Pest pressure on field crops is not unusual, but with this season's
abnormally dry conditions, any extensive feeding injury could put stressed
plants in further jeopardy.
In periods of extended dry conditions and heat, the economic thresholds
of insects might be lower than usual, said Ron Hammond, an Ohio State
University Extension entomologist.
"Injury from insects can make problems even worse for crops already
stressed under severe environmental conditions," Hammond said. "Sometimes
those thresholds need to be lowered to help protect the crop. A good example
is the potato leafhopper on alfalfa.
"The rule of thumb in determining potato leafhopper thresholds is, if out
of a 10-sweep sample, the number of leafhoppers present is greater than
plant height, treatment is warranted. Under hot, dry conditions, that
threshold might be lowered by a third because plant development might not be
as far along as it should be."
Hammond recommends that growers scout their fields for pests and apply
insecticide treatments if warranted.
Some pests appearing now include:
* The first brood of European corn borer -- "Even though much of the
corn may contain the Bt corn borer gene, fields without the gene should be
checked for first-brood borers," Hammond said. "If whorl injury is apparent,
then about 20 plants should be inspected at five locations to determine the
percentage of stand exhibiting whorl injury. If the number of larvae found
exceeds an average of one or more per plant and the larvae have not yet
begun to burrow into the stalks, treatment may be warranted."
* Armyworms on corn -- "Growers should be watching their cornfields
adjacent to wheatfields or planted into grass cover crops, especially rye,
for the presence of armyworm injury," Hammond said. "Detection of foliar
feeding injury by armyworm on 15-20% of a stand should be regarded
as an indicator of a potential problem, and treatment is warranted if
infestation is greater than 50% and the larvae are not mature."
* Potato leafhopper on alfalfa -- "Growers should now be sampling their
fields for leafhoppers and paying attention to the threshold," Hammond said.
"For potato leafhopper-resistant varieties of alfalfa, the economic
threshold is three leafhoppers per inch of growth. In areas with little
moisture and the alfalfa is beginning to show stress, we recommend lowering
* Soybean aphid -- Aphids are beginning to appear. Hammond said that the
insect is emerging in northern states several weeks earlier than normal,
indicating that Ohio infestations also could occur earlier than the normal
July or August emergence. "Growers in more northern areas are advised to
check their fields so as not to be caught off-guard by high aphid
populations," he said. "My colleagues to our north are surprised soybean
aphid is showing up already. Infestations by mid-June are a bit unusual."
As the season progresses, soybean leaf defoliators such as bean leaf
beetle, Japanese beetle and Mexican bean beetle will make an appearance. If
hot, dry conditions continue these pests also could cause excessive injury
to underdeveloped, stressed plants.
According to the National Drought Mitigation Center, most of Ohio and
Indiana are experiencing either abnormally dry conditions or a moderate