Tuesday, April 7, 2015

Fighting slime with slime

Or: the tale of two gastropods, slime and punishment, etc etc...
A curious brown garden snail
I've often noticed large round snails clinging to plant stems in the shade or on wet mornings. They seem to be everywhere. Known as the the brown garden snail, these beefy vegetarians are native to the mediterranean. They were originally imported to North America to serve as escargot. Edible, though perhaps not palatable to American tastes, these snails now range across most of the southeast and California.

These introduced snails are a minor garden nuisance, but high populations in California have vexed the citrus industry. The best snail management plan has a multi-pronged approach. Removal of daytime hide-outs, snail baits and traps, using copper barriers, and releasing another non-native mollusk: the decollate snail [1, 2] are used in tandem to knock back populations. Unlike the brown garden snail, the decollate snail is a predator. It hunts other snails.

Releasing one non-native to control another is called "classic" biological control, and has a storied history. In Australia the cane toad was introduced to control a pest beetle, but has since become a much larger threat to ecological stability than the beetle ever was. Closer to home, in North America a parasitic fly introduced to control gypsy moths has directly contributed to the scarcity of our own beautiful silk moths. The luna, the cecropia, the polyphemus, and the promethea are all much rarer than they once were. A little bit of magic left our eastern forests. On the other hand, biological control is our last best hope in the case of the hemlock woolly adelgid and purple loosestrife , so it cannot be discounted completely.

While the UCANR Integrated Pest Management (IPM) guide noted that releases of the carnivorous decollate snail are prohibited outside certain areas (because the decollate snail is a generalist, and will eat native snail eggs as readily as those of a pest), I have not heard of the decollate snail causing problems. Of course, how would we know. I can count the number of individuals I know who can even identify snails on one hand, let alone track and monitor native population dynamics.

But, what's done is done. When I turn the soil in my garden bed, the shells of both non-native snails surface in abundance. Eradicating either species is likely an exercise in futility at this point. However, their history piques my interest in our native snails! I'll have to keep a look out for these tough survivors.

Foreground: decollate snail shell. Background: brown garden snail shells
1. http://entnemdept.ufl.edu/creatures/misc/gastro/brown_garden_snail.htm
2. http://www.ipm.ucanr.edu/PMG/PESTNOTES/pn7427.html

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