Too Hot to Spray? Too Wet to Wait? Managing Pesticide Decisions During Extreme Weather

Weather has always been part of pest management. But as Ontario experiences more frequent extreme weather, such as intense rainfall or flooding, prolonged heat, drought, and strong winds, it is becoming an increasingly important factor in deciding when and how to spray. For example, heavy rainfall and wind can impede insect movement (think mites, moth flight, etc), challenge pesticide persistence (i.e., wash off, coverage) and increase disease pressure or spread. Meanwhile, prolonged periods of heat and drought can alter tree physiology, accelerate or cease pest development, affect spray performance, and increase the risk of crop injury.

With difficult conditions, come difficult questions: Should a product be reapplied after rain? Will pests continue developing during a heat wave? When does tree stress impact efficacy of a product?

The answer is rarely as simple as looking at the thermometer or rain gauge (or asking your favourite weather app). Successful pest management during extreme weather requires consideration of three interacting factors:

1. How weather affects the pest.
2. How weather affects the product.
3. How weather affects the tree.

Rain: Protection Meets Infection

Rain is perhaps the most obvious weather factor affecting product performance. But rainfall has two sides: it can reduce the amount of product on the tree, while also creating conditions that affect pest pressure.

Rain Vs Pest

For insects, rainfall can alter movement, activity and the timing of some development events. For example, spider mites can be washed off in heavy rainfall, moth pests don’t fly in wet conditions, movement of plum curculio into orchards often follow a warm thunderstorm after petal fall, and apple maggot emergence depends strongly on sufficient soil moisture.

For disease, the relationship with rainfall is much more direct. Many pathogens depend on water for some part of their disease cycle. Rain can trigger spore release, dispersal and infection, while prolonged wetting can extend infection periods. Rain splash can move spores from infected tissue to healthy leaves and fruit, while wind and rain together can distribute inoculum throughout the canopy and to neighboring blocks.

Apple scab is a classic example: infection requires a wetting event, with free water on the leaf surface initiating germination and temperature determining how quickly the infection progresses. Other diseases, like black rot, bitter rot and fire blight are spread through rain splash during warm, humid conditions.

This means rainy periods can simultaneously increase disease pressure and decrease the protection provided by previous sprays. The combination is particularly important after extreme rainfall. During the flooding events in Niagara this summer, rainfall totals exceeded the amounts used in many published rainfastness studies (meaning everything was washed off), while flooded conditions allowed for sitting water around roots and delayed orchard access. That delay can give pathogens additional time to sporulate and infect tissue.

Rain Vs Product

Rainfall can reduce product residues through wash-off, but the amount of rain alone does not determine whether an application has failed.

The greatest loss of residue generally occurs when rain falls shortly after application (within 24 hours), before the spray has had sufficient time to dry, adhere to the plant surface or move into plant tissue, depending on the product. Once a residue has become established, its ability to withstand rainfall depends on several factors, including:

• chemistry and formulation of the product
• plant penetration
• amount and timing of rainfall
• age of the residue
• environmental persistence
• inherent activity of the pesticide against the target pest

Research conducted by Dr. John Wise at Michigan State University has demonstrated that insecticides differ considerably in their susceptibility to wash-off. In general, products that remain primarily on the plant surface such as organophosphates are more vulnerable to rainfall than products that move into plant tissue such as spinosyns, diamides, avermectins, and some insect growth regulators. Even so, locally systemic or translaminar products including neonicotinoids are not immune to wash-off because some residue remains on the plant surface.

Tables 1 & 2 serve as a guide for general rainfastness to common insecticide groups.

Table 1. General rainfast characteristics for common insecticide groups registered in apples

Insecticide Group	Rainfastness ≤ 0.5 inch(1.25 cm)		Rainfastness ≤ 1 inch (2.5 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

The same distinction is important with fungicides. Contact fungicides such as Group M and many biologicals depend heavily on maintaining protective surface residue, while systemic or translaminar products such as DMI, SDHI and SIs may be less vulnerable once absorbed. But similar to insecticides, neither category should be considered completely “rainproof”. Refer to Table 3 for type of fungicide activity of commonly registered products.

