Ambrosia beetles are among the most damaging pests of nursery. The main culprits are granulate ambrosia beetle (Xylosandrus crassiusculus) and the black stem borer (X. germanus). We have worked on ambrosia beetles for several years to find ways to reduce damage and insecticide use. Recently though, in collaboration with Chris Ranger at USDA ARS in Wooster and other scientists, we finally got to the root of the problem: why do ambrosia beetles attack nursery stock to begin with?
Chris and other collaborators started down this road with the realization that when trees get too much water they release ethanol through their bark. And what is the lure in ambrosia beetle traps?: ethanol.
The question then became “how wet is too wet?” If nurseries could prevent trees from getting ‘too wet’ water maybe they could stop ambrosia beetle attacks all together. To answer this question we grew Florida dogwoods in containers and maintained 10, 30, 50, 70, or 90% media moisture throughout the spring. It turns out that trees above 50% media moisture got hammered by ambrosia beetles and those below did not get attacked at all. So, at least for dogwoods, we recommend a media moisture threshold of 50% or below to reduce ambrosia beetle attacks.
Next we measured media moisture at 6 cooperating nurseries each spring for two years. Unfortunately, containers at most nurseries were above the 50% threshold during peak ambrosia beetle activity in early spring. Thus, trees in these nurseries smelled sick to ambrosia beetles and were susceptible to attack.
Reducing media moisture in spring is a challenge because the weather is cool and often wet and trees are not transpiring. That said, sometimes we found irrigation systems were operating during ambrosia beetle season when they probably weren’t required. More research is required to develop horticultural tactics, such as new substrates, to reduce spring media moisture. In the mean time though growers can add the media moisture threshold of 50% to their IPM toolbox to use in combination with other tactics like trapping and insecticide applications.
Frank, S. D., and C. M. Ranger. 2016. Developing a media moisture threshold for nurseries to reduce tree stress and ambrosia beetle attacks. Environmental Entomology 45: 1040-1048.