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Slugs

To sum a slug, it is magnificently designed to deconstruct. This can be a little unsettling to those who like to produce. This difference in activities often leads to conflict. If you battle these voracious consumers, you may benefit by learning more about these creatures.

They generally have a bad reputation, slugs. It's the slime thing. And probably that they are basically a stomach on one large foot. Spineless, rapacious, how can one like these loathsome creatures? They do have a vital role to play in their place and they can be quite fascinating. Slugs (or shell-less snails) occupy the phylum Mollusca in the class known as Gastropoda. We are interested, in this case, in the slugs on land, the pulmonates, which have lungs rather than gills. Unlike their brethren the snails, slugs can occupy an ecological range without rich calcium deposits required to build shells. Of course, the down side to running naked of shell is the risk of desiccation. Thus the attraction to moist climates such as found in the Pacific Northwest.

As David George Gordon mentions in the Field Guide to the Slug, the "consuming passion" of slugs is eating. Various species can eat algae, animal feces, carrion, centipedes, fungi, green plants, insects, lichens, worms, and other slugs. Many slugs consume several times their own body weight each day. Tools of the trade include a guillotine-like jaw and mouth equipped with a radula (Latin: a scraper). The radula is a ribbon affixed with thousands of backward-pointing, replaceable teeth. It is also used in slug-to-slug combat and scars of prior battles can be seen on some species.

It can be useful to know your way around a slug. Check out Slug Anatomy 101

Slugs are trans-gender, hermaphroditic with both male and female sex organs. They are first males, developing female reproductive organs when mature. Slug courtship is not for the faint-hearted. It is elaborate, sustained, and usually results in the transfer of genetic material between two slugs. Slugs serve up their young as eggs. The eggs are quite small, around 1/4 inch (6mm) in diameter. They can be round or oval shaped, transparent, golden, or white. Some species lay their eggs singly while others lay eggs in clutches of 3-50 (the European garden slug lays as many as 500 eggs per year). Eggs can be found beneath pieces of wood or debris or in small crevices or holes in the ground. Eggs generally hatch in a few weeks although those eggs laid in late October or November usually overwinter and hatch as early as January if warm rains occur. Hatchlings are extremely vulnerable to predation and other mortality factors.

Admittedly, slugs are well lubricated. Slime, though a rather prejudicial term, can be quite handy. Mucus is used for locomotion, self-defense, moisture control, trail finding, and mating. Produced by various glands on the slug body including the pedal gland, some slime is very slippery easing movement while another type of slime produced is actually viscous and can increase traction. The mucus absorbs water so it is best wipe it off first rather than washing with soap and water.

Slugs differ from snails in their preference for time spent underground. This means slugs are also more of a problem with root crops, buried seeds, and seedlings. There are reports of slug damage on many ornamentals including Aconitum, Adiantum, Aesculus, African violets, asters, chrysanthemum, coleus, cyclamen, dahlias, dogwood, hibiscus, ivy, orchids, poppies, primroses, rhododendrons, roses, Solanum spp., Viburnum, and zinnias. Bulbs and tuber crops such as amaryllis, gladioli, irises, calla lilies, lilies, narcissus, and tulips are damaged. Young conifer trees can also be damaged or killed by aggressive slug attacks. Most damage comes from slugs on site rather than from immigrants. During the summer, only five percent of the slug population will be above ground. Immigration can occur. Edges of fields, particularly near fallow or weedy areas often have the most damage. Banana slugs have been clocked at 6 ½ inches (16.5 cm) per minute (about the speed of a line in the grocery store). The milky slug will travel up to 40 feet (12.2 m) in a single night.

Monitor for slugs to assess activity, damage, susceptible plants, and population increases, especially of young slugs or snails. Slugs are nocturnal preferring the nightlife. One can use flashlights or headlamps for night monitoring, use baited traps, or check likely daytime habitat under wood, moist protected areas, tall grass, and under pots. Signs of slug damage can range from the obvious such as slime trails near feeding damage to the more subtle such as fecal residue. In the Northwest, March, April, May, June, and October are considered the best months to monitor for slugs. They are more abundant after light rains and when night temperatures are above 50 degrees F (10 C). Many people use beer-baited slug traps made with plastic cottage cheese-type containers with lids, sunk into the soil and with one inch square holes cut in the sides at soil level. Leaving dead slug bodies in the traps may also attract slugs. Handle slugs with gloves as they may carry parasites potentially harmful to human health.

