Rainfast characteristics of fruit crop insecticides

Precipitation can impact insecticide performance, but some compounds resist wash-off.

The rainfall events experienced in Michigan have prompted questions about the relative “rainfastness” of the insecticides used in fruit production. In 2006, AgBioResearch provided funds to purchase and install a state-of-the-art rainfall simulation chamber at the MSU Trevor Nichols Research Center (TNRC), after which Michigan State University Extension has conducted trials – with generous funding support from Michigan fruit commodity groups – on fruit crops for a range of insecticides.

There are several critical factors that influence impact of precipitation on a pesticide’s performance. First is the plant penetrative characteristic of the various compounds. Some pesticide chemistries, like organophosphates, have limited penetrative potential in plant tissue, and thus are considered primarily as surface materials. Some compounds, such as carbamates, oxadiazines and pyrethroids, penetrate plant cuticles, providing some resistance to wash-off. Many newer compounds, such as spinosyns, diamides, avermectins, and Insect Growth Regulators (IGR) readily penetrate plant cuticles and have translaminar movement in leaf tissue. Others, like the neonicotinoid insecticides, are systemic and can have translaminar as well as acropetal movement in the plant’s vascular system. Penetration of plant tissue is generally expected to enhance rainfastness of pesticides.

The second factor is the inherent toxicity of an insecticide to the target pest and the persistence of the compound in the environment. In some cases, a compound may be highly susceptible to wash-off, but its persistence and inherent toxicity to the target pest compensates for the loss of residue, thus delaying the need for immediate re-application.

The third factor is the amount of precipitation. In general, organophosphate insecticides have the highest susceptibility to wash-off from precipitation, but their high level of toxicity to most insect pests overcomes the necessity for an immediate re-application. Neonicotinoid insecticides are moderately susceptible to wash-off with residues that have moved systemically into plant tissue being highly rainfast, and surface residues less so. Carbamate, IGR and oxadiazine insecticides are moderately susceptible to wash-off, and vary in their toxicity to the range of relevant fruit pests. Diamide, spinosyn, avermectin and pyrethroid insecticides have proven to be moderate to highly rainfast on most fruit crops.

For most insecticides, a drying time of two to six hours is sufficient to “set” the compound in the plant. With neonicotinoids, for which plant penetration is important, drying time can significantly influence rainfastness. For neonicotinoids, up to 24 hours is needed for optimal plant penetration, thus the time proximity of precipitation after application should be considered carefully. Spray adjuvants, materials intended to aid the retention, penetration or spread on the plant, can also improve the performance of insecticides.

Based on the results from the current studies, the following charts have been developed to serve as a guide for general rainfastness characteristics and re-application recommendations for certain insect pests (also printed in the MSU Extension E-154 bulletin, “2013 Michigan Fruit Management Guide”). Note that these recommendations should not supersede insecticide label restrictions or farm-level knowledge based on site-specific pest scouting, but rather are meant to compliment a comprehensive pest management decision-making process.

Rainfastness rating chart: General characteristics for insecticide chemical classes.

Insecticide class

Rainfastness ≤ 0.5 inch

Rainfastness ≤ 1.0 inch

Rainfastness ≤ 2.0 inch

Fruit

Leaves

Fruit

Leaves

Fruit

Leaves

Organophosphates

L

M

L

M

L

L

Pyrethroids

M/H

M/H

M

M

L

L

Carbamates

M

M/H

M

M

L

L

IGRs

M

M/H

M

M

 

 

Oxadiazines

M

M/H

M

M

L

L

Neonicotinoids

M,S

H,S

L,S

L,S

L,S

L,S

Spinosyns

H

H

H

M

M

L

Diamides

H

H

H

M

M

L

Avermectins

M,S

H,S

L,S

M,S

L

L

* H – highly rainfast (≤ 30 percent residue wash-off); M – moderately rainfast (≤ 50 percent residue wash-off); L – low rainfast (≤ 70 percent residue wash-off); S-systemic residues remain within plant tissue

Apple insecticide precipitation wash-off re-application decision chart: Expected codling moth control in apples, based on each compound’s inherent toxicity to codling moth larvae, maximum residual and wash-off potential from rainfall.

