September, 2010

Large Milkweed Bug

Any gardener who cultivates tropical milkweed for monarch butterflies has probably noticed that several other insects also relish the plant. The large milkweed bug (Oncopeltus fasciatus) is one of the most conspicuous and numerous of these. This is a true bug in the family Lygaeidae, collectively known as the seed bugs. It is one of the largest members of the family and decidedly gaudy compared to the many small brown species to which it is related.

While the pictured insect looks like it is perfectly camouflaged, the choice of sitting on orange flowers is just a coincidence; this same kind of bug is often found on the white or pink flowers of other milkweeds, or on the green foliage. The large milkweed bug wears its brilliant colors (ranging from yellow to red) with good reason though: its preferred food plant contains toxic chemicals. Any insect adapted to feeding on the milky white sap will absorb the toxins and become distasteful to predators. In arthropod language, black and yellow/orange/red means "Beware! I am poisonous!" and most predators understand. The nymphs are mostly red and tend to feed in groups, where their bright warning colors are most effective.

This bug is a vegetarian, using its tube-like mouthparts to suck the juice from plants. Unless there are a large number of individuals on one plant, they usually do little damage and so are not of much concern to gardeners. This species will also sometimes feed on dead insects, especially caterpillars, but it merely does so after they have died from other causes.