Two invasive ambrosia beetle species are serious pests in nursery and landscape trees and shrubs: granulate ambrosia beetle (Figure 1), and black stem borer (Figure 2). Ambrosia beetles are about 2 to 4 mm in length and spend most of the time in the tunnels they build in the sapwood of trees. In spring, when temperatures reach 68o F for at least 3 days in a row, female ambrosia beetles start to fly out of their galleries to colonize new hosts. Stressed plants are more susceptible to ambrosia beetle attack. For this reason, ambrosia beetles are considered opportunistic pests.
Monitoring using alcohol-baited traps (Figure 3) is the best approach to detect ambrosia beetle presence and estimate seasonal populations.
In 2017, due to the mild winter conditions, we started monitoring these pests on February 16 in Todd, Calloway, Graves, and Caldwell counties in western Kentucky. The numbers of captures of granulate ambrosia beetles were low and inconsistent in all locations up until the second week of March. Granulate ambrosia beetle populations increased substantially the third week of March in Calloway, Graves, and Todd counties: 105, 236, and 92 beetles in each location, respectively. Black stem borer has been captured in low numbers.
Currently, the management practice to control these beetles is through multiple insecticide sprays directed to the trunks when populations are high to prevent infestations. For more information, consult county agents close to you.
If toothpick-like structures protruding from stems are observed (Figure 4), the beetles are already inside plants and nothing can be done because insecticide sprays cannot reach them. In nursery stock, infested trees must be removed from nursery and burned.
- Granulate Ambrosia Beetle (University of Maryland, Entomology Bulletin, 2008) link
- Managing Black Stem Borer in Michigan Tree Fruits (Michigan State University) link
- Granulate Ambrosia Beetle (Xylosandrus crassiusculus) (Indiana Department of Natural Resources) link
By Zenaida Viloria, Extension Associate for Nursery Crops; Ginny Travis, Horticulture Technician; Win Dunwell, Extension Horticulturist; and Raul T. Villanueva, Extension Entomologist