Forest Tent Caterpillars (FTCs), Malacosoma disstria, are native North American insects that become pests during cyclical outbreaks. These caterpillars can completely defoliate deciduous trees, such as maples, oaks, and poplars, over a five-to-six-week period in the spring. While healthy, mature trees often survive a few years of defoliation by producing a second flush of leaves, repeated attacks weaken them, making them vulnerable to other pests and diseases. Effective management depends on accurate identification and targeting the most vulnerable stages of the insect’s life cycle.
Identifying the Pest and Vulnerable Stages
Identifying the Forest Tent Caterpillar (FTC) is crucial, especially distinguishing it from the Eastern Tent Caterpillar. FTC larvae are primarily blue and black, featuring a distinctive row of white or pale yellow markings down the center of their backs that resemble keyholes or footprints. This pattern contrasts with the single, continuous white stripe found on the Eastern Tent Caterpillar.
Despite their name, FTCs do not construct the large, conspicuous silken tents in tree crotches that Eastern Tent Caterpillars do. Instead, they spin small, unnoticeable silken mats on the trunk or branches where they congregate to rest or molt. Timing control measures to coincide with the vulnerable stages is crucial for the most effective reduction in pest numbers.
Vulnerable Stages for Control
- Overwintering egg masses
- Newly hatched larvae in early spring
- Actively feeding mature larvae
- Pupae
Immediate Manual Removal Techniques
Manual removal is the safest and most immediate way to control Forest Tent Caterpillars, especially on smaller, accessible trees. The best time to act is during the dormant season by removing the egg masses. These masses appear as silvery-brown, cylindrical bands that completely encircle small twigs and are coated in a hard, frothy substance called spumaline.
You can remove these egg masses by scraping them off the branch with a dull knife or pruning the infested twig entirely. The removed masses should be destroyed by crushing them or submerging them in a bucket of soapy water to prevent hatching. Once the larvae hatch in early spring, they can be removed when they congregate on the trunk or branches.
Larvae can be scraped off the tree bark or dislodged from lower branches using a forceful spray of water. Collected caterpillars should be dropped into a container of soapy water, which kills them quickly. To prevent caterpillars from climbing back up the tree, a sticky barrier product, like Tanglefoot, can be applied to a protective band wrapped around the trunk. The sticky material must be applied to the band and not directly to the bark to avoid potential damage to the tree.
Using Targeted Biological Controls
For larger trees or widespread infestations, a targeted biological control offers an effective and environmentally conscious solution. The primary option is Bacillus thuringiensis subspecies kurstaki (Btk), a naturally occurring soil bacterium. Btk is a highly specific insecticide because it must be ingested by the caterpillar to be effective.
Once consumed, the alkaline environment within the caterpillar’s gut activates a protein crystal in the Btk, creating a toxin that paralyzes and destroys the insect’s digestive system. This mechanism ensures that Btk is harmless to humans, pets, birds, and beneficial insects like bees, as they lack the necessary gut chemistry.
Timing the application is crucial, as Btk is most effective only against young, actively feeding larvae, specifically the first and second instars, which are less than one inch long. Application should occur shortly after the eggs hatch in early to mid-May, typically when the tree buds are just beginning to open. Mature caterpillars are much harder to kill, making a late application largely ineffective.
Since Btk degrades quickly when exposed to sunlight, it is best applied in the late afternoon or evening. For a significant population reduction, two applications spaced about one to two weeks apart are often recommended to ensure all newly hatched larvae are targeted.
Preventing Future Outbreaks
Long-term management focuses on monitoring and maintaining tree health to minimize the impact of future cyclical outbreaks. FTC populations naturally rise and fall, with outbreaks typically lasting two to four years before collapsing due to natural factors like disease, starvation, and predators. The goal of management is to help trees survive these periods.
Maintaining overall tree vigor is a preventative measure, as healthy trees better withstand defoliation and rapidly produce new leaves. This includes ensuring proper watering, especially during dry periods, and appropriate fertilization. Avoiding the use of broad-spectrum chemical insecticides helps to encourage natural predators, such as parasitic wasps and flies, which are highly effective at regulating FTC populations.
Annual monitoring for the distinctive egg masses during the winter and early spring is the most proactive way to break the cycle. Removing these masses before they hatch reduces the initial population density and serves as an early warning system for a potential outbreak year. Turning off outdoor lights at night during the adult moth stage in July can also help, as the moths are attracted to light and tend to lay eggs on nearby trees.