Is Fall Really Here? The Slug Moths Say So

August, September and October – fall, to us. We enjoy pumpkin flavored beverages and sweets, pick apples at orchards, bundle up boots and in warm-tone knit sweaters. But what do the animals – who are not Starbucks rewards members – do? They’re prepping for winter. We know some settle in burrows for hibernation, some migrate thousands of miles south, and some, like slug moth caterpillars, are approaching pupation at just the right time. While relatively few lepidoptera overwinter as adults, hidden away in a brush pile (question marks, Eastern commas, mourning cloaks), or even as eggs, more spend the cold, dark, seemingly endless months as larvae or pupa. Some of our coolest caterpillars, aptly named slug moths, will overwinter as pupa – but before they do, they’re more easily found and seen than all summer long!

Most often, caterpillars prefer to feed on the underside of leaves to help stay hidden and avoid the ever-present, searching eyes of their most common predators, birds. But, once they’ve reached their final instar (i.e. stage of growth), some, such as the skiff moth, will begin exploring the leaf upperside or even leave the host plant altogether (how many times have you seen a black and brown wooly worm crossing the road?). They’re also the largest they’ll ever be as caterpillars, and that helps in spotting them, too.

Recently at Wahkeena, we’ve found nine species of slug moth! Three have been found on redbud (yellow-shouldered, elegant-tailed, and monkey slugs), and six on beech (saddleback, button slug sp., smaller parasa, jewel-tailed slug, crowned slug, and skiff moth).

Literature often cites oak as the most common host for slug moths, as well as lepidoptera in general, and for good reason – oaks in North American support over 500 species of butterflies and moths! However, we’ve learned something interesting at the preserve about our caterpillars’ preferences. We have plenty of oak here, but it’s all in the canopy, and the slug moths in particular are usually found between knee and head height. Our understory is essentially entirely dominated by beech – a trend we’ll come back to – and its what we’ve found the majority of our caterpillars on. Putting two and two together, we can assess that as important as the simple presence of a host plant is, it might just be that vegetation height/size is just as important.

This finding is something to think about when we consider that in the eastern United States, our oak-hickory forests are slowly but steadily being replaced by beech-maple and have been for some time. Other animals beyond caterpillars need oak, too; squirrels, deer, turkey, and a number of other vertebrates reply on the hard mast produced by oaks to survive. While some butterfly and moth species may be able to simply switch to another host plant, others may not, and what does that mean for the future of our insect life? With insects in trouble, serious problems with far-reaching effects loom on the horizon.

If you’ve some land, we suggest planting any suitable native oaks to your area that you can, and in the meantime, take a peek below to see what these odd cats look like.

Here, you can see just how slug-like these caterpillars have adapted to be. Of all the slug larvae encountered this year, the skiff moth seemed to be the most mobile.

Of these nine species, six have stinging hairs or stinging spines (though the yellow-shouldered and elegant-tailed slugs lose them in their last instar) and in fact, most slug moth caterpillars have them. Recall that their prolegs have been modified into a slimy belly pad – they glide or undulate on this pad along the surface of leaves, and for that exact reason, they prefer leaves with a smooth surface over one that is hairy. This also means they move a little slower than most other caterpillars, which would make them more vulnerable to predators. To defend themselves, they developed these stinging apparatuses that pack a serious punch – sometimes as bad or worse than a bee sting, even resulting in bubbling blisters! The skiff moth slug and elegant-tailed slug are the only two lacking these threatening, elongated tubercles.

The slug moths have adapted in other ways, too. Their eggshells are particularly thin, so they lack any and all spiny structures in their first instar (the one that emerges from the egg) to avoid breaking that fragile chorion. Their heads are also entirely recessed, protecting the most important and most vulnerable part of them. Still, like other caterpillars, many fall victim to parasitoid wasps and flies, whose larvae will burrow into the caterpillar and feed on it from the side out.

While we’ve found nine slug moth species in caterpillar form of the 18 that inhabit Ohio, our mothing records show we’ve had a whole lot more historically, and the season of slugs is just beginning, so we’ll keep looking! Who knows, maybe we’ll turn up a spun glass slug…

– Naturalist Leah