Bug of the Week

Tent Caterpillar (Family Lasiocampidae)


Greetings, BugFans,

In the spirit of the season, the BugLady has retooled this episode from the spring of 2008. Last year, an over-zealous article in a state newsletter advocated aggressive push-back against tent caterpillars. True – the BugLady has been accused of being pro-bug, but she’s never seen a scourge of Eastern tent caterpillars that defoliated the landscape. There’s some temporary cosmetic impact, but trees readily grow new leaves.

Eastern tent caterpillars (Malacosoma americanum) are known for the dense, whitish webs they spin in the forks of their food trees, Rose family members like apple, cherry and hawthorn. Eggs are laid in late spring/early summer in egg cases that encircle the twigs, and then it gets interesting. The larvae start developing in their eggs immediately, but then they aestivate in there for nine or ten months, emerging in the following spring.

Tent Caterpillar

About a week after hatching, the fuzzy gang of ETCs gets together and starts spinning a communal, webby enclosure in the crotch of a trunk or branch. The webs of the tent caterpillar are their dwellings; they leave for food and return for shelter. The shelter that they weave is made of layers of tough silk, spun in great sheets with multiple “doors.” If you look closely at a web, you can see frass (bugspeak for “poop”) and accordion-like shed skins inside. As the caterpillars grow, the old layers get crowded and littered with empty skins and frass, and a new, roomier layer is added outside the old.

The climate-control technology they use would make a Master HVAC practitioner proud. The tent is situated so as to catch the sun’s rays – a necessity for a cold-blooded critter that gets its start before the final frosts of spring. A cold caterpillar can’t digest its leaves. The layers of silk with air spaces in between provide the caterpillars with effective insulation; the sunny side of the web on a chilly May morning may be up to 50 degrees warmer than the ambient air. As spring warms up, caterpillars retreat to the shaded side of the tent, and a very warm and humid day may drive the sweaty inhabitants outside. The tent also provides some protection from predators.

Tent Caterpillar

The caterpillars spin (in mid-morning and early evening) and shed (five times) and eat leaves (often at night, and they spin web-trails so they can get back home). According to Donald Stokes in A Guide to Observing Insect Lives, the web trails, which can be observed on twigs, communicate via pheromones the excellence of the cuisine, and a “five-star rated” trail gets the most traffic.

Tent Caterpillar

Their adolescence is brief – they are designed to eat only the young leaves. After their fifth molt, the caterpillars leave their web (sometimes they take off on solo cross-country trips before pupating), and they pupate for two to three weeks in a sheltered spot. Their cocoon is described in several references as imbued with a whitish powder and “flocculent,” (the BugLady’s (only) favorite word from high school chemistry). The web starts to deteriorate after the larvae bail. The unspectacular brown moths they metamorphose into produce more egg masses, and so it goes http://bugguide.net/node/view/522365/bgimage. There is a single generation per summer, and the adults (moths) do not feed.

Tent Caterpillar

The closely-related Forest Tent Caterpillar (M. disstria) (not pictured) moves about, feeds and roosts in crowds of 100 or more, but even though they guide themselves from roost (tree trunks) to pantry (the leaves of sugar maple, ash, basswood, and aspen, among others) and back with pheromone-laced web trails, they are “tent-less” tent caterpillars. Forest tent caterpillars do have some pretty impressive population booms, and maple defoliation ensues.

ETCs are often lumped with Fall Webworms (Hyphantria cunea), web-makers that spin at the other end of summer. FWs also construct communal webs, but they throw their webs around their food supply, encasing the ends of branches loosely in silk and feeding within. When they’ve eaten everything within the web, they move to happier hunting grounds, leaving a ragged web behind in fall.

Tent Caterpillar

Lepidopterists have an adage that “the homelier the caterpillar, the more spectacular the adult,” but the BugLady gives both the larva and adult Eastern tent caterpillar a “5.”

After last week’s episode, recognizing BugFan Elaine for her recent Fond du Lac lightning beetles, BugFan Tom reminded the BugLady that he had seen LBs 2 ½ months earlier in (roughly) central Mississippi. Phenology is phenology, but the BugLady didn’t think that her audience was ready to hear about LBs in mid-March.

The Bug Lady