Ministry of Environment
Integrated Pest Management
All butterflies and their less glamorous relatives, the moths, spend part of their life as caterpillars. They hatch from eggs laid on plants by the female butterfly or moth. After hatching, caterpillars feed on the plant until they grow to their full size. Then they change into an immobile stage called a pupa, which is called a chrysalis in the case of butterflies. Moth pupae are usually protected by a silk cocoon spun by the caterpillar just before it changes. The gradual transformation into an adult insect takes place inside the pupa. A week or two, or even a year later, depending on the species, the fully formed butterfly or moth forces its way out of the pupa case and spreads it damp wings to dry. It then flies away to mate and lay eggs.
These caterpillars feed together in groups on deciduous trees. Tent caterpillars are hairy and have various color markings. They cannot be mistaken for gypsy moth caterpillars, however, which are distinctly marked with 6 pairs of red dots and 5 pairs of blue dots on their backs. Northern tent caterpillars spin large, conspicuous silk nests in crotches of tree branches in spring and early summer; forest tent caterpillars do not spin these silken mats. Tent caterpillar populations rise and fall in 5-10 year cycles. When their numbers become too high, the caterpillars are eventually killed off by diseases and parasitic insects.
For home gardeners, the best control for tent caterpillars is to pull the nests out of trees (wear gloves) or prune them out and drop the caterpillars in a bucket of soapy water. Start inspecting for tents or colonies of small caterpillars in early spring before they cause much damage and while they are easier to remove. On trees with severe or inaccessible infestations, spray with BTK or products containing pyrethrins and insecticidal soap. Before spraying, make sure caterpillars are still present—the empty webbing remains in the trees long after the caterpillars are gone. Old webs can be removed with a strong stream of water from a hose. In the winter, scrape or prune away eggs masses, which look like a silvery-brown, hardened band of foam (about 1 cm wide), partially encircling small tree branches. Dormant oil sprays also kill tent caterpillar eggs.
- Fall Webworm: These are similar to tent caterpillars, but occur in late summer and fall. They make large silken nests covering the leaves at the ends of branches. Use the same control methods as for tent caterpillars.
- Ermine Moth: Apple ermine moths, which only feed on apples, first became a problem in the lower mainland of B.C. several years ago. There is now another species present that feeds on hawthorn, cherry and other plants in addition to apple. The caterpillars are present from late April to mid-June. They feed first in the tips of leaves, then in a silken nest of fine webbing along tree branches. Inside the nest are smooth, wiggly, white to greyish-green caterpillars with dark heads and a pair of black dots on each segment. Although often mistaken for tent caterpillars, the ermine moth webs are much smaller, finer and less thickly woven. Unlike tent caterpillars, ermine moth larvae drop quickly on a silk thread when the nest is disturbed. Most trees suffer little damage even when there are large numbers of ermine moth caterpillars. The nests are easy to pick off of branches and destroy and BTK sprays are very effective while the caterpillars are still small.
- Imported Cabbageworm: Slender, velvety green caterpillars found chewing large holes in leaves of cabbage family plants are cabbageworms. They eventually become white cabbage butterflies. In some areas and some years, they are common enough to cause significant damage. BTK sprays are very effective against these caterpillars. Sticky yellow traps placed among the plants are also useful to intercept female butterflies, which are attracted to yellow when they are ready to lay eggs. In future, try planting purple cultivars of cabbage, which seem to be less attacked.
- Winter Moth: These caterpillars are bright green with three narrow, white stripes on each side of the body. Beginning in April, they feed on flowers, leaves and buds of apple, blueberry, fruit and ornamental trees such as maple and poplar. Sprays of BTK or pyrethrins will provide reasonable control of caterpillars. Prevent damage the following year by using sticky tree bands. The bands trap the female moths, which are wingless, as they crawl up the trees in late fall and winter to lay eggs.
- Leafrollers: These small green caterpillars with brown or black heads are named after their habit of rolling the tips of leaves together around themselves for protection. They also bind together blossoms and buds and feed on them. Leafrollers are common on fruit trees in early spring about the time apples bloom. To control leafrollers, spray with BTK just after the flower petals fall.
- Tomato Hornworm: Hornworms are huge green caterpillars (up to 5 cm long). They have diagonal white stripes on the sides and a single horn pointing backward from the last segment. They feed on tomato leaves and bite into fruit. The caterpillars are so large that they can be readily hand-picked from leaves. BTK sprays are effective while the caterpillars are small.
