Northern Prairie Wildlife Research Center
Species with physical or chemical defenses tend to be highly conspicuous, advertising their unpalatability with bright colorsreds, oranges, and yellows offset with black and white. They also tend to be "messy eaters" that leave a telltale path of damaged foliage and excreta or frass, which further facilitates their discovery. Often they feed by day, seemingly unconcerned about the legions of insectivorous birds that share the forest with them. Palatable species, on the other hand, tend to be far less conspicuous in both habit and designmore often than not they are cryptically rendered in greens and browns. Many feed only at night. As a group they are "neat feeders" and may be quick to move off damaged leaves. Sphinx and underwing caterpillars are especially tidy, "covering up their tracks" by clipping away leaves upon which they have fed. After finishing with a leaf, they chew through the petiole and drop incriminating foliage to the forest floor, leaving birds with little that would reveal their whereabouts.
An accumulation of frass pellets or damaged leaves are always good cluesthe fresher the signs the more likely that the caterpillar can be located nearby. Many cryptic species play a shell game with their avian predators, moving off damaged leaves onto intact leaves several times each day. Thus any hint of browning or other signs of leaf necrosis often indicate that the perpetrator is long gone. Searches are usually best focused on young foliage, saplings, isolated plants, and plants growing along a forest edge. Because many species feed at night, nighttime walks with a flashlight can be very productive and rewarding for locating caterpillars.
"Beating" is perhaps the most time-efficient method for securing caterpillars from trees or shrubs. The idea is to abruptly jar caterpillars from their resting or feeding sites and collect the dislodged individuals from sheets or drop clothes held below. The sheet can be a hand-held construction (available through biological supply houses) or simply a bed sheet spread over the ground. A short baseball bat or a stout stick rapped against a series of branches will almost always dislodge a few caterpillars. Working only 1 plant species during a collecting episode allows host plant associations to be noted and recorded accurately. In fields and other habitats where the vegetation is low, sweep nets can be used to dislodge caterpillars in an analogous fashion. Collections made during the night will often yield a different set of species than those obtained during daytime collections.
Burlap bands stapled to tree trunks can be used to sample forest lepidopterans that descend from the forest canopy to shelter during the day. A strip of burlap about 8 to 10 inches in width is wrapped about the trunk once or twice, stapled along the top edge, and then slit vertically so that flaps can be lifted and the bark beneath inspected for resting caterpillars. Diversity and abundance of caterpillars peaks in late spring when there is an abundance of young, tender foliage. Except for gypsy moth and other outbreak species, expect no more than 2 to 4 caterpillars for every 10 bands examined. Groups that may be especially well represented under burlap include tussock moths, tent caterpillars, underwing noctuids and their kin, and lithosiine arctiids.
An effort should be made to collect and rear several individuals of each species because many are likely to fall victim to parasitoids and pathogens. Such collections should be cross-labeled so that an adult emergence can be tied to any photographs, preserved specimens, and reared parasitoids. Field-collected larvae that yield natural enemies provide important data on host ranges and hence efforts should be made to preserve and label issuing parasitoids and cadavers killed by pathogens. There is an immediate need to know the host ranges (and impacts) of several introduced parasitoids on our native butterfly and moth fauna. One gypsy moth dipteran parasitoidCompsilura concinnata (Family Tachinidae)that was still being imported into the United States as recently as 1975 as a classic biological control agent has been reared from more than 200 species of native Lepidoptera.