The leaves on a hardwood tree make the food, so it stands to reason that any loss of foliage would be detrimental to tree health…right? Well, not always. Unlike the impacts of girdling (phloem loss) which are detrimental whenever they occur, the impact of foliage loss depends on the season in which it occurs. In early spring, when trees have used stored resources to reflush and grow new leaves, defoliation has the biggest impact because the loss of these newly formed leaves before they have had a chance to make any food means the tree has to use MORE stored resources to make new leaves. Think of it like taking food out of your pantry, going through all the work to make a nice meal, and then someone else comes in and eats it all. Now if you want to eat, you have to go back to the pantry and start over.
Defoliation later in the summer, however, removes leaves that have already been “working” for a while, contributing energy to the roots and essentially “filling the pantry”. This type of defoliation is much less detrimental to a tree. However, several types of caterpillars are known to heavily defoliate branches or trees in the summertime, and their feeding can be quite conspicuous, often looking worse than it really is. Let’s go over several common summertime defoliators on hardwood trees in South Carolina.
Fall webworms are probably the most obvious summer defoliators, as their whiteish webs can encompass entire branches. Starting as tiny caterpillars that wrap a single leaf together, as the caterpillars grow they increase their web size. While smaller trees may be injured, most of this defoliation is cosmetic only, and won’t seriously harm a tree. Common hosts include pecan, sweetgum, walnut, and many others. Adults are about an inch across and white (sometimes with black spots).
Several other non-webmaking caterpillars often cause noticeable defoliation on hardwood trees in natural and managed areas. The rosy maple moth adult is a bright yellow and pink moth, and these caterpillars feed on maple trees. Rarely a tree health issue, individual branches can be completely stripped of foliage. Yellownecked caterpillars are known for the U-shaped defensive posture they make when disturbed. These will feed upon many different tree species, as well as shrubs and blueberry bushes (I know this from experience, unfortunately). Adults are brownish, and look like the end of a stick. There are many species of oakworms, all of which feed on oak trees. These caterpillars have little horn-like structures on the front and back end, likely to confuse bird predators. Adults are silk moths, closely related to larger, more showy moths like the luna moth.
All of these caterpillars can cause noticeable damage on hardwood trees during the summer months, but rarely is this damage harmful to a tree. Of course, there are exceptions, as newly transplanted trees or young trees are more susceptible, but established trees probably won’t show any lingering ill-effects. In most cases management isn’t necessary, as there are plenty of natural predators that love to make these big, plump caterpillars into a juicy meal. However, if management is necessary nearly any labeled contact insecticide will provide control, as will physically removing caterpillars by hand or with a strong stream of water.
Find out more about oak pests at https://hgic.clemson.edu/factsheet/oak-diseases-insect-pests/
Find out more about fall webworms at https://hgic.clemson.edu/factsheet/fall-webworm/
Find out more about maple pests at https://hgic.clemson.edu/factsheet/maple-diseases-insect-pests/ and https://hgic.clemson.edu/factsheet/rosy-maple-moth-dryocampa-rubicunda/
Colorful adult rosy maple moth. Photo credit: Dave Coyle, Clemson University. 
Caterpillar of the rosy maple moth feeding on a maple leaf. Photo credit: Dave Coyle, Clemson University. 
Oakworm moth. Photo credit: Dave Coyle, Clemson University. 
Oakworm moth caterpillars with horn-like structures on the front and back end. Photo credit: Dave Coyle, Clemson University. 
Yellownecked caterpillar make a u-shaped defensive posture when disturbed. Photo credit: Dave Coyle, Clemson University. 
Yellownecked caterpillars grouped together feeding on a leaf. Photo credit: Dave Coyle, Clemson University.
Author
Dave Coyle, Cooperative Extension, Forestry and Wildlife Specialist
This information is supplied with the understanding that no discrimination is intended and no endorsement of brand names or registered trademarks by the Clemson University Cooperative Extension Service is implied, nor is any discrimination intended by the exclusion of products or manufacturers not named. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.
Clemson University Cooperative Extension Service offers its programs to people of all ages, regardless of race, color, gender, religion, national origin, disability, political beliefs, sexual orientation, gender identity, marital or family status and is an equal opportunity employer.
