Last Thursday (7/23), we checked several cotton fields in historical silverleaf whitefly (SLWF) areas with NO positive detections of infestations. Still, though, anyone in areas that have had them in previous years may want to keep SLWF in mind while scouting fields.
People are reporting high numbers of stink bugs – primarily southern greens – moving into some cotton fields in southern Alabama, while others report low populations of stink bugs. These differences likely are due to the size of the field and also proximity of the cotton to corn, from which stink bugs are leaving.
The larger the field, the more spread out stink bugs can be, so you are less likely we are to find high numbers. This is part of why we sample quarter (0.9 to 1.1 inch) sized bolls to scout for stink bugs. Remember, our threshold is 10% internal injury from the 3rd week of bloom to the 6th week.
A Word On Wash Offs
We have reached the time of year where we seemingly have a 30% to 60% chance of rain every day. The threat of “pop up” storms complicates application scheduling. Several factors go into the efficacy of an insecticide application after a rainfall event. For example, how much did it rain? How hard did it rain? What was the target pest? What type of insecticide was used?
Systemic (moves into the plant) insecticides tend to be more rainfast than contact insecticides because once they enter the plant they cannot be “washed off.” Generally, plant uptake of systemic insecticides happens in 2 to 4 hours.
Another consideration is the behavior of the target pest. In situations where insects are exposed to contact insecticides at application, control should be better since residual is less of an issue. On the flipside, if targeting insects hidden behind the bracts of squares or in white blooms, contact insecticide residual is more important because the likelihood of exposure is not as high at the time of application.
In short, the answer to “do I need to respray?” is not always a black-and-white decision. Sometimes, an application that results in 60% of the expected efficacy may be enough to reduce the population below threshold, saving the need for another application.
The amount and intensity of rain combined with target pest and insecticide type used play important roles in control after a rain.
We can generally expect adequate control from an application that goes out 2 to 4 hours prior to a rainfall event, keeping in mind there are always situations that don’t play by the rules. One caveat is acephate (e.g., Orthene) which is among the least rainfast insecticides we use and needs a minimum of 8 to 12 hours prior to rain.