Caterpillars, the larval/immature stage of moths and butterflies, are generally not considered a major pest of greenhouse-grown crops. However, during summer through fall, moths can enter greenhouses through doors, vents and sidewalls (which are usually open) and lay eggs that hatch into caterpillars. These caterpillars have chewing mouthparts and will feed on a variety of plants. If left unchecked, caterpillars can severely damage a crop; herbaceous plants, such as annuals and perennials located outdoors, are especially susceptible to attack.
Caterpillars that are most commonly encountered and most problematic on plants grown both indoors and outdoors include beet armyworm (Spodoptera exigua), cabbage looper (Trichoplusia ni), imported cabbageworm (Artogeia rapae), diamondback moth (Plutella xylostella), European corn borer (Ostrinia nubilalis), cutworms and leafrollers. Some caterpillars feed on particular plant types or feed on crops in a certain plant family. For example, imported cabbageworm, diamondback moth and cabbage looper primarily feed on plants in the cole crop family (Cruciferae), which includes ornamental cabbage and kale.
Identifying the Culprit
Imported cabbageworm is a velvety green caterpillar approximately 1 1/4 inches long with a yellow stripe down the back and a broken line of yellow spots along each side. Diamondback moth caterpillars are small, approximately 1/8-inch long, light green and mine leaves. Cabbage looper caterpillars are also light green, approximately 1 1/2 inches long, with white stripes down the back and along the side. In contrast, the imported cabbageworm caterpillar is a deeper green with yellow stripes. In addition, it has three pairs of legs near the head and three additional pairs, referred to as prolegs, near the rear.
The lifecycle consists of an egg, caterpillar or larva, pupa and adult. Adult female moths, which are generally most active at night but can be seen during the daytime, lay eggs on leaf undersides. The number of eggs laid depends on the species, with females laying anywhere between 20-100 eggs during their lifetime. Eggs hatch into caterpillars that consume plant foliage. Caterpillars go through 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 anywhere from 3-5 instars, depending on the species. Caterpillars feed voraciously at first, then less as they prepare for pupation.
The caterpillar stage can last one week to 10 days. Eventually, the caterpillar undergoes a transitional or pupal stage, with some species spinning cocoons. Pupation may occur on plants, in stems or on the growing medium surface. After approximately one week, adults emerge from the pupae. The life cycle from egg to adult takes approximately 3-4 weeks, depending on temperature.
Controlling the Damage
The majority of caterpillars damage plants by eating plant parts, including leaves and flowers. They may eat the entire leaf or parts of it, leaving the mid-vein. The presence of fecal deposits (frass) on plant leaves is an indication of caterpillar activity. Several species will roll leaves with silken threads, whereas other species will actually tunnel into plant stems. If caterpillar populations are high and the damage is not Á 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, occur at higher numbers outside the greenhouse. These natural enemies may provide sufficient control of caterpillars on outdoor-grown crops.
Adult moths are attracted to lights at night located in and around greenhouse facilities, so reducing lighting during peak moth activity will avoid luring females into areas where they could lay eggs. Managing weeds inside and outside greenhouses will also reduce problems with caterpillars, because weeds serve as hosts for adult females to lay eggs. In addition, cleaning up plant debris will remove any sources of overwintering pupae. Pheromone or blacklight traps may be located outdoors and are helpful in detecting peak adult activity. Inspecting plants on a regular basis when adult moths are flying will also avoid damage to the crop. Placing sticky cards near plants inside and outside the greenhouse will capture adults; this can serve as a guide to help time applications of pest control materials. When scouting, be sure to check plants near openings (e.g., vents, doors and sidewalls) because these are areas where adult moths will enter, especially locations near corn, soybean or vegetable fields that are in decline or have been harvested.
Pest control materials are directed primarily at the caterpillar stage. Most of these materials have contact activity only, so thorough coverage of all plant parts is essential. Systemic insecticides are generally not effective in controlling caterpillars. The microbial insecticide Dipel (Bacillus thuringiensis var. kurstaki) is the most commonly used pest control material for caterpillars. While very effective, it must be applied when caterpillars are young. The active ingredient has to be consumed to be effective and young caterpillars don’t have to consume as much material before they die; however, larger caterpillars must eat more before the active Á
ingredient inhibits feeding. In the meantime, the caterpillar still can cause plant damage. Dipel may need to be reapplied more frequently when used outdoors, as environmental conditions, including UV light and rainfall, may shorten residual activity. Conserve (spinosad), which is widely used for thrips control, also has activity on various caterpillar pests. A recently registered pest control material called Confirm (tebufenozide) is an insect growth regulator (juvenile hormone mimic) that also has activity specifically on caterpillars. Additional pest control materials, including those in the pyrethroid chemical class, such as Talstar, Decathlon, Tame, Mavrik and Astro, are labeled for caterpillars. However, these materials are harmful to natural enemies; be sure to consult the label before use if you are using biocontrols or if your crop is outside. Pest control materials labeled for controlling caterpillars are listed in Table 1.
An alternative management strategy available to growers experiencing problems with caterpillars on a regular basis is to purchase natural enemies for release into the greenhouse. Parasitic wasps in the genus Trichogramma attack the egg stage of various caterpillar species, including diamondback moth, cabbage looper and imported cabbageworm. The life span of the parasitoids is approximately seven days as it matures within the egg, and then up to 10 days as adults. Several species of Trichogramma wasps, including T. minutum and T. pretiosum, are available from commercial insectaries. For more information, consult a biological control supplier.