Table 3. Type of activity of common fungicides registered in apples

Fungicide Group	Trade Name	Type of Activity
M	Copper, Copper Spray, Cueva, Dithane, Follow, Folpan, Kumulus, Maestro, Manzate, Microscopic Sulphur, Microthiol Disperss, Parasol, Penncozeb, Supra Captan	Contact
1	Senator	Locally systemic
3	Cevya, Fullback, Nova, Aprovia Top (3+7), Inspire Super (3+9)	Locally systemic
7	Aprovia, Excalia, Fontelis, Kenja, Sercadis, Luna Tranquility (7+9), Merivon (7+11), Pristine (7+11)	Locally systemic
9	Scala	Locally systemic
11	Flint	Locally systemic
19	Diplomat	Systemic
24	Kasumin	Locally systemic
25	Streptomycin	Locally systemic
29	Allegro, Downforce, Vantana	Locally systemic
39	Magister	Locally systemic
50	Property	Locally systemic
52	Migiwa	Locally systemic
BM	Blossom Protect, Double Nickel, Serenade, Serifel	Contact
BM	Ecoswing	Locally systemic
NC	Buran, Cyclone Plus, Milstop, Oxidate, Purespray Green Spray Oil, Suffoil-X, Vegol Crop Oil	Contact
P5	Regalia	Locally systemic
P6	Lifegard	Locally systemic
P7	Phostrol	Systemic
U12	Syllit	Locally systemic
U13	Gatten	Locally systemic

A general rule of thumb often used is that 1 inch (2.5 cm) of rain removes approximately 50% of protectant fungicide residues and over 2 inches (5 cm) of rain will remove most of the residue.

While the above is generally true, rainfastness can also be influenced by formulation, application rate, coverage, adjuvants and the amount of time between application and rainfall.

Rainfastness is not the same as residual activity.

A pesticide may remain on the surface after a rain event yet still have little effective residue remaining because the product was already near the end of its activity period. Most labels provide the interval between applications, or the length of residual activity under ideal conditions.

For more information on rainfastness of insecticides and fungicides, see Pesticides & Rain: Ensuring Strengths in Storms on the ONfruit blog.

Rain Vs Tree

Rain doesn’t just remove what is already on the tree. It can change what needs protection. Rapid growth following good moisture can encourage expansion of new leaf, shoot, bloom or fruit tissue that wasn’t present when the previous application was made. At the same time, prolonged wetness can increase the conditions conducive to disease development both within the canopy and in the roots. The result is a moving target: some existing protection may have been reduced while new susceptible tissue has developed.

Wind: Where Did the Spray Go?

Wind is often treated primarily as a drift concern. But in an orchard, it is also a coverage and deposition problem.

Wind Vs Pest

Wind can influence the movement and distribution of insects and can help disperse airborne pathogens, spreading spores from inoculum sources into nearby susceptible blocks, up to many kilometers away on strong currents. It’s an environmental factor that can turn orchard IPM into a regional or neighbourhood issue.

A good example of how wind can aid in dispersal was the ash many regions saw in the air coming from the wildfires in the north this summer. With the ability to carry large particulates like that, imagine how easy a microscopic spore can travel.

Wind Vs Product

With orchard sprayers, wind interacts with fan-generated airflow, canopy density, travel speed, nozzle configuration and droplet behaviour. Strong or gusty winds can carry spray beyond the intended target – or short of it, reducing deposition where it is needed and increasing off-target movement.

Wet or gusty conditions can seriously reduce coverage while increasing drift and runoff. It is important to adjust airflow and assess the actual coverage (in all parts of the tree!) rather than assuming the spray is reaching its target. For more information, refer to Airblast Spraying in Poor Conditions, Ten Tips for Spraying in the Wind (herbicide specific) and Fundamentals of Spray Drift on Sprayers101.com.

