Description
Newly emerged adult Mexican bean beetles are round, about 1/4
inch long, and yellow. Their color changes to copper with age.
They are marked by sixteen black spots, eight on each wing cover.
Six branching spines are present on each segment of the bright
yellow, 1/3 inch-long larvae. Pupae are also bright yellow and
have only remnants of larval spines. Eggs are yellow and oval
shaped. |
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Figure 1. Mexican Bean Beetle Larva
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Figure 2. Mexican Bean Beetle Adult
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Life
Cycle
Adults overwinter along fence rows, woodlots, or in stubble
and can usually be found within 1/4 mile of the host plants.
The beetles become active in the spring, when they fly to host
plants, feed for a week or two, and then mate. Normally, 400
to 500 eggs (but occasionally three times this number) are laid
in clusters on the undersides of leaves over a period of 3 to
6 weeks. Larvae hatch in 5 to 14 days and feed for 2 to 5 weeks
before pupating on the undersurfaces of leaves. Adults emerge
7 to 10 days later and live from 4 to 6 weeks. There are approximately
three generations per year in Illinois. |
Damage
The Mexican bean beetle is one of only two North American species
of destructive insects in an otherwise beneficial family (ladybird
beetles) that contains over 400 species. Adult Mexican bean
beetles feed on seedlings early in the season. The larvae feed
on leaves; in their early growth stages, they feed exclusively
on the lower surface of the leaf. Bean pods may also be scarred,
but this damage is seldom considered economic. Soybeans near
woodlots, alfalfa fields, and fields where residues have not
been plowed are most likely to incur damage. Though the Mexican
bean beetle has mandibles that are typical of chewing insects,
it does not swallow bits of food. Rather, it masticates its
food and consumes the resultant juices. The foliage of garden
beans such as snap, kidney, pinto, and lima are preferred, but
Mexican bean beetles can also be serious pests of soybeans.
The beetles also feed on alfalfa, clover, peanut, okra, eggplant,
squash, and various weeds. Both larvae and adults impart a skeletonized
or lacy appearance to leaves by consuming the leaves' epidermal
layers. Heavily infested soybean fields take on a dusty appearance
as leaves shrivel and turn brown. |
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Figure 3. Mexican Bean Beetle Damage to Soybean
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Threshold
The
growth stage of the insect (young larvae are easiest to control),
cost of treatment, market price of the crop, growth stage of the
crop, and percentage of defoliation should all be considered when
making control decisions. In soybeans, treatment may be warranted
when defoliation reaches 30 percent before bloom and 7 or more adults
and larvae can be found in one foot of beans in a row. For blooming
or postbloom beans, treatment may be warranted when defoliation
reaches 20 percent and S or more adults and larvae are present in
one foot of beans per row. To determine the number o£ Mexican
bean beetles per foot of row, spread a ground cloth between two
rows of beans. Shake the plants overhanging the cloth in one row
only, and count the number of adults and larvae on the cloth. You
can vary the width of the ground cloth by unrolling it to accommodate
differences in space between the rows. The typical length of a ground
cloth is one yard. Thus, in dividing the count by three, one can
easily determine the number of Mexican bean beetles per foot of
row. As the practice of double cropping soybeans after wheat increases,
problems with the Mexican bean beetle may increase. Leaves of conventionally
planted beans turn yellow before those of double-cropped beans.
Thus, double-cropped soybeans and late-maturing varieties readily
attract the beetles. Supplemental food sources such as these increase
the survival of overwintering beetles.
Authors:
Susan T. Ratcliffe (sratclif@uiuc.edu)
Michael E. Gray (m-gray4@uiuc.edu)
Kevin L. Steffey (ksteffey@uiuc.edu)
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