JoAnne Skelly: Do systemic insecticides work against bark beetles in pines?

My friend Roni and I were discussing an upcoming heavy pruning on her pine trees, which are growing into power lines.

She said she was planning on using a systemic insecticide containing imidacloprid well in advance of the pruning to keep borers and bark beetles from infesting the trees, which will be stressed after a hard pruning.

My reaction was, “Let me check to see if that chemical even works on borers and bark beetles” since this active ingredient is primarily effective against foliage feeding insects. It is also highly toxic to bees, other pollinators and beneficial insects.

Borers and bark beetles feed under tree bark in the phloem, the food-relaying vascular tissue that carries the food produced in the leaves throughout the tree. Both insects damage phloem.

Imidacloprid dissolved in water and applied to the soil around the tree is absorbed by the roots. It is then translocated or moved in the xylem, the water-carrying vascular tissue, from soil into leaves, fruit, pollen and nectar.

Xylem is not where these pests eat. Depending on the size of tree, it can take one week to three months to distribute through the tree. While the imidacloprid label says round headed borers, such as longhorn beetles, and flatheaded borers can be managed with this chemical, neither of these are likely to be pests of Austrian pines.

University of California, Davis Integrated Pest Management program states: “Systemic insecticides, meaning those that are implanted or injected through the bark or applied to soil beneath trees, have not been shown to prevent attack or control populations of bark beetles. Insecticide products available to home users are not effective for bark beetle control. Most home gardeners also lack the high-pressure spray equipment and experience to effectively treat large trees. Protective spraying for bark beetles must be done by a licensed pesticide applicator.”

Once the beetles are under the bark and have girdled the tree, there are no chemicals that will save the tree. Preventive spray insecticides should be applied by a professional before beetles emerge in spring or early summer.

Then, when they do emerge, they will chew through the insecticide-coated bark and die. The moral to the story is that unless you really research any insecticide or other pesticide you are thinking of using, starting with a detailed reading of the label, you may be spending a fortune on chemicals that don’t do the job, but can damage the environment.

JoAnne Skelly is Associate Professor & Extension Educator Emerita at University of Nevada Cooperative Extension. Email skellyj@unr.edu.