But two Texas Agricultural Experiment Station entomologists think they can reduce these losses with a new model to predict and better target the pests.
The model developed by Dr. Jerry Michels, Experiment Station entomologist in Bushland, and Dr. Marvin Harris, Experiment Station entomologist in College Station, is based on temperature and number of adults emerged.
While crop rotation is the best way to control the corn rootworm, many corn growers don't have that option. So producers spray. Timing of the spray is critical, Michels said.
As much as 25 percent of the yield loss is from the adults clipping the silks, but the greater damage is done by the larvae, chewing off the roots of the corn and causing it to fall over, he said. As much as 75 percent of the crop can fall over.
"The idea behind this is to control more adults before the fall, so the number of eggs and resulting larvae the next spring will be lessened," Michels said.
He started collecting data on adult western corn rootworm emergence in 1996. Nine years of catching adult beetles in traps and comparing the data with temperature data from the North Plains potential evapotranspiration network helped the researchers determine emergence patterns.
"The emergence pattern of the beetles is quite variable for a given year," Michels said. "However, because it is temperature-driven, the model is flexible and can account for differences in emergence due to normal, cool or hot years."
This flexibility can improve Western corn rootworm management decisions by showing when an insecticide application will do the most good, Michels said.
The model has been incorporated into the North Plains PET weather station network. Daily model output can be accessed by going to http://amarillo2.tamu.edu/nppet/station.htm, the weather station network site. Select a station, then go to faxes and then to the date.
The recommendation is for producers to wait until they see about 50 percent emergence, he said. Because residual of most of the chemicals is a week to two weeks, a producer would miss a large part of the population by spraying too early, Michels said.
"We can control adults and if we get rid of enough of them, it will lessen the impact," Michels said. "If we can have a model to tell our producers when we have about 50 percent adult emergence, the producer can make an application at that time."
Over the years, he said, the model is showing 50 percent emergence can come anytime from July 13 to Aug. 15.
"If it is a hot year, emergence will be earlier than a cool year," Michels said. "So rather than saying we need to spray on July 15, some years it might need to be delayed."
"We feel this is a robust model," Harris said. "We think of this physiological model as providing us a bus schedule.
"When we have the bus schedule saying 25 percent of the adults will be out and active, we can go to the field and meet that bus and actually census the number of adult beetles there," he said.
A field census of beetles represents about one-fourth of what is actually there, Harris said.
"If the number is significant enough, we institute management. It lets us orient in time to make management decisions," he said. "When we incorporate this to overall management, we will limit pesticide usage to when it is needed."