Corn growers need to scout their fields to determine wireworm populations before planting so they don't miss the optimum treatment window, a Purdue Extension entomologist says.
Wireworms, or click beetle larvae, feed on corn seed germ and may prevent plants from sprouting. The pest becomes active when soil temperatures warm into the upper 40s.
"There are two trapping methods to determine the potential risk from wireworm to this year's crop, but they will need to be implemented very soon," Christian Krupke said.
Growers can bait the pests with either solar or flour bait stations. With either method, traps should be placed in at least five representative areas of a field, Krupke said.
Solar bait stations are created by digging holes in the soil 9 inches in diameter and 6 inches deep. Each hole should have a handful of untreated corn and wheat seeds, then be refilled with soil, covered with a piece of black plastic (holding down the sides with soil) and marked with a flag or stake.
"The black plastic acts as a solar collector, warming the soil surrounding the bait, providing heat for germination of the corn and wheat seeds," Krupke said.
The second method is similar, but replaces the corn and wheat seeds with flour. The black plastic isn't necessary, but marking the spot is.
"Gasses given off during fermentation of the flour are attractive to the wireworms, and they will move to the bait to feed," Krupke said. "The bait should be left in the fields as long as possible (two-week maximum) up to the time of planting. At that time, the bait at each station should be dug up and examined for wireworms. If they are in the field, you will find them feeding on the flour ball."
Once growers know whether their corn crops will be at risk and the extent of the risk, they can decide on management strategies.
According to Krupke, one or two wireworms found per bait station might warrant higher rates of seed-applied insecticides, possibly at the rootworm rate. Higher populations could render seed-applied treatments ineffective.
Krupke also pointed out that while seed-applied insecticides will protect the crop, they often do not kill wireworms, causing populations to build up over the years.
"In this scenario, granular soil insecticides would afford better protection of the corn seed and seedling, especially during slow-growing environmental conditions after planting," he said.
Another option would be to plant soybeans where wireworm populations could be higher. While the pest feeds on soybeans, the plants are typically planted a little later into warmer soils and also have the ability to compensate for thinner stands with higher seed production, unlike corn.
The bottom line, Krupke said, is to keep in mind that pre-planting is the last opportunity to manage wireworms.
"Remember that there is no rescue treatment for wireworms once the crop has been planted," he said.