The Rainfastness of Pesticides

By Mario Lanthier

I just finished spraying and the rain started.  Darn.  Do I have to spray again?

Here is a slightly different story.  I started early this morning so I could spray while nobody was around.  We forgot to tell the irrigation crew.  Now the water is running where I sprayed.  Will it wash away the spray residue?

All of us working with pesticides are faced with these questions at one time or another.  Pesticides are crucial in current commercial programs.  Yet the products are expensive and spraying is not a fun job.

Rain and irrigation can remove the pesticide residue from the plant surface or dilute the product making it less effective.  Research done in the 1960s related to older products no longer sold or no longer registered for ornamental plants, led to guidelines that may still be appropriate today: a rainfall of 2.5 cm (1 inch) will decrease the residual activity by 50% (if the material was to last 10 days, it will now last only 5 days).  The more recent research examined the newer pesticides which are often built with adjuvants to enhance their rainfastness.

This article will review recent information on the topic and offer guidelines to make proper decisions.

About rainfastness
A pesticide is considered “rainfast” when it has dried on the plant surface or was absorbed into the plant tissue and it remains effective even after a rainfall or irrigation.  

The degree of rainfastness depends on many factors.  The most important is the rain.  All pesticide products need a certain amount of drying time between application and rainfall.  Loss of pesticide effectiveness is greatest when rainfall occurs within 24 hours of spraying.  

Pesticide effectiveness is also lost after an intense rainfall of short duration: the large droplets have a mechanical impact on the plant surface while the rapid water accumulation can dislodge the pesticide deposits.  

Some pesticide products are designed with adjuvants, materials added to the formulation to increase the effectiveness.  Some adjuvants improve rainfastness by enhancing the capacity of the active ingredient to bind to the plant surface or by helping the systemic activity of the compound.  

Not all pesticides are impacted the same way by rain.  Pesticides formulated as dusts are easily washed away.  Wettable powders (WP) and granules (G) are less rainfast than flowables (F) and suspensions (SC).  Pesticides formulated as emulsifiable concentrate (EC) stay on the plant longer during rainfall as they are designed with an emulsifier to help mix in water.  Thus, the same product in EC formulation will be more rainfast than the WP formulation.

The key points

- Rainfastness is the property of a pesticide to withstand rainfall.
- For most pesticides, rain immediately after an application will remove most of the pesticide residue.  A product staying longer on the leaf before the rain will have adhered to the leaf surface or penetrated into the leaf cuticle.
- For fungicides and insecticides, expect a severe loss of efficacy after 5 cm (2 inches) of rain.
- It is the amount of rainfall that is a concern, not the duration.  A rainfall of 2.5 cm (1 inch) over 1 hour will remove more pesticide residue than a small drizzle or misting over several hours.
- At 24 hours after application, most products can sustain up to 2.5 cm (1 inch) of rain without loss of effectiveness.  However, a rainfall of 5 cm (2 inches) will remove enough pesticide residue that a reapplication will be necessary.
- Rainfastness is lost over time.  At 7 days after a spray, the residue of most products will be lost with a rainfall of 2.5 cm (as little as 1 cm for some products).
- Systemic fungicides perform better than contact fungicides during rainy periods.  Systemic products penetrate the leaf cuticle within 20 to 40 minutes after application.
- Most insecticides require 2 to 6 hours of drying time to “set” on the plant surface and become rainfast.  Up to 24 hours is required for optimal uptake of systemic products.
- In the first 24 hours, contact insecticides are more rainfast than systemic insecticides.  After 24 hours, systemic insecticides are more rainfast than contact insecticides (the product is now inside the plant).
- For herbicides, many labels provide information about rainfastness.
- The rainfastness of a pesticide mixture can be improved by adding to the spray tank an adjuvant with sticker or penetrant properties.

Rainfastness of Fungicides
These pesticides are applied for management of plant diseases.  Contact fungicides are applied before the infection and provide a film on the plant surface to kill the fungus spores and hyphae.  Examples are captan, chlorothalonil and quintozene.  

Systemic fungicides are absorbed by the foliage, killing the fungus that has penetrated the plant surface.  Examples include azoxystrobin, boscalid, metalaxyl, propiconazole and thiophanate-methyl.

It is generally better to apply systemic fungicides under humid and cloudy conditions.  The leaf cuticles will be swollen, allowing quicker absorbtion of the pesticide active ingredient.  By contrast, protectant fungicides should be applied during dry sunny weather.  The product will be better absorbed on the leaf surface and become rainfast over several days after application.

Research on the rainfastness of fungicides is extensive in food production.  Here are some examples.  

