Issue 5, May 28, 2013

Tree and Shrub Borers

Most borers attack trees that are under stress. Roundheaded beetle borers typically chew niches through the bark to lay their eggs, although flat-headed beetle borers lay their eggs in bark crevices and under loose bark flakes. Moth borers typically lay their eggs in pruning and other wounds. Healthy trees have enough sap flow that borer eggs are flushed out of niches and young borer larvae drown. It is thought that the internal sap pressure in healthy trees squashes soft-bodied borer larvae.

Trees that are under stress produce aromatic chemicals that borer adults find attractive, so one reason that healthy trees are not attacked is that they do not attract borers to them. Trees are under stress and produce borer-attractive chemicals when they are reaching the end of their life span and when they have been recently replanted. Borers attacking dying trees shorten their lives, but not significantly. Insecticide application against borers attacking dying trees essentially only prolongs the death of the tree a couple of years or so and is usually not recommended.  However, insecticide applications to young, recently transplanted trees can get them through this susceptible time, resulting in long lives. Once the trees become established, borer treatments are usually no longer needed. This can be determined by looking for normal yearly twig growth, as a stressed tree adapting to site will exhibit greatly reduced growth.

The above pertains primarily to native insect borers of native trees and shrubs. This is a relationship that allows both to survive. If an insect borer were to attack healthy trees, it would soon kill all of the trees of that species and die itself due to lack of food. It is to the borer’s advantage to only attack trees that have already reproduced and are at the end of their natural lives.

Exotic insects or trees upset this balance. Exotic trees, such as European white-barked birches, commonly do not have host plant resistance to a native borer, such as the bronze birch borer, when they are introduced into the borer’s geographic range. More commonly, an exotic borer finds its way to another part of the world where the native plants in that area are not resistant. This is the case with emerald ash borer and Asian longhorned beetle.

Generally, imidacloprid, sold as Merit, Xytect, Optrol, and others, is effective against beetle borers. Imidacloprid is systemic and will last inside of the tree for about one year, making the time of application less critical. Because most borer adults are laying eggs in the spring and tree leaves are most active in transpiration in the spring, application of imidacloprid or other systemic insecticide is most effective at that time. Transpiration carries systemic insecticide up into the tree. 

Because imidacloprid is not very effective against caterpillars, permethrin, sold as Astro, is recommended for moth borer control. Permethrin is not systemic and only lasts two to four weeks on the outside bark so timing is more critical to kill hatching larvae of moth borers before they get into the tree. (Phil Nixon)

Phil Nixon

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