Introduction

Scott Strain (alias Daffy Duck) sent this message to OLD: "Help-Slug Attack! I am having a severe slug invasion. Anyone know a good way to get rid of them? I grow my orchids in lava rock so the slimy balls hide down in the rock. So far, they are only eating the Phal. leaves but they hide out in any of the pots (Dendrobium, Vanda, Oncidium, etc.). I have tried some commercial slug bait but it does not seem to be of much help [ 1]".

This note compiles the seventeen responses received in OLD from May 24 to June 13, 1997, to this "orchids in distress" call. Responses given similar "extermination techniques" were placed together to facilitate the judgements between them. Then, answers are not necessarily listed in the order they were sent to OLD. Also, different parts of one entry may be cited under various subjects of this note, depending on its contents. A few entries have been selected to prepare the Remarks section at the end of this article giving instructions on how to take care personally of the slugs .

Let's now take a look at the slug control methods sent by these OLDers.

1. Diatomaceous earth (DE)

From Lauderhill, Florida, Sal Cherch sent the first response on how to deal with the slug problem: "Try sprinkling some DE on the plants and around the growing area. It was recommended to me and worked[2]". Natural grades of DE are made of the fossilized remains of one-celled algae, known as diatoms, that were common inhabitants of prehistoric seas several million years ago. It looks and feels like a fine talcum powder, but to an insect it is a lethal dust that both scratches and absorbs the wax layer on the bug's surface, leaving it to die from dehydration[25].

A member of the San Francisco Orchid Society, Assana Fard, agreed with the use of DE to control slugs. The product, Celatom Diatomite (a chemically treated, swimming pool filter grade DE), was suggested to her by someone at OSH (Orchard Hardware Supply) to control slug infestations: "It's worked great for me and my neighbors. We've beenusing it for the last couple of months. We just sprinkle the powder (wearing gloves) over the flowerbeds and flowers and it's good till it gets all washed away (about a few weeks) and then we reapply". However, she recommended a combination of methods to get a more effective control: "I've come to the conclusion that no one method works all the way all the time. I use the beer method (see below) periodically and the combination of the two has saved all my plants [14]".

Other materials with similar dehydrating properties as the DE -like wood ashes and gypsum- also seem to dissuade slugs from crossing an area sprinkled with them[19].

2. Slug Pubs

Setting out shallow pans filled with stale beer or any fermenting liquid for this matter (yeast and water, spoiled yogurt), buried with the lip flush with the soil surface, is one of the oldest methods of protecting plants from slugs. They cannot resist the scent and they will be found drowned the next morning. The beer method is the "die happy" slug solution suggested by Jessica who wrote: "I have heard, but never tried, that uttering out dishes of beer works great. The slugs are supposedly unable to resist the scent and end up drowing in it. If you try it , please, let me know if it works since it seems nicer than chemicals [ 3]". Sandra, the "Elegant Bee", answered Jessica's question: "Yes, putting out little dishes of beer does help get rid of slugs. You have to set them into the earth, or low enough so the slugs can get into them easily. Sprinkling salt on them also destroys them quickly, they dehydrate immediately (not a pretty sight) but it works [4]".

3. Mechanical Barriers

Donna M. Ullian, now a member of the Triangle Orchid Society, North Carolina, but still with many fond memories of her old orchid society in Atlanta, suggested: "I recently was at Chadwick and Son Orchids, outside Richmond, Virginia. Art uses all "natural pesticides" in his greenhouses. He came up with a solution to slugs ... copper flashing. Secure copper flashing to your benches (bottom of upper ledge). It works, he swears the slugs do not like copper. They crawl up and do an about-face when them come up to the copper flashing. One drawback: it is a bit expensive but your collection is worth more! I use it on my benches and I do not have a slug problem [6]". A similar approach is also indicated by Charles Sowle Price, Plant Chair of Northern Nevada Orchid Society: "I have wrapped the legs of my wood benches with bare copper wire. No more slugs! [5]".

J.L. Spitzer was experimenting with the effectiveness of mechanical barriers: "One experiment involves copper tape around the edges of the pots, another has copper pennies stuck to the pot rims, and the third has all the pots that are outside sitting on racks of stumps covered with heavy duty aluminum foil. So far, no signs of slug damage, in spite of our very wet weather [18]". She also tried thin copper foil (the one that comes in rolls from stained glass supplies) and wrapped it around the edges of her pots: "Result: all of the untreated pots were invaded at one time or another, usually after every rainstorm, by slugs. But NONE of the treated pots were bothered at all [24]".

