Inchworms are the larval stage of moths belonging to the family Geometridae. These caterpillars are easily identified by their distinct movement, where they arch their bodies into a loop as they bring their rear prolegs forward to meet their front true legs. Large numbers of inchworms are considered agricultural and horticultural pests because they are voracious leaf-eaters that can cause significant defoliation of trees, shrubs, and garden vegetables. Effective control relies on combining immediate actions with long-term preventative care.
Manual Removal and Physical Barriers
Dealing with localized infestations often begins with direct, non-chemical methods. Hand-picking inchworms from plant foliage is an effective tactic for smaller trees and garden plants. The collected larvae can be dropped into a container of soapy water, which quickly eliminates them. This method is most practical when caterpillar numbers are low or when the infestation is limited to easily accessible areas.
Targeted pruning of infested branches and twigs can also reduce the overall population before the larvae cause widespread damage. After removal, these plant parts should be destroyed or tightly sealed in a garbage bag to prevent the pests from crawling back to the host plant. This physical removal is particularly useful in the early stages of an infestation when the caterpillars are still small and concentrated.
Physical barriers applied to tree trunks are effective against certain inchworm species, such as cankerworms, whose wingless females must crawl up the trunk to lay eggs. A band of sticky material, often applied over duct tape or a specialized tree wrap, traps these females before they can ascend and deposit their eggs. This barrier should be applied before the adult moths emerge in the late fall or early spring to interrupt the reproductive cycle.
Applying Biological Control Agents
For broader treatment, biological control agents offer a method of managing inchworm populations. The most widely used biological control product is Bacillus thuringiensis (Bt), a naturally occurring soil bacterium. Specifically, the kurstaki strain (Btk) is effective against caterpillars and other chewing insects.
Btk works as a stomach poison, meaning the inchworm must ingest the treated foliage for the bacteria to be effective. Once consumed, the bacteria produce a crystalline protein endotoxin that dissolves the lining of the caterpillar’s gut, causing it to stop feeding within hours. Timing the application is important, as Btk is most effective against young, actively feeding larvae, typically when they are less than 3/8-inch long.
Horticultural oils and neem oil also serve as effective organic controls for inchworms. Horticultural oils can be sprayed on tree trunks during the dormant season to suffocate overwintering eggs laid by adult moths. Neem oil acts as an insect growth regulator and a repellent, interfering with the inchworm’s ability to feed and develop. When using these oils, care must be taken to ensure the specific host plant, such as certain maple varieties, is not sensitive to oil applications.
Using Targeted Chemical Treatments
Chemical treatments are reserved for infestations where manual and biological controls have failed to reduce the pest population. These insecticides offer a rapid knockdown of active larvae. Common chemical options include products containing pyrethrins or their synthetic counterparts, pyrethroids, such as deltamethrin or permethrin.
Pyrethrins and pyrethroids work by attacking the insect’s nervous system, quickly causing paralysis and death. Because these chemicals are broad-spectrum, they can affect beneficial insects, so targeted application is necessary. Always strictly follow the manufacturer’s directions, ensuring that pets and people are kept away from the treated area until the product has completely dried.
Long-Term Prevention Strategies
A proactive approach is the most sustainable way to manage inchworm populations. Regular monitoring of plants, especially in the early spring when eggs typically hatch, allows for the detection of small larvae before they can cause extensive damage. Shaking branches and inspecting the foliage for the telltale signs of small, irregularly shaped holes helps in early identification.
Encouraging natural predators to inhabit the garden environment provides a continuous, self-regulating control method. Birds, ground beetles, yellow jackets, and parasitic wasps, particularly Trichogramma species, all prey on inchworms or their eggs. Installing bird feeders and houses can attract songbirds, which are natural enemies of the caterpillars.
Sanitation practices also help disrupt the inchworm life cycle by removing overwintering sites. Many moth species whose larvae are inchworms overwinter as pupae or eggs in leaf debris and soil beneath host plants. Clearing leaf litter and maintaining general garden hygiene eliminates these hiding places and reduces the number of pests that emerge in the spring.