Rainfastness of Insecticides

This article is dated but applies very well today given the pop up showers across most of the state lately. Every year when afternoon thunderstorms start popping up, the phone starts ringing about rainfastness.  Rainfastness simply means the amount of time needed after application before a rainfall event for the product to still be effective.  One thing I have figured out about rainfastness is that everyone has their own opinion based on their personal experiences of past performance.  This is not surprising since most labels do not address this at all.  Over the years, this has been one of the top requested research areas from consultants and producers.  Table (1) is a list I put together with many of the most common insecticides with comments from their label about rainfastness.  You can see why it can be confusing.  Keep in mind that if the label does not list rainfastness that does not mean that the product cannot stand rainfall, it more likely means that their has not been data generated.

Below are results from a few studies that Jeff Gore and I have conducted with respect to insecticide performance after a rainfall event.  In both of these test, we were expecting rainfall and went to the field making applications at set intervels until the rain arrived.  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, however Orthene alone needed 6 hours for similar results.  Figure 2 shows results from a bioassy test we conducted on 6/22/2011 in a similar manner.  In the test in Figure 2, we recieved 0.4 inches of hard rain that came in about 30 minutes.  Instead of using established field populations we made the application then pulled leaf tissue and placed suscetable lab colony adults on the tissue and rated mortality 48 hours after treatment.  Bioassays like these are not ideal in my opinion but they do reflect some gross differences, however, they do not really reflect expected field efficacy.  The “standout” is the bifenthrin but remember this is pyrethroid susceptible plant bugs.

Figure 1. Rainfast Field Study on Tarnished Plant Bug. Click to Enlarge

Figure 2. Rainfastness of Insecticides Bioassy on Tarnished Plant Bugs. Click to Enlarge

Figure 3 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.

Figure 3. Soybean Looper Efficacy after a Rainfall event. Click to Enlarge

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 has been a hard topic over the years 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.  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. That does not mean if you get 2 hours that you will get no control but it will likely be reduced.  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 will likely be required.