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Finding Slug Caterpillars

I would be hard-pressed to choose a favorite group of insects, but slug caterpillars, family Limacodidae, would be strong candidates for that distinction. Their bizarre beauty is unmatched. Ultimately, my goal is to find and photograph all of the species native to my area. I hope that this article will encourage others to seek out and enjoy these amazing creatures, and perhaps those discoveries will generate data points that will aid me in my own search. If anyone has any advice or additional information to add on the subject, I would be most grateful if you’d leave a comment below.

Resources:

It really helps to know something of the habits of your quarry before you start looking. For some of the more common species of slug caterpillars, I was able to use BugGuide records to get a rough idea of when to start looking and which potential host plants to prioritize. Unfortunately, many species are poorly represented on the site at this time. Probably the most helpful overall resource I’ve found is David Wagner's Caterpillars of Eastern North America. The Limacodidae section is full of valuable information, and I won’t try to reproduce all of it here. This is a wonderful book, and anyone with even a casual interest in caterpillars should really consider picking up a copy.

For my location specifically, Northeast Georgia, I looked to the website of naturalist Giff Beaton for a better idea of what to look for and when. I even wrote to him a few years ago to see if he had any advice on finding slug caterpillars, but he replied that almost all of the ones he had seen were prepupal and crawling on the ground. (Of course, this is potentially useful information as well.) Lastly, I happened across this report by Sam Jaffe on the caterpillars of Massachusetts. It contains an abundance of region-specific notes on the habits of Limacodidae, some of which are more broadly applicable.

General tips:

1. The basic method of searching for Limacodidae is really simple: either turn over low branches or stand under higher branches and look up. It helps if the sun is shining down through the leaves. As a general rule, slug caterpillars prefer the underside of smooth, hairless leaves, but occasionally I'll encountered individuals of various species on leaf upper-sides (not just the Skiff moth, which Wagner associates with this behavior in his book.) If you’re looking up through overhead leaves, the caterpillar will appear as a symmetrical shadow, lying flush against the leaf surface. I always stop to investigate any such shadows, though many of them will turn out to be caused by fallen leaflets or other bits of debris. I have also found other interesting creatures this way, such as bolas spiders.

2. Most species of slug caterpillars can be found on a wide variety of woody plants; however, host plant preference seems to vary by region. This became really obvious as I compared my own observations to the writings of Wagner and Jaffe. The tree species that I expected to be the most productive (mainly oaks and maples) gave me an extremely poor return on the time and energy I spent searching them. I had much better luck with Carpinus caroliniana, persimmon, beech, and even sweetgum. Keep this in mind as you read about likely host plants.

3. The best season to search also depends on where you are located. Here on the Georgia piedmont, “peak” slug season seems to be early September. However, all species are not active at exactly the same time, and it’s likely that various conditions can also shift that active period around a bit from year to year. I’ve seen many more species in the last two years than ever before, and it’s mostly because I’ve started looking earlier.

4. I have also found certain habitats to be more favorable than others. The vast majority of the slug caterpillars I’ve found have been located along shady, wooded trails not too far from water. Field-forest edges have proven somewhat productive, but even then, the caterpillars are usually found on shaded branches. I’ve had almost no luck searching landscape trees and roadsides; dry, sunny spots like rock outcrops have likewise been extremely poor producers. My impression is that many of the species I’ve encountered to date avoid heat and direct sunlight (see species notes below for exceptions.) Going off-trail in really thick woods typically fails to produce caterpillars also, for whatever reason. It’s possible that this is more a matter of difficult search conditions than an actual absence of Limacodidae. Update: It's becoming clear to me that most of these species prefer saplings to full-sized trees. It makes sense then that they wouldn't be as abundant in habitats with a relative lack of young trees (mowed roadsides, suburban landscapes, and woodlands with a privet-infested understory to name a few.) This may very well be a more important factor than moisture or exposure to sunlight.

5. It is sometimes possible to zero in on a slug caterpillar by observing its feeding sign. Jaffe describes many species “eat[ing] leaves straight across, like they have been cut by scissors.” I’ve seen this feeding pattern myself, but I’ve also observed slugs leaving right-angled and scalloped edges as they eat. Young caterpillars “skeletonize patches of leaf tissue from either leaf surface,” according to Wagner, and I’ve witnessed this as well. Any of these signs may warrant flipping over the branch and taking a look. Don’t rely on this one indicator to guide your search, though. I’ve often found caterpillars resting under leaves that were completely intact. I’m not entirely sure why, but perhaps this is something they do prior to molting. Limacodidae also have unusual frass; each pellet has a “dent” pressed into one side. Since these pellets are relatively small, however, it’s unlikely that one would be able to locate a caterpillar by frass alone.

