Adult Mexican bean beetles appear like large (one-quarter inch
long), copper-colored lady beetles that have 16 black spots on their
wings. Immature Mexican bean beetles (larvae) are yellow and covered
with black spines. Adults and larvae feed on the undersides of
leaves, eating leaf tissue but leaving the veins intact. This type
of feeding damage is known as skeletonizing. They can also damage
stems and pods when populations are high.
Adults overwinter on the ground in garden debris, tall grass and
weeds or other sheltered locations. They emerge from the winter
hiding places when beans begin growing and remain active for roughly
two months. Adults must feed for about two weeks before beginning to
lay eggs. Mexican bean beetle eggs appear as small yellow dots laid
in groups on the undersides of host plant leaves. It takes eggs 5
days to hatch, depending on temperature and moisture. Larvae feed
for two to five weeks before pupating. We have two generations a
year in our climate. The most severe damage from this pest occurs in
July and August.
Control options include planting succession crops of bush beans.
Bush beans do not take as long as pole beans to mature and produce a
crop. Those growing and producing prior to or after peak Mexican
bean beetle activity are less likely to be damaged.
You can also
exclude Mexican bean beetles by covering bush beans with floating
row covers, something that would be more difficult with pole beans. This thin, lightweight fabric allows air, water and light to pass
easily, but excludes insects from their host plants. It is light
enough that the growing plants easily push it up as they get taller.
Floating row covers are sold under trade names such as Reemay or
Garden Blanket. Beans are largely self-fruitful, so it is not
critical that floating row covers keep bees from pollinating the
You can also check plants regularly for eggs and destroy
them before they hatch. Handpicking adults and larvae and dropping
them into a pail of soapy water can reduce their population to a
tolerable level in the home garden.
Insecticides labeled to control
Mexican bean beetles include BioNeem (azadirachtin), pyrethrins and
Sevin (carbaryl). Repeat applications will be necessary for
season-long control. Always read and follow label directions for
mixing and application intervals.
gardens - Mulching for weed control
Bean seeds saved for next year