Caterpillar control

Caterpillars, which are the larval or immature stage of moths and butterflies, can be major insect pests of greenhouse-grown crops including vegetables and ornamentals during late spring through fall depending on geographic location. Moths can enter greenhouses through openings such as doors, vents, louvers and sidewalls, and lay eggs that hatch into caterpillars. Caterpillars have chewing mouthparts and will feed on a variety of horticultural plants grown in greenhouses. If left unchecked, caterpillars can severely damage a crop.

Caterpillars that may be encountered feeding on horticultural crops grown indoors or under cover include the beet armyworm (Spodoptera exigua), cabbage looper (Trichoplusia ni), corn earworm (Helicoverpa zea), diamondback moth (Plutella xylostella), European corn borer (Ostrinia nubilalis), imported cabbageworm [Artogeia (Pieris) rapae], leafrollers (Choristoneura spp.), and cutworms. Some caterpillars feed on particular plant types or crops in a certain plant family. For example, imported cabbageworm, diamondback moth, and cabbage looper feed primarily on plants in the cole crop family (Cruciferae), which includes vegetables and ornamental cabbage and kale. Imported cabbageworm is a velvety green caterpillar approximately 1¼ inches long with a yellow stripe extending down the back and a fragmented line of yellow spots along each side. Diamondback moth caterpillars are small, approximately ? inch long, light green, and either chew or mine leaves. Cabbage looper caterpillars are light green, approximately 1½ inches long with white stripes extending down the back and along the side, which is in contrast to the deeper green and yellow stripes of the imported cabbageworm caterpillar. Furthermore, cabbage loopers have three pairs of legs near the head and three additional pairs, which are referred to as prolegs, near the last abdominal segment.

The life cycle entails an egg, caterpillar or larva, pupa, and adult. Adult female moths are generally active at night although they may be observed during the daytime. Females fly around and lay eggs on leaf undersides. The number of eggs laid depends on the species with females laying anywhere from 20 to 100 eggs during their lifetime. Eggs hatch into caterpillars that consume plant leaves. Caterpillars undergo a series of stages referred to as instars, in which there is an increase in size from one instar to the next. There may be 3 to 5 instars depending on the species. Caterpillars feed voraciously at first then consume less as they prepare for pupation. The caterpillar stage can last a week to 10 days. Eventually, the caterpillar undergoes a transitional or pupae stage. Some caterpillar species spin cocoons whereas others do not. Pupation may occur on plants, in stems, or on the growing medium surface. After one week, adults emerge from the pupae. The life cycle from egg to adult takes approximately 3 to 4 weeks depending on temperature.

Most caterpillars cause damage by eating plant parts including leaves and sometimes flowers. They may consume the entire leaf or leave the mid-vein. The presence of fecal deposits (frass) on plant leaves is an indication of caterpillar activity. Several caterpillar species will roll leaves together with silken threads whereas other species will actually tunnel into plant stems. If caterpillar populations are extensive and damage is not noticed in time, this can reduce crop quality or result in crop losses. Plants grown inside greenhouses may suffer more damage from caterpillars than plants grown outdoors because natural enemies including parasitoids and predators tend to occur at higher numbers outdoors and there is less migration of natural enemies into greenhouses.

Reduce weeds, plant debris
Adult moths are attracted to lights (at night) that are located in and around greenhouse facilities, so minimizing lighting during peak moth activity will avoid luring females into areas where they can lay eggs. Weed management inside and outside greenhouses will reduce problems with caterpillars because many weeds serve as hosts for adult females to lay eggs. In addition, cleaning-up plant debris will remove sources for overwintering pupae. Pheromone or blacklight traps located outdoors may be helpful in detecting peak adult activity. Inspecting plants on a regular basis when adult moths are active (flying around) will also avoid damage to the crop from caterpillar feeding. Placing yellow sticky cards near susceptible crops inside the greenhouse will capture adults, which will help in the timing of insecticide applications. When scouting, be sure to check plants near openings including vents, doors, louvers, and sidewalls because these are where adult moths will enter, especially those openings near corn, soybean, or vegetable fields that are in decline or have been harvested.

Control measures
Insecticides are used primarily on the caterpillar stage with most materials having contact activity only, so thorough coverage of all plant parts is essential. A microbial insecticide with the active ingredient Bacillus thuringiensis subsp. kurstaki is commonly used for controlling caterpillars. It is effective but must be applied when caterpillars are young. The active ingredient has to be ingested in order to be effective and young caterpillars don’t have to consume as much material before they die; however, larger caterpillars must eat more material before the active ingredient inhibits feeding. In the meantime, the caterpillar can still cause plant damage. The insecticide may need to be re-applied frequently depending on the time of year and implementation of cultural practices as environmental conditions including ultra-violet (UV) light and overhead irrigation may shorten residual activity. Products with the active ingredient spinosad, which is widely used for thrips control, also has activity on various caterpillar pests. Spinosad works as both a contact and stomach poison. A number of insecticides are labeled for use against caterpillars on horticultural crops. However, always read the label of all insecticides before use to make sure they can be applied to horticultural crops.

Another management strategy that is available to greenhouse producers, who experience problems with caterpillars on a regular basis, is to purchase natural enemies for release into the greenhouse. Parasitoids (parasitic wasps) in the genus Trichogramma attack the egg stage of various caterpillar species including diamondback moth, cabbage looper, and imported cabbageworm. The lifespan of the parasitoids is approximately seven days as immatures within the egg, and then up to 10 days as adults. Several species of Trichogramma wasps may be available from commercial insectaries; however, be sure to contact your biological control supplier in advance in order to determine availability of these parasitoids.

Ray Cloyd is professor and extension specialist in horticultural entomology and integrated pest management at Kansas State University; rcloyd@ksu.edu.

Photos courtesy of Raymond Cloyd