A product cannot protect tissue it never reaches.

In the last few years, I have commonly heard from growers surprised to find scab in their orchard, despite a strong fungicide program. Often, looking further into where this infection began suggested consistent wind and poor coverage to be playing a role in some escapes. Sprays weren’t always reaching the upper canopy or penetrating into the dense canopy even with a well-calibrated sprayer.

Wind Vs Tree

Moving air increases evaporative demand of a tree and can increase water loss from leaves. In a well-watered orchard, this may be relatively manageable. In a drought-stressed tree, however, additional atmospheric demand can further increase stress.

Wind also changes how the canopy behaves during an airblast application. A sparse canopy provides less resistance to airflow, while a dense canopy intercepts more of the air and spray. As the tree canopy changes through the season, the same sprayer settings may therefore produce different coverage patterns.

Heat: Behaviour Changes

When high temperatures persist, the orchard’s biological processes and environmental conditions begin to shift. Warm nights can keep pest activity going, while intense daytime heat and solar radiation place

additional demands on the tree. Humidity and soil moisture further influence how the orchard responds. In other words, a heat wave can change the conditions under which pest activity, product performance and crop response occurs.

Heat Vs Pest

When it comes to pest activity, heat does not necessarily mean slower. Our hot days with temperatures reaching 27-30°C are still well within the favourable range for development for many pests. In fact, these conditions can accelerate degree day accumulation and likely advance pest development.

Insects and mites are ectothermic, meaning their development is dependent on external temperature. For common insect pests, including aphids, moths, leafhoppers, and scale, warm temperatures are more likely to increase development than suppress it. Warm nights are particularly important, allowing insects to continue accumulating degree days around the clock.

However, insects do not continue developing faster indefinitely as temperatures rise. While some insects adjust to heat by being less active during the hottest parts of the day such as plum curculio, temperatures generally need to exceed 32-35°C for extended periods before significant reductions in activity or survival occur.

If running developmental models, consider including the maximum temperature thresholds when days are above 30°C.

Some approximate maximum thresholds for common orchards pests include:

• Codling moth – 31°C
• Oriental fruit moth – 32°C
• Obliquebanded leafroller – 30°C
• San Jose scale – 32°C
• European red mite – 30°C
• Two-spotted spider mite – 34°C

In addition to impacting development, reproduction or survival, temperatures can also influence metabolic activity that can increase insecticide detoxification or toleration. For instance, toxicity to some organophosphate insecticides can increase with temperature, while pyrethroid toxicity decreases.

Disease responses are even more nuanced…

Apple scab, for example, still requires leaf wetness for infection. Infection is generally most efficient under moderate temperatures. Sustained temperatures over 28°C become less favourable for ascospore survival, germination, and infection. However, there does not seem to be a specific temperature threshold above which infection ceases. If wetting events occur, infection remains possible, particularly during cooler overnight periods.

Bitter rot is a different story. Warm temperatures, high humidity, and mild nights create ideal conditions for infection. Temperatures between 25-30°C are near optimal for disease development, especially if thunderstorms are in the forecast. Susceptibility studies led by University of Guelph in recent years have shown fruitlets can be infected with bitter rot shortly after petal fall; however, symptoms often do not become apparent for several weeks.

Heat Vs Product

Heat- and drought-stressed trees may have a reduced capacity to tolerate additional stress. Under these conditions, products that are normally safe can present a greater risk of leaf injury, fruit injury or other phytotoxic effects.

Risk can increase when:

• Temperatures are high and solar radiation is intense
• Warm nights provide little opportunity for trees to recover from daytime stress
• Trees are moisture stressed
• Fruit or foliage is directly exposed to intense sunlight
• Oils, penetrant adjuvants or other materials with known crop safety considerations are included in the tank mix
• Products are applied outside label-recommended environmental conditions

There is no single temperature at which every pesticide becomes unsafe. Crop safety depends on the product, formulation, rate, tank mix, cultivar, crop stage and environmental conditions.

