Michigan State University's John Wise uses a rain chamber to measure the rainfastness of insecticides sprayed on apple trees in the field, with the fruit and leaf clusters brought into the chamber and exposed to simulated rain.

Michigan State University’s John Wise uses a rain chamber to measure the rainfastness of insecticides sprayed on apple trees in the field, with the fruit and leaf clusters brought into the chamber and exposed to simulated rain.

Though insecticide labels provide plenty of dos and don’ts, growers are often left in the dark when it comes to the chemical’s rainfastness and when reapplication may be needed if rain occurs after treatment. A Michigan State University entomologist is working to develop a rainfastness database that will guide growers in deciding when retreatment for codling moth and other insect control may be necessary.

If you spray for codling moth today and it rains 24 hours later, is it necessary to repeat the treatment, or is there enough residual insecticide activity to kill the target pest for another week? How about if you spray today and it rains a week later? How does the amount of rain received influence the rainfastness of an insecticide?

Dr. John Wise, MSU entomologist and coordinator of the university’s Trevor Nichols Research Complex in Fennville, is developing a series of charts that will show the impact of precipitation on insecticide sprays and help growers decide when retreatment is necessary. His goal is to have the information included in the MSU Fruit Management Guide (MSUE Bulletin E-154).

For the last two years, Wise has been evaluating the rainfastness of new and old codling moth insecticides, using a rain chamber to simulate precipitation events. Results are still preliminary as he continues to look at varying amounts, times, and duration of rain and its impact on the residual activity of insecticides.


When insecticides are sprayed on a fruit tree, a lot of things are going on, he explained. There is ultraviolet degradation of the chemical; if rain occurs, there is some wash-off; and for some compounds, there is translocation into the plant. "All of these things are factors that influence the subsequent performance of the insecticides," he said, adding that factors directly influencing the wash-off include the amount of rainfall, duration, size, and velocity of raindrops. "One rain is not the same as another."

There are also dynamics between the plant and chemical. Most of the older insecticides are largely surface-residue compounds; however, many of the newer ones have penetrative characteristics, he said.

"Rainfastness is not a yes or no issue," Wise stated, noting that there is variability by chemical compound and by plant type.

Moreover, he said that the inherent toxicity of the chemical and residual decline by ultraviolet degradation will drive whether wash-off matters. "The more inherently effective the compound is, the more forgiveness you’ll have in relationship to weather conditions."

Rain chamber

Wise evaluated the rainfastness at varying amounts of simulated rain—.1, .5, 1, and 2 inches. The length of time after insecticide application varied between 24 hours and seven days after spraying. The procedure was to spray codling moth insecticides in the orchard with a ground sprayer and then bring leaves and fruit/leaf clusters into the rainfall chamber for simulated rain.

He also conducted bioassays on the fruit/leaf clusters, exposing codling moth larvae to treated apple tissues after different simulated rainfall events. This was to quantify how rainfall impacted the pesticide performance on the pest. He also conducted residue analysis for each treatment compound to measure the loss of residue (surface and inside the plant tissue) resulting from simulated rainfall events.

Preliminary results

Codling moth insecticides under study included different chemical families and modes of action, from organophosphates Guthion (azinphos-methyl) and Imidan (phosmet) and synthetic pyrethroids like Asana (esfenvalerate), to the newer groups like the neonicotinoids Assail (acetamiprid) and Calypso (thiacloprid), microbial fermentation products like Proclaim (emamectin benzoate), and insect growth regulators, such as Rimon (novaluron).

When simulating .1 inch of rain, 24 hours after spraying, all of the tested chemicals performed well. However, Assail, Proclaim, and Asana began showing some performance loss at .5 inch (after 24 hours of drying). "But all insecticides were better than no treatment at all," Wise said.

The bigger risk came when rain occurred seven days after treatment and field aging. All materials showed a drop in performance after two inches of rain when residues had aged seven days before the simulated event.

Wise is carefully studying the data to ensure he puts together reliable charts that will be dependable as a guide to indicate when it is necessary to re-treat. He envisions a chart in which growers can decide, "If you have freshly applied product sprayed yesterday and it rains today, do you or don’t you have to reapply?" he asks. Based on current data, he suggests that if the rain is .1 inch, the expense of re-treating is not justified unless sprays were made more than seven days ago. But with a half-inch of rain, Asana, Assail, and Proclaim could probably justify another treatment for codling moth.

As the rain amount increases, the number of chemicals that need to be reapplied also increases. At one inch of rain, most chemicals need reapplication, while at two inches, the list gets even bigger.

For applications that occurred seven days ago, based on preliminary data, Wise said that with .5-inch rain, Asana, Rimon, Assail, and Proclaim would need retreatment. At one inch and seven days, all except Delegate (spinetoram) and Altacor (rynaxypyr) would need reapplication, while at two inches, everything would need to be reapplied.


"I’ve learned that all compounds really maintain good activity with that .1 inch of rain, and there’s really hardly any justification to go out and spend that money again," he said. "Even with .5 inch of rain on fresh residues, you still have life left.

"But I did get the feel that the risk of residue wash-off on fruit is more than with leaves. If the pest is a foliar pest, you can take more risks than you can with fruit pests."

Additionally, he learned that although the organophosphate residues appear to be more susceptible to wash-off when the residue data is analyzed, the OPs tend to get the job done because of their inherent toxicity, even with a minimal amount of residue left.

"My picture of the neonicotinoids, right now, is that they have a unique characteristic in that the amount of product that moved into the plant tissue appears to be quite rainfast, but there’s some susceptibility to the residue that remains on the surface, especially given that they tend to be highly water soluble."