WEST LAFAYETTE, Ind. — Corn growers should start scouting in
late July for signs of western bean cutworm, a potentially yield-reducing pest,
two Purdue Extension entomologists said.
First found in Indiana in 2006, western bean cutworms feed
on corn, legumes and dry beans, such as kidney and black beans. Despite the
name, they do not feed on soybeans.
The cutworms can be identified by the black rectangles that
develop behind the head of the later instars. These are commonly found feeding
within the ear in late summer.
Western bean cutworm populations are tracked and roughly
estimated each year using pheromone traps placed throughout the state.
The traps capture the grayish-white moths, which usually lay
eggs on Indiana corn plants in July. Feeding damage can appear a few weeks
So far this year, moth captures in northern Indiana are
lower than previous years, said John Obermeyer, Purdue Extension entomologist
and Indiana trap coordinator.
But the number of moth catches does not necessarily give
entomologists an accurate number of larvae and generally is used as a presence
or absence tool to initiate scouting.
Larvae begin emergence after a period of warm weather, but
extreme conditions, such as last year’s drought and excessive heat, are hard on
western bean cutworm. With the cooler, wet Indiana spring this year, Purdue
Extension entomologist Christian Krupke said the insects likely are
“We’ve had a cool spring, so we’d expect emergence a little
later than last year,” he said. “It is likely that the hot, dry weather that we
had last year was not good for them.”
In the early stages of development, western bean cutworms
feed on pollen caught in leaf axils of corn plants, where the leaf meets the
stem. This is when growers should apply insecticides to fields to prevent
Once the cutworm is inside the husk of the corn plant and
begins feeding on kernels, insecticides no longer are effective. When pollen is
gone, kernel feeding will begin and continue until the first frost.
According to Krupke, western bean cutworms also are
sometimes indirect pests because they can increase the risk of ear rot
development in corn plants by feeding on corn kernels. Some ear rots produce
toxins that can be fatal if consumed by livestock, especially swine.
“When you take your grain to the elevator, they will test
for mycotoxins. If it is over the threshold for that particular toxin, they
might reject the whole batch,” Krupke said. “That’s why this insect is a
concern, because it makes the fungal pathogen colonize the ear more fully, which
in turn raises toxin levels.”
Some Bacillus thuringiensis, or Bt, corn hybrids are very
effective in resisting western bean cutworm damage, but growers are urged to
check the label of their particular hybrids. Scouting and insecticide sprays are
effective, as well, but must take place before larvae enter corn ears.