By Ken Johnson
Lady beetles are widely considered a gardener’s friend. Sure, multi-colored Asian lady beetles can be a nuisance when they enter our homes during the fall. Still, they are all predatory, beneficial insects that feed on various pest insects such as aphids and mealy bugs, right? Two species of lady beetle, the Mexican bean beetle and the squash beetle, feed on plants and are considered pests.
What do they look like?
Mexican bean and squash beetles have the typical round, dome-shaped bodies of lady beetles. Adult squash beetles are yellow or orange, and Mexican bean beetles are more copper colored. Squash beetles have seven black spots on each wing covering and four black spots on their thorax. Mexican bean beetles have eight on each wing covering and none on their thorax.
Mexican bean beetles are about the same size as ‘typical’ lady beetles (about 1/4 inch long), while squash beetles are much larger, about twice the size.
The larvae of both species are yellow and are covered in spines, while the eggs are yellow and laid in clusters on the undersides of leaves.
Mexican bean beetle
The adult and larval Mexican bean beetles (Epilachna varivestis) cause problems in snap (aka bush, string and green) and lima beans. They feed primarily on leaves, causing them to have a skeletonized appearance. They will also feed on the leaves, flowers and bean pods. The way they feed is unique for beetles. They will scrape the surface of a leaf to gather a clump of tissue. They will then squeeze the tissue with their mandibles to remove the liquid, which they will drink, and then discard the solid plant material (like a juicer).
Squash beetles (Epilachna borealis) feed on cucurbit plants (squash, pumpkins, cucumbers, melons, gourds). The larvae will feed on the undersides of leaves, while adults feed primarily on the upper sides of leaves.
While feeding, the adults and larvae will create a trench in the leaves around where they will feed. This trenching slows the spread of defensive chemicals (cucurbitacin) the plants release in response to feeding damage (similar to young monarch caterpillars).
Managing Mexican bean and squash beetles
Fortunately, squash beetle populations rarely get high enough to cause enough damage to cucurbits where management would be needed. If you do have squash beetles, hand-picking the adults and larvae can be an effective way to manage populations in home gardens. Pesticides can be used if populations do get high enough (and they rarely do).
Unfortunately, Mexican bean beetles can reach levels where they can damage plants. Beans will vary in susceptibility to the beetles, with wax beans being more susceptible than other snap beans and lima beans being the most resistant. So, if you consistently have issues with Mexican bean beetles, consider growing less susceptible cultivars.
As with squash beetles, floating row covers can be used to protect plants; since beans can self-pollinate, they don’t have to come off until harvest. Beetles can also be hand-picked from plants, and insecticides can be used If populations get too high.
Good Growing Fact of the Week: Like other lady beetles, Mexican bean and squash beetles produce chemicals that make them distasteful to many predators. When disturbed, they will release an amber liquid from their legs that is bitter tasting.