Plant Health: Combating caterpillars

Caterpillars, which are the larval or immature stage of moths and butterflies, are generally not considered major insect pests of greenhouse-grown crops. However, during summer through fall, adults (moths or butterflies) can enter greenhouses through openings such as doors, vents, louvers and sidewalls and lay eggs on plants that hatch into caterpillars. Caterpillars have chewing mouthparts and will feed on a variety of plants grown in greenhouses. If left unchecked, caterpillars can severely damage a crop. Furthermore, herbaceous plants such as annuals and perennials located outdoors are highly susceptible to attack by caterpillars. Here are some common questions and answers about these caterpillars.
 

What types of caterpillar pests may I encounter during the growing season?
Caterpillars that you may encounter, depending on your location, feeding on ornamental crops grown both indoors and outdoors include the beet armyworm (Spodoptera exigua), cabbage looper (Trichoplusia ni), imported cabbageworm [Artogeia (Pieris) rapae], diamondback moth (Plutella xylostella), European corn borer (Ostrinia nubilalis), corn earworm (Helicoverpa zea), leafrollers (Choristoneura spp.) and cutworms. Some caterpillars feed on certain plant types or on crops in particular plant families. 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.
 

How much damage can caterpillars cause?
Most caterpillars cause damage by eating plant parts including foliage and flowers; either consuming the entire leaf or leaving the mid-vein. The presence of fecal deposits (frass) or caterpillar “poop” on plant leaves is an indication of caterpillar activity. Several caterpillar species will roll leaves together with silken threads whereas other species will 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 be more abundant outside, thus possibly providing sufficient regulation of caterpillars on outdoor grown crops.
 

What is the best way to holistically manage caterpillars?
It is important to devise a management strategy that targets both the caterpillars and adults. Adults are attracted to lights (at night for moths) located in and around greenhouse facilities, so reducing lighting during peak adult activity or installing a less attractive light source will avoid luring females into areas where they could lay eggs. The management of weeds both inside and outside greenhouses will alleviate problems with caterpillars as many weeds serve as hosts for adult females to lay eggs. In addition, cleaning up plant debris will remove any overwintering pupae. Pheromone or blacklight traps located outdoors may help in detecting peak adult activity. Placing yellow sticky cards among plants both inside and outside the greenhouse will capture adults, thus helping to time insecticide applications. Visually inspecting plants regularly when adults are flying will also avoid crop damage from caterpillar feeding. When scouting, be sure to check those plants nearest openings such as vents, doors, louvers, and sidewalls, because this is where adults may enter, especially those openings facing corn, soybean, or vegetable fields that are in decline or have been harvested. Another option is to simply hand-pick caterpillars (those individuals that are squeamish can use gloves) from plants and dispose of them outside for the birds to eat.

Insecticides are directed primarily at the caterpillar stage. Most insecticides have contact activity only, so thorough coverage of all plant parts is essential. Systemic insecticides are generally less effective against caterpillars. The microbial insecticide, Dipel (active ingredient=Bacillus thuringiensis spp. kurstaki) is commonly used for controlling caterpillars. It is very effective but has to be applied when caterpillars are young. The active ingredient must be consumed in order to be effective and young caterpillars don’t have to consume as much material before they die; however, larger caterpillars must eat (“munch on”) 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 more frequently when used outdoors, as environmental conditions including ultra-violet (UV) light and rainfall may shorten residual activity. Conserve (spinosad), which is widely used for thrips control, also has activity on various caterpillar pests. Conserve works as both a contact and stomach poison. Additional insecticides, including those in the pyrethroid chemical class such as Talstar/Attain (bifenthrin), Decathlon (cyfluthrin), Tame (fenpropathrin), and Mavrik (fluvalinate), are labeled for caterpillars. However, these materials are harmful to many natural enemies.

Diamondback Moth larvae and feeding damage.

If I want to use biological control, what are my options?
Biological control is an alternative management strategy that is available to greenhouse producers that experience problems with caterpillars on a regular basis. It is possible 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 including T. minutum, T. brassicae, T. pretiosum, and T. plantneri are available from most commercial insectaries; however, be sure to contact your biological control supplier in advance in order to determine availability of these parasitoids.

Have a question? You can write Raymond at rcloyd@ksu.edu.