Large tobacco hornworms can quickly strip leaves from backyard tomato plants. Prevent heavy damage by controlling caterpillars while they are still small. This one is 3 inches long.
Caterpillars are vexing pests to many of the plants we grow in our home landscapes and vegetable gardens. There are numerous different species of pest caterpillars, most of which specialize in feeding on a particular group of plants: azalea caterpillars sometimes defoliate whole plantings of azaleas; heavy infestations of bagworms destroy arborvitae trees; tobacco hornworms strip the leaves from homegrown tomatoes; squash borers kill squash and pumpkin vines. And the list goes on.
What is the best way to control caterpillar pests and keep them from causing so much damage? The key to successful caterpillar control is to treat while they are small. Newly hatched caterpillars are much easier to kill than caterpillars that are an inch or more in length and almost ready to pupate. More importantly, by controlling the critters while they are small, you avoid most of the damage they would otherwise cause. Most caterpillar pests do about 80 percent of their feeding in their last few days as a caterpillar. Wait too late to treat an outbreak of caterpillars, and you may get revenge — but you won’t prevent most of the damage.
Squash vine borers kill squash and pumpkins by boring into the stem of the plant. Successful control requires treatment before newly hatched caterpillars bore into the plant. This one is 1 inch long.
The problem is that small caterpillars are tough to spot. So how do you know when it is time to treat? With some crops you have to treat preventively based on plant development or time of year. Tomatoes are a good example. Unless you treat preventively once your plants begin setting fruit, you could be disappointed at harvest. “Oh, no, this tomato has been ruined by fruitworms, so has this one, and here’s another!” Preventive treatments are also necessary to control pests such as squash vine borers and peach tree borers. Because such pests are safe from insecticide sprays once they are inside the plant, it is necessary to treat before newly hatched caterpillars have bored in — you need to have the insecticide residue on the plant before the eggs hatch so hatching caterpillars have to crawl over treated surfaces. With peach tree borer, this can be accomplished with a couple of well-timed treatments applied to the lower trunk after harvest is over, but preventing squash vine borers requires spraying plants weekly once plants begin to bloom.
Fall webworms build unsightly webs in pecan and other trees, but it is not always safe and practical to spray large trees in urban settings.
Fortunately, preventive treatment is not necessary for all caterpillar pests. In many cases, it is possible to take a more reactive approach and wait until you see early warning signs of a caterpillar infestation before spraying. Newly hatched leaf-feeding caterpillars often begin by feeding on the undersides of leaves without chewing through the clear upper epidermis. This results in small windowpane-like spots that should alert observant gardeners to potential caterpillar infestations. Watch for these windowpanes or other early feeding signs; check the undersides of the leaves to verify the presence of caterpillars, and treat if necessary. This approach works best for leaf-feeding caterpillars in vegetable crops, ornamental shrubs and annuals.
There are also situations where the “do nothing” approach may be the best plan. Hardwood trees can be attacked by a variety of different caterpillars, and heavy outbreaks may sometimes cause severe defoliation. But most home gardeners do not have the equipment to treat a 60-foot tree and, even if you hire a commercial applicator, there are still drift and liability issues to consider. Fortunately, mature hardwood trees can tolerate a single heavy defoliation without suffering serious long-term consequences. Is it really worth the time and expense to spray large shade trees for an outbreak of defoliating caterpillars? By the time the problem is noticed, it’s likely that the caterpillars are almost fully grown and nearly ready to pupate. Small, recently planted trees are a different matter. If a tree is still small enough, you can treat it safely and effectively; if you can detect and treat an infestation in time to prevent severe defoliation, it is usually worth doing so. This will keep the young tree growing and protect it from unnecessary stress.
Newly hatched caterpillars, like these cross-striped cabbageworms, often leave telltale “windowpanes” on leaves where they feed, an early sign of caterpillar infestation. This one is a quarter-inch long.
Bagworms only have one generation per year. Insecticide sprays must be applied while caterpillars are active, before they have pupated for the year. This one is 2 inches long.
What insecticides work best for caterpillar control? One of the most effective active ingredients available to home gardeners today is spinosad. Spinosad is sold under many different brand names, and products containing spinosad are readily available in local lawn and garden centers. Spinosad is labeled for use on most vegetable crops and ornamental plants, and some formulations are even approved for organic gardening. Primarily for caterpillar pests, spinosad also controls thrips and some leaf-feeding beetles, but it is not effective on sucking insects like stink bugs and aphids. Organic gardeners may wonder what happened to the Bacillus thuringiensis (Bt) products. Bt products are still available, but they are not nearly as effective as spinosad.
Pyrethroid insecticides, with active ingredients like bifenthrin, cyfluthrin, permethrin, and lambda-cyhalothrin, are also very effective on most caterpillar pests. Pyrethroids are broad-spectrum insecticides that control a wide range of insect pests, but they also have the potential to trigger outbreaks of pests such as spider mites, whiteflies or aphids. This happens because these three groups of pests tend to be less susceptible to pyrethroids than the beneficial insects that help control them. Use pyrethroids when you need to control multiple pests, but don’t count on them to control mites, whiteflies or aphids. For example, pyrethroids are a good choice for treating tomatoes for tomato fruitworms, because they work well on fruitworms and will also control stink bugs and hornworms. Spinosad is a better choice for treating arborvitae for bagworms, because it is less likely to trigger a mite outbreak.
A version of this article appeared in an April 2012 print edition of State-by-State Gardening.
Photography courtesy of Blake Layton.