Mortal combat against slugs can be waged on several fronts: cultural, biological, and chemical. Cultural controls include reduction of habitat, trap crops such as marigolds, use of barriers such as copper strips, and cultivation. Freezing and flooding greatly reduces slug populations at least temporarily. Predators include: small mammals, snakes, spiders, amphibians, birds, carnivorous beetles, other slugs, and humans. A predatory nematode, Phasmarhabditis hermaphrodita, is being sold in Europe and Britain for slug control.

There are relatively few registered chemical controls for slug management. Historically people have used heavy metals and substances such as lime to control slugs. The chemical age ushered in new chemical classes to use in the assault on slugs. One of the most active classes against slugs is that of the carbamates. Carbamate insecticides used for slug control include carbaryl, metaldehyde (numerous products) and methiocarb (Mesurol). The molluscicidal effects of metaldehyde were discovered by accident in the 1930's in South Africa. Metaldehyde was used as camp stove fuel. It was noted that slugs died after consuming the material and word soon spread through gardening circles in Europe about the effect. The mode of action with metaldehyde is destruction of the mucus production abilities of the slugs reducing mobility and digestion. Metaldehyde is destroyed by sunlight and water. Irrigation and rain favor conditions that allow fungal growth or physical breakdown of the bait reducing slug feeding. When mixed with wet soil, 25% is destroyed in 2 hours and 50% is destroyed in 24 hours. Breakdown is faster in alkaline than acidic soil. Baits are distasteful to slugs at the levels of active ingredient required for a good kill and sublethal effects and subsequent bait aversion are common. Work by Glenn Fisher and colleagues at Oregon State University has shown bait with carbaryl alone does not control the gray garden slug as effectively as bait with metaldehyde alone, or bait with metaldehyde and carbaryl. Both carbaryl and methiocarb in a bait greatly reduces populations of earthworms and predatory beetles. One of the newest molluscicides on the block is Sluggo, of which the active ingredient is iron phosphate. In Fisher's slug control trials in grass seed crops, Sluggo performed statistically comparable with metaldehyde.

Whatever the choice of poison, coordinating baiting with the slug life cycle can achieve more effective results. According to Fisher, the best time to bait in non-irrigated cropping systems of western Oregon is in late September or early October, as the first rains occur. Slug activity increases as the cool temperatures and moist nights stimulate them to mate and lay eggs. This timing is key to disrupting the life cycle. Apply baits before the eggs are laid if possible. The eggs that were laid should hatch in 2-4 weeks and a second application should help control the newly hatched slugs. Small compact pellets are probably better than the larger and loosely compacted pellets after rains start in the fall. Timing is not as fixed in irrigated systems as slugs don't hide underground as much. Given the low thresholds for damage in ornamental crops, late winter/early spring applications may be required once slug activity is noted.

Humans have done much for the success of many of the very slugs we battle. Our dispersal of exotic slug species throughout the world continues at an alarming rate. We will likely have more opportunity to become familiar with these slippery horticultural foes.

There are 2000 species of pulmonate slugs and snails west of the Rockies with a conservative estimate of 23 different slug species in the Olympic Peninsula. There are thought to be 10 species of pest slugs in Oregon. All but the marsh slug are exotic imports. Some of the most problematic slug families in the Pacific Northwest include: Arionidae with the European red slug, Arion rufus, and the grey garden slug, A. circumscriptus. Limacidae including the great gray garden slug or tiger slug, Limax maximus and the tawny garden slug, L. flavus; Agriolimacidae which includes the field slug or milky slug, Deroceras reticulatum and D. laevae, the marsh slug; and Milacidae which includes the greenhouse slug, Milax gagetes.

