This summer has been pretty dry but when we get rains, they seem to be doozies! Understanding rainfastness of insecticides is important in deciding whether they require re-application after a rain. Here’s a reprise of an article posted in 2019 with some new products included.
Insecticide Rainfastness
The following information has been adapted from John Wise, Michigan State University. For the complete article, refer to https://www.canr.msu.edu/news/rainfast_characteristics_of_insecticides_on_fruit. Note that some products listed in this article may not be registered for use in Canada. Check with your local supplier or refer to Publication 360 2020 at http://www.omafra.gov.on.ca/english/crops/pub360/p360toc.htm for a complete list of registered products for your crops.
According to Wise, the impact of rain on an insecticide’s performance can be influenced by the following:
Penetration into plant tissue is generally expected to enhance rainfastness.
- Organophosphates have limited penetrative potential, and thus considered primarily surface materials.
- Carbamates and pyrethroids penetrate the cuticle, providing some resistance to wash-off.
- Spinosyns, diamides, avermectins and some insect growth regulators (IGR) readily penetrate the cuticle and move translaminar (top to bottom) in the leaf tissue.
- Neonicotinoids are considered systemic or locally systemic, moving translaminarily as well as through the vascular system to the growing tips of leaves (acropetal movement).
- For products that are systemic or translaminar, portions of the active ingredient move into and within the plant tissue, but there is always a portion remaining on the surface or bound to the waxy cuticle that is susceptible to wash-off.
Environmental persistence and inherent toxicity to the target pest can compensate for wash-off and delay the need for immediate re-application.
- Organophosphates are highly susceptible to wash-off, but are highly toxic to most target pests, which means re-application can be delayed.
- Carbamates and IGRs are moderately susceptible to wash-off and vary widely in toxicity to target pests.
- Neonicotinoids are moderately susceptible to wash-off, with residues that have moved systemically into tissue being highly rainfast, and surface residues less so.
- Spinosyns, diamides, avermectins and pyrethroids are moderate to highly rainfast.
Drying time can significantly influence rainfastness, especially when plant penetration is important. For instance, while 2 to 6 hours is sufficient drying time for many insecticides, neonicotinoids require up to 24 hours for optimal penetration prior to a rain event.
Spray adjuvants that aid in the retention, penetration or spread will enhance the performance of an insecticide.
The following tables can serve as a guide for general rainfastness to compliment a comprehensive pest management decision-making process.
*Tables adapted from “Rainfast characteristics of insecticides on fruit” by John Wise, Michigan State University Extension, https://www.canr.msu.edu/news/rainfast_characteristics_of_insecticides_on_fruit
Based on simulated rainfall studies to combine rainfastness with residual performance after field-aging of various insecticides, including carbamates (Lannate), organophosphates (Imidan, Malathion), pyrethroids (Capture), neonicotinoids (Assail, Actara, Admire), IGRs (Rimon, Intrepid), spinosyns (Delegate) and diamides (Altacor), Wise recommends the following re-application decisions for apples. Additional work was done on grapes and blueberries; see Wise’s article for this information. Among the crops, variation in rainfastness of a specific insecticide occurs since the fruit and leaves of each crop have unique attributes that influence the binding affinity and penetrative potential.
- ½ inch rainfall: All products with 1-day old residues could withstand ½ inch of rain. However, if the residues have aged 7 days, immediate re-application would be needed for all products but Assail, Rimon, Delegate or Altacor.
- 1-inch rainfall: In general, most products would need re-application following a 1-inch rainfall with 7-day old residues, whereas Delegate and Altacor could withstand this amount of rain on apples and would not need to be immediately re-applied. Some products such as Imidan on apples could withstand 1 inch of rain with 1-day old residues.
- 2-inch rainfall: For all products, 2 inches of rain will remove enough insecticide to make immediate re-application necessary.
It is important to note, not all products registered for the selected pests were included in this study. According to the Bayer CropScience infosheet on Movento: when applied in combination with recommended tankmix additives (non-ionic surfactant, methylated seed or horticultural oil), the active ingredient in Movento rapidly moves into the leaf to the vascular tissues where it is fully protected from environmental effects. Subsequent rainfall has negligible effects on residual performance.