Another Blow BoxwoodAgainst : Box Tree Moth

By Carol Allen

In our deer-plagued region, a versatile, evergreen shrub that is deer-resistant is a cherished garden staple. Boxwood has its detractors, but it is still popular, particularly with the new disease-resistant varieties entering the marketplace.

As with any commonly used plant, new issues will arise. The most recent is the box tree moth (Cydalima perspectalis). This pest has not been reported yet in Maryland, DC, or Virginia, but has been recently reported in Kent County, Delaware, and in Erie, Pennsylvania. If you see it in Maryland, be sure to report it to Ask Extension (https://extension.umd.edu/ask-extension/).

It is of concern because it can rapidly defoliate and potentially kill boxwood. Damage occurs through chewed leaves and bark, causing leaf browning, leaf drop, and possible girdling of the stems. If boxwood have been observed with chewing damage, box tree moth may be the cause.

It winters over as a caterpillar protected by several leaves held together by silk. Now would be a good time to look for that peculiar structure and pick or prune it off. The caterpillars can survive -22 degrees Fahrenheit, so an exceptionally cold winter will not affect the population.

Feeding begins about April in our region or when temperatures reach 46–53°F. Caterpillars take around 28 days to mature and three to five generations per year are possible. Adult moths emerge in June, mate, and lay eggs. They do not feed during this time. This is one window for dispersal, since they are reported to fly up to 4–5 miles.

Eggs are pale yellow and are laid in a shingled pattern in groups of five to 20. Look for the very small, 1 mm-sized eggs on the underside of the leaves in June. The tiny caterpillars can hatch in three to five days. Emerging larvae have black head capsules, and green and yellow bodies with white, yellow, and black stripes. They are the only caterpillars that feed on boxwood.

As the caterpillars mature, they enclose themselves and their feeding area in silk. This decreases the efficacy of pesticide sprays. It is important to note that preventive sprays of a pesticide are not recommended. Handpicking and dropping into a bowl of soapy water or a strong spray of water will dislodge them. They will not be able to climb up and resume feeding.

To help control the spread of this invasive pest, monitor existing boxwood, and closely examine newly purchased plants. Be aware that the entire state of Delaware and Erie County, Pennsylvania, have joined counties in southeastern Michigan, southwestern Ohio, western New York, and the entire state of Massachusetts under federal quarantine.

As you travel this spring and possibly purchase plants, be aware of these quarantine restrictions.

For more information and an Insect Alert poster, go to the UMD extension page here: https://extension.umd.edu/ resource/box-tree-moth/. o

Carol Allen describes herself as a committable plant-a-holic. She has more than 25 years’ experience in the horticulture industry, with a special interest in plant pests and diseases; is a Licensed Pesticide Applicator in the state of Maryland; and is an ISACertified Arborist. She can be contacted at carolallen@erols.com.

Box tree moth caterpillar photo by Luc Hoogenstein, CC BY-SA 4.0 , via Wikimedia Commons.