How Soon Did It Rain After You Sprayed?

Over the past week we have been getting a significant amount of rain across most of Mississippi. Unfortunately, most insects don’t mind the rain.  Other than spider mites and aphids, we will likely have to continue managing insects in all crops despite the rain. Whenever we get into a rainy period, there are often numerous questions about rainfastness.  Rainfastness refers to the amount of time after an application needed before a rainfall event for the insecticide to still provide acceptable control. 

Table (1) is a list that Angus put together last year based on what is printed on the labels of the insecticides listed. As you can see, most of the common insecticides that we use don’t have anything about rainfastness on the label. That does not necessarily mean that they have poor rainfastness,  it more than likely means that sufficient data has not been generated.

Below are results from a few studies that we have done in Mississippi with respect to insecticide performance after a rainfall event.  Figure 1, is a field study that recieved 1.15 inches of rain.  Treatments were made 6, 4, 3, 2, and 1 hour prior to rain.  In this test, Centric and Orthene + Diamond were fairly rainfast after 1 hour and gave good control against tarnished plant bugs in cotton, however Orthene alone needed 6 hours for similar results. 

Figure 1. Rainfast Field Study on Tarnished Plant Bug.

 

The next question is, can rainfastness be improved with an adjuvant?  We did some tests this week looking at multiple adjuvants and their impact on improving the activity of various insecticides against plant bugs in cotton. In these tests, we made the applications between 2:30 and 3:00 pm.  These plots received a little over 1 inch of rain that started at about 5:30 pm (2.5-3 hrs after application). 

The graph below shows the results of these trials.  The maroon bars show the numbers of tarnished plant bug nymphs on a drop cloth in the untreated, the gray bars show the insecticide alone, and the black bars show the insecticide plus an adjuvant.  We averaged across all of the adjuvants, because the results were similar for all of the adjuvants we tested.  As you can see, the addition of an adjuvant improved the level of control for all of the insecticides tested compared to the insecticide alone.  The bottom line is that a quality adjuvant can improve the rainfastness of an insecticide and should be used when rain is in the forcast.

Figure 3 shows results from a bioassay we conducted in 2011.  We received 0.4 inches of hard rain over a 30 minute period.  Instead of using established field populations we made the application then pulled leaf tissue and placed adults from a susceptible lab colony on the tissue and rated mortality 48 hours after treatment.  Bioassays like these are not ideal. They do reflect some gross differences, however, they do not really reflect expected field efficacy.  The “standout” is the bifenthrin but remember these are pyrethroid susceptible plant bugs.

 Figure 4 below shows efficacyof selected insecticides on soybean loopers.  This trial received 0.5 inches of rain within an hour of application.  Products like Belt, Intrepid, and Coragen still preformed very well despite the rainfall.

In summary, very little actual data exists comparing product efficacy after a rainfall event.  Most of the IGR and contact materials like the pyrethroids have been more rainfast.  This is a difficult topic to address because there are so many different scenerios to test.  For example, how much rain falls, duration of the rain, how hard the rain falls etc. can all impact   You can see  how complicated it is to give a reliable answer.  The bottom line is the longer the better no matter what anybody tells you.  Generally I like 4 hours with most products.  Orthene is one product where I really like a minimum of 8 hours, but longer is better.  I feel pretty good with the pyrethroids particulary in soybeans with less than one hour.  For most other products if you get rainfall within 1 hour of application retreatment may be needed. 

Don’t give up the fight when the rain starts, there are several insecticides that have good rainfastness and will provide some benefits even if sprayed before a rain.  Results from this year show that a quality adjuvant can significantly improve the rainfastness; and therefore, the performance of several insecticides.