That said, when daytime temperatures climb into the 30s, careful consideration needs to be made when applying sprays. Making applications during the evening or overnight when temperatures are below 25°C can reduce the risk of heat-related phytotoxicity.

To reduce the risk:

• Aim for 6-8 hours of drying time before temperatures begin to rise the next morning.
• Evening applications (e.g., 9:00pm – midnight) as temperatures begin to drop generally provide a greater safety margin than spraying just before sunrise.
• Keep tank mixes as simple as possible during periods of prolonged heat.

Heat does not automatically cause phytotoxicity. The risk increases when spray deposits remain wet as temperatures rise, trees are already heat- or drought-stressed, or products (especially captan, copper, or sulphur) are applied with penetrating adjuvants, crop oils or certain foliar nutrients that facilitate uptake.

Many systemic pesticides are generally well tolerated when applied according to the label. However, these products are absorbed most efficiently when the plant cuticle is hydrated. During prolonged hot, dry weather, uptake may be slower, making good spray coverage and appropriate application timing even more important.

Biological product efficacy may also be affected by prolonged hot, dry weather. Cooler evening temperatures and higher overnight humidity can improve survival and establishment of beneficial pathogens (ie., active ingredients in the biological products) on tissue surfaces, potentially enhancing their effectiveness.

Heat Vs Tree

Under hot, dry conditions, apple trees may reduce stomatal opening and transpiration to conserve water. At the same time, exposed leaves and fruit can become substantially warmer than the surrounding air, affecting photosynthetic activity and immune function.

Importantly, a tree can experience heat stress even when soil moisture appears adequate. High temperatures, intense solar radiation and dry air can create a high evaporative demand that challenges the tree’s ability to maintain water balance.

Extended heat can contribute to:

• Leaf scorching
• Fruit sunburn
• Premature fruit drop
• Reduced shoot growth
• Reduced overall physiological resilience (ie., winter hardiness, etc)

A stressed tree therefore has a smaller margin for additional stress. This is particularly important when considering products or combinations that may cause crop injury under certain environmental conditions. For instance, products that activate a tree’s own defence response, such as systemic acquired resistance (SAR) inducers, require the tree to divert energy toward defense rather than growth or stress recovery. During periods of prolonged heat or drought stress, when trees are already under physiological strain, consider whether the benefits of inducing defense outweigh the additional metabolic stress, particularly if disease pressure is low.

Induced resistance works best when the tree has the resources to mount a defense response.

Drought: Stress Adds Up

Drought is different from heat, although the two frequently occur together. A tree can experience high temperatures without severe water stress if adequate moisture is available. Conversely, moderate temperatures following a prolonged dry period can still leave trees physiologically stressed.

Drought Vs Pest

Many of our common orchard pathogens depend on rainfall, humidity or prolonged leaf wetness, so dry conditions can reduce opportunities for infection. But that does not mean disease risk disappears. The arrival of rain following a dry period can rapidly change the disease environment, particularly when inoculum is already present.

Insects and mites can respond differently. Hot, dry conditions can favour certain mite populations, while drought-stressed plants may also alter the severity of insect pests that attack stressed trees such as borers.

Drought Vs Product

Under hot and dry conditions, changes in leaf hydration and cuticle characteristics can influence how droplets spread, dry and penetrate the plant surface. Rapid evaporation can also concentrate spray residues. The magnitude and importance of these effects will vary depending on the product formulation, plant condition and application environment.

Foliar-applied pesticides generally enter plant tissues primarily through the cuticle rather than through open stomata. Once absorbed, movement within the plant depends on the properties of the active ingredient and whether it is capable of movement in xylem, phloem or only within treated tissues. Because drought stress can reduce transpiration, growth and vascular flow, it may influence the movement and redistribution of products that depend on these processes.

At the same time, reduced shoot growth can also change the need for protection of newly developing tissues.

