Black stem borer: An opportunistic pest of young fruit trees under stress

Trees with winter injury or drought stress can attract black stem borers. Proper identification, orchard sanitation and timing of control measures using ethanol-baited traps will help minimize its spread.

A female black stem borer. Photo credit: Brad Barnd, BugGuide.net

A female black stem borer. Photo credit: Brad Barnd, BugGuide.net

Identification

The black stem borer is a very small – about 2 millimeters – ambrosia beetle (Xylosandrus germanus) that attacks stressed and apparently healthy trees, and in particular young trees, with trunk diameters of less than 2.5 inches. The insect is rarely seen outside of its galleries and only females emerge from the galleries they create to infest new trees.

Signs of infestation include round entrance holes that are approximately 1 millimater in diameter, toothpick-like strings of compacted boring dust and frass emerging from the holes, and sometimes weeping or oozing of plant sap from the holes. These signs are similar to other boring insects, such as a bark beetle called the shothole borer (Scolytus rugulosus) that feeds on tree sap.

History

Black stem borers were first detected in the United States around 1930. By 1980 they were detected in Michigan, but until recently they have rarely been seen in commercial orchards. Reports of the pest from ornamental nurseries were followed by detections in several young apricot and plum orchards in the southwest region in 2010 and 2011. More recently, this pest has been found in young apple trees in the Grand Rapids, Michigan area. In addition to reports in Michigan, black stem borers have been recently reported as a pest in commercial high density orchards in New York.

Hosts

The black stem borer will infest a wide variety of woody plant species including all tree fruit species that are grown commercially in Michigan. Ambrosia beetles as a group are attracted to ethylene, which is naturally produced by injured trees. After a harsh winter, some trees that are in fact injured, but look uninjured, will produce ethylene, which attracts the beetle. Typically ambrosia beetles do not attack healthy, unstressed, older trees because healthy trees produce resin to kill potential trunk invaders; however, young saplings produce less resin than mature trees and are vulnerable to attack.

Life cycle

Females emerge in spring to find new hosts, boring a tunnel and one or more brood chambers in the sapwood or sometimes the heartwood of a tree. Ambrosia beetles carry fungal spores that they use to cultivate fungal “gardens” in the tunnels they create inside tree trunks that they bore. Eggs laid in the brood chamber hatch, and larvae and adults feed on the fungus growing on the gallery walls. Each gallery can contain up to 100 larvae with up to two generations per year in temperate climates like Michigan. Females may overwinter in galleries or in leaf litter near the base of trees.

Boring dust and frass Apricot sapling oozing
Left, Characteristic boring dust and frass sticking out of the entrance hole made by a black stem borer in a plum tree. Right, An apricot sapling oozing from holes made by a black stem borer. Photo credit: Bill Shane, MSU Extension

Apple damage Black stem borer damage
Left, Holes made by a black stem borer in apple with bark cut away to show damage. Right, A young apricot sapling split open to reveal damage caused by a black stem borer and signs of the fungus it was cultivating in its galleries. Arrow points to fungus. Photo credits: Amy Irish-Brown (left) and Bill Shane (right), MSU Extension

Scouting

To monitor for the pest, look for the signs of infestation described above within 1 meter of the ground and use a simple trap to capture females. Cut two to four windows in the body of a plastic 1 or 2 liter bottle that has a cap. Hang it in the orchard upside down at a height of 0.5-1 meter, or 1.5 to 3 feet, near wooded areas or in low areas where trees are prone to cold injury and where there are trees with signs of infestation.

Bait the trap with ethanol using one of the following three methods:

  1. Squirt about a quarter cup of ethanol-based hand sanitizer (unscented) into the cap end (bottom) of your trap.
  2. With the bottle capped, pour in a cup of cheap vodka through one of the holes made in the side of the trap.
  3. Purchase a ready-made ethanol lure to hang inside the trap and fill the bottom of the trap with soapy water.

If using hand sanitizer, traps must be checked daily because the sanitizer will form a crust on the surface after 24 hours. If using vodka or a purchased lure, traps should be checked at least once per week. 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.

Black stem borer trap
Example of a trap used to monitor for black stem borers. Photo credit: Amy Irish-Brown, MSU Extension

Management

Unlike other borers, trunk sprays of systemic insecticides will have very little to no effect because the insect feeds on the fungus that it cultivates, as opposed to plant tissue, and is well protected in the galleries it creates. The only potential time that a contact insecticide spray might have an impact is when females are emerging in the spring. Because they are so tiny, they are difficult to monitor to determine the optimum time to apply an insecticide, but a trap as described above can be used to aide in timing a spray in the spring.

A recent three-year trapping study conducted near Wooster, Ohio by the USDA Agricultural Research Service suggests that the first incidence of female activity in the spring will coincide with the accumulation of 100 growing degree days (GDD) base 50 degrees Fahrenheit from Jan. 1. When females are active, pyrethroid insecticides, at least in nursery settings, have shown the most promise in reducing the number caught in traps post-treatment. In another study, researchers found that the application of a bio-repellent applied to the trunks of stressed trees significantly reduced infestation.

Later in the season, the best management strategy is to remove trees with extensive symptoms of decline, i.e., 75 percent or more of the tree is dead or dying, and burn them. It is also important to make sure that all large prunings and brush piles are either flailed or burned. Both intact but stressed trees and fresh cut, large diameter prunings have been implicated as sources of new infestation. Trees that still seem healthy with a few borer holes in them might survive and should be treated for the pest using the scouting techniques outlined above to time the application of an insecticide.

If you suspect you have this beetle infesting young orchard blocks, please contact your local MSU Extension educator. With all the winter injury this season and drought stress last season, it would be easy to miss this pest or misidentify tree decline as this pest.

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