Slow but Steady

Found this guy on the trail at Illinois Beach today:

Meadow Slug (Deroceras laeve), Illinois Beach State Park, Lake Co, IL  4/13/2014

This is Deroceras laeve, the Meadow Slug. (At least according to the INHS -- there doesn't appear to be a standardized source for molluscan common names.) It's apparently naturally Holarctic in distribution, as all of the sources I checked stated that it was native here.

Slugs are, of course, snails that have evolved right out of their shells. Snails are pretty diverse ecologically, with some eating mostly plants and others mostly animals, often other molluscs. (Let's face it, catching active prey is not likely to be a snail's strong point!) It shouldn't be surprising, then, that slugs sometimes prey on animals as well, and this species has been observed feeding on a wide range of items, including beetle grubs and sawfly eggs and larvae. (1) This sort of generalized behavior probably helps explain their wide distribution.

Another factor in that distribution appears to be their tolerance to the cold temperatures expected in the northern half of our hemisphere, as D. laeve turns out to be notably more freeze-resistant than the introduced D. reticulatum and Arion circumscriptus. (2)

When we think of slugs at all, it's usually in the context of their damage to our vegetable gardens, but a native, freeze-tolerant species with abundant enemies of its own, that's able and willing to attack insect pests, seems like an ideal biological control agent - Sweetman suggested this back in 1958! (3) But we do need to be careful with this idea, as we can see from the tale of Achatina fulica, a very large snail from Africa sometimes introduced as a food source. When it was released onto the Marquesas Islands, it outcompeted a number of endemic tree snails, to the point of extinction, and there are concerns that it will do the same in Hawaii. (4)

(1) FOX, L., & Landis, B. J. (1973). Notes on the predaceous habits of the gray field slug, Deroceras laeve. Environmental entomology, 2(2), 306-307.

(2) Storey, K. B., Storey, J. M., & Churchill, T. A. (2007). Freezing and anoxia tolerance of slugs: a metabolic perspective. Journal of Comparative Physiology B, 177(8), 833-840.

(3) Sweetman, H. L. (1958). The Principles of Biological Control. Interrelation of Hosts and Pests and Utilization in Regulation of Animal and Plant Populations.The Principles of Biological Control. Interrelation of Hosts and Pests and Utilization in Regulation of Animal and Plant Populations., (Revd. edn. 11 1/4× 8 1/2).

(4) Elton, C. C. (1958). The reasons for conservation (pp. 143-153). Springer Netherlands.