The Red Humped Caterpillar, Schizura concinna, is a common garden pest known for causing significant, rapid defoliation, particularly in late summer and early fall. These pests feed in large groups, quickly skeletonizing leaves and stressing host plants. Prompt, targeted intervention is necessary to protect vulnerable trees and shrubs. This guide details practical strategies, from immediate physical removal to long-term life cycle disruption, for eliminating and preventing future infestations.
Identifying the Red Humped Caterpillar and Host Plants
Correct identification is the first step in managing this pest. The Red Humped Caterpillar is easily recognizable by its striking appearance, which includes a bright red head capsule and a prominent, reddish-orange hump on the first abdominal segment. The body is predominantly yellow with lengthwise black and white stripes, and it is covered in numerous black projections called tubercles. Mature larvae can reach a length of up to one and a half inches.
Young caterpillars feed side-by-side in distinct clusters, initially skeletonizing leaves by consuming tissue but leaving the veins intact. As they grow, they disperse slightly but continue to feed voraciously, sometimes consuming the entire leaf. They target a wide array of woody host plants, frequently damaging fruit and nut trees like apple, plum, cherry, and walnut. Ornamental trees and shrubs are also susceptible, including willow, birch, sweetgum (Liquidambar), and redbud.
Immediate Non-Chemical Removal Methods
For localized or small-scale infestations, physical methods offer immediate, non-toxic control. Since young Red Humped Caterpillars remain clustered, an effective strategy is to prune the infested branches from the host plant. Use sharp, clean pruning shears to remove the entire shoot or branch section where the cluster is visible. Promptly destroy the removed material to prevent the pests from returning.
Manual removal, or hand-picking, is a straightforward option for accessible parts of the plant. Wear gloves when performing this task, as the caterpillars can spray a defensive chemical, formic acid. The collected caterpillars should be dropped into a bucket of soapy water, which causes them to drown quickly.
Using a strong jet of water can dislodge the caterpillars from the foliage. While this method does not eliminate the pest entirely, knocking them to the ground exposes them to natural ground-dwelling predators. This technique is most effective when the caterpillars are young and the infestation is not widespread.
Strategic Use of Biological and Targeted Controls
When manual removal is impractical due to the plant’s size or the extent of the infestation, targeted treatments are necessary. The microbial insecticide Bacillus thuringiensis (Bt) is a highly selective biological control agent for caterpillars. Bt is a naturally occurring soil bacterium that must be ingested to be effective, acting as a stomach poison that stops feeding within a day.
Bt is particularly effective against younger, smaller caterpillars, which are actively feeding and more likely to consume the treated foliage. The product, often available as subspecies kurstaki, is safe for humans, pets, and beneficial insects, including bees and parasitic wasps. Thorough coverage of the leaves, especially the undersides, is important. A second application may be needed after seven to ten days because Bt degrades quickly.
For severe outbreaks or when dealing with larger, mature caterpillars, an insecticide containing Spinosad provides a powerful targeted alternative. Spinosad is derived from the fermentation products of a soil bacterium and kills insects through both contact and ingestion. It is highly effective against caterpillars, often killing a larger percentage of older larvae than Bt.
While Spinosad has low toxicity to mammals and birds, it can be toxic to pollinators, particularly bees, for several hours after application. Application should be carefully timed to avoid periods when the host plant is flowering or when pollinators are most active. Always treat locally, focusing only on the infested areas, which is often sufficient given the localized nature of the feeding.
Disrupting the Life Cycle for Long-Term Prevention
Long-term management focuses on disrupting the pest’s life cycle to prevent future generations. The moth overwinters as a mature larva inside a cocoon in the soil or leaf litter at the base of the host tree. In the spring, pupae transform into adult moths, which emerge and lay eggs in clusters of 25 to 100 on the underside of leaves.
Scouting for these cream-colored or yellow egg masses in late spring and early summer is a proactive measure to prevent infestation. Destroying the egg clusters or pruning the leaves with the eggs is a simple, effective form of cultural control. Monitoring should be repeated later in the season, typically late June and July, to intercept the second generation of eggs and larvae.
Encouraging the presence of natural enemies is a significant component of long-term prevention. Parasitic wasps, such as Hyposoter fugitivus and Cotesia schizurae, are important natural controls that lay eggs inside the caterpillars. Generalist predators like spiders, green lacewings, and bigeyed bugs also consume the eggs and small larvae. Maintaining a diverse garden environment, including planting flowering species, helps conserve these beneficial insects, reducing the likelihood of damaging caterpillar populations.