Copper is very toxic to slugs and snails. Some studies indicated that copper is effective because slugs and snails actually get an electric shock when they touch it. It's theorized that the slug's slimy coating interacts chemically with the copper, creating an electric current. To securely protect plants on greenhouse benches by placing copper strips around bench legs, be sure that the benches don't touch the greenhouse wall or glazing. Securing strips of copper as a permanent edging for greenhouse tables is an effective but expensive way to keep slugs at bay [25].

Aluminum sheets, bent in U-shapes with the arms of the U "large enough", are, purportedly, a good remedy since slugs may not crawl the horizontal overheads of this barrier [19].

4. Heat Method

For those who grow organic, like Bod Dudley from Lehigh Valley, Philadelphia, getting rid of slugs without employing commercial slug baits is very hard. He prefers potting material that has been steam treated or, since it is difficult to find this material nowadays, he sterilizes small batches of medium at a time (Caution: it does smell!). He recommended to Scott this solution: "Slug and snails eggs are in the potting material and these materials should be heat treated to kill the eggs. The plants and pots should be inspected every time they are watered and if there is a suspicion of snail and slug damage the plant should be unpotted [7]". Bob also indicated another "interpretation" on how the beer method should work, which is explained in Remarks.

5. Metaldehyde

"Unfortunately, the only effective control that I have seen is metaldehyde [8]". This observation, first made by Eric Muehlbauer while criticizing some of the other methods (see Critics), soon received more support:

a) From Willis Dair: "I would have to agree. I like Deadline and Deadline type products. They are liquid metaldehyde. We use it all over the garden [10]".

b) From Alan Rossing, newsletter editor of the Carmel Orchid Society: "I have found the best is Deadline or its equivalent. Cut up a sheet of writing paper into 2" squares and apply a small amount of Deadline to the paper. Then, place these around the pots on the bench and on top of the mix. It will dry out but still be effective, plus every time you water the Deadline is reactivated just when the critters are coming out. If you can find paper that they like to eat, even better, as that will attract them [11]".

c) From Troy C. Meyers, living in Poulsbo, Washington: "Like Eric Muehlbauer and Willis Dair, the only really successful slug control that I have found is the liquid metaldehyde such as Deadline. I do use liquid metaldehyde in "traps" in my greenhouse, which are covered trays with open sides, allowing slug passage but not dilution of the bait by falling water [9]". Troy, who lives near woods in the Evergreen State, also contributed with a personalized way to eliminate slugs from his surroundings but, unfortunately, he's never yet won a battle against them (See Remarks).

d) From Harry Tolen, newsletter editor of the San Diego County Orchid Society: "Best all around slug and snail bait is metaldehyde granules. Looks like sand, strength is 7.5% and it works! After the second distribution you will not have a slug or snail problem. I get it from the local wholesale nursery supply [12]". Harry, with the experience of managing large collections of different orchid species for many years, also explained why the beer method didn't work for him: "I also have been told about putting out saucers of stale beer to attract and drown slugs. It will also drown roaches who have the habit! But, I never have had any beer go stale around here so have never been able to try".

e) From Kay Little: "Tried the beer, tried the bait and nothing worked, especially in the Phals. OFE (a USA horticultural supply company - Ed.) suggested Slug-Fest. It's a liquid concentrate that's fairly expensive, however, it works! I haven't seen a slug or snail anywhere [13]".

6. Cupric Hydroxide

The original entry sent by Charles Sowle Price, Northern Nevada Orchid Society, also indicated this other product to combat slugs: "I have had good luck with Kocide 101 (Cupric Hydroxide) available from Hummert International or E.C. Geiger (USA suppliers - Ed.)[5]".

7. Predatory Snails

Predatory decollate snails (Rumina decollata) have a varied menu to choose from: brown garden snails (bush snails), common slugs, decaying vegetable matter, and, if those food sources are scarce, living seedlings and transplants. Therefore, there is a potential risk that these snails may become a serious pest in their own right, particularly, because there are no natural predators that eat them. These snails are used very successfully in commercial citrus groves in California where they do no harm to established trees and provide excellent control of the brown snails[25]. Brown snails, although commonly not a serious pest for orchids since they feed mostly on algae and mosses, they are difficult to eradicate. They are not attracted to slug bait and metaldehyde liquid products do not seem to affect them. Predatory snails might be an effective way to control them and, as a bonus, common slugs may also be included as part of their normal diet.

A commercial supplier in Montana, Planet Natural, offers decollate snails to control slugs and common garden snails. Eric Muehlbauer decided to try them to protect his orchids from slugs and he provided us with some information in this sense: "According to the catalog, they can eventually achieve 100% reduction of pest snails (bush snails) and 50% reduction of slugs. I would assume that the lower percentage for slugs is based on size. These snails (Rumina decollata) are about 3/4 inch, none more than 1 inch. I would assume that they must do a good job on small slugs, but are unable to eat large slugs. The snails follow the slime trails produced by their prey, track them down and eat them. It's too soon for me to be talking from experience. All my information has come second hand, from the catalogue. By the way, these snails are banned in parts of California, and perhaps other areas of the West coast, as they are considered a threat to native species[20]".

