Caterpillars you are likely to run across at some time in your gardening career
Story and Photos by Blake Layton, Ph.D.
Where there are plants there are caterpillars. As an avid gardener, you are probably familiar with several species of caterpillars, particularly those that damage some of your favorite plants, such as tobacco hornworms, cabbage loopers, and tomato fruitworms. But our gardens and landscapes are host to hundreds of other caterpillar species. Many of these are so small and inconspicuous they are rarely seen. Others are large and even colorful, but because they occur sporadically or do not usually damage prized plants, they are less familiar. We see them occasionally, but we don’t really know much about them. Let’s become more familiar with a few of these caterpillars. The species discussed here are all moths as adults and most are rarely serious pests, though there are some exceptions. You probably won’t see all of these caterpillars during any given growing season, but you are likely to encounter most of these during your gardening career.
These striking caterpillars vary from brown to green, but the spots and other markings are fairly consistent. The “horn” on the rear identifies this as one of the sphinx moth caterpillars, the same family as the tobacco hornworms that plague backyard tomato growers. Because they have a fairly narrow host range, tersa sphinx caterpillars are not especially common, but they are fond of Pentas, and can severely defoliate those planted in landscape beds. This can present a minor moral dilemma for butterfly gardeners. Do you control these caterpillars in order to have more pentas blooms for visiting butterflies, or do you let the caterpillars have the pentas so they can develop into moths? Tersa sphinx moths are quite sporty looking, with a streamlined appearance and approximately 3-inch wingspan, but like most moths, they only fly at night, which makes them harder to observe. An insecticide that contains the active ingredient spinosad, applied according to label directions, is a good option if you choose to protect the pentas.
Walnut caterpillars undergo a significant change in appearance as they grow. Young caterpillars are red with longitudinal yellow stripes, while older caterpillars that are nearing time to pupate are black with long, fine white hairs. These caterpillars specialize in feeding on walnut (Juglans spp.) and pecan (Carya illinoinensis) trees where they cause a rather unusual defoliation pattern. Often they will strip all the leaves from a particular limb without affecting the rest of the tree. This is because the eggs are laid in masses and walnut caterpillars like to remain near their siblings when feeding. All caterpillars molt, or shed their skin, several times as they grow. When it is time for walnut caterpillars to molt, the entire family group crawls to the trunk or a large limb, clusters together, sheds their skins, and then moves away, leaving a hairy mass of shed caterpillar skins behind. Gardeners are sometimes perplexed to discover what appears from a distance to be the skin of a dead possum or some other small mammal stuck to the trunk of their tree.
These hairy caterpillars do not sting, but they occasionally occur in outbreak numbers and can damage field crops, vegetables, and tender ornamental plants. They are an exception to the rule that caterpillars do not normally feed when in the wandering phase. Larger saltmarsh caterpillars actively move around on low-growing vegetation in search of food. Outbreaks usually begin in crop fields or weedy areas, but wandering caterpillars will occasionally appear in managed landscapes. Saltmarsh caterpillars vary in color, from light tan to black, and fully mature caterpillars are about 2 inches long. Adults are medium-sized, heavy-bodied moths with white forewings speckled with black.
Giant Leopard Moth
Most gardeners encounter this insect in fall or winter when they move an item that has been lying about for a while and discover a big, hairy caterpillar curled underneath. These caterpillars seek out protected sites to overwinter but remain as larvae until the following spring. Despite those stiff black hairs, this is not a “stinging caterpillar.” Look past the hairs and you will notice narrow red bands on the skin. Mature caterpillars are about 3 inches long; young caterpillars are black with orange bands and the hairs are not as thick. Giant leopard moth caterpillars have a fairly wide host range, including some ornamental and vegetable plants, but they are rarely numerous enough to cause serious damage. The heavy-bodied moths have white wings covered with large black spots. Often there are a few blue spots on the thorax and the upper surface of the abdomen is covered with iridescent blue and orange markings.
