Gorgeous larger elm leaf beetles often frequent elms in riparian forests in the eastern United States.
Elm leaf beetles remove tender leaf tissue leaving behind epidermis and leaf veins. These skeletonized leaves turn brown and crispy in the heat of summer.
While strolling along Cedar Creek in the wilds of Virginia, we noticed several gnarly leaves on American elm trees lining a trail by the creek. Once verdant and unscathed foliage now bore the telltale signatures of mandibulate insects dining on them. The usual suspects for defoliation of elms include caterpillars like the spiny elm caterpillar which turns into the lovely mourning cloak butterfly or the elm spanworm whose adult stage is a rather pretty white moth. However, two species of beetles, one native to North America and one from Europe, also find American elms delectable. In this natural setting, defoliation was the handiwork of the gorgeous larger elm leaf beetle. Larger elm leaf beetles emerge from their wintering refuge in the soil and move to leaves of elms and other woodland trees to feed and lay eggs in spring. Larvae and adults feed on soft leaf tissue between tough leaf veins, leaving behind a reticulate network of leaf tissue in a characteristic pattern called skeletonization. Larvae develop through the summer and autumn before moving to the soil, where they overwinter and molt to the pupal stage. This pretty native beetle is only an occasional pest of elms in landscapes. When populations of larger elm leaf beetles increase, parasitic tachinid flies attack and kill the beetles, thereby reducing damage caused by this interesting leaf-muncher.
A beautiful larger elm leaf beetle surveys the possibilities from the edge of a leaf while nearby another adult battles a forest of hairs on the stem of a wild berry plant. Larger elm leaf beetle larvae chew ragged holes in leaves, often leaving behind tough leaf veins.
Adult elm leaf beetles make ragged holes in leaves.
That was the wild, but let’s visit the city where American elms and other species of elms face a far more serious pest called the elm leaf beetle. Elm leaf beetle is native in Europe and first arrived in the US in the 1930s, where it now occupies states from coast to coast and border to border wherever elm trees grow. After spending the leafless winter in protected locations on the ground, in spring adults move to new leaves to feed, mate, and lay eggs on leaves. Each female can lay between 600 to 800 eggs. Larvae hatch from eggs, eat soft leaf tissues, and skeletonize leaves in a manner similar to their larger elm leaf beetle cousins. When development is complete, larvae move to bark crevices on the tree or soil beneath the tree to pupate. Later in summer a new generation of adults move to the canopy to dine, mate, and lay eggs. These eggs hatch into a second generation of larvae which continue to ravage leaves of the elms. As summer wanes, larvae complete development and overwinter as adults. In warmer parts of our land, there may be 3 or 4 generations of elm leaf beetles annually. With climate change, new regions of the US could someday experience additional generations.
Here an elm leaf beetle larva removes leaf tissue.
Elm leaf beetles are found on every continent except Antarctica and punish several species of elms as well as Japanese zelkova. However, with the importation and release of a tiny egg-killing wasp and a small parasitic fly that attacks other life stages of the beetle, populations of elm leaf beetle have declined dramatically in several areas of the US. Here in the DMV the status of elm leaf beetles has changed dramatically from major pest status in the ‘70s to minor pest status today. Reuniting a non-native pest with its complement of non-native parasites in an invaded land has proven to be a highly effective management tactic for several alien invaders in the US and in other lands.
Female elm leaf beetles can lay hundreds of eggs in clusters like these on elm leaves. Notice the small holes where larvae have chewed their way out.
Pupae and prepupae like these can be found on tree bark or in soil beneath elm trees.
Acknowledgements
Bug of the Week thanks an anonymous Virginia Park Ranger who spotted the larger elm leaf beetles at Natural Bridge State Park, which inspired this episode. References used for this story include “The larger elm leaf beetle, Monocesta coryli (Say)” by D. Anderson and C.S. Papp, “The Larger Elm Leaf Beetle, Monocesta coryli (Say), an Occasional Pest of Elms in Florida (Coleoptera: Chrysomelidae)” by Michael C. Thomas, “The Elm Leaf Beetle, Xanthogallerucae luteola” by Andrew Lawson, and “Larger Elm Leaf Beetle” by Steven Frank and James Baker.