PSI: The case of the caterpillar

Features - Pest Control

The PSI team’s file on caterpillars is extensive, as there are many different kinds that can destroy many different crops. Here’s how to spot them, treat them and prevent them.

February 16, 2015

The Caterpillar

Big mouth: Caterpillars chew, chew, chew right through flowers, leaves, shoots and even roots. The most common caterpillars are foliage eaters, says entomologist and OHP technical manager Carlos Bográn. “They feed on the leaves of the plant, and depending on the size of the plant, they can be very devastating or cause just aesthetic, minor damage,” he says.

On the lookout: Different caterpillars damage different plants at different stages. “For example, field-grown seedlings can be affected by cotton worms when the seed is germinating, or the cutting is rooting or being transported into the field,” Bográn says. “It can be very damaging for flower feeders as well, so the damage they cause will depend on the value of the crop.”

Common types: Some of the most common types of caterpillars include beet armyworms, which are green with a white stripe down the middle of its sides. It feeds primarily on vegetables and flowers. Tobacco hornworms, which are similar in color to beet armyworms but rounder in shape, will feed on tomatoes. And the western tussock moth has a reddish body with grey, spiky, fur-like hairs, and will feed on ornamental and fruit tree species, according to the University of California Agriculture & Natural Resources.

Spot ‘Em Out

What’s left behind: Aside from the visible holes in your foliage and stems, “Typical signs of caterpillar activity would be the fecal pellets, the excrement that the caterpillars leave behind after feeding,” Bográn says. The fecal pellets— which look almost like peppercorn— will be soft, and found on older, bottom leaves of the plant. Because caterpillars camouflage so well into the foliage, sometimes finding their leftovers may be your best bet to discover them.

Getting Rid of the Pest

Pluck ‘em: Caterpillars can be literally picked right off the plant, though it may be difficult to keep up with their breeding.

Spray ‘em away: “Control of caterpillars should include both the control of the immature insects, the larvae and the adults. And therefore, if a chemical control is selected, you can use different products to detect the different life stages,” Bográn says. Biological products like insecticides can also be used to rid the pests, such as Dipel and Thuricide. “The control should be targeting the young immatures or the eggs, because at that point the damage is minimal and the insecticides are much more effective in young larva in general than old larvae,” he says.


Know your pest: “To determine what type of caterpillar the damage is being caused by, [making] a good identification is the start of any management program. If we’re able to determine what species, what caterpillar it is, we’re able to find the target solutions for that problem,” Bográn says.

Once you identify: Scout and monitor frequently, as female butterflies can lay eggs quickly and abundantly.

Control it: “One advantage that we have against caterpillars is that the many species communicate through pheromones…so we can take advantage of those mating pheromones to detect or disrupt mating of the pest population,” Bográn says. Setting out pheromone traps can re-direct mating butterflies during egg-laying periods to divert the damage they’d cause.

Create a barrier: There are screens that can be used to block the pests — almost like a greenhouse — and can resist the colonization from the inside, Bográn says. They reduce wind current and movement in the growing areas.