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Crop Rotation No-Cost Way to Manage Rootworms 

           

Corn rootworms will be getting a lot of attention in the next few months as seed companies
advertise their new biotechnology-enhanced corn hybrids that will be resistant to damage
caused by corn rootworm larvae.

What will usually get overlooked in all the commotion is crop rotation, a proven means of
managing corn rootworms in South Dakota.

Larvae of corn rootworms feed almost exclusively on roots of corn. They cannot normally
survive on roots of soybeans, sunflower, or alfalfa.

This weak link in their biology has enabled growers to avoid rootworm damage through
crop rotation.  Remove corn from the field and the rootworm larvae will starve to death
in the spring after they hatch from eggs.

Crop rotation remains the most effective means of managing rootworms.  Best of all,
it is free.

Even the so-called "extended diapause" corn rootworms can be effectively managed by
rotating out of corn for two seasons.

"Extended diapause" is an adaptive response of corn rootworms to corn-soybean-corn
rotations. The eggs laid by the beetles when corn was on the field do not hatch when soybean
is on the field the following season.

The eggs will then hatch when corn is again planted on the field thereby making crop rotation
ineffective as a means of controlling rootworms.

To date, no economic populations of rootworm eggs ever have been shown to survive in the
soil for more than two growing seasons. Thus, rotating out of corn for at least two seasons
will eliminate "extended diapause" rootworms in the soil.

Corn rootworm larvae prune and hollow out corn roots causing corn plants to lodge,
"gooseneck," or be stunted in growth. Damage due to corn rootworm larvae can range from
slight to total loss of yield depending on infestation levels, moisture availability, soil type, and
root characteristics of the corn hybrid planted.

Most of the damage done by corn rootworms in South Dakota has been observed on fields
where corn is the only crop for several growing seasons (continuous corn).

Corn, following soybean-with-numerous-volunteer-corn also may suffer rootworm damage,
not because crop rotation failed, but because the volunteer corn attracted a lot of egg-laying
beetles to the field the previous year.

Fully-grown corn rootworm larvae are white in color, about half-inch long and found in the soil.
Adult corn rootworms are beetles about a quarter-inch long and primarily feed on silks, pollen,
and ears of corn. They can also feed on cucurbits--the cucumber, squash, and gourd family--
and many other plants.

Northern corn rootworm beetles are uniformly green to yellow green in color. Western corn
rootworm beetles are tan in color with two to three black stripes on their wings. Beetles lay
eggs in the soil from late summer to fall. Eggs overwinter in the soil then hatch in the spring.

Variants of the western corn rootworm that appear to have circumvented crop rotation by
laying eggs directly on soybean fields have not been found in South Dakota or Nebraska.

To date, these variants have only been documented in certain regions of Indiana, Illinois,
Ohio, Michigan, and eastern Iowa.

 Last Updated on Monday, March 24, 2003 by Mike Catangui.