Tent caterpillars are out once again this year and they seem as hungry as ever for our landscape trees and shrubs.
But are they really a menace and should we do anything about them? The answer is nuanced between the number of caterpillars, the age of the trees/shrubs, and your personal tolerance. On mature trees, even if the infestation is heavy, most trees that are not overly stressed from other conditions, i.e. drought, will recover nicely once the caterpillars stop feeding even if the trees are mostly defoliated.
But if your trees are younger, more caution should be taken because defoliating a tree may kill the tree. Obviously, if you just invested thousands of dollars in nice looking trees you wouldn’t want a colony of caterpillars to kill your trees so some measure of control needs to be taken.
There is a human-safe biological product available that utilizes Bacillus thuringiensis (Bt) a naturally occurring bacterium. Yes, the name may look familiar if you follow agriculture because it is the same bacterium that can be found in GMO crops. You apply Bt to the leaves of the host plants and when the caterpillars feed on the leaves they ingest the bacterium and kills them in 3 to 4 days as it messes with their digestive system.
The product is deemed so safe that we recently applied the brand Dipel Pro to Wilkes Elementary School on Bainbridge Island. We have found that using our FertileTea (a compost tea derivative) as the carrier is a highly effective way to coat the leaves to run off and the plants get the added benefit of worm castings biology and the added sea kelp.
Let’s not forget that while using a biological insecticide is considered safer than their chemical counterparts we are still killing insects. This needs to be taken seriously. There are 2 steps to consider before you reach for an insecticide… 1) Reconsider your tolerance levels. While Tent Caterpillars may cause cosmetic damage healthy, mature trees can withstand their presence and recover nicely after they stop feeding. For Wilkes Elementary School theses are new plantings and loss was a real possibility. And 2) Consider cultural controls. Can the nests be cut out and disposed of without doing structural damage to the trees?
And of course, should you reach for a biological insecticide, wear proper clothing, mix strictly to directions and only apply to host plants.
We have seen such good results with Dipel Pro and our application method that we believe there is no longer a need for stronger chemicals in the control of Tent Caterpillars.