2

non-listed freshwater and estuarine/marine invertebrates (risk quotients or RQs = 11 and 20, 

respectively) when the aerial ULV spray was modeled at 100 ft flight altitude with 500 ft swath 

width under 1 mph wind speed. To further explore these risk concerns, EFED performed an 

additional wind speed analysis for prallethrin typical end-use product (TEP) application by a 

fixed wing aircraft using AGDISP (v8.26). The prallethrin percent deposition into a water body 

was modeled based on the scenarios of 5 wind speeds (1, 2, 3, 4 and 5 mph), 2 flight altitudes (75 

and 100 ft) and 2 spray lines (1 and 2 swaths). 

Based on the current analysis, it is concluded that prallethrin deposition into a water body can be 

minimized by increasing the wind speed and manipulating spray altitudes. When wind speed 

reached 5 mph at the flight altitude of 75 ft or ≥ 3 mph at the flight altitude 100 ft, the percent 

deposition (or percent application efficacy) under a single 500 ft flight swath (or percent aquatic 

deposition) approaches zero (Table 1) with corresponding RQ value ≤ 0.001, a level which is 

less than level of concern (LOC = 0.05) for both non-listed and listed estuarine/marine and 

freshwater invertebrates, respectively (Table 2).

However, for two flight swaths over a water body, when the wind speed reaches 5 mph at 100 ft 

flight altitude, the percent aquatic deposition is 1.19 (Table 1), which is equivalent to 0.91 ppb in 

water with corresponding RQ values 0.62 (>LOC = 0.5) for estuarine/marine and 0.39 (>LOC = 

0.05) for freshwater invertebrates respectively (Table 2). The analysis indicates that multiple 

swaths (≥ 2 swaths) over a water body may pose a risk to aquatic invertebrates. One uncertainty

related to the AGDISP modeling is that the model predictions on multi-swath deposition have 

not been validated. 

Additional Analysis on the Mosquito Adulticide Deposition 

Using AGDISP (v8.26) Model

AGDISP (v8.26) Modeling Method 

AGDISP (version 8.26) is a science-based model that predicts spray drift from application sites. 

The model was developed by the USDA Forest Service. AGDISP was designed to optimize 

agricultural spraying operations and has detailed algorithms for characterizing the release, 

dispersion, and deposition over and downwind of the application area. This model can be used in 

estimating downwind deposition of spray drift from aerial and ground boom applications. In 

addition, it can be used in estimating downwind deposition of spray drift from forestry and 

adulticide/mosquitocide applications.

The prallethrin percent deposition into a water body was simulated by a fixed wing aircraft based 

on scenarios of 5 wind speed (1, 2, 3, 4 and 5 mph), 2 flight altitudes (75 and 100 ft) and 2 spray 

lines (1 and 2 swaths). One spray line (single swath) was assumed to be 500 ft. The methods for 

calculations of surface water estimated environmental concentrations (EECs) and risk quotients 

(RQs) were same as the previous assessment (USEPA 2014, DPs 417461, 417581, 417582). 
3

Results

Flight Altitude

The highest percent of 

prallethrin deposition in the 

application sites are 51% and 

73% at a 75 ft altitude with 1 

mph wind speed for a single 

swath and two flight swaths 

respectively. The percent of 

deposition decreases

following an increase of the 

spraying altitude (Fig. 1). 

Flight altitude above 100ft is 

not modeled due to the 

limitations of AGDISP 

(v8.26). 

Wind Speed

The percent of prallethrin application efficiency (deposition) under spray swath(s) is heavily 

influenced by the wind speed as well as spraying altitude discussed above. The deposition 

decreases as wind speed increases (Fig .1 and Table 1). The simulation modeling shows that a 

single swath of the mosquito aerial ULV spray under ≤ 2 mph wind speed at 100 ft spray 

altitude, and ≤ 4 mph at 75 ft produces substantial deposition, which results RQ values above the 

level of concerns for both listed (LOC = 0.05) and non-listed species (LOC = 0.5) (see Table 2

for EECs and RQ calculations) for aquatic invertebrates. However, when the wind speed reaches

5 mph at 75 ft flight altitude or ≥ 3 mph at 100 ft, the deposition in the water body approaches

zero for a single flight swath (Fig 1. and Table 2) with corresponding RQ values (≤0.001) below 

the level of concerns for both listed (LOC = 0.05) and non-listed (LOC = 0.5) aquatic 

invertebrates (Table 3).

Flight Swaths over Water Body

As indicated above, there are not risk concerns for a single swath application over the water body 

when the wind speed reaches 5 mph at 75 ft flight altitude or ≥ 3 mph at 100 ft. However, the 

analysis also indicates that multiple swaths (≥ 2 swaths) over a water body may pose a risk to 

aquatic invertebrates. Table 2 shows that 2-swaths of adulticide application over a water body at 

5 mph wind speed and 100 ft flight altitude, the aquatic deposition is reduced to 1.19% with 

corresponding RQ values 0.62 (≥ LOC = 0.5) and 0.39 (≥ LOC = 0.05) for estuarine/marine and 

freshwater invertebrates (Table 3). There is an uncertainty, however, related to multiple swaths 

using the AGDISP model to predict adulticide deposition behavior. The model prediction has 

not been validated for multiple swaths.

