Know the conditions favorable for these yield robbers and what to look for when scouting.
• By Amanda Huber,
Soil insects are somewhat unpredictable, but producers are likely to see one or more of them in their fields this year. The primary pests to be prepared for are the peanut burrower bug, lesser cornstalk borer, southern corn rootworm and wireworms.
While burrower bug is typically thought of as causing problems in dry years or in non-irrigated fields, irrigation and/or adequate rainfall do not preclude problems.
“We are working very hard to learn more about the factors that contribute to burrower bug injury,” says Mark Abney, University of Georgia Extension entomologist.
Although they have tested the efficacy of a wide range of potential management tactics, Abney says deep tillage and the use of granular chlorpyrifos are still the only management tools that can be recommended for this pest at this time.
When scouting, there is a bug that looks similar to burrower bug. The “other” burrower bug that is commonly seen this time of year is the white-lined burrower bug, named so because of the white line on the margin of its body.
This cousin to the peanut burrower bug can occur in large numbers and is often seen on weeds as temperatures begin to rise and land preparation and planting get underway.
Fortunately, the white-lined burrower bug is not considered a pest of peanut, and growers should not be alarmed by its presence. The peanut burrower bug is solid dark brown to black and has very distinct spines on its legs. If in doubt about which bug you are finding in the field, you can always call your Extension agent to help.
Southern Corn Rootworm
The southern corn rootworm, of which the adult is the banded cucumber beetle, require moist soil for survival. Abney says conditions were perfect for rootworm development in peanut for much of the 2018 growing season in many Georgia counties.
“There were rootworm infestations and pod damage in my research plots last year in fields near Tifton where I had never observed these pests in the past,” Abney says. In Plains, where rootworms are more common, infestations were severe.
Rootworm larvae feed directly on pods. They can penetrate the wall of young pods and will feed on the developing seed. Abney says when this happens, the pods generally rot and will not make it into the combine at harvest.
“Feeding on older pods can result in direct injury to seed and/or provide an entry point for pathogens.” These more mature pods will make it into the combine where it can result in grade deductions.
Southern corn rootworm larvae cannot survive in dry soil and live entirely below ground. To scout, dig adjacent to peanut rows or remove plants to examine pods for damage and check the soil for larvae.
The only insecticide that has shown consistent efficacy against burrower bug and rootworm is granular chlorpyrifos. Abney says recent research at UGA indicates that chlorpyrifos can reduce rootworm damage when applied to existing infestations, but the study is not complete.
“Because burrower bug and rootworm infestations are sporadic, growers who have never had a problem should not change their practices, and those who experienced damage for the first time in 2018 may not see problems in 2019,” Abney says.
“Scouting remains the most important tool a grower can use to assess the potential for and extent of insect problems in the field.”
For lesser cornstalk borer (LCB), Abney says he does not expect it to be a problem in fields after the canopy closes if there is adequate rainfall or irrigation. “We should be looking for LCB especially in fields where vines have not lapped the row middles and any fields where drought stress becomes evident regardless of canopy closure.”
When scouting for LCB, look for wilted stems and silk tubes, remove plants and check tap root, pods, and stems for feeding injury and larvae. Moths are a good sign of LCB infestation. Plants in a “skip” or at the ends of rows with bare soil around them will usually be attacked first.
New Insecticides Needed
Research into the development of thresholds for insect pests continues, but Abney is certain that one of the currently recommended products, chlorpyrifos, will likely be lost soon.
“We are going to lose chlorpyrifos,” he says. “The prospect of new active ingredients for soil pests does not look good, and there are no new chemistries for insect pests in 2019.”
Abney’s take-home message for soil insect management is that it will be a challenge with limited options. Scouting will help you avoid mistakes. Use economic thresholds, and peanut fields do not always have to be insect free. Finally, he says to choose the correct insecticide for the job.