In 2014, researchers in Brazil published the results of work done in potatoes.  The longer the time between application and rain, the greater the control of the disease.  Systemic fungicides were less influenced by rain than contact fungicides.  

In 2010, researchers in the United States published the results of work done in grapes.  The greater the amount of rainfall, the more fungicide residue was removed and the lower the control of disease.  However, the reduction in disease control was not substantial until rain accumulation was 2.5 to 5 cm (1 to 2 inches).  The fungicides azoxystrobin and boscalid were more affected than captan and trifumizole.  

In 2007, researchers in Spain published the results of work done in citrus.  The fungicides mancozeb, iprodione and pyraclostrobin maintained their effectiveness when 9 cm of simulated rain was applied 24 hours after spraying.  The effectiveness was much lower when the rain was applied 22 days after spraying.  The longest residual activity under rainfall was observed with copper products.  

Research has also been done in turf.  Work was published in 2009 at the University of Maryland examining dollar spot on creeping bentgrass.  Simulated rainfall of 2.5 and 3.2 cm (1 and 1.5 inch) was applied 30 minutes after the pesticide application.  Effectiveness to prevent dollar spot was reduced for all fungicides tested.  The loss of effectiveness was most pronounced for chlorothalonil (Daconil); less pronounced for iprodione (Rovral) and propizonazole (Banner); whereas the most rainfast fungicide tested was boscalid (Cadence).  

Most fungicides used in natural programs are contact products.  They must be applied ahead of disease infection and during dry weather.

Rainfastness of Insecticides
Organophosphate insecticides have contact activity and are mostly surface materials.  They are more prone to wash off in rain but their higher toxicity compensates for the loss of residue.  Newer compounds made with spinosyns (Delegate, Entrust and Success), avermectin (Agri-Mek) and insect growth regulators (Citation, Distance) have translaminar activity (they penetrate the plant cuticles of leaf tissue).  After drying, these products are less susceptible to washing by rainfall.

 Research on the newer insecticides has been conducted since 2006 at Michigan State University.  In one study, the products tested were carbaryl (Sevin), phosmet (Imidan), thiamethoxam (Actara) and bifenthrin (Brigade).  

Here are some conclusions:
- A rainfall of 5 cm (2 inches) removed enough residue that all insecticides tested needed to be reapplied.
- A rainfall of 2.5 cm (1 inch) was not a concern for products applied 24 hours before the rain, but products applied 7 days before the rain lost enough residue to require a reapplication.  For more toxic insecticides such as carbaryl, sufficient residue persisted to remain effective.  

Over time, degradation by sunlight reduces the effectiveness of insecticides.  After 7 days, the combination of photodegradation and rainfall results in a loss of disease control.  

Insecticidal soap is used in “natural” programs and in cities with a pesticide restriction bylaw.  This is a contact product that likely removes protective waxes that cover the insect body, resulting in dehydration.  Soap has no residual activity, thus rainfastness is not a preoccupation.  To increase effectiveness, it is best to apply the product in early morning of a bright sunny day.

Rainfastness of Herbicides
Post-emergent herbicides are applied on weeds that have already emerged.  An example is glyphosate, a product used extensively around the world and sold under many different trade names (such as Round-up).  

Traditional formulations of glyphosate required a dry period of 6 to 12 hours after application.  Rain or irrigation within that period would reduce effectiveness and require a second application.  

Newer formulations are packaged with an adjuvant that increases absorption into the plant.  A dry period of 30 to 60 minutes is sufficient between application and rainfall.  

The user should consult the label for rainfastness of the product.  For products where the information is not provided, guidelines were prepared by specialists of corn and soybean crops.  For example, it is generally accepted the rainfastness of 2,4-D Amine is 6 to 8 hours, 2,4-D Ester is 1 to 3 hours and Tordon is 2 hours.  

Herbicides made from acetic acid are used in “natural” programs and in cities with a pesticide restriction bylaw.  When sprayed on growing weeds, these products dissolve the cell membranes, causing a loss of water and death of the leaves.  Best results are obtained on a sunny day with warm temperatures.  

Pre-emergent herbicides are applied to bare soil to control weeds as they germinate.  Unlike post-emergent herbicids, the pre-emergent do require 2 to 5 cm of rain or irrigation within a few days after application to become adsorbed to soil particles.  

In the absence of rain, some herbicides tend to dissipate in the air (volatilization), or break down in sunlight (photodegradation).  Two examples are the products dichlobenil (Casoron) and napropamide (Devrinol).  They should be applied during cool temperatures and watered into the soil promptly after application.

Mario Lanthier is the principal of CropHealth Advising & Research and a regular WCTA conference speaker
www.crophealth.com