Caterpillar Pests

Background

Caterpillars are, collectively, the larvae of moths and butterflies in the order Lepidoptera. With over 110,000 species described, nearly every grower will encounter them — especially in outdoor crops and gardens. Some species also invade greenhouses and other structures.

Common names vary: cabbage loopers, tent caterpillars, hornworms, inchworms, soil-feeding cutworms, and stem-boring borers. Despite the different labels, one fact remains constant — they are always the larvae of moths or butterflies, and they all feed the same destructive way.

How They Become a Problem

Infestations begin at different times of the year depending on the life cycle of the particular moth or butterfly. Many start their lives by hatching from overwintered eggs late in the spring, and some species cycle multiple times across a growing season while others have just one annual flush. Exponential reproductive growth — like that seen with aphids — should not be expected from caterpillars.

What caterpillars lack in reproductive speed they make up for with raw appetite. Their cutting mouthparts can chew gaping holes in leaves, chop into fruits, or tunnel through stems. In adverse weather, large temperature swings or wet conditions can knock back wild populations — but in protected culture, conditions stay favorable and caterpillars can do enormous damage quickly.

The key to caterpillar control isn’t how often you scout — it’s knowing your foe, and timing your response before the larvae reach late instars.

A Serious Pest

Caterpillars devour host plants with their cutting mouthparts, causing severe damage if they’re not treated early or prevented. The key to caterpillar pest control doesn’t necessarily lie in how thoroughly you scout, but in knowing your foe and timing your response. Your local cooperative extension office — since caterpillars are usually regional in distribution — can tell you which species are problematic in your area and when they tend to be active.

Extension staff can also recommend whether Bacillus thuringiensis (Bt), botanical insecticides, soil-applied agents, parasitic wasps, predatory bugs, pheromone traps, beneficial nematodes or even bird-friendly plantings are the right approach. Caterpillars can be detected, controlled and even prevented — you just need to know what you’re dealing with.

Some Advice on Caterpillar Control

Caterpillars can be fast-feeding and merciless. Damage to vulnerable crops can range from severe to total loss through complete defoliation. Most medium-to-large trees can handle two to three consecutive years of defoliation, but most organic-garden crops and cole crops are not that resilient. A large caterpillar infestation can mean disaster — and although caterpillars themselves are not very hard to spot, the irreversible damage they cause can develop in just a few days.

During the active part of the season for the species you’re tracking, intensive scouting and rapid response are critical. Once you understand a species’ lifecycle and feeding behavior, your control program writes itself.

For the Scout

For a species that overwinters as an egg, start by looking for eggs in the immediate area, then look for the destructive larvae. Caterpillars are usually apparent by their presence (if they aren’t nocturnal), their feeding damage, or their frass (droppings). Frass can be surprisingly large — tomato hornworms grow to three or four inches and produce droppings the size of cooked rice grains.

Some species are best monitored with pheromone traps, which lure male moths and let you time your release of egg parasitoids or your Bt sprays for maximum effect. Note that for nocturnal species, daytime scouting won’t reveal active larvae — check for fresh damage and droppings instead.

What to look for, week by week

Egg masses on leaf undersides, skeletonized or windowed leaves (early instars), large ragged holes (late instars), heavy frass on lower leaves, and rolled or webbed foliage all indicate active caterpillar feeding. Catch the early signs and your control options open up dramatically.

Some Solutions for Caterpillar Control

The good news: caterpillars — regardless of size, color or feeding area — are usually identifiable and can be controlled when you take multiple, aggressive steps quickly. Knowing the species you expect to deal with is fundamental, but it’s not difficult.

Available tools include neem oil and other botanical insecticides, Trichogramma egg parasitoids (released to attack moth eggs before they hatch), Bacillus thuringiensis Kurstaki (Bt) sprays or granules (a stomach poison specific to caterpillars), spined soldier bugs (Podisus maculiventris), and pheromone-baited traps for monitoring and mating disruption.

Caterpillar control is achievable — but only if you know the species, you scout early, and you commit to acting on the first instar, not the fifth.

In our experience, if scouting is done right and if Bt Kurstaki, Trichogramma and pheromone traps are deployed together at the right stage, caterpillars become a manageable problem rather than a crop-ending one.