- Cutworm: Cutworms are the plump, grey, greasy-looking caterpillars of several species of moths. They chew the stems of young plants at night and hide in the soil around the base of the attacked plant during the day. Gardeners can usually find and destroy a cutworm by searching through the soil near a damaged stem. Prevent damage to transplants and seedlings by using protective collars around the stems. Collars should be 8-10 cm high and set 2-4 cm deep in the soil. Sections of cardboard toilet paper rolls, small tin cans (open at both ends) or cardboard strips stapled into a ring, work well. Unfortunately, barriers do not foil the climbing species of cutworms, which climb up stems and chew large holes in leaves of a wide variety of garden plants. Pyrethrins sprays are likely to be more effective than BTK sprays to control them. Where cutworms are a serious problem, it might be worth it next year to treat the soil before planting with insect parasitic nematodes, now available at garden centres.
- Painted Lady Caterpillar: These dark, spiny caterpillars feed among the leaves of artichoke, sunflower, artemisia, everlasting and many common weeds, such as burdock and thistles. They develop into beautiful reddish-brown and -black patterned painted lady or butterflies. In some years they are common enough to cause concern, usually because large numbers have migrated from the south into Canada. If there are too many on a favoured plant, break off a piece of leaf with the caterpillar and transfer it to a more robust plant of the same kind that can tolerate more feeding.
- Parsleyworm: If you find a large, fat green caterpillar (up to 4 cm long) with black and yellow stripes encircling each segment feeding among the parsley, dill or lovage — let it be. In a few weeks it will become a beautiful swallowtail butterfly. It is extremely rare for parsleyworms numbers to be high enough to cause significant damage.
- Currant Sawfly: Smooth, green worms with black spots found feeding on currants or gooseberries look a lot like caterpillars, but they are actually sawfly larvae (sawflies are insects related to wasps). Hand-pick them or spray with insecticidal soap or pyrethrins. Because they aren't true caterpillars, BTK sprays have no effect.
To decide whether control measures are necessary, first inspect plants to find out whether caterpillars are still present. Their damage is often noticed only after they have finished feeding and left the plant. Where possible, use preventative measures, such as cutworm collars and sticky tree bands. If sprays are necessary, minimize the impact on beneficial insects by spraying only plants that are being attacked.
Caterpillars have many native enemies including birds, yellowjackets and other predatory wasps, parasitic flies, tiny parasitic wasps and predatory bugs as well as viruses, bacteria and fungus diseases. Encouraging these native enemies is the most environmentally sound (and often the simplest and least expensive) method of suppressing pest caterpillars. (For more information see Beneficial Insects).
Handpicking: Removing caterpillars and egg masses by hand is feasible and effective in a home garden. If you have trouble finding the caterpillars, look for signs of fresh leaf injury and dark green pellets of excrement to tell you where they are feeding. Destroy caterpillars by stepping on them or drop them in soapy water.
Tree Bands: Wrap one or two 15-cm wide bands of plastic packaging tape around the trunk, 30 and 60 cm above the ground. Coat the band with a sticky glue, such as Tanglefootreg (available at garden centres). To catch winter moths, traps should be in place from late October through February.
BTK Sprays: Products containing a naturally occurring biological control for caterpillars are widely available in garden centres. They contain the highly specialized protein crystals and dormant spores produced by the bacterium Bacillus thuringiensis kurstaki (BTK). The product begins to work only after a caterpillar eats a piece of leaf with BTK crystal proteins and spores on it. The proteins dissolve in the highly alkaline conditions found in a caterpillar gut and this paralyzes their digestive tract. This causes the caterpillar to stop feeding immediately and eventually to starve. BTK is non-toxic to humans, other mammals, birds, snakes, fish, earthworms and most other insects. It is an excellent choice for caterpillar control because it does not harm the beneficial insects and other animals that keep caterpillar numbers low.
BTK must be eaten by caterpillars to have an effect, therefore it should only be used when caterpillars are actively feeding. It does not work on eggs, pupae or adult stages. For best results, spray in the evening, when no rain is expected. Use a fine spray and ensure that both sides of leaves are thoroughly covered.
Dormant Oil Sprays: Dormant oil sprays kill overwintering eggs and pupae of leafrollers, codling moths, tent caterpillars, webworms and other caterpillars, as well as scale, mites and aphid eggs. Apply to trees on a dry winter day above freezing.
Pyrethrins: Leaf-eating caterpillars can be controlled by sprays containing pyrethrins, the active ingredients extracted from flowers of pyrethrum daisies. Pyrethrins break down in a few days in the environment and are less disruptive to the natural enemies of caterpillars than most synthetic pesticides. Products containing pyrethrins mixed with insecticidal soap are also effective.
Note: Always follow directions on the label for mixing and using any pesticide.