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Garden Q&A: Don’t worry about the wasps, they’re friendly unless threatened

Q: These large wasps are chewing bark off of certain plants in my yard. What are they doing and why, and should I be worried?

A: This is a European Hornet, a non-native social wasp that’s been in the U.S. for well over a century. They are not aggressive towards people, but can be defensive around their nest or another perceived threat, so observe from a distance. Given the sensationalization around the Asian Giant Hornet, the two might be confused based on size alone unless you look for the features that easily separate them. (You can see comparisons with other large wasps on our webpage. For now and the foreseeable future, one key point is that these two hornet species occur on opposite sides of the country.)

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The European Hornet, a non-native social wasp that’s been in the U.S. for well over a century. They are not aggressive towards people.
The European Hornet, a non-native social wasp that’s been in the U.S. for well over a century. They are not aggressive towards people. (Miri Talabac)

Adult social wasps only consume liquid food — usually flower nectar, but also sap, overripe fruit juices, or honeydew. They hunt other insects solely to feed to their young — some prey we consider beneficial, like other pollinators, and others we consider pests. While probably focused on the sap flowing from the chewed wounds, they may also be hunting insects that are drawn to it, plus perhaps using the wood fibers for augmenting their paper nest. With the colony at its largest size now, there are many mouths to feed, which may be why this behavior goes unnoticed earlier in the year.

Nests tend to be tucked into hollow trees, and are best left alone unless posing an acute risk to people or pets. They do not reuse nests the following year, and workers do not survive the winter. These wasps have an unusual preference for flying and hunting at night and may be drawn to lights and, in turn, end up inside when a door is opened. If any hornets accidentally enter your home as they explore, consider turning off outdoor night lights (also appreciated by migrating birds).

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You don’t need to do anything now, as plants generally survive this treatment. No need to spray the plants with repellents or insecticides. Any branch dieback that may occur as a result of bark loss can be trimmed away later. Cooling temperatures will soon put an end to their visits to the garden.

Q: I put some houseplants outside to bask in summer’s humidity, which they have definitely enjoyed. When do I need to bring them back in before winter?

A: The answer depends a bit on how temperature-sensitive each plant species is, but it’s usually sometime in October when overnight temperatures regularly fall into the mid-50s. Many aroids and prayer plants (like alocasia, philodendron, and calathea) could experience chilling injuries below 60 degrees. Others, like Boston fern, gardenia, and citrus will be fine with temperatures into the 50s. Often native to lowland tropical habitats, few houseplants tolerate frost, so get them in well before our nights get that chilly. Our Overwintering Tropical Plants page also provides tips for palms, mandevilla, banana, and other patio tropicals.

As preparation for moving them back inside, inspect plants carefully for pests, and give leaves a good hosing-off to clean off air pollutant dust and pollen. Common pests are aphids, mealybugs, thrips, whiteflies, spider mites, and scale. While beneficial insects help provide free pest suppression while outdoors, it’s not guaranteed plants will be pest-free, so be vigilant. Any needed treatments are easiest to apply while the plants can drip-dry outside. Give yourself enough time to inspect and treat, since multiple applications may be needed to achieve good control. Our web pages on these various pests will provide more information about management options.

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University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.

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