Like most species in this family, the milkweed tussock moth's larvae obtain a chemical defense from their host plants, in this case, cardiac glycosides from milkweed or dogbane. These are retained into the adult stage and deter predators. Only very high cardiac glycoside concentrations deterred bats, however (Hristov and Conner 2005). Unpalatability is also advertised with clicks from the tymbal organs (Simmons and Conner 1996).

Sporting tufts of black, white and orange (sometimes yellow) setae, larvae skeletonize whole leaves gregariously, leaving a lacy remnant. The head capsule is black. The later instars wander much more, and may appear alone or in small clusters. Mature caterpillars are found from June onwards (Wagner 2005). Grows to a length of 35 mm. [2]

Erebidae (Tiger and Lichen Moths) is a large and diverse family of moths with around 11,000 species found all over the world, with 6000 Neotropical species (Scoble 1995). This family includes the groups commonly known as tiger moths (or tigers), which usually have bright colors, footmen (which are usually much drabber), lichen moths and wasp moths. Many species have 'hairy' caterpillars which are popularly known as woolly bears.

The most distinctive feature of the family is a tymbal organ on the metathorax (Scoble 1995). This organ has membranes which are vibrated to produce ultrasonic sounds. They also have thoracic tympanal organs for hearing, a trait which has a fairly broad distribution in the Lepidoptera but the location and structure is distinctive to the family. Other distinctive traits are particular setae ('hairs') on the larvae, wing venation, and a pair of glands near the ovipositor (Scoble, 1995). The sounds are used in mating (Simmons and Conner 1996) and defense against predators (Fullard et al, 1994).

Adult Milkweed Tussock Moth
Photo: Patrick Coin

Many of the caterpillars and adults are active during the daytime. If disturbed, woolly bear caterpillars will roll into a tight spiral. Common folklore has it that the forthcoming severity of a winter can be predicted by the amount of black on the Isabella tiger moth's caterpillar, the most familiar woolly bear in North America; however the relative width of the black band varies among instars, not according to weather (Wagner 2005). Isabella tiger moths (Pyrrharctia isabella) overwinter in the caterpillar stage. They can survive freezing at moderate subzero temperatures by producing a cryo-protectant chemical (Layne and Kuharsky 2000).