Protecting your trees from Mountain Pine Beetles

What works, what doesn’t

Irene Shonle

05/08/2008 - As spring progresses, we will see the trees that were attacked by Mountain Pine Beetle last fall begin to turn red and dead. It is a painful process to watch our beautiful trees die, and many of us are in various stages of the grief process (denial, anger, bargaining, depression, and acceptance). The pine beetle infestation is spreading more quickly than has ever been seen in the past, largely due to a “perfect storm” of densely stocked forests, older forests, and warmer, drier winters. Nora Rasure, the acting Deputy Regional Forester, has declared it to be an epidemic along the Front Range, meaning that it is significantly in excess of the normal levels. Aerial surveys from last fall show that pine beetle has killed lodgepole pine on at least 155,000 acres of the Arapahoe/ Roosevelt National Forest, according to a USDA Forest Service report.

  With the increasing densities of the pine beetle, many homeowners would like to protect their trees from being killed. There are many products out there that claim to protect trees from mountain pine beetle, and some of them have more science and field-testing backing them than others. Colorado State Forest Service Entomologist Ingrid Aguayo investigated these different products, and came up with some thoughts that are summarized below.

Pheromones are a hot topic right now. These are essentially powerful scents that insects use to communicate. One is an antiaggregation pheromone called verbenone, which acts to tell other beetles that “this tree is full.”  Research on this has shown that it works pretty well while beetle populations are low. However, the pheromones don’t kill the beetle – they merely divert them to a neighboring property where the populations will continue to build. When populations increase (epidemic levels), the synthetic material is not able to 'mask' the communication system of the beetle, and it loses efficacy. Additionally, research has shown mixed results on the use of these pheromones in a natural forest setting. Verbenone is commercially available, and registered in Colorado; landowners should have their property assessed by a professional forester before deciding to use verbenone. It is not a “silver bullet,” but it may temporarily help in an area where the beetle populations have not built up. It should be used in conjunction with an aggressive plan of searching out and treating infested trees.

Tree injections have also received a certain amount of media attention. With this system, a systemic insecticide called emamectin benzoate (EB) is injected into the trunks of trees. While EB has been shown to be effective in protecting ash trees from emerald ash borer infestation in the eastern U.S, EB is not registered for use against mountain pine beetle and cannot currently be used for this purpose on a commercial basis. Further, there are no research reports available that demonstrate that injecting pine trees with EB (or even any other insecticide) will protect trees from pine beetle infestation. The reason for this is that pine beetle larvae feed in the phloem tissue of the tree, and there is limited movement of the insecticide to the phloem area. Preliminary results of research carried out with EB in conifers against mountain pine beetle by CSU show treated trees have become infested.

Spraying trees is the only mechanism for protecting trees that consistently showed effectiveness on the Western Slope. Because there are environmental consequences to spraying, landowners will have to weigh their options. If you do decide to spray, choose only a few high value trees, and get a certified applicator to spray. It will need to be done each year for as long as the epidemic lasts (10 years or more). Spraying should occur on a dry day in May or June before the beetles fly; call the contractor as soon as possible to get on their list. You can look in the phone book under “tree services” to find a qualified applicator.

  Irene Shonle is the director of Colorado State University Extension in Gilpin County. If you have any questions on pine beetles, please call or stop by the CSU Extension office in Gilpin County Extension at the Exhibit Barn, 230 Norton Drive, Golden CO 80403 (right next to the Gilpin Community Center). Website: www.ext.colostate.edu/gilpin, phone: 303-582-9106, email: irene.shonle@colostate.edu. Extension programs are available to all without discrimination.