Mexican Bean Beetle

The Mexican bean beetle belongs to the ladybird beetle family (Coccinellidae), but unlike the vast majority of its relatives, which are predatory and beneficial, this species is a dedicated plant feeder. It is one of only a small handful of herbivorous ladybird beetles found in North America.

Native to the highlands of Mexico and Central America, it has spread into much of the eastern and central United States, where it is closely tied to cultivated and wild legumes, especially bean plants. It is notable as an ecological outlier among ladybirds, illustrating that the family's familiar predatory reputation does not apply universally.

Within agricultural and garden ecosystems it functions as a specialist herbivore of legumes, and its feeding can be recognized by the distinctive lace-like damage left on bean foliage.

• Oval, domed body about 6–7 mm long, copper to yellowish-brown in color.
• Sixteen small black spots arranged in three rows across the wing covers.
• Body shape and size closely resemble a large ladybird beetle, but the coloring is duller, coppery-tan rather than bright red or orange.
• Larvae are pale yellow, oval, and covered in branching spines, giving them a fuzzy or bristly look very different from a typical ladybird larva.
• Lookalikes include the squash beetle, a very close relative with a similar spot pattern found on cucurbits instead of beans.

Found across the eastern and central United States as well as its native range in Mexico and Central America, wherever bean plants (including soybeans, snap beans, and lima beans) are grown. Adults and larvae are active on host foliage from late spring through summer and into early fall in warmer climates, and adults seek shelter in garden debris, mulch, or nearby vegetation to pass the colder months.

Both adults and larvae feed on the underside of bean leaves, scraping away the lower leaf tissue and leaving the upper surface intact, which produces a distinctive lacy, skeletonized appearance. They are day-active, relatively slow-moving insects that rely on host-plant chemical cues to find legumes. As a specialist herbivore, the Mexican bean beetle plays a role in regulating legume foliage biomass and serves as prey for various generalist predators such as spiders and predatory bugs. It has little defense beyond mild reflex bleeding of distasteful hemolymph from its leg joints when disturbed, a trait shared with many ladybird beetles.

Development is complete metamorphosis: egg, larva, pupa, adult. Yellow-orange egg clusters are laid on the underside of bean leaves; the spiny larvae pass through four instars while feeding on foliage before pupating attached to the leaf surface. Multiple generations, typically two to three, occur per growing season in warm regions. Adults overwinter in sheltered plant debris and emerge in late spring to begin feeding and laying eggs on new bean growth.