The simplest way to find a willow leaf and cottonwood leaf beetle is to look for them on the plants they love to eat. Both are commonly found on willows, either trees or shrubs, that grow in wet areas such as the edge of a pond or stream. These beetles also love cottonwoods, poplars, aspens, and even alders.
Almost immediately after any of these trees begin to sprout three tiny beetles make their presence known by nibbling away at the new growth. They will be among the first to attack any new leaves. Commonly you may find the willow leaf beetle or Chrysomela interrupt, the cottonwood leaf beetle or C. scripta, and the willow leaf beetle or Plagiodera versicolara.
The willow leaf beetle and cottonwood leaf beetle look very similar in appearance. Both are about one-fourth inch long with patterns of black lines and dots on the yellow background of their bodies. The willow leaf beetle Plagiodera Versicolor is even smaller with a body that is only one-eighth inch long and a coloring of shiny steel blue.
The first thing you might see when looking for these beetles is the tiny, irregular holes in the leaves of the tree or shrub. But if you continue to look you will find the beetles. Since all of these beetles openly feed on leaves during their adulthood the specimens you find will most likely be adults. They will continue feeding for around two weeks before mating, so if they are found in pairs the mating season has begun.
Once mating has taken place the female will begin laying eggs is tight clusters of approximately fifty eggs per cluster. This is usually done one the underside of the leaves. Around a little more than a week, the larvae will hatch and immediately start feeding. You can tell when this occurs since when feeding they tend to skeletonize the leaves by eating just the bottom surface between the veins. This causes the leaves to appear almost transparent with only the veins showing through.
The larvae, when closely observed, look like tiny lizards with the exception of their three pairs of legs. In the two larger species have two white dots on each side of the thoraxes from which they can emit a milky juice that is said to be quite distasteful when they are disturbed. Once the larvae are full-grown it will pupate while still attached to the leaves by enclosing themselves in small black cases that are attached to the end of leaves.
Although many of these beetles are common there are over fourteen hundred species in the family, most of which are under a one-half inch long, oval-shaped, and have antennas that no longer than half their body length. When fall arrives the willow and cottonwood leaf beetles will hide among the leaf litter at the base of their food source. When spring announces its presence they will emerge to feed and mate.
Willow leaf and cottonwood leaf beetles are know to spend the majority of their life cycle on or among the leaves of the trees where they are born only leaving the tree only to spend winters at the base. Each female will usually produce around five broods, with the last one reaching the adult stage before crawling down the tree or shrub to hibernate in the leaf litter.
Because of this, both the willow and the cottonwood leaf beetles are known to be great subjects for giving one the opportunity to observe the different stages of complete metamorphosis at its best.