"Know thy enemy and know thy self and you will win a hundred battles" is the well-known advice of Sun Tzu Wu in The Art of War. Ask most gardeners who grow dahlias what pest gives them the most grief and inevitably the answer is slugs. The war on slugs wages hot with most of us who want the brilliance of dahlias in our gardens.
Nursery experts at Oregon State University call these land mollusks "basically a stomach on one large foot" that can consume several times their own body weight each day. A jaw like a guillotine and a scraper-like mouth part with thousands of backward-pointing replaceable teeth equip the slug with all it needs to reduce your beds of precious emerging dahlias to stubs and slime-tracks. The author of the Oregon State University Pacific Northwest Nursery Pest Management Site says of this nastiest of dahlia pests, "To sum a slug: It is magnificently designed to deconstruct."
The personal lives of slugs can be fascinating. Slugs are trans-gender hermaphrodites which begin life as males but develop female reproductive organs at maturity. Slug courtship is an elaborate and sustained intertwining. Female slugs lay eggs, about a quarter of an inch or less in diameter, beneath wood or garden debris and sometimes in crevices or holes in the ground. Late-laid eggs can overwinter and hatch when the first warm rains occur. The slime that accompanies a slug is of two sorts; a slippery mucus offers ease of movement while a more viscuous version actually increases traction.
What species of slugs you have to cope with depends on where you live. In the Northwest, the main culprits include the European red slug Arion rufus, and the great gray garden slug, tiger slug or spotted leopard slug Limax maximus. The Northwest's iconic banana slug, a slow-moving forest cleanup committee, rarely invades the garden.
Controlling These Dahlia Pests:
A preference for underground habitation makes slugs sneaky, and allows them to have at those tender parts of your dahlia where it emerges from the tuber. Slugs are largely nocturnal, and like rain. OSU nursery advisors suggest monitoring your slugs' activities, monitoring your gardens with a flashlight at night and checking daytime habitat under flower pots, debris or even stepping stones as well as under tall grass, ivy or other sheltering foliage.
Controls include barriers such as copper strips, encouragement of predators including some birds and snakes, manual picking or stabbing, small boys with salt shakers, diatomaceous earth and lime, baiting with beer, and poisons including carbaryl, metaldehyde and methiocarb. The poisons, of course, are more effective than the organic methods. Lynch Creek Farm's growers suggest circling dahlia plants with a poison barrier as they begin to sprout. Slug poisons are most effective for the long term when used in late summer when cooling temperatures trigger mating and egg-laying activities. The side effects of the poisons, of course, include loss of beneficial critters such as earthworms, and the danger of poisoning birds and pets.
Attempting to expanding my arsenal of nontoxic dahlia pest control weapons, I came across a recommendation by Valerie Easton, a Seattle Times magazine section writer. She rejects poisons and finds most of the other methods too time consuming; her ammo is a spray of one part ammonia and two parts water which, she claims, "disintegrates slugs almost instantly." Her ultimate solution, she adds, is to garden using only those plants that aren't subject to slug predation. If you want dahlias, you have to reject that option.
The most effective garden treatment I have encountered was the most natural. When Mount Saint Helens erupted in 1980, one of the ash-plume events coincided with a rare southeast wind that brought a visible dusting of ash to Mason County. That summer our garden was virtually slug-free, and it was a couple of years before the slug population was back to its annoyingly normal level.
Wouldn't it be nice to harness a mountain's power and have volcano-on-demand for your garden?