Nursery Notes: How to manage ambrosia beetles
Ambrosia beetle attacks have been sporadic, unpredictable and extremely costly to a few local producers over recent years. Weakened or stressed plants are attacked. They have killed cherry, chestnut, hydrangea, lilac, red maple cultivars, especially October Glory, weeping mulberry, and redbud locally.
Adult beetles bore into the wood and form a gallery, which the female inoculates with an ambrosia fungus to feed herself and her larvae. Toothpick-like filaments composed of frass (excrement and sawdust) protrude from the holes several inches. Wind and rain will remove these tell tale signs. Attacked trees usually die. The time to prevent damage is prior to the adult boring into the tree. The annual trigger for ambrosia beetle activity is generally one to three consecutive warm/sunny days. Ambrosia beetles have been trapped in Warren County as early as February during mild winters but typically not before late March with peak flights occurring in April. The peak flight period may vary by several weeks among different years. Ambrosia beetles overwinter as adults, so a flush of flight activity can occur as soon as the weather becomes suitable.
Early detection of ambrosia beetle activity is the key to successful management. Traps are an essential tool as an early warning system. Ambrosia beetles begin attacking trees as they appear in traps. Therefore, beetles in the trap signal to spray ASAP.
The closer spray treatments can be timed to initial beetle activity, the greater the likelihood that sufficient pesticide residues will be present on the tree surface to kill or repel ambrosia beetles. Thorough wetting of the trunk is needed to prevent ambrosia beetles from boring into the plant. The effectiveness of air blast sprayers for delivery of sufficient pesticide residues to tree trunks, is questionable at this time.
Permethrin-based insecticides (Ambush, Astro, Pounce, Tempo) have been the only effective products for preventing Asian ambrosia beetle attacks. Ambush and Pounce are restricted use pesticides.
There is no way to predict in advance which plants will be attacked by ambrosia beetles. Trees that have not broken bud are at the greatest risk of ambrosia beetle attack.
Traps can be made from soft drink bottles. Plans and directions are available upon request.
Traps should be placed near expected ambrosia beetle habitats, woods adjacent to nursery crops. Place the traps about 2 to 3 feet above the ground. A single trap placed in the right location can provide a great deal of information about ambrosia beetle activity on your property. However, until you have gained personal experience with the most effective trap sites on your property, we would recommend several traps be used. A good pattern might be one trap on the north, east, south, and west sides of the crop.
Check traps daily from mid-February through April. A trap can catch several hundred beetles in only a few hours when beetle flights begin. Insecticide treatments should begin as soon as beetle activity is detected and continue at 10-14 day intervals until trees break dormancy. Asian ambrosia beetles are reported to fly at dusk, so a good time to check traps might be right after dark. The best indicator that an ambrosia beetle flight has begun will be large numbers of small beetles suddenly showing up in the trap.