0

10

20

30

40

50

60

1 2 3 4 5

% Aquatic Deposition

Wind Speed (mph)

% Aquatic Deposition Influenced by 

Wind Speed and Spray Altitude

75 ft 100 ft
4

Table 1. Comparison of Application Efficiency of Fixed Wing Aircraft at Five Wind Speeds 

and Two Spray Altitudes with 500 ft Swath Using AGDISP Model (v8.26)

Swath 

width 

(ft)

No. of 

Swath

Release 

Height (ft)

Wind 

Speed 

(mph)

Application 

efficiency (%)a

Downwind 

Deposition (%)b

Airborne 

Drift (%)c

500 1 75 1 51.33 48.67 0.0

500 1 75 2 24.14 75.86 0.0

500 1 75 3 5.59 82.6 11.87

500 1 75 4 1.14 80.88 17.99

500 1 75 5 0

d 76.12 24.42

500 1 100 1 36.04 63.96 0

500 1 100 2 19.19 80.81 0

500 1 100 3 0

d 81.77 18.66

500 1 100 4 0

d 73.82 28.15

500 1 100 5 0

d 65.21 37.08

500 2 75 1 73.18 26.82 0

500 2 75 2 49.72 50.28 0

500 2 75 3 27.93 62.93 9.14

500 2 75 4 15.57 68.31 16.12

500 2 75 5 9.81 68.25 21.94

500 2 100 1 63.92 36.08 0

500 2 100 2 39.89 60.11 0

500 2 100 3 13.05 71.01 15.94

500 2 100 4 4.18 70.58 25.24

500 2 100 5 1.19 65.15 33.66

a % Application Efficacy –the percent of prallethrin deposited directly under the 500 ft flight swath or deposited into 

the water when an aircraft sprays over a water body. The focus of EFED’s analysis is to reduce the percent of 

aquatic deposition (or the % application efficacy), which is used to calculate the aquatic EEC and RQ. 

b % Downwind Deposition –the percent of prallethrin drift downwind and deposited on land. For this assessment, 

EFED did not use percent drift deposition in the aquatic RQ calculation. However, this parameter is still considered 

because a higher percent downwind deposition may represent a lower percent mosquito control efficacy because of 

reduced opportunities to come into contact with flying mosquitoes. 

c % Airborne Drift –the percent of prallethrin fine droplets that stay aloft downwind. A higher percent of airborne 

drift may indicate a higher amount of prallethrin retained in the air for better control efficacy against flying 

mosquitoes. 
5

dBolded application scenarios with estimated RQs below the LOC while the spray deposit approaching to zero. 

Table 2. Estimate Environmental Concentrations (EECs) and Risk Quotients (RQs) after 

Mosquito Adulticide ULV Applicationa over a Water Body (Rice Paddy)

No. of 

Swath

Release 

Height (ft)

Wind 

Speed 

(mph)

Application 

efficiency 

(%)

Peak EECsb

(μg/L)

RQs Freshwater 

Invertebrate 

EEC / EC50 (EC50(TEP)

= 2.32 μg /L)c

RQs Estuarine/ 

marine 

Invertebrate

EED / EC50 (EC50(TEP)

= 1.46 μg /L)d

1 75 1 51.33 39.18 16.89** 26.83**

1 75 2 24.14 18.43 7.94** 12.62**

1 75 3 5.59 4.27 1.84** 2.92**

1 75 4 1.14 0.87 0.38* 0.60**

1 75 5 0 ≤ 0.001 ≤ 0.001 ≤ 0.001

1 100 1 36.04 27.51 11.86** 18.84**

1 100 2 19.19 14.65 6.31** 10.03**

1 100 ≥3 0 ≤ 0.001 ≤ 0.001 ≤ 0.001

2 75 1 73.18 55.86 24.08** 38.26**

2 75 2 49.72 37.95 16.36** 25.99**

2 75 3 27.93 21.32 9.19** 14.60**

2 75 4 15.57 11.88 5.12** 8.14**

2 75 5 9.81 7.49 3.23** 5.13**

2 100 1 63.92 48.79 21.03** 33.42**

2 100 2 39.89 30.45 13.12** 20.85**

2 100 3 13.05 9.96 4.29** 6.82**

2 100 4 4.18 3.19 1.38** 2.19**

2 100 5 1.19 0.91 0.39* 0.62**

a Maximum single application rate at 0.07 lb formulation (Multicide 2922) per acre

bCalculation based on: 0.17 (total lb formulation applied to 1 ha) x (application efficiency/100) x 

(453.6*1,000,000)(gram *10 ha water pond volume). For multiple sprays (≥2 spray lines), only a single application 

rate (0.17 lb per ha) is used to calculate the EEC. 

c endpoint for water flea (D. magna) to acute MULTICIDE Fogging Concentrate 2798

d endpoint for mysid (M. bahia) to acute MULTICIDE Fogging Concentrate 2922

e estimated RQ (< 0.01) with simulated zero deposition in water

* exceeds aquatic listed species acute LOC (=0.05)

**exceeds aquatic non-listed species acute LOC (=0.5)
6

References

USEPA. 2014. Prallethrin: Revised ecological risk assessment to support proposed new use 

over, near, and around agricultural areas. Environmental Fate and Effects Division, USEPA, 

Washington, DC. PC Code 128722; DP Barcodes 417461, 417581, 417582; February, 2014.