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Northern
corn rootworm beetles are about 1/4 inch long, but they have no
distinct markings. Newly emerged northern corn rootworms are cream colored or
tan, but they become green as they age. Corn pollination, and hence yield, can
be adversely affected if silk clipping and pollen feeding by rootworm beetles is
excessive. When cornfields are not silking or
pollinating as the beetles begin seeking food, northern corn rootworms will
feed on the epidermal layer of corn leaves and partially or totally strip the
leaves of green tissue. This type of injury seldom is economically important,
but it looks fairly ugly.
The first objective of scouting for
rootworm adults is to
determine their potential to interfere with pollination. In commercial field
corn, treatment may be justified if you find five or more beetles per plant,
pollination is not complete, and silk clipping is observed. A second objective
of scouting for rootworm beetles is to determine the potential for rootworm larval
injury problems next year, either in corn after corn or corn after soybeans. The
third objective of scouting for rootworm beetles is to determine whether you should
prevent them from laying eggs this year to prevent rootworm larval injury next
year.
Populations of
the northern corn rootworm in western areas have adapted to crop
rotation, resulting in damage to corn following soybeans, a phenomenon referred
to as extended diapause. Studies and observations
indicate that the problem is expanding. Management options for the northern corn
rootworm in extended
diapause areas have included longer rotations, soil insecticides and utilization
of YieldGard™ Rootworm Bt hybrids.
A primary
concern for producers is the damage caused by corn rootworm beetle larvae as
they hatch from eggs in the spring and feed on the roots of corn plants (see Corn
Rootworm Larvae).
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