive species) exceeded the spray drift LOC 

up to a distance of 328 feet (based on spray drift EECs for plants inhabiting terrestrial areas) beyond the edge of 

field. Along with monocots, the least susceptible dicots were not affected at the edge of the field.

Proposed Risk Mitigation Measures

EPA is supportive of thidiazuron’s continued use, as it is a chemical with low risks. EPA is primarily interested in 

reducing spray drift risk to non-target plants, especially sensitive dicots such as lettuce. We propose that the 

following language, which already appears on some labels, appear on all labels:

1. No aerial application within 0.5 miles of lettuce (this language already appears on some labels)

2. No aerial application within 5 miles upwind of citrus in flush (this language already appears on some labels)

3. No ground application within 100 feet of lettuce (this language already appears on some labels)

4. No ground application within 0.5 miles upwind from citrus (this language already appears on some labels)

5. Apply with medium or coarser spray (according to ASAE standard 572) for standard nozzles for both aerial and 

ground applications (this language already appears on some labels)

6. When using ground application, apply with nozzle height no more than 2 feet above the ground or crop canopy 

(this language already appears on some labels)

7. Mixtures with organophosphates can increase non-target crop phytotoxicity. Additional care should be 

exercised when lettuce, citrus, cantaloupes or any other edible crops are adjacent to treated cotton fields (this 

language already appears on some labels).

8. Maximum application rate of 0.3 pounds of active ingredient per acre per year (this was RED risk mitigation, this 

language already appears on most labels, all registrants need to check that this appears on their label).

9. Maximum of 2 applications, but not to exceed 0.3 pounds of active ingredient per acre per year (this language 

already appears on some labels)

10. Minimum retreatment interval of 21 days (this language already appears on some labels)
Rationale

o Including label advisory for sensitive crops such as lettuce and citrus helps prevent unnecessary harm to nontarget crops and off-site native species

o Adding stronger spray drift precautions on labels for ground and aerial application helps to reduce spray drift

risk to non-target crops and off-site native species. 

o Noting that mixtures can increase phytoxicity may reduce non-target adverse effects

Considerations regarding benefits

o Thidiazuron is less toxic than some alternative cotton defoliants

o There are no risk concerns for most taxa

Current Data gaps

o Storage stability portion of terrestrial field dissipation study

o Enforcement analytical method for water

o Enforcement analytical method for soil 

o Honeybee larval acute oral toxicity study (tier 1)

o Honeybee adult 10-day chronic oral toxicity (tier 1)

o Honeybee larval repeat dose study (tier 1)

o Semi-field testing for pollinators (tier 2)

o Field testing for pollinators (tier 3)

o Field trial of residues in pollen and nectar (tier 3)

Note: tier 2 and tier 3 honeybee testing should not be initiated until EPA has completed tier 1 pollinator screening for 

thidiazuron.