European red slug
The European red slug, Arion rufus L., is common to sites in the Pacific Northwest. It can be distinguished from other slugs by the placement of the pneumostome before the mid-point of the mantle; a back without a keel; and heavily wrinkled skin. It is generally 70-100 mm in length and variable in color ranging from reddish, orange, brown or black. The foot fringe is mostly red with stripes. The optics tentacles are dark.

Great Gray Garden Slug, Tiger slug, Spotted leopard slug
Limax maximus is found commonly in West Coast gardens. It has tiger stripes on the mantle and upper surface of its foot, smooth, furrowless body, pneumostome placement located near the rear edge of the mantle. It's length is about 4 inches (10 cm). It is four times faster than banana slug which is useful as it is predatory on other slugs.

 

References:
Airey, W.J., I.F. Henderson, J. A. Pickett, G.C. Scott, J.W. Stephenson, and C.M Woodcock. Novel Chemical Approaches to Mullusc Control. In Henderson pp. 301-307.

Baker, G.H. 1989. Damage, Population Dynamics, Movement and Control of Pest Helicid Snails in Southern Autstralia. In Henderson. pp. 175-185.

Cook, R. B.J. Thomas, and K.A. Mizen. 1989. Dissemination of white clover mosaic virus and stem nematode, Ditylenchus dipsaci, by the slug Deroceras reticulatum. In Slugs and Snails in World Agriculture. Ed. Ian Henderson. British Crop Protection Council.

Crowel, H.H. 1977. Chemical Control of Terrestrial Slugs and Snails. Bull. No. 628. Agricultural Experiment Station. Oregon State University, Corvallis.

Crowel, H.H., B.C. Simko, J. Capizzi, and J.D. DeAngelis. 1991. How to Control Slugs. FS 277 Oregon State University Extension Service.

Dreves, Amy, Nicole Anderson, and Glenn Fisher. 2014. Slug Control. PNW Insect Handbook.

Duval, A. and G. Banville. 1989. Ecology of Deroceras reticulatum (mull.)(Stlylommatophora, limacidae) in Quebec Strawberry Fields. In Henderson. pp . 147-160.

Fisher, T.W., RI. Moore, E.F. Legner, R.E. Orth. 1976. Ocypus olens, a Predator of the Brown Garden Snail, Helix aspersa. California Agriculture. Vol 30, no. 3, 20-21.

Gordon, David George. 1994. Field Guide to the Slug. Western Society of Malacologists, Sasquatch Books, Seattle, WA. 48 pp.

Graveland, J. and R. van der Wal. 1996. Decline in Snail Abundance Due to Soil Acidification Causes Eggshell Defects in Forest Passerines. Oecologia Vol 105, 351-360.

Harper, Alice Bryant. 1988. The Banana Slug. Bay Leaves Press. Aptos, CA. 32 pp.

Kelley, J.R. and T.J. Martin. 1989. Twenty-one years experience with methiocarb bait. 1989. In: Henderson, pp. 131-145.

Terrestrial Gastropods of the Columbia Basin, British Columbia This is a great site for those in the Pacific Northwest to see what has been found in surveys. The site includes a key (slug portion of the key) and species accounts.

Pestline. 1991. Pestline: Material Safety Data Sheets for Pesticides and Related Chemicals. Van Nostrand/Reinhold, New York.

Purvis, G. and J.W. Bannon. 1992. Non-target effects of repeated methiocarb slug pettet application on carabid beetle (Coleopter:Carabidae) activity in winter-sown cereals. Ann. Appl. Biol. 121:401-422.

Quarles, William. 1997. Slugs and Snails in Gardens and Fields. Common Sense Pest Control. Vol. 13, no. 1, 5-15.

Savonen, Carol. 1997. Slug Wars. Oregon's Agricultural Progress. Vol. 43, no. 2, 16-21.

http://home.nc.rr.com/slugs/index.html

South, A. 1992. Terrestrial Slugs: Biology and Control. Chapman and Hall, London. ISBN: 0-412-36810-2. 428pp.

 

Slug courtship (video clip - warning large file)
slug courtship
Photo: Rosetta
Website editor:
Robin Rosetta
Page last modified 6/29/05
 

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