Black Stem Borer
Common Name: Black stem borer aka Asian ambrosia beetle
Order: Coleoptera
Family: Curculionidae
Scientific Name: Xylosandrus germanus (Blandford)
The black stem borer (BSB; Figure 1) is an invasive beetle from Asia (primarily Japan, Korea, Vietnam, China, and Taiwan), which is a serious pest of a wide range of nursery and landscape trees (over 200 species), including oak, elm, red maple, beech, hickory, chestnut, magnolia, pear, dogwood, black cherry, tupelo, and black walnut, to name a few. It now occurs in central Europe and North America, and was first detected in New York grapes in the 1930’s. In the past seven years or so, black stem borer has become a pest of tree fruit in New York and Michigan. The reason for this movement from ornamentals into orchards is unclear, as is the level of potential damage from this pest in the future.
In Wisconsin, DATCP has been monitoring the presence of the BSB in Wisconsin and has detected the BSB in 21 counties since 2013 (Figure 2). Until this year, the BSB had not been found in any agricultural crop, but only in lumberyards and wood waste disposal sites. However, on June 4, 2019, an apple grower in Lafayette County reported symptoms characteristic of the BSB (Figure 3) on one apple tree. The grower noted the presence of about 20 holes in the trunk, some with the sawdust “toothpicks”, accompanied by an overall tree decline, including wilting and drooping leaves. The tree was cut down and the trunk brought to the UW-Madison Insect Diagnostic lab where PJ Liesch extracted over a dozen adult beetles that he identified as black stem borers.
Figure 2. DATCP’s distribution map of BSB detections as of June 2019 in Wisconsin. Map credit: Krista Hamilton, WI DATCP.
Life Cycle
The BSB is a small insect about 2mm (0.08”) in length (Figure 1). The BSB overwinters as an adult, primarily females, in the host plant, and has two generations per year. Overwintering adults become active in late April-early May, following 1-2 consecutive days at ≥68°F. Females usually outnumber males by 10 to 1 and are the only ones known to fly. After mating, females will search for a new host and bore a hole about 1mm in diameter (Figure 4) typically towards the bottom 2-3 feet of the tree trunk. Females excavate into the wood of trunks or limbs, creating galleries made of entrance tunnels, brood chambers containing immatures, and branch tunnels in which the immatures will develop. Females carry in mycangia (internal pouches on their bodies) a symbiotic fungus (Ambrosia hartigii) that they will disseminate in the galleries, hence the name Ambrosia beetle. The adults and larvae do not feed on the plant tissue, but rather on this fungus that the females carry with them. Larvae develop through three instars and it takes about 30 days from egg to adult. Females lay one egg per day and may deposit up to 18 eggs. The next generation of adults should occur during the month of July and the last generation, which is the one that overwinters, should appear around the end of August and into September.
Damage Symptoms
The BSB prefers trees that are 3 inches or less in diameter and are primarily attracted to stressed trees. Stressors such as flooding, frost injury, and drought have been associated with BSB attraction. Injured trees produce several types of volatile chemicals, including ethanol, which is attractive to the BSB. The BSB is usually found in higher numbers in neighboring woods and moves into orchards from the edges with woods. They can fly more than 100 m (330 feet) to colonize new trees. Their presence inside a tree will trigger the tree to defend itself by walling off its vascular system, which will result in wilting, dieback, tree decline and eventually death. In New York, they observed some of the BSB present in trees with fire blight symptoms (as well as seemingly healthy looking trees) and so did the Lafayette grower who reported the first injury by the BSB in apple in Wisconsin. Another concern associated with the BSB is the fact that this beetle may be spreading pathogens, such as fire blight, canker-causing Fusarium solani, and others.
Figure 3. Compressed sawdust “toothpicks” caused by black stem borer. Photo credit: Chris & Juli McGuire, Two Onion Farm, Wisconsin. 
Figure 4. Black stem borer entry hole. Photo credit: Deborah Breth, Cornell Cooperative Extension
Monitoring
Look for signs of infestation, including the presence of sawdust toothpicks protruding from holes (Figure 3), the presence of entry holes (Figure 4), dark discoloration on the bark, oozing sap, and dry, blistery bark.
Ethanol-baited traps can be used to monitor the flight activity of adult females in the spring. Traps can be made of a plastic 1 or 2-liter juice bottle into which two to four windows are cut out of the sides (Figure 5). The trap is capped and hung upside down in the orchard at a height of 0.5 m (1.6 feet), at the orchard edge near wooded areas. The trap can be baited with ethanol by pouring a cup of vodka into the trap which will serve as an attractant and drowning solution or by purchasing a ready-made ethanol lure (Standard Release ethanol lures, AgBio, Inc., Westminster, CO), hanging the lure inside the trap and filling the bottom of the trap with soapy water or a small amount of antifreeze. Do not use rubbing alcohol. Traps should be checked at least once a week. Proper identification of tiny BSBs will require the use of a microscope and training to determine species.
Management
The best time to spray an insecticide is in the spring when the overwintering females are emerging and searching for new suitable hosts. Pay special attention to young trees near woodlot edges as these are at greatest risk of injury. Because the beetles do not actually feed on the woody tissue but rather on the Ambrosia fungus, systemic insecticides that work on other borers will not be effective against the BSB. Studies on ornamental trees have shown that applying pyrethroids as trunk sprays reduced the number of new infestations. Permethrin-based insecticides are the most effective at controlling the BSB, though they will reduce but not prevent attack by the BSB on stressed trees. While permethrin products do not have the BSB on their label, they are registered in apple in Wisconsin and can be used in apple. They will also help control green fruitworm and spotted tentiform leafminers, as these are active at the same time than the BSB in the spring. Spring is also the time of apple bloom and protecting pollinators should also be taken into account. It is recommended to immediately remove and burn infested trees that are 75% or more dying. Large pruning’s and brush piles should be flailed or burned as they may harbor the beetles and provide a source for new infestations.
At this time, we recommend to pay special attention to stressed trees showing symptoms of wilting and to make sure that you can identify symptoms that may indicate a BSB infestation.
Happy growing season!
This article was posted in Apples and tagged black stem borer, BSB.