PSI: The case of the crafty caterpillar

The suspect

Caterpillars are the larval or immature stage of moths and butterflies, and can be major insect pests of greenhouse-grown horticultural crops, including vegetables from late spring through fall. However, this is contingent on geographic location. Moths (adults) enter greenhouses through openings such as doors, vents, louvers and sidewalls, and lay eggs that hatch into caterpillars. Caterpillars possess chewing mouthparts and feed on a variety of horticultural plants grown in greenhouses. Caterpillars can severely damage a crop if left unchecked.

The victims

Caterpillars that may be encountered feeding on horticultural crops in greenhouses 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. A number of caterpillars feed on particular plant types or crops in a certain plant family.

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 in color, 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.

The life cycle includes an egg, caterpillar or larva, pupa, and adult. Adult female moths are typically active at night although they may be observed during daytime. Females fly around and lay eggs on leaf undersides. Number of eggs laid depends on the species with females laying between 20 and 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 three to five instars depending on the species. Caterpillars feed extensively at first then consume less as they prepare for pupation. Caterpillar stage may last a week to 10 days. The caterpillar undergoes a transitional or pupae stage. Some caterpillar species spin cocoons, which are located on leaf undersides. Pupation may occur on plants, in stems, or on the growing medium surface.

How they do damage

Caterpillars primarily cause damage by feeding on 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 tunnel directly into plant stems. If caterpillar populations are extensive and damage is not noticed in time, this may reduce crop quality or result in crop losses. Plants inside greenhouses may suffer more damage from caterpillars than plants that are grown outdoors because natural enemies, including parasitoids and predators, tend to be more abundant outdoors whereas there is less migration of natural enemies into greenhouses.
 

Prevention

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. The removal of weeds both inside and outside greenhouses will subsequently reduce problems with caterpillars because many weeds serve as either primary or alternative hosts for adult females to lay eggs. Furthermore, cleaning up plant debris will remove any sources for overwintering pupae. If feasible, hand-picking caterpillars and placing into a container of soapy water will quickly kill them and consequently prevent future damage. Pheromone or blacklight traps located outdoors may help detect peak adult activity. The routine inspection of plants when adult moths are active and flying around will also alleviate problems associated with 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, as these are where adult moths are likely to enter, especially those openings near corn, soybean or vegetable fields that are in decline or have been harvested. Routine visual inspections of susceptible crops will be helpful in detecting eggs and young caterpillars early.
 

Stopping the pest

Insecticides are primarily used to kill the caterpillar stage with most materials having contact activity only, so thorough coverage of all plant parts is essential. The microbial insecticide, Dipel (active ingredient=Bacillus thuringiensis subsp. kurstaki) is commonly used for controlling caterpillars. There are a number of other insecticides labeled for use against caterpillars on horticultural crops. Always read the label of insecticides before use to insure that they can be applied to horticultural crops.

Growers may also release natural enemies into the greenhouse. Parasitoids 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 7 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.

Raymond Cloyd is a professor and extension specialist in horticultural entomology/plant protection in the Department of Entomology at Kansas State University. His research and extension program involves plant protection in greenhouses, nurseries, landscapes, conservatories and vegetables and fruits.