Since 2006, when growers first began treating limited acreages of Mississippi soybeans for bollworms, the pest has increasingly become a problem, says Angus Catchot, associate Extension professor of entomology and plant pathology at Mississippi State University.

“We didn’t have a lot of bollworms in soybeans in the Mississippi Delta in the early 2000s,” he said at the annual meeting of the Mississippi Agricultural Consultants Association. “We sprayed almost no bollworms in soybeans until 2006, but now we’re on an incline where we’re spraying more and more — last year we treated around 750,000 acres an average of two times. The acreage is growing every year, and we’re seeing more pyrethroid-resistant bollworms.”

The last two years were somewhat unique, Catchot says, in that populations were sustained over most of the season.

“Normally, we have well-defined peaks and valleys in our moth trap catches, but that wasn’t the case in 2010 and 2011. Last year, I first started getting calls about bollworms in soybeans the third week of June. At end of September, we were still constantly getting calls about bollworms in late-planted beans. It never quit. We dealt with eight to 10 weeks of constantly high pressures, which is very unusual.”

There have been various suggestions as to reasons for the sustained flights, Catchot says, “but we don’t really know for sure what’s behind it

“Most of these worms — about 98 percent of them —will come through corn at some point. We estimate 40 percent of our acreage is planted to VT3P varieties containing two Bt genes for lepidopterous pests. Some of the worms coming through this corn are delayed seven to nine days, compared to non-Bt corn or regular YieldGard corn.

“So, even if you planted all your corn at the same time, with all these different technologies there is a span of a couple of weeks when worms will be coming out of the corn at different times because the Bt is slowing them down. When you start adding in variations in planting dates, you get these periods of long emergence.”

As bollworm numbers have increased, control problems have also multiplied, Catchot says.

“Since we started spraying soybeans for bollworms in about 2007, with just over 100,000 acres, I could have counted on one hand the number of control failures we had. Even in 2008, there were very few failures, and most of those we could explain.

“Now it seems seven of every 10 calls we get are for some kind of control failure where pyrethroid insecticides are used. That doesn’t mean you’re not getting any control — but that it’s not acceptable control.”