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It’s Crunch Time For Cotton Insects

• By Scott Stewart •

clouded plant bug adult

Clouded plant bug adult — photos courtesy University of Tennessee

The three to four weeks after first bloom, and typically the last two weeks of July and the first two weeks of August, is a critical window to scout and manage insect pests. This is when we are setting the bolls that will contribute most to yield, and it’s also a time insects are most attracted to the crop (with plenty of susceptible fruiting structures to feed on).

Most of the comments in this article will focus on plant bug and stink bug management. However, bollworm management will become a bigger factor by the end of the month. I’ll hit on that topic in the near future. Bollworm moth trap catches continue to be low at this time.

So far, plant bug pressure as been below average, but there are clear signs that infestations are rapidly on the rise in many areas. The drop cloth becomes an important tool during bloom to scout for immature plant bugs.

The suggested treatment threshold for tarnished plant bug is an average of three per drop cloth. The threshold for stink bugs is one per drop cloth, and thus, you can essentially count a stink bug as equivalent to three tarnished plant bugs when making a treatment decision.

tarnished plant bug adult

Tarnished plant bug adult

We are also seeing more clouded plant bugs than usual in some areas, and as always, I suggest counting them as equivalent to 1.5 tarnished plant bugs (because they are more likely to feed on bigger bolls).

As for insecticide selection, this is the time for “go to” products, either by themselves or in combination. I like mixing in many situations, and it is sometimes needed to maximize control of both stink bugs and plant bugs (and bollworms later, of course).

I’m mostly talking about Acephate (Orthene), Transform, Diamond and Bidrin. If applying these product by themselves, my suggested standard rates are Acephate at 0.75 lb ai/a, Transform at 1.5 oz/a, Diamond at 6 oz/a, and Bidrin (6-8 oz/acre).

However, Diamond does not kill adult insects and 4-6 oz/a is typically tankmixed with other insecticides. If stink bugs are present at threshold numbers, Transform or Diamond should be tankmixed with products like Acephate or Bidrin.

I’ve had good results from mixing a reduced rate of Transform (e.g., 1 oz) with Acephate (0.5 lb) or Bidrin (4 oz). Bifenthrin or another pyrethroid is an option as a tankmix partner with any of these products, especially if there are many stink bugs in the field. But never use a pyrethroid insecticide by itself for control of tarnished plant bug.

Green stunk bug adult

Green stunk bug adult

Rain can muck up the works. I lean toward Diamond when there is a high chance of rain within 12-24 hours of application. It has good rainfastness after only a few hours.

Any rain within the first 12-24 hours after application will reduce the efficacy of most insecticides. However, if rain holds off for four to six hours after application, I would not automatically retreat a field. In other words, you can probably afford to wait until the next scheduled scouting time to decide if retreatment is needed.

Obviously, some of this depends on how bad the infestation was to begin with. But if it rains within a few hours of application of Orthene or Bidrin, it’s just bad luck and time to respray for plant bugs.

Dr. Scott Stewart is a University of Tennessee integrated pest management specialist. He may be reached at sdstewart@utk.edu.