Web of life: Caterpillars in trees usually not a cause for concern
During our long growing season, I get calls about caterpillars in trees and what to do about them. Caterpillars, the larvae of moths and butterflies, use tree leaves as a source of food. In turn, many other things use caterpillars as food. This is often referred to as part of the web of life.
When I answer calls about caterpillars in trees, my first two questions are: What is the caterpillar and what is the host plant?
Once this is determined, the next question is the effect on the host plant and is it a problem. This can be solved by several ways of looking at the situation.
Fall webworms form nests in trees in the fall, and are only a concern for small, less established trees.
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Some caterpillars only affect a small part of the tree. As an example of this, in spring we often spot the tents of the eastern tent caterpillar (Malacosoma americanum) high up in trees. This type of tent caterpillar is a social species that forms communal nests in the branches of trees.
They generally defoliate a small area of the tree and since they only produce one generation per year, they are not a significant problem for a large healthy tree. Thus, the only time that you might want to do something about them would be for a newly planted and/or small tree.
In late summer and into the fall, we will often see a similar nest created by fall webworms (Hyphantria cunea). This is another gregarious insect that feeds on tree foliage. Like the eastern tent caterpillar, these larvae consume small areas of large trees, so they are only a concern with smaller trees.
An added consideration on why not to worry about fall webworms is that they are consuming leaves at the end of the year. This is less of a problem for the tree, as the leaves are soon to be discarded during dormancy, making the loss of leaves for caterpillar food production much less harmful.
A similar case is when I observed a large Shumard oak that had been partially defoliated by orange-striped oakworms (Anisota senatoria).
This caterpillar is also a fall defoliator, so the impact of one defoliation was not a big deal for this tree this time. We do, however, need to keep watching an infested tree such as this, as I have seen successive years of caterpillar defoliation actually kill a tree.
If the caterpillars are a problem for the tree, treatments include spraying with an insecticide. Choices can include synthetic insecticides or organic insecticides such as Bt (Bacillus thuringiensis).
Another option is to provide a sticky barrier band on the tree if the caterpillar needs to access leaves by crawling up its trunk. Finally, on smaller trees, caterpillars can be simply picked or shaken off a tree.
But it is always good to consider if all this is necessary. I have let caterpillars kill a tree as the tree was in a wooded area and this was just a natural process. The loss of a tree can be nature’s way of making room for other trees to grow. A common forestry practice of thinning the forest.
Another thing to keep in mind is that many of us purchase bird food. Birds need other food sources besides seeds. Insect larvae is an important source of protein that many birds need, especially when feeding their young.
Finally, while many of these caterpillars become moths, some become beautiful butterflies. Different types of swallowtails feed on sassafras, hoptree and tulip poplar. Thus, I often view caterpillars in my trees as butterfly gardens and bird feeders.
Diagnosing a tree problem and finding a cure is not always easy. Generally, the best thing to do to keep trees healthy is to provide a good mulch bed underneath their crowns so the soil is protected and the nutrients that are in leaves and seeds, etc., are recycled.
Also, growing trees in groups with proper spacing allows trees to have protection from each other, but minimizes competition between them. Trees in this situation are often less stressed and thus less likely to succumb to insect and disease problems.
Stan Rosenthal is a forester with Natural Resources Planning Services and Forestry Agent Emeritus with UF/IFAS Extension Leon County, an Equal Opportunity Institution. For gardening questions, email the extension office at AskAMasterGardener@ifas.ufl.edu.
This article originally appeared on Tallahassee Democrat: Caterpillars in your trees? Most do very little harm
