With the continued wet season across the province this year, understanding rainfastness, or the ability of a pesticide to withstand rainfall, is important to ensure proper efficacy. Coverage will need to be considered right up until harvest during rainy seasons like this. This will protect fruit from any late disease infection periods from scab or rots as well as insect activity that extends into the fall such as apple maggot or codling moth. Not only does the amount of rain impact rainfastness but also the age of the spray.

All pesticides require a certain amount of drying time between application and a rain event. Typically, residue loss by wash-off is greatest when rain occurs within 24 hours of spraying. After this point, the rainfastness of a product will depend on formulation, adjuvants and length of time since application.

Rainfastness of Insecticides

John Wise, Michigan State University has studied rainfastness of various common tree fruit insecticide groups and is summarized below. For the complete article, click here.

Note that some products listed by Wise may not be registered for use in Canada. Check with your local supplier or refer to the Ontario Crop Protection Hub 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:

1. 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.
2. 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.
3. 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.
4. 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 (1.25 cm)	Rainfastness ≤ 0.5 inch (1.25 cm)	Rainfastness ≤ 1 inch (2.5 cm)	Rainfastness ≤ 1 inch (2.5 cm)	Rainfastness ≤ 2 inches (5 cm)	Rainfastness ≤ 2 inches (5 cm)
	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) Danitol, Decis, Labamba, Matador, Perm-Up, Poleci, Pounce, Ship, Silencer, Up-Cyde, Zivata	M/H	M/H	M	M	L	L
Neonicotinoids (4A) Aceta, Assail, Calypso, Cormoran, Theme *(Closer, Sivanto may be similar)	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, Vayego	H	H	H	M	M	L

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) Danitol, Decis, Labamba, Matador, Perm-Up, Poleci, Pounce, Ship, Silencer, Up-Cyde, Zivata	Short	Cuticle	Moderate-high
Neonicotinoids (4A) Aceta, Assail, Calypso, Cormoran, Theme *(Closer, Sivanto may be similar)	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, Vayego	Medium-long	Translaminar	Moderate-high

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:

• ½ inch (1.25 cm) 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 (2.5 cm) 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 could withstand 1 inch of rain with 1-day old residues.
• 2-inch (5 cm) rainfall: For all products, 2 inches of rain will remove enough insecticide to make immediate re-application necessary.

Rainfastness of Fungicides

There is no comparable research on rainfastness of fungicides and few labels provide this kind of information. A general rule of thumb often used is that 1 inch (2.5 cm) of rain removes approximately 50% of protectant fungicide residue and over 2 inches (5 cm) of rain will remove most of the residue. However, many newer formulations or with the addition of spreader-stickers, some products may be more resistant to wash-off. However, avoid putting on fungicides within several hours before a rainstorm as much can be lost to wash-off regardless of formulation. As well, there are exceptions to the general rule in regard to truly systemic fungicides such as Aliette and Phostrol.

The effectiveness of sticker-spreaders with fungicides is variable and product/crop specific. Penetrating agents don’t help strobilurins; in fact, some fungicide/crop combinations have been associated with minor phytotoxicity due to excessive uptake. 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. 

Consult labels for minimum drying times for individual products and recommendations for using surfactants. 

Consider the following to improve fungicide efficacy during wet weather:

• During rainy periods, systemic fungicides tend to perform better than protectant (or contact) fungicides since they are less prone to wash-off.
• Applying a higher labelled rate can extend the residual period.
• Apply protectant fungicides such as captan (Supra Captan, Maestro), mancozeb (Manzate, Dithane, Penncozeb) and folpet (Folpan) during sunny, dry conditions to allow for quick drying on the leaves. These types of fungicides are better absorbed and become rainfast over several days after application.
• Similar to the above protectants, fluazinam (Allegro) is not systemic but does have good sticker properties during times of minor rainfall.
• Apply systemic fungicides such as sterol inhibitors (Cevya, Nova, Fullback, Inspire), SDHI (Excalia, Fontelis, Sercadis, Kenja, Aprovia, Luna Tranquility) and strobilurins (Flint, Pristine, Merivon) under humid, cloudy conditions. The leaf cuticle will be swollen, allowing quicker absorption. In dry, hot conditions, the cuticle can become flattened and less permeable, so product can breakdown in sunlight, heat or microbial activity or be washed off by rain.
• In general, most protectant biological products such as Buran or Serenade are prone to wash-off. Do not apply Buran if rainfall is imminent. However, this product can be used as a post-infection rescue treatment for apple scab.
• SAR/ISR products such as Regalia or Lifegard are rainfast after 1-3 hours but do not provide immediate protection. Maximum protection from these products is achieved in 3-5 days following application so timing these products in anticipation to infection events is critical.

For more information, see the article How to Get the Most Out of Your Fungicide Sprays by Annemiek Schilder, Michigan State University.