6. I can’t say that time of day has been a big consideration. Years ago, I’d gotten the idea from some of the photos posted here on BugGuide that slug caterpillars might be more active at night, but every one I’ve found since then has been openly going about its business during the day. I may still experiment with nighttime searches at some point in the future.

7. Be patient. Even in exactly the right habitat, at the right time of year, and on the right host plants, these caterpillars are relatively uncommon.

Species-specific observations:

Button slug, Tortricidia spp.
Sample size: 1 (or possibly 2)
Earliest date observed: June 2 (2012)
Latest date observed: June 24 (2012)
Host plant species: Redbud- Cercis canadensis
How high off the ground: head-height
Additional notes: When I first found this caterpillar, it had dark blotches on both sides of its body, and I assumed it had been parasitized. However, the blotches disappeared after its next molt, and it continued to grow and develop normally. While eating, it never consumed an entire leaf before moving on to the next one.

Yellow-shouldered slug, Lithacodes fasciola
Sample size: 7
Earliest date observed: August 20 (2011)
Latest date observed: October 10 (2013)
Host plant species: Persimmon- Diospyros virginiana, American beech- Fagus grandifolia, Black cherry- Prunus serotina, Redbud- Cercis canadensis
How high off the ground: knee-height to overhead
Additional notes: The first three Yellow-shouldered slugs I observed were found just a couple of feet off the ground on Persimmon saplings. One of these had been parasitized by braconid wasps. The fourth and fifth caterpillars defied this trend by turning up overhead on a full-sized Beech tree, and the sixth was overhead on Black cherry.

Skiff moth, Prolimacodes badia
Sample size: 12+
Earliest date observed: August 21 (2012)
Latest date observed: October 8 (2013)
Host plant species: Sweetgum- Liquidambar styraciflua, American beech- Fagus grandifolia, Boxelder- Acer negundo, Elm- Ulmus spp., American hornbeam- Carpinus caroliniana, White oak- Quercus alba, Silverbell- Halesia carolina
How high off the ground: head-height to far overhead
Additional notes: As mentioned previously, these will typically be found on leaf uppersides. Moreso than most species, I've frequently spotted them on full-sized trees (not saplings). They are also commonly parasitized by tachinid flies.

Spun glass slug, Isochaetes beutenmuelleri
Sample size: 1
Earliest date observed: August 25 (2013)
Latest date observed: August 25 (2013)
Host plant species: American hornbeam- Carpinus caroliniana
How high off the ground: overhead
Additional notes: I was surprised to find one of these, at long last, on Carpinus instead of the beech and oak trees I've been searching for years. Unfortunately, it had been parasitized by braconid wasps.

Monkey slug, Phobetron pithecium
Sample size: 12
Earliest date observed: August 11 (2012)
Latest date observed: September 11 (2012)
Host plant species: American beech- Fagus grandifolia, White oak- Quercus alba, Black cherry- Prunus serotina, Redbud- Cercis canadensis, Northern red oak- Quercus rubra, Hickory- Carya spp., American hornbeam- Carpinus caroliniana, Sweetgum- Liquidambar styraciflua
How high off the ground: waist-height to overhead
Additional notes: Eleven of the Monkey slugs I've seen to date have been at the same location, and eight of them were found in 2012. Only one showed signs of possible parasitism. The degree of hairiness seems to vary somewhat.

Nason’s slug, Natada nasoni
Sample size: 25+
Earliest date observed: July 21 (2012)
Latest date observed: September 25 (2011)
Host plant species: Sweetgum- Liquidambar styraciflua, Persimmon- Diospyros virginiana, Water oak- Quercus nigra, American hornbeam- Carpinus caroliniana, American beech- Fagus grandifolia, Blackgum- Nyssa sylvatica, Silverbell- Halesia carolina
How high off the ground: knee-height to overhead
Additional notes: Unlike many of these other species, I often encounter Nason's slugs in relatively open habitats (forest clearings, roadsides, etc.) Sweetgum seems to be a preferred host plant. Although most slug caterpillars I observe move very little, I once saw a Nason's slug repeatedly twitch from side to side when I disturbed its leaf.

Crowned slug, Isa textula
Sample size: 11
Earliest date observed: August 13 (2013)
Latest date observed: October 27 (2012)
Host plant species: Water oak- Quercus nigra, Willow oak- Quercus phellos, Sweetgum- Liquidambar styraciflua, Sweetshrub- Calycanthus floridus, American beech- Fagus grandifolia, White oak- Quercus alba, American hornbeam- Carpinus caroliniana, Silverbell- Halesia carolina
How high off the ground: knee-height to overhead
Additional notes: Generally found on saplings, sometimes in fairly sunny spots. Two were seen together on the same Willow oak tree. A couple of the late-season finds were just husks, completely consumed internally by parasites.

