Tarnished plant bug numbers are picking up in cotton and with pop-up rainfall we experience this time of year, we are getting calls on how long an insecticide needs to be on the plant before it is rainfast. The longer an insecticide is on the plant, the better. Most products need to be on the plant for at least 6 to 12 hours before it rains without losing too much of their effectiveness.
If rain is in the forecast, what should a grower do? Studies were conducted several years ago 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).
It was discovered that adding a surfactant can dramatically increase the rainfastness of these materials. The graphs below shows the results for Transform and Bidrin. Rainfastness was increased dramatically just one hour after application. Similar results were found with other insecticides tested. It also appeared that the type of surfactant did 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 when applying an insecticide.