This does not mean systemic pesticides stop working during drought. Rather, environmental stress can reduce the predictability of uptake and movement and reinforce the importance of good coverage, appropriate timing and maintaining tree water status wherever possible.

Drought Vs Tree

For the tree, drought is fundamentally a water-balance problem. Water stress can reduce cell turgor, alter photosynthesis and limit growth. It can also increase susceptibility to heat injury, cause growth cracks or splitting during moisture fluctuations that are susceptible to secondary infection, and reduce the tree’s ability to recover from additional environmental or chemical stress.

The tree’s condition matters when interpreting risk.

The same product applied to a healthy, well-watered tree and a severely stressed tree may not have the same crop safety outcome.

Before You (Re)Spray After Extreme Weather

When weather conditions disrupt a pest management program, consider the following before making the next application:

1. What is the current pest risk?

Don’t make a management decision based on weather alone. Use scouting, traps, disease forecasting and pest development models or lifecycle information to determine whether the target is currently active and whether treatment is still justified.

2. What happened to the previous application?

Consider the age of the residue, drying time before rainfall, rainfall amount and the product’s expected rainfastness and residual activity.

3. What condition is the tree in?

Look for signs of moisture stress, heat stress, sunburn or reduced growth. A stressed tree may respond differently to an application than a healthy, actively growing tree.

4. Are conditions suitable for another application?

Consider temperature, relative humidity, wind, expected rainfall and any specific environmental conditions that may impact product performance.

Weathering the Storm

Heavy rainfall can reduce pesticide residues while simultaneously increasing disease pressure and promoting new growth. Heat can accelerate the development of some pests and narrow management windows. Hot, dry conditions can affect droplet behaviour, spray deposits, plant uptake and the movement of some systemic products. At the same time, environmental stress can increase the risk of phytotoxicity and reduce the tree’s ability to tolerate additional stress.

The most useful framework for making spray decisions during extreme weather is to consider three interacting factors:

1. How does the weather affect the pest?
2. How does the weather affect the product?
3. How does the weather affect the tree?

There is rarely a single answer that applies to every product, orchard or weather event. Understanding the interaction between these three factors — and using labels, scouting and pest forecasting tools to guide decisions — will become increasingly important as Ontario orchards continue to experience periods of intense rainfall, prolonged heat and summer drought.

References

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Fronk, L. 2026. Fungicides & Rain. PennState Extension.

Garcia-Salazar, C. 2012. Counteracting Adverse Weather Effects on Deposition and Degradation of Insecticides Used in Berries. Michigan State University Extension.

Grigg-McGuffin, K. 2024. Timing of Infection and Management of Bitter Rot in Ontario. ONcore Newsletter. 28(1): 20-24.

Grigg-McGuffin, K & McFadden-Smith, W. 2024. Pesticides & Rain: Ensuring Strength in Storms. ONfruit.

Giuliani, N., Wegher, M., Asensio, D., Zanotelli, D., Andreotti, C. & Tagliavini, M. 2025. Impact of Soil Water Availability on Apple Tree Physiology During Heatwaves and on Post-Stress Recovery. Environmental and Experimental Botany. 235: 106161.

Murray, M. 2008. Using Degree Days to Time Treatments for Insect Pests. Utah Pests Fact Sheet IPM-05-08. Utah State University Extension.

Niu, G., Yao, Y., Li, L., Ji, M., Liu, H., Gao, L., Wang, X., Xu, H., Zhang, D., Wang, Y. Xu, J. & Hao, B. 2026. Advances in Physiological and Molecular Mechanisms of Heat Stress in Apple and Pear. Plants. 15(16): 2429

Schilder, A. 2014. How to Get the Most Out of Your Fungicide Sprays. Michigan State University Extension.

Wise, J. 2019. Rainfast Characteristics of Insecticides on Fruit. Michigan State University Extension.

Wolf, T. 2021. Fundamentals of Spray Drift. Sprayers101.

Wolf, T. 2021. Ten Tips for Spraying in the Wind. Sprayers101.