The black stem borer (Xylosandrus germanus) is a small (2 millimeters) ambrosia beetle that becomes active in spring after one or two consecutive days of temperatures 68 degrees Fahrenheit or higher, often coinciding with forsythia bloom. The list of host plants it will infest includes many forest and ornamental trees as well as apple and stone fruit.

Simple black stem borer trap: inverted juice container with sides cut away containing an ethanol lure in the top and soapy water in the bottom. Photo credit: Mike Haas, MSU.

Black stem borer adults are attracted to stressed trees although they have been known to infest trees that do not appear to be stressed. Young trees near the perimeter of orchards, especially near woodlots, are at greatest risk of injury. Signs of infestation include 1 millimeter diameter entrance holes, sawdust “toothpicks” protruding from the holes, dark discoloration on the bark, oozing sap and dry, blistery bark.

Monitoring
Use a simple trap to monitor female activity in the spring. To construct a trap, cut two to four windows in the body of a plastic 1- or 2-liter bottle that has a cap (see photo). Hang it in the orchard upside down at a height of 0.5 meters (1.6 feet) at the orchard edge near wooded areas. Bait the trap with ethanol using one of the following methods: Pour a cup of vodka into the cap end of the trap, or purchase a ready-made ethanol lure (Standard Release ethanol lures, AgBio, Inc., Westminster, CO) to hang inside the trap and fill the bottom of the trap with soapy water or a small amount of antifreeze.

Check traps at least once per week. The beetles are very tiny and require the use of a microscope and training to identify them correctly to species. Your local Michigan State University Extension fruit educator can help.

Management
The time to spray an insecticide for this pest is when females are colonizing new trees in the spring. Pyrethroid insecticides applied as trunk sprays, at least in nursery settings, have shown the most promise in reducing the number of new infestations within a season. The application of bio-repellents applied to the trunks of stressed trees may also reduce infestation. Although not registered specifically for black stem borer control in Michigan, permethrin is an effective material for reducing the number of new trees attacked in the spring. In apples, green fruitworm and spotted tentiform leafminers are insects listed on the permethrin label and these pests commonly occur around the same time as the spring black stem borer flight. Apple bloom occurs near this time also, so precautions must be taken to protect pollinators.

Unlike sprays for other borers, systemic insecticides are not recommended against this pest because larvae and adults in brood galleries feed on the ambrosia fungus, not plant tissue.

Later in the season, the best management strategy is to remove and burn trees that are 75% or more dead or dying. It is also important to make sure all large prunings and brush piles are either flailed or burned. Infested trees, either intact or freshly cut into piles, have been implicated as sources of new infestations.

For more information, please download the free fact sheet: Managing Black Stem Borer in Michigan Tree Fruits.