A Poisonous Interlude

Here's a nifty little critter from this morning:

Arge humeralis, Illinois Beach State Park, Lake Co, IL 6/7/2014

This is Arge humeralis, a sawfly in the family Argidae. While many of the hymenopterans are predators or parasites as larvae, this guy started off as a vegetarian. In fact, he was a very picky eater -- here's his breakfast, lunch, and dinner:

Poison Ivy (Toxicodendron radicans),
Sedge Meadow FP, Lake Co, 10/6/2011

Anyone who's grown up in the Midwest should know this plant. (If not, learn it soon!) This is Poison Ivy, Toxicodendron radicans. It's called Poison Ivy because it produces an oil called urushiol, which in many people can start an allergic reaction resulting in a nasty, itch, weeping rash. It's not fun. (If you're extra-sensitive to Poison Ivy, you might want to consider avoiding Mangos, perhaps even Cashews. They're all in the same family, and Mangos actually produce a small amount of urushiol in their skin.) I can only assume it has the same effect on deer and other mammalian herbivores. The plant is certainly common enough around here. But most such secondary compounds that plants use are primarily aimed at insects rather than vertebrates.

Many of our insects are specialist herbivores as larvae - many moths, for example, only use one or two host plants. Being a specialist means that you can fine-tune your larval digestive enzymes and greatly simplifies your search for a useful place to lay your eggs. Specializing in a plant with effective defenses also seriously reduces your competition. All of these points explain why a specialist individual might be favored within a population, and therefore why so many insects do so.

But there is one potential issue with being a specialist. If your host plant declines, so do you, and if it goes extinct, well...

One potential solution is to periodically make "mistakes" -- after all, biology is always a bit messy. If every once in a while you get an odd individual who lays her eggs in the wrong place, then once in an even longer while that new spot will work -- and now you have an escape hatch if your initial host plant has troubles. Owen-Smith & Novellie predicted something along these lines in 1982. (1) Since then, Poison Ivy Sawflies (that's the common name for A. humeralis) appear to have fulfilled that prediction, according to Benda, et al. (2) , being found feeding on Poisonwood, Metopium toxiferum. (Another member of the Cashew family, Poisonwood also produces urushiol.) Owen-Smith was talking about large herbivores, and the constraints on smaller herbivores are admittedly different, but the same basic logic seems to apply.

This isn't to say that every specialist species manages to find such an out - the trouble with messy processes is that sometimes they don't work. They also tend to be easily outpaced when faced with rapid changes. This is unfortunate, these days, since one things humans are very good at is producing rapid change in a landscape. For every species like an Ailanthus Webworm Moth, we really don't know how many are unable to adapt in time. It's very difficult to say how many species are disappearing when we don't really know how much was out there to begin with. (Incidentally, this is not a call for inaction -- while it's hard to say how many are disappearing, what we do know about the distributions of little critters, especially tropical ones, strongly suggests that our estimates of extinction are biased low rather than high.) The only way to solve this is to get out there and find these critters, which is something that, in many groups, is surprisingly within the reach of amateur naturalists.

(1) Owen-Smith, N., & Novellie, P. (1982). What should a clever ungulate eat?. American Naturalist, 151-178.

 

(2) Benda, N., Possley, J., Powell, D., Buchanan-Mcgrath, C., & Cuda, J. (2012). New host plant record for the poison ivy sawfly, Arge humeralis (Hymenoptera: Argidae), and its performance on two host plant species. Florida Entomologist, 95(2), 529-531.