In previous episodes of Bug of the Week like “Home invader turned garden helper” and “White, waxy ladies” we met helpful lady beetles that put a beat-down on sap-sucking aphids in the garden and phloem draining scale insects on magnolia trees. These highly beneficial insects keep many pests in our gardens and landscapes at bay. But like many large families, part of the clan sometimes goes astray and lady beetles in the genus Epilachna evolved to eat plants rather than other insects.
The aboriginal home of the Mexican bean beetle is the high southern plateau of Mexico where many members of the bean plant family grow. As beans became widely cultivated in the United States, the Mexican bean beetle crossed the border and spread throughout the eastern half of the US where moist conditions and many types of cultivated beans favor its survival. In most years, Mexican bean beetles are relatively scarce due to depredations of a small parasitic wasp, Pediobius faveolatus, released decades ago in the mid-Atlantic region. However, in a local community garden, this year’s wet spring and warm summer seem to have provided the perfect growing conditions for the bean beetle.
As I meandered through the garden plots, almost every leaf of the snap and lima beans was skeletonized by the feeding of the larvae and adults of this mini-jawed monster. Bristly, bright yellow larvae fed alone or in groups of 2 or 3. Orange and yellow adults with black spots contentedly scraped and munched the green tissue between the tougher veins of bean leaves. This nutritious food is converted into eggs and each female can produce as many as 600 eggs during the course of her lifetime. Laid by the dozen, these bright yellow eggs hatch in about a week.
Bean leaves are good to the last mouthful for this Mexican bean beetle.
Larvae complete development and form into pupae, which hang suspended from the leaves of bean plants. As summer wanes, adult beetles seek shelter in decaying vegetation or leaf litter to survive the chill of winter. In spring, adults emerge from their refuge and move back to the bean fields to resume feeding and producing pestiferous spawn.
One way to reduce problems caused by these hungry bean-eaters is to remove plant refuse from the garden plot at the end of the growing season. This forces overwintering beetles to take a longer and more perilous journey to and from hibernal refuges. Some gardeners use row covers with a fine mesh to prevent beetles from reaching the bean leaves. These should be put in place early and kept in good repair. Another strategy to confound this pest is to plant a small “trap crop” of a favored food like snap beans in the garden early in the season. This early plot will attract and collect overwintering beetles as they emerge from their refuges. Once the beetles have moved in, the trap is set, and by annihilating the trap crop loaded with beetles, the number of beetles remaining to attack beans planted later in the growing season will be reduced.
It’s upside down dining for larvae that usually feed on the underside of bean leaves.
Another method of control is a bit more direct and involves crushing eggs, larvae, and adults on the plants. Seems foolproof, but be forewarned that fingers may turn orange after encountering defensive secretions produced by the beetles. And if you go the crushing route, please avoid the urge to lick your fingers, as the defensive secretions produced by the larvae and adults is a witches’ brew of noxious alkaloidal compounds that defend against would-be predators. If you want to go the biological control route, the parasitic wasp Pediobius faveolatus can be purchased and released in your garden, but be sure to follow directions as timing is critical for this method of intervention. While it may be a little late to save the beans this season, make a note of the health of your beans this year and consider your options for next year. Remember, beans are good.
Information used in preparing this Bug of the Week came from the marvelous article “The chemistry of phyletic dominance” by J. Meinwald and T. Eisner. Bug of the Week thanks community gardener Barbara for demonstrating the value of row covers.
More information on Mexican bean beetle and its management can be found at the following website: http://extension.umd.edu/growit/insects/mexican-bean-beetle