Insecticides

Rainfall = 0.5 inches

Rainfall = 1 inch

Rainfall = 2 inches

*1 day

*7 days

*1 day

*7 days

*1 day

*7 days

Guthion

 

 

 

X

X

X

Imidan

 

X

 

X

X

X

Asana

 

X

X

X

X

X

Calypso

 

 

X

X

X

X

Assail

 

 

X

X

X

X

Proclaim

 

X

 

X

X

X

Rimon

 

 

X

X

X

X

Delegate

 

 

 

 

X

X

Altacor

 

 

 

 

X

X

Belt

 

 

 

 

X

X

* Number of days after insecticide application that the precipitation event occurred.
X – Insufficient insecticide residue remains to provide significant activity on the target pest, and thus re-application is recommended.
- An un-marked cell suggests that there is sufficient insecticide residue remaining to provide significant activity on the target pest, although residual activity may be reduced.

Grape insecticide precipitation wash-off re-application decision chart: Expected Japanese beetle control in juice grapes, based on each compound’s inherent toxicity to Japanese beetle adults, maximum residual, and wash-off potential from rainfall.

Insecticides

Rainfall = 0.5 inches

Rainfall = 1 inch

Rainfall = 2 inches

*1 day

*7 days

*1 day

*7 days

*1 day

*7 days

Imidan

 

X

X

X

X

X

Sevin

 

 

X

X

X

X

Capture

 

 

 

X

X

X

Actara

 

X

 

X

X

X

Avaunt

 

X

 

X

X

X

* Number of days after insecticide application that the precipitation event occurred.
X – Insufficient insecticide residue remains to provide significant activity on the target pest, and thus re-application is recommended.
- An un-marked cell suggests that there is sufficient insecticide residue remaining to provide significant activity on the target pest, although residual activity may be reduced.

Blueberry insecticide precipitation wash-off re-application decision chart: Expected cranberry fruitworm control in blueberries, based on each compound’s inherent toxicity to cranberry fruitworm larvae, maximum residual, and wash-off potential from rainfall.

Insecticides

Rainfall = 0.5 inches

Rainfall = 1 inch

Rainfall = 2 inches

*1 day

*7 days

*1 day

*7 days

*1 day

*7 days

Guthion

 

X

X

X

X

X

Asana

 

X

X

X

X

X

Intrepid

 

X

X

X

X

X

Assail

 

X

 

X

X

X

Delegate

 

X

 

X

X

X

* Number of days after insecticide application that the precipitation event occurred.
X – Insufficient insecticide residue remains to provide significant activity on the target pest, and thus re-application is recommended.
- An un-marked cell suggests that there is sufficient insecticide residue remaining to provide significant activity on the target pest, although residual activity may be reduced.

Blueberry insecticide precipitation wash-off re-application decision chart: Expected Japanese beetle control in blueberries, based on each compound’s inherent toxicity to Japanese beetle adults, maximum residual, and wash-off potential from rainfall.

Insecticides

Rainfall = 0.5 inches

Rainfall = 1 inch

Rainfall = 2 inches

*1 day

*7 days

*1 day

*7 days

*1 day

*7 days

Imidan

X

X

X

X

X

X

Mustang Max

 

X

 

X

X

X

Sevin

 

X

X

X

X

X

Provado

 

X

X

X

X

X

* Number of days after insecticide application that the precipitation event occurred.
X – Insufficient insecticide residue remains to provide significant activity on the target pest, and thus re-application is recommended.
- An un-marked cell suggests that there is sufficient insecticide residue remaining to provide significant activity on the target pest, although residual activity may be reduced.

Insecticide persistence, plant penetration and rainfastness rating

Compound class

Persistence (residual on plant)

Plant penetration characteristics

Rainfast rating

Organophosphates

Medium - Long

Surface

Low

Carbamates

Short

Cuticle Penetration

Moderate

Pyrethroids

Short

Cuticle Penetration

Moderate - High

Neonicotinoids

Medium

Translaminar & Acropetal

Moderate

Oxadiazines

Medium

Cuticle Penetration

Moderate

Avermectins

Medium

Translaminar

Moderate

IGRs

Medium - Long

Translaminar

Moderate

Spinosyns

Short - Medium

Translaminar

Moderate - High

Diamides

Medium - Long

Translaminar

Moderate - High

 Dr. Wise’s work is funded in part by MSU’s AgBioResearch.