Caterpillars in the Canopy

Earth’s approximately 180,000 species of butterflies, moths, and skippers all belong to the family Lepidoptera. With a few exceptions, the adult insects are characterized by two sets of scale-covered wings that are wildly diverse in size and color and often strikingly beautiful. Mostly herbivorous — having a taste for plant matter — Lepidoptera share a four-stage life cycle: egg, larva, pupa, and adult. In their larval feeding stage, they are known as caterpillars — also with incredibly diverse evolutionary characteristics. 

Some of Missouri’s most visible native tree-eating caterpillars are vital players in our shared ecosystem and not the munching miscreants they are perceived to be. Eastern tent caterpillars, fall webworms, and walnut caterpillars dine on trees where the “ick” factor accompanying their evolved habits makes the furry little wigglers seem more menacing than is warranted. Save your ick for bagworms. 

Bagworms

Feeding on more than 100 species of trees and shrubs, bagworms (Thyridopteryx ephemeraeformis) have a distinct preference for evergreens, including eastern red cedar, juniper, arborvitae, pine, spruce, and more. Deciduous menu items include maple, black locust, viburnum, and sycamore, among others. Evergreens are most susceptible to damage and can be killed by high numbers of bagworms.

“You need to take bagworms seriously on evergreens,” said Robbie Doerhoff, MDC’s forest entomologist.

Bagworms create 2-inch dangling silken bags on their host plant, adorned with camouflaging bits of the plant’s foliage, branches, and sometimes even berries. Mobile larvae accumulate the camouflage as they feed and grow. In late summer, they seal their silken armor and attach themselves to a branch to pupate. 

In seven to 10 days, the male emerges as a short-lived, furry-bodied black moth whose wing scales were shed as he emerged from his bag, leaving his wings translucent. Females do not develop wings, remaining maggotlike within their bags and releasing a pheromone that attracts a mate. The female lays eggs within the bag and dies. Larvae hatch in spring and travel short distances or “balloon” to suitable hosts by way of a silken thread. 

From late fall to early spring, MDC recommends removing the silken bags into a bucket of soapy water and disposing of them in a sealed bag. Heavily infested branches may be pruned and burned. In cases of large numbers of bagworms, the organic pesticide Bacillus thuringiensis var. kurstaki (Btk) can be sprayed in early June to kill small larvae as they feed.

Eastern Tent Caterpillars and Fall Webworms 

Hatching at about the same time as their host tree’s buds break, the larvae of eastern tent caterpillars (Malacosoma americanum) form a colony and construct a white webbed tent in the fork of a tree, venturing out to feed in the morning, evening, or at night. The tent protects them from predators, rain, and temperature fluctuations until they are ready to crawl away to pupate.

The best first defense against eastern tent caterpillars is spotting and removing their eggs, which appear as shiny black masses of 150 to 400 eggs encircling small branches on their favored wild plum, black cherry, apple, crabapple, and other fruit trees.

With a preference for nut trees, fall webworms (Hyphantria cunea) also enjoy persimmons and other fruit trees, but happily dine on more than 120 species. 

In the late spring and early summer, the first of two to three generations of emerging adult webworm moths lay hairy masses of several hundred eggs on the bottoms of leaves. Hatching larvae form colonies that spin nests of silk at the ends of tree branches. Because they never leave its protection, the nest expands to encompass more foliage as they grow larger — until they abandon the web to pupate. 

Eastern tent caterpillar moths have feathery antennae and are brown or gray shades with lighter stripes that allow them to blend in with natural surfaces. Their larvae are black with a white stripe down the back, both brown and yellow lines, and a row of oval blue spots along their sides. 

Fall webworm moths are white with black spots and larvae are yellow-green with black heads or tan with red heads, both with gray hairs and pairs of wartlike black spots on their backs.

When caterpillars can be easily reached, handpick them from their webs with gloved hands and drop them into a bucket of soapy water. It’s a great way to eliminate them. 

“Tearing their nests open exposes them to insect and bird predation,” Doerhoff said. “For nests higher up, pound a nail into the end of a long stick and use that to tear the nest open.”

Twirling the stick in the nest can help clear it.

Btk will not penetrate the insects’ silken structures, so catching larvae as they hatch — or move in and out of their tents — is key to its effectiveness.

Large numbers of colonies can stress trees and are unsightly, but damage is mainly cosmetic, and numbers of insects fluctuate year to year. Doerhoff warns the common practice of burning the nests is harmful to the tree.

“Because of the ecological damage that insecticides can cause, we only recommend chemical controls for specific issues,” she said. “There’s no need to think of most native defoliators as problematic or dangerous. Healthy trees will survive. 

“Think of them as early Halloween decorations,” she joked.

Large numbers of caterpillars migrating away from the trees to pupate can be a nuisance and are quite alarming, but not a problem for people or pets.

Walnut Caterpillars 

Walnut caterpillar (Datana integerrima) pupae overwinter in the soil, and the 1- to 2- inch adult moths, which are tan to reddish-brown with lines on their forewings and lighter hindwings, emerge to lay masses of 600 or more eggs on the backs of leaves. Larvae, which feed together, skeletonizing leaves, are a light-yellow green, turning reddish-brown with white stripes on their sides.

The larvae molt together in groups on main branches or the trunk of a tree. After their final molt, they emerge as white-tufted black caterpillars and feed ravenously as individuals in the canopy. They consume 80 percent of their entire lifetime of meals in three to five days before dropping to the ground to pupate. Walnut caterpillars prefer mature foliage.

