In view of the frequent heavy rains in many regions this season, understanding rainfastness of insecticides is important to ensure proper efficacy. 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 the 2018-2019 Publication 360: Guide to Fruit Production and 2019 Supplement for a complete list of registered products.
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 translaminar 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.
Table 1. General characteristics for insecticide chemical classes
| Insecticide Group | Rainfastness ≤ 0.5 inch | Rainfastness ≤ 1 inch | Rainfastness ≤ 2 inches | |||
| Fruit | Leaves | Fruit | Leaves | Fruit | Leaves | |
| Carbamates (1A)
Lannate |
M | M/H | M | M | L | L |
| Organophosphates (1B)
Imidan, Malathion |
L | M | L | M | L | L |
| Pyrethroids (3A)
Decis, Mako, Matador, Perm-Up, Pounce, Silencer, Up-Cyde, Ambush |
M/H | M/H | M | M | L | L |
| Neonicotinoids (4A)
Actara, Admire, Alias, Assail, Calypso, Closer, Clutch, Cormoran, Sivanto Prime |
M,S | H,S | L,S | L,S | L,S | L,S |
| Spinosyns (5)
Delegate, Entrust, Success, TwinGuard |
H | H | H | M | M | L |
| Avermectins (6)
Agri-Mek, Minecto Pro |
M,S | H,S | L,S | M,S | L | L |
| IGRs (15 & 18)
Rimon, Cormoran, Confirm, Intrepid |
M | M/H | M | M | L | L |
| Diamides (28)
Altacor, Exirel, Harvanta |
H | H | H | M | M | L |
H –highly rainfast (≤30% residue wash-off), M –moderately rainfast (≤50% residue wash-off), L –low rainfast (≤70% residue wash-off), S –systemic residues remain with plant tissue
Table 2. Insecticide persistence, plant penetration and rainfastness rating
Insecticide Group |
Persistence |
Penetration |
Rainfast rating |
Carbamates (1A)
Lannate |
Short |
Cuticle |
Moderate |
Organophosphates (1B)
Imidan, Malathion |
Medium-long |
Surface |
Low |
Pyrethroids (3A)
Decis, Mako, Matador, Perm-Up, Pounce, Silencer, Up-Cyde, Ambush |
Short |
Cuticle |
Moderate-high |
Neonicotinoids (4A)
Actara, Admire, Alias, Assail, Calypso, Closer, Clutch, Cormoran, Sivanto |
Medium |
Translaminar, acropetal |
Moderate |
Spinosyns (5)
Delegate, Entrust, Success, TwinGuard |
Short-medium |
Translaminar |
Moderate-high |
Avermectins (6)
Agri-Mek, Minecto Pro |
Medium |
Translaminar |
Moderate |
IGRs (15 & 18)
Rimon, Cormoran, Confirm, Intrepid |
Medium-long |
Translaminar |
Moderate |
Diamides (28)
Altacor, Exirel, Harvanta |
Medium-long |
Translaminar |
Moderate-high |
*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.
Refer to Publication 360 for a complete list of management options.
There is no comparable research on rainfastness of fungicides and few of the labels specify the period for rainfastness. However, the common rule of thumb for rainfall and protectant fungicides is the same as above for insecticides. With the heavy rainfalls we’ve had, it is necessary to reapply protectant fungicides (sulphur, copper, captan, mancozeb, biologicals) as well as insecticides.
The effectiveness of sticker-spreaders with fungicides is variable and product/crop specific. For example, in grape research trials, Wayne Wilcox has consistently better results with sulphur when a spread sticker is added. Rovral and Indar give better brown rot control when used with a non-ionic surfactant or other penetrating agent. However, captan, which is intended to stay on the surface, is notorious for causing injury when mixed with oils or some penetrating surfactants that cause them to penetrate the waxy cuticle. Penetrating agents don’t help strobilurins; in fact, some fungicide/crop combinations have been associated with minor phytotoxicity due to excessive uptake. Consult labels for minimum drying times for individual products and recommendations for using surfactants.
Be aware that phosphorous acid products penetrate very rapidly – anecdotally (from Wayne Wilcox) as quickly as 20 minutes. Reapplication after the rain (the same day) resulted in severe foliar injury.
Annemiek Schilder did some interesting research on the effects of rainfall on fungicide residues. The following is excerpted from one of her articles:
Apply protectant fungicides like during sunny, dry conditions to allow for quick drying onto the leaves. In fact, it appears that protectant fungicides become better adsorbed to the plant surface and more rainfast over several days after application. While it is better to have protectant fungicide applications on before a rain or heavy dew event which could represent an infection period, avoid putting on protectant fungicides within several hours before a rainstorm as you may lose much of it to wash-off.
Apply systemic fungicides under humid, cloudy conditions when the soil is moist. That way, the cuticle, or waxy layer covering the plant surface, will be swelled up and allows the active ingredients to quickly pass through. Under extended hot, dry conditions, the cuticle becomes flattened and less permeable; any product that is not absorbed may remain on the plant surface and break down due to UV light or microbes or get washed off by rain.
Website: https://www.canr.msu.edu/news/how_to_get_the_most_out_of_your_fungicide_sprays