Forest Tent Caterpillar
Despite their name, forest tent caterpillars do not build tents; they spin inconspicuous silk mats on the bark of their host tree where they molt and rest when not feeding. At feeding time, all the caterpillars move out together, following trails of silk to the leaves and returning, again all together, along the same trails. Populations of forest tent caterpillars vary greatly from year to year. During heavy outbreak years, these caterpillars can be so numerous that they totally defoliate oaks (Quercus spp.) and other hardwood trees. This can include individual trees in home landscapes, as well as thousands of acres in large forests. Fortunately, trees defoliated this early in the year will produce new leaves and suffer little long-term adverse effect. Most trees can survive several successive years of such defoliation, though there may be a reduction in trunk growth rate. During heavy outbreaks the large numbers of wandering caterpillars, combined with their fecal droppings and severe defoliation of shade trees, can be quite disconcerting to homeowners and landscape managers. Fortunately this is a short-lived phenomenon, so control measures are generally not recommended. There is only one generation per year and this occurs in early spring, with eggs hatching shortly after trees leaf out. The caterpillars pupate in late spring and emerge as moths 10 to 14 days later to mate and lay eggs. Eggs are deposited in a mass that encircles twigs of host trees and these eggs hatch the following spring.
Cecropia Moth Caterpillar
Cecropia moths belong to a group known as the “giant silkworm moths,” which also includes polyphemus and luna moths, along with several other species. As large and colorful as cecropia caterpillars are, you might wonder why you don’t see them more often. It’s because they spend their lives feeding overhead on the leaves of trees such as maples (Acer spp.), birch (Betula spp.), and wild cherry (Prunus spp.). Mature caterpillars are over 4 inches long and form their cocoons on twigs and leaves in the tree canopy. Big caterpillars grow up to be big moths, and cecropia moths are some of the largest moths in the country, with wingspans up to 6 inches. You might not think a moth that’s primarily marked with various shades of brown could be described as colorful – until you see one!
This descriptively named caterpillar feeds primarily on maple trees. Most years they go largely unnoticed, but in years when populations are unusually high, they can cause heavy or complete defoliation. Fortunately, trees usually recover with little long-term effect. In Southern states there are two or three generations each year. Mature caterpillars pupate in the soil underneath their host tree. The surprisingly beautiful adults are called rosy maple moths.
Banded Woolly Bear
You have probably seen banded woolly bears crawling across a road or driveway, and you are probably familiar with the folk tale that the width of the rust-colored middle band foretells the severity of the coming winter. Banded woolly bears are not really good at predicting severe winters, but they are very good at surviving them. They have special compounds in their blood that help them survive being frozen, even when ice crystals form inside their bodies, and even when exposed to repeated cycles of freezing and thawing. Like the giant leopard moth, banded woolly bears overwinter as nearly grown caterpillars and pupate the following spring. Mature caterpillars are about 2 inches long and the width of the rust-colored band is quite variable. Although these insects complete two or three generations each year, they are most commonly seen in late fall when they are in search of a place to overwinter. Banded woolly bears have a wide host range, feeding mostly on herbaceous plants and weeds, but rarely damage ornamental plants. This is another hairy caterpillar that is not a stinging caterpillar.
NOT ALL WHO WANDER ARE LOST
Most caterpillars develop wanderlust as they approach the transition between youth and adulthood – once they are fully grown caterpillars and are ready to pupate. Rather than forming their cocoon where they ate their last meal, and where they would be much easier to find by parasites and predators, they crawl some distance away and then pupate. This is when we first notice caterpillars that have been present in our landscape for several weeks. After they enter this wandering phase, most caterpillars are no longer interested in eating, especially after they have wandered away from their host plant, but there are exceptions to this rule. Caterpillars that are forced to wander because they have eaten all their original host plant before they are fully grown are still in feeding mode and will often feed on plants they would not normally eat.