Slugs are not the Loveliest of Creatures
Ro Wauer, August 15, 2004, The Victoria Advocate, © 2004
Most non-biologists or gardeners think of a slug as the guy next door who spends most of his time drinking beer and watching TV, and rarely if ever has anything nice to say. We all know some slugs, and none are very pleasant. And what is fascinating is that the slow moving creature that crawls across the roadway, invades gardens, and is a beer-lover - also known as a slug - fits the human profile pretty well.
The outdoor slug is closely related to snails, even possessing a shell. Those of snails are obvious, but slug shells are much smaller and rarely visible as they are located underneath the flesh on their back. Their mouthparts are located in front of the head and contain a sharp rasping strap, called the radula. It is used to tear food into small pieces, which are then passed back through a cavity to be digested.
Slugs move across the terrain by gliding on a ribbon of mucous secreted by their own bodies. They are able to follow slime trails secreted earlier by themselves or other slugs. In this manner they are able to utilize a network of host plants. Dee Kennedy, in "Nature's Outcasts," wrote that "slugs aren't just slimy - they're connoisseurs of slime. They secrete and employ different kinds of mucous for use in locomotion, self-defense, temperature regulation, and mating."
But what is the strangest slug characteristic is that it is a hermaphrodite, having both male and females reproductive organs. They go through male and female stages at different time. In a sense, they can mate with themselves. Eggs are laid in the soil in clusters of two dozen or more. Adults usually overwinter in the soil and can live for several years. When warm weather arrives, they soon are gliding about in search for food.
There are about 40 species of slugs in the United States. Three of the more common types include grey field, black, and common garden slugs. In the Northwest there is a huge yellow slug known as a banana slug because of its banana-like appearance. The slug in the Mission Valley area where I live is the common garden slug. I believe that it has only moved into the area during the last couple years; I never saw one until about two years ago. And now it seems to be on the increase; it is most abundant during rainy periods. During dry periods, like what we have experienced the last few weeks, slugs remain burrowed in the soil or in other moist sites. Since slugs are composed mostly of water, dehydration is dangerous. The common garden slug is one to two inches in length with a lighter side-stripe. Like all slugs, it has two pairs of tentacles, a longer upper pair that has eyes on the end and a shorter lower pair that is utilized for smell.
Most slugs are vegetarians and can cause serious damage to garden plants, but a few are carnivorous and prey on snails and earthworms. The latter group may have little effect on gardens, but the vegetarians can be a real pest. Those species feed on many kinds of leaves, stems, bulbs, and even tubers, often digging deep into the soil for a choice meal. They are able to detect food at a distance, and Kennedy points out that "if you change the location of the food while they're en route, they'll reorient and set off in a new direction."
Does this suggest that our gardens are doomed once slugs invade? Although many unattended gardens can be impacted, slug control is possible. Controls can range from direct elimination, which takes a concerted effort once the slugs have invaded, to a number of ideas from maintaining soil free from decaying materials, establishing a barrier, to various chemical controls. I once attracted slugs to beer bait and physically killed those that I found in the saucer of beer. C. Malcolm Beck and John Howard Garrett, in the "Texas Big Book," suggest dusting "dry hot pepper in problem areas," or using citrus oil spray or coffee grounds "sprinkled on top of mulch." They also suggest some natural controls like planting "stands of clover and mulches to favor ground beetles and rover beetles (which eat slugs). Centipedes also eat slug eggs." And they point out that "in the insect world their biggest enemy in the larva of the lightening bug." The abundance of lightening bugs in our area is on the side of our gardens.