Though rarely necessary, Btk can be used when the larvae are small.

One to Watch

Larvae of the European spongy moth (Lymantria dispar), formerly known as the gypsy moth, defoliate more than a million acres of forest in the northeastern U.S. each year. 

Introduced into Massachusetts in 1869, the moth has slowly spread — but not yet into Missouri. Small populations have been observed but eradicated due to a diligent 60-year cooperative partnership to detect their presence. MDC shares the credit for the effort with the Missouri Department of Agriculture and the USDA. 

“Statewide, partners on this effort place over 6,000 traps a year to detect their presence,” Doerhoff said of the bright orange, triangular cardboard boxes — benign to humans — that use female spongy moth pheromones to lure and trap the flighted male spongy moths. “If it were to establish here, the spongy moth could decimate our oak forests.” 

Thirteen of the moth’s favorite host species are common in Missouri. A loss of these trees would adversely affect entire ecosystems of insects, birds, and animals across the state, as well as the state’s economy. 

Control — Naturally or Chemically

Tree-feeding caterpillars have many predators, from birds to wasps and assassin bugs, and they also have a diverse set of parasitic insect species to help keep their numbers in check.

But when it is deemed necessary to remove them quicker than nature provides, Doerhoff said manual removal is the best practice to prevent harming off-target insect species. Insecticides, many of which are broad-spectrum and can kill a wide range of insects we know and love, should only be considered in cases where a tree’s or shrub’s life is endangered. When warranted, she recommends Btk, which only targets larvae of Lepidoptera.

Healthy trees are better equipped to handle foliage loss. Keeping trees watered during long rain-free periods is advised. 

“Drought is very hard on trees, causing stress and root damage, which can weaken them over time,” Doerhoff said. “Even though the weather is ultimately to blame for a tree’s decline, the defoliators can be the last straw for an unhealthy tree.”

Trees and shrubs most often regrow foliage that is munched in spring, which adds extra stress on a tree; late season defoliation is less troublesome since trees have already fruited, stored energy for the year, and are on a path to normal dormancy.

 MDC urges Missourians to take note of insect activity in your area, especially heavy leaf-feeding on oaks. If you spot caterpillars with pairs of red and blue spots on their backs, contact your local MDC office or send an email to Forest.Health@mdc.mo.gov. If possible, collect specimens or take pictures of the caterpillars. 

Catalpa Worms

Catalpa trees are best known for two things — their gorgeous gold-and-purple throated white clusters of trumpet-shaped blooms and the fat, 3-inch yellow-and-black larvae of the catalpa sphinx moth that dine freely on their leaves. Commonly known as catalpa worms, there are those who call the boom-and-bust moth larvae “catfish candy” as they are a prized fishing bait for catfish, bluegill, and largemouth bass. Whereas some folks are worried about caterpillars that defoliate trees in their landscapes, others have planted catalpa trees specifically for the larval activity. Plump, smooth catfish candy caterpillars freeze well for later use if blanched first.

Sawflies

European and native Loblolly pine sawflies (Neodiprion sertifer and Neodiprion taedae) are common defoliating insects in Missouri, capable of defoliating and stressing native shortleaf and other pine trees.

In recent years, there have been localized outbreaks of sawflies in the state, Doerhoff said. 

“Weather conditions, like abnormally warm winters, are likely to be a contributing factor,” she said.

At first glance, the adult sawfly somewhat resembles a housefly but is a broad-waisted wasp from the Hymenoptera family, which includes sawflies, wasps, ants, and bees. Adults have four membranous wings. What looks like a stinger on the sawfly is a sawlike ovipositor that the adult female uses to deposit her eggs, the source of the insect’s name.

Like their cousins the Lepidoptera, sawflies’ ravenous larvae — resembling caterpillars — are the host plant defoliators. 

In late summer to early fall, sawflies emerge from cocoons to mate. The female “saws” through the outer layer of 10 to 12 different pine needles and lays six to eight eggs into the slits she makes on each of the needles. 

Eggs hatch April through May and the larvae feed in groups for three to four weeks before dropping off the tree and spinning a cocoon in leaf litter or soil. European pine sawflies have black heads and grayish-green bodies with light stripes and feed primarily on the outer epidermal layer of older needles on nonnative pines. Loblolly pine sawflies have brown heads, dull green bodies, and heavy black stripes and will munch on all but the newest needles on shortleaf and loblolly pine. 

One way to easily identify conifer-loving sawfly larvae is to wave your hand near them. When disturbed, they strike a defensive pose, curling their heads and tails back over their bodies. 

When pine needles lose their outer layer to sawfly feeding, they become desiccated and turn brown. Keeping an eye out for this telltale “flagging,” as it is called, allows for mechanical removal by knocking the larvae into a bucket of soapy water.

An organic approach to control them is to spray pine trees with insecticidal soap or neem oil when the larvae are small. 

Sawflies have several natural predators including yellowjackets, bald-faced hornets, paper wasps, and a parasitic fly. Some birds and rodents also have a taste for the larvae. 

Chemical control may be warranted after two consecutive years of defoliation on the same pine. Note that because they are not caterpillars, the organic, larvae-killing Btk insecticide has no effect on them, but another organically derived insecticide, Spinosad, is effective.