Insecticide Rainfastness

DR. SCOTT STEWART

JACKSON, TENN.
   With scattered rains in the forecast and plant bugs in our cotton, the question of rainfastness always comes up. This most common question this year has been … should I use Acephate/Orthene to get rapid knock down of plant bugs before the rain hits? The short answer is not necessarily, and let me explain why.
   Few insecticides list a rainfastness time on the label, even those that do have some better rainfastness qualities. Examples of insecticides that, in my experience, have good rainfastness include products like Belt, Intrepid, and Prevathon. This is one reason these products provide relatively good residual control. Unfortunately, none of these products would control plant bugs.
   Most insecticides will lose considerable efficacy if it rains shortly after application. The shorter the interval, the worse the wash off. Don’t think of insecticides like systemic herbicides that enter and kill the plant. It is mostly the contact action of insecticides that kills plant bugs. Even for products that have some systemic or ‘translaminar’ activity, systemic activity diminishes greatly as the plants get larger.
   Some of my counterparts in Mississippi and Arkansas have done some recent testing of how rainfall or irrigation effects the control of plant bugs. In short, there was a substantial drop-off of efficacy if significant rainfall or irrigation occurred within 4-6 hours of application. This was true for all the products tested (Centric, Acephate, Bidrin). These data suggested, if anything, Acephate needed a longer rain-free period than the other products.
   For pre-flowering applications, I would NOT switch to Orthene/Acephate under the false assumption that it does not much of a rain-free period. I generally discourage acephate applications until we are very close to or during bloom. This helps preserve beneficial insects and also preserves the efficacy of acephate.
   You should consider re-application of insecticide if rainfall occurs within 3-4 hours of treatment. If rainfall occurs within 4-8 hours, be sure to re-evaluate the field within 3-4 days and treat accordingly.
   Should you hold off a treatment if a rainfall is imminent? A tough one! It depends on how bad the infestation is, how likley is the rain, and whether you will be able to treat the field within a few days if it does rain. It’s a judgment call. If plant bug populations are well above thresholds, I’d rather risk a wash-off and only partial control then risk not being able to treat for a week because I don’t have access to a plane and the field is too wet. ∆
   DR. SCOTT STEWART: IPM Extension Specialist, University of Tennessee