Tarnished plant bug numbers are picking up in cotton and with the rainfall we’ve been experiencing, we are getting calls on how long an insecticide needs to be on the plant before it is rainfast. Of course the longer the insecticide is on the plant, the better. However, with rain in the forecast, what can a grower do to help his insecticide application last longer? Studies have been conducted for the last couple of years looking at the effect of adding a surfactant to an insecticide application and how it can increase the rainfastness of the insecticide. Several types of surfactants were tested along with the some of the most commonly used tarnished plant bug insecticides (Transform, Bidrin, Acephate and Centric). Basically, adding a surfactant can dramatically increase the rainfastness of these materials. It also appears that the type of surfactant does not matter. However, in the absence of rain, there was very little increase in insecticide activity by adding a surfactant. Long story short, if rain is in the immediate forecast, add a surfactant to your insecticide. If no rain is in the forecast, it is doubtful that adding a surfactant will be of benefit.
The treatment threshold for tarnished plant bug in cotton is when 3 plant bugs are found per 5 row feet. Growers should also strive to maintain small square set above 80%. If small square set drops below this level, the field should be treated for tarnished plant bugs if they are present even at low levels. For a list of recommended insecticides and rates consult MP144 Insecticide Recommendations for Arkansas. Always follow label directions on the insecticide.