At the Big Lake (Shasta County CA) boat launch hundreds of wooly bears were crawling on the ground and nearby low vegetation. Wooly bears are the caterpillar or larval stage of the various tiger moths, characterized by dense hairs over their bodies. Wooly bears also curl into balls and “play dead” when stressed or threatened.
The life cycle of moths consists of four stages: 1) egg, 2) larva or caterpillar, which is the feeding or growth stage, 3) cocoon, when the caterpillar morphs into a moth, and 4) adult, the reproductive stage. The garden tiger moth (Arctia caja) has one generation per year and overwinters as a larva. (Other types of moths overwinter in different life stages.) The wooly bears at Big Lake were garden tiger moths emerging from their winter hibernation.
Garden tiger moth caterpillars are a reddish-brown on the underside and black on the top with whitish hairs on the back. Their coloration is variable with different amounts of brown visible on the back and diverse quantities of white hairs. The adult moth is quite striking with a chocolate-brown and white mosaic on the forward wings and orange rear wings with dark blue spots. Later in the season I hope to share a garden tiger moth pictures.
Tiger moth caterpillars survive subfreezing winter temperatures by seeking areas protected from the prevailing weather. Decaying plant matter and snow also provide insulation. In addition these caterpillars produce a glycerol “antifreeze” in their bodies that enables them to survive temperatures as low as 15° F without damage.
Decreasing snow cover and the warming trend in recent years has subjected tiger moth larvae to more frequent cycles of freezing and thawing over the winter, as opposed to a single, winter-long freeze. These freeze/thaw cycles subject these insects to periods of elevated metabolism during the winter forcing them to use stored nutrients and carbohydrates. Marshall and Sinclair, writing in the Journal of Experimental Biology, found that caterpillars subjected to multiple freeze/thaw cycles had an increased concentration of glycerol in their bodies, a higher mortality rate and increased damage to certain tissues.
Although our winter was relatively mild this year and there were periods of freezing temperatures alternating with warm spells, these garden tiger moth wooly bears seem to have survived in large numbers. By now they are ravenously eating and growing in preparation for their next life stage.