Purple-crested slug, Adoneta spinuloides
Sample size: 5
Earliest date observed: August 18 (2012)
Latest date observed: October 4 (2012)
Host plant species: American hornbeam- Carpinus caroliniana, Persimmon- Diospyros virginiana, White oak- Quercus alba, Possumhaw- Ilex decidua, Silverbell- Halesia carolina
How high off the ground: knee-height to head-height
Additional notes: Three of five specimens were found on wooded slopes overlooking a lake shoreline. Another was a very early instar on an uneaten White oak leaf that had disappeared when I returned a few days later. Two were parasitized by braconid wasps and one by flies.

Adoneta bicaudata
Sample size: 2
Earliest date observed: October 8 (2010)
Latest date observed: October 29 (2011)
Host plant species: American hornbeam- Carpinus caroliniana
How high off the ground: head-height
Additional notes: I was able to observe the development of one of these caterpillars in situ, and other than growing larger, its appearance didn't change very much between instars.

Spiny oak slug, Euclea spp.
Sample size: 6
Earliest date observed: August 22 (2013)
Latest date observed: September 4 (2011)
Host plant species: American hornbeam- Carpinus caroliniana, American beech- Fagus grandifolia
How high off the ground: eye-level to overhead
Additional notes: Most of the individuals I've encountered were well above head height, on full-sized trees. Four of them appeared to be harboring fly larvae.

Smaller parasa, Parasa chloris
Sample size: 25+
Earliest date observed: July 22 (2012)
Latest date observed: October 15 (2008)
Host plant species: American hornbeam- Carpinus caroliniana, Sourwood- Oxydendrum arboreum, White oak- Quercus alba, Northern red oak- Quercus rubra, Black cherry- Prunus serotina, American beech- Fagus grandifolia, Elm- Ulmus spp., Hickory- Carya spp.
How high off the ground: knee-height to head-height
Additional notes: I once observed two early-instar Smaller parasas together on Black cherry. Although I've found this species on many different host plants, American hornbeam seems to be the most common. Many of these caterpillars have been parasitized by wasps, flies, or both. I tried rearing one several years ago, but it never emerged from its cocoon.

Stinging rose slug, Parasa indetermina
Sample size: 12
Earliest date observed: August 10 (2012)
Latest date observed: September 11 (2011)
Host plant species: Silverbell- Halesia carolina, American hornbeam- Carpinus caroliniana, Black cherry- Prunus serotina, Persimmon- Diospyros virginiana
How high off the ground: head-height to overhead
Additional notes: I sometimes see two or three Stinging rose slugs on the same sapling. In 2011, they were almost always on Silverbell; however, my subsequent observations have not fit that pattern, so it may have been coincidental. I have found them with fly and wasp parasitoids.

Saddleback caterpillar, Acharia stimulea
Sample size: dozens
Earliest date observed: August 21 (2006)
Latest date observed: October 24 (2008)
Host plant species: Sweetgum- Liquidambar styraciflua, Boxelder- Acer negundo, Persimmon- Diospyros virginiana, Scarlet oak- Quercus coccinea, Silverbell- Halesia carolina, American beech- Fagus grandifolia, others. This is the only species that I have also seen on vines and herbaceous plants including Greenbriar- Smilax spp., Groundnut- Apios americana, Crownbeard- Verbesina spp., and Hibiscus.
How high off the ground: almost touching the ground to far overhead
Additional notes: Saddlebacks are the most commonly observed of the slug caterpillars in this area, and for years it was the only species I had ever come across. I'm not certain whether they are really more abundant than the other slugs, or if their indiscriminate tastes for host plant and habitat just make them harder to miss. In any case, I've found them along wooded trails, roadsides, waterways, in forest openings, fields, gardens, and everywhere in between. Early instar caterpillars sometimes cluster together. They seem less vulnerable to parasitism than some of the other species, but I occasionally find individuals with misshapen lobes (usually an indicator of parasitoids within), and I once found a Saddleback whose body was almost completely obscured by braconid cocoons.

Great Info
Slug caterpillars are a personal favorite of mine as well. I share the goal of photographing all my local ones.

I have lots of photos from my area (not far from you). I'll try to make an effort to get more of those on BugGuide, complete with info about where, when, and how it was found.

Don't give up on landscape trees, roadsides, etc. Most of what I find is from casual walks in my backyard and local parks.

I'd love to find this one again:

 
I would be thrilled...
... to hear anything and everything you wish to share about the circumstances of your finds. I'm really starting to realize that host plant is just one piece of the picture. I'll continue to update this article with new sightings going forward. It's been a pretty good year for slug caterpillars already, but I haven't yet found any of the new-for-me species I've been actively seeking (mainly Apoda biguttata and your lovely Isochaetes beutenmuelleri, as I saw adults of both in July.)

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