There are some large, scary wasps that keep flying to the lilac bush in our front yard. They look like they’re chewing on the trunk. Are they going to kill the bush? P. O., Marblehead, MA

You’re witnessing a behavior typical of the European hornet, Vespa crabro. These hornets are social wasps similar to yellowjackets with a colony containing queens, males, and sterile female workers. Inseminated queens overwinter and start new nests in the spring. It’s at this time that the queens and workers will chew up rotting wood and bark and mix it with saliva to build the brown protective paper envelope around the nest. They may be getting nest material from the bark of your lilac, but it’s more likely that they are feeding on its sap.

The European hornet is large, about an inch long, and is rust-colored with yellow and brown markings. The cicada killer wasp is another large wasp that is sometimes confused with the European hornet. The cicada killer, though, is a solitary wasp that nests alone in the ground. Neither wasp is aggressive but they will defend their nest if threatened. There are two curious things about the behavior of the European hornet: it is active mostly at night, and it flies to lights, sometimes banging into windows (see Large Wasps That Bang on Your Windows!)

These Wasps Feed on Tree Sap and Ripe Fruit

Even though they are predators on insects, these big hornets have a sweet tooth. They are notorious for girdling certain shrubs and flowering trees in order to feed on the sap. They seek out lilacs, dogwoods, rhododendrons, boxwood, birch, and ash, to mention a few. They will also gnaw holes in the bark and might strip bark from the trees to use for their paper nests. They can damage ripe fruit trying to reach the juice. Damage to the trees is most evident in late summer or fall when the leaves on dying branches brown prematurely. If there is enough girdling of branches, it could kill small trees.

Nests Can Be in Hollow Trees or in Attics, or Anywhere

European hornets nest in a variety of sites and nests are not always visible. They prefer to build their nests inside hollow trees. In this case, the layers of combs in the nest may be open and uncovered. They sometimes nest in structures in wall voids or attics, or in sheds, bird houses, or similar sites. The more exposed the nest site, the more likely that the nest will have a paper covering over the combs. At the end of the summer, a mature nest can contain 6-9 combs with a total of 1,500 to 3,000 cells. Each cell once contained a developing wasp larva. But at its peak, a typical nest will contain 200-400 worker wasps.

European hornets are imposing but away from their nest they are much more docile than yellowjackets. They are generally considered beneficial since they collect various insects such as grasshoppers, flies, and even yellowjackets to feed to their larvae. Even so, if they are seriously damaging trees or are nesting in sites where they could cause a problem, control and removal of the nest is warranted. If you can’t live with European hornets near your house, give Colonial Pest a call. We can track down and treat the nest.

Photo Credit : “European hornet lateral view” by Trancelius – Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons.