Howdy, BugFans,
The BugLady was out taking pictures on the beach when she spied this spiffy Cottonwood leaf beetle, a member of the very large (probably 50,000 species worldwide) leaf beetle family Chrysomelidae). It was actually a few feet away from a cottonwood tree!
Chrysomelids are vegetarians throughout their various life stages, and some species are dedicated to a single plant or group of plants (which they may be named after). On the plus side, this means that some species of Chrysomelids act as biological controls of unwanted plants; on the minus side, there are cereal leaf beetles, mustard beetles, asparagus beetles, Colorado potato beetles, etc.
COTTONWOOD LEAF BEETLES (Chrysomela scripta) (“scripta” means “written over”) are found in much of the US and Alaska where their host plants – cottonwood, poplar, alder, and willow – grow. Bugguide.net doesn’t have them listed from Wisconsin, but there it was.
Chrysomela is a genus that includes 17 species in North America and more elsewhere. Identification can be tricky, and the life stories of many are sketchy.
Adults are about ¼” long, and the patterns on their elytra (wing covers) and the color of their body and of the edges of their thorax are variable. The elytra protect a pair of flying wings that are folded up beneath them.
Their life cycle begins in spring, when the beetles that overwintered as adults abandon their shelters under tree bark or in leaf litter/debris and find some tender, new leaves, shoots, and bark to nibble. Boy meets girl and romance ensues. Afterward, the female lays clusters of yellow eggs on the undersurface of leaves. The eggs hatch, the larvae stay together and skeletonize leaves gregariously for a while, and by the time the siblings venture away from the group to seek their fortunes, a hungry larva is able to consume most of a leaf all by itself.
After about two weeks, the mature larvae pupate, attached to the underside of leaves. Like lady beetles, they spend their pupal stage “naked” and exposed and vulnerable to predators.
The larvae are chemically protected (cottonwood leaves themselves are chemically protected, but Chrysomela beetles are attracted to them rather than being repelled by them, and feeding on cottonwood leaves seems to enhance the larva’s personal chemicals). Two white scent glands on each side, at the front of the larva’s body above the legs release pungently scented droplets when it’s alarmed (and reabsorb them when danger passes). Neither the adult nor the pupa possess defensive chemicals.
Despite the chemicals, the larvae are eaten by a few stink bugs, ants, lacewings, tachinid flies, lady beetles, wasps, spiders, and assassin bugs. Stink bugs and lady beetles prey on the pupae.
Sources agree that the larvae stay together during part of their larval stage before going their separate ways, but the BugLady also found a number of pictures of what looks like “group pupating” but could find no explanation for it. Here’s a larva (right) and a pupa, and here’s a recently-emerged adult next to its pupal skin.
There are multiple generations per year over much of their range.
Some say that control is not needed, and others disagree. For mature forest trees, the arrival of Cottonwood leaf beetles isn’t a huge problem (their feeding is tougher on saplings), but their presence in nurseries that grow ornamental trees (and that grow willows to make willow baskets) is most definitely unwelcome.
The BugLady