Pyrrharctia Isabella (Isabella Tiger Moth)

Pyrrharctia isabella (Isabella Tiger Moth)

This beautiful erebid moth is the only member of the genus Pyrrharctia. It’s larvae are the common banded wooly bear caterpillars, known to kids and bug enthusiasts of all ages. There is a popular myth that the proportion of reddish brown central band to black ends can predict the severity of the coming winter, with a larger reddish portion denoting a milder winter. But this is more-or-less fallacious, because the central band lengthens with the age and feeding success of the caterpillar. However it does appear that wetter conditions may produce longer black ends, and may also force the caterpillar into hibernation at a younger age, which would result in a shorter central section. This might foretell an earlier start to winter, though not necessarily a more severe one. 

Pyrrharctia isabella (Isabella Tiger Moth)

But there are other very fascinating things about these larvae. One is that since they overwinter as larvae in diapause, rather than in a pupal state, and exist in a hibernaculum in the relatively stable, but often very cold, environment between the leaf litter and the ground, they produce glycerol, a cryoprotectant that slows down freezing by allowing their bodily fluids to supercool without forming ice crystals. This is called ‘freeze avoidance’, and is actually a relatively common adaptation in the insect world. 

Pyrrharctia isabella (Isabella Tiger Moth)

But it can be negated by extremely low temperatures. Pyrrharctia isabella works around this by being freeze tolerant. To accomplish this they allow ice crystals to form, but at higher temperatures when they grow more slowly, so that the cell walls can accommodate them without injury, and the interior of the cell remains unaffected due to a high glycerol hemolymph. They can actually freeze almost solid, and still revive when temperatures rise!

Pyrrharctia isabella (Isabella Tiger Moth)

And it is likely (though not as yet documented in this species) that, like certain closely related Grammia spp., they will selectively ingest plants containing pyrrolizidine alkaloids (like groundsels, hounds tongue, coltsfoot, etc) when they are infected by the parasitoid larvae of wasps and tachinid flies. These alkaloids have no effect on the moth larvae, but are toxic to the parasitoid larvae. They are also lower in nutritional value, which is why only infected moths engage in eating them. This sort of self medicating, adaptive plasticity, is as yet unknown in the insect world outside of the tiger moths. 

Pyrrharctia isabella (Isabella Tiger Moth)

I might not have found this particular moth on our trip to the Ridge, without the help of William Sondergaard, my friend Craig’s grandson. It blended into some low mosses for me, due to my colorblindness, but William spotted it right away on the ground near the base of the sheet. I’ve now found Isabella Tiger Moths in elevations from sea level to almost 4,000’, and in habitats as diverse as floodplain bottomland, oak scrub, mixed deciduous and coniferous low elevation forest, and subalpine fir dominated forest. Which is exactly the sort of highly adaptable niche exploitation one would expect for a creature found nearly coast to coast in northern North America. 

Pyrrharctia isabella (Isabella Tiger Moth)

Description-Large (forewing length 22-26mm) orange to brownish orange moth with black speckling at mid fw, and a line of black dots near the fw apex; hindwing lighter orange; legs black. 

Similar species-Nothing else in our region has this combination of orange coloring, large size, and small black dots. 

Pyrrharctia isabella (Isabella Tiger Moth)

Habitat-Moist to mesic forests and mixed woodlands, coastal grasslands and wet meadows, up to 5,000’ elevation. 

Range-Northern North America; region wide in appropriate habitat; absent from the most arid parts of the interior. 

Eats-Larval hosts include a wide variety of grasses and leafy vegetation; larvae will resort to cannibalism in the absence of other food sources; adults do not eat. 

Eaten by-Larvae and adults are preyed upon by insectivores of all classes, including birds, reptiles, amphibians, mammals, insects, and arachnids. 

Pyrrharctia isabella (Isabella Tiger Moth)

Reproduction-Univoltine in our region; overwinters as larvae in diapause, and pupates in spring. 

Adults active-April into September, with the peak from mid May to mid July. 

Etymology of namesPyrrarctia is from the Greek for ‘flame colored’, plus the genus Arctia, which is from the Greek for ‘bear’, and refers to the wooly caterpillar; the specific epithet isabella appears to be a lady’s name, but which lady is unknown. 

Pyrrharctia isabella (Isabella Tiger Moth)

https://www.nationalgeographic.com/science/article/self-medicating-caterpillars-use-toxic-plants-to-kill-parasites

http://pnwmoths.biol.wwu.edu/browse/family-erebidae/subfamily-arctiinae/tribe-arctiini/pyrrharctia/pyrrharctia-isabella/

https://www.weather.gov/arx/woollybear

https://www.jstor.org/stable/2426919?seq=10#metadata_info_tab_contents

https://bugguide.net/node/view/539

http://mothphotographersgroup.msstate.edu/species.php?hodges=8129

Pyrrharctia isabella (Isabella Tiger Moth)

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