8. Ducks and Rent-a-Duck Services

The initial entry recommending ducks to control slugs came from Max Redman, living in Sydney, the city of the 2000 Olympics, and it was as follows: "There is one thing that I have not seen anyone mention and, believe me, it really works. A couple of Indian Runner ducks! Don't laugh! A couple or more ducks running around will completely clear the place of slugs and snails and, as an added bonus, the eggs are pretty good also. The only problem is that one needs to have an area that can be fenced off as, otherwise, the ducks tend to disappear. Also, I don't know how they would go if you grow in a basement. Of course, in that case you may not have any problems. Friends of mine grew something like 5000 cymbids as well as other genera and after putting in the ducks they never had any more problems. By the way, the plants were all on benches but the ducks did not attack the plants at all. Try it, you may like it, and it also gives the ducks a chance [15]".

Max's suggestion received the support of Manfred Schmucker, newsletter editor of the Vienna Orchid Society, Austria. He sent this entry based on his personal observations of some of the ducks in a friend's garden: "I can only support the message of Max Redman on the Indian Runner ducks. A friend here in Austria has some of them in his garden and no more slugs since then! They are outside all year round, even in snow and frost. They have a little dog-house as a home and a submerged old bathtub as a pool and receive in the winter time some additional food (no slugs). Highly recommended [16]".

An upgrade in the use of ducks to control the slug population was also contributed by Manfred: "The interesting thing is that this summer the concept of slug control by ducks went out of hiding and the first rent-a-duck service by breeders has appeared in Northern Austria. The method for duck renting described (from newspaper features and personal communications) is as follows: for an amount of approx. US$10 per week you can rent a pair of ducks, including safety cage and some corn fodder. One to two weeks is recommended to get the slug population down. The safety cage is provided to protect the animals during travel and, mainly, during the night from stray cats, minks and other predatory-minded animals in the neighborhood. As far as I know, the customer has to fetch and bring back the ducks. So far, I have not talked to anybody who has actually tried the service. I only know one owner, a friend in Tyrol, who has been happy with them for more than two years [21]".

Critics

Voices of discontents with the effectiveness showed by some of the above methods soon appeared:

1. "I have yet to find an organic method of slug control that actually works. I have always tried beer traps in the garden (Why waste good beer? Yeast and water will do the same job). I find plenty of slugs dead in the beer, and plenty more alive and well beneath the container. And lots more alive and well and very well fed on the plants they stopped to munch on along the way! As for DE, it is totally useless. I have seen slugs completely devour lettuce and pepper plants that were literally dusted all over with it, and before rain or watering washed it away. It doesn't seem to work on insects, either. I had a student do an experiment using DE on both crickets and mealworms. Neither group, experimental or control, had any fatalities[8]".

2. "Beer does indeed attract and drown some slugs, but I'm afraid many just slide home crooked. Copper wire and foil tends to repel slugs, but they are very clever about finding alternate routes, and some "stomach" actual contact with it, seemingly knowing that temporary discomfort is worth the goal. Furthermore, barriers such as copper will keep a slug in as well as out, so they may be inclined to take up residence in a pot [9]".

3. "I've tried the beer and I don't find it effective on my slugs. Have used slug baits, metaldehyde and pelleted stuff, with minimal effectiveness -it gets stale, washes away, etc- [18]".

4. The main concern for the use of "slug killers" containing metaldehyde is the risk of toxicity produced by accidental ingestion of this pesticide in domestic pets and, as the next entry explains, for wildlife animals too. Lets take a look at Troy C. Meyers observation: "I do use liquid metaldehyde in "traps" in my greenhouse, which are covered trays with open sides, allowing slug passage but not dilution of the bait by falling water. The traps also keep dogs and cats from getting the bait, which smells good to them but is very, very toxic. I heard a report on the news recently that stated that the last year over 500 dogs were poisoned in the Seattle area by metaldehyde. Until recently, I used the "traps" in the outdoor areas, but I had to quit. It is true that the traps prevent dogs and cats from getting into the bait, but usually the slugs, after eating their fill of bait, crawl out and lose muscular control near the traps. Also, the ones that die in and out of the traps are consumed by maggots, beetle larvae, sow bugs, and earwigs. With shock one day I saw a robin and a varied thrush eating both the maimed slugs and some of the other invertebrates just named. These birds were collecting food for their nestlings. I don't have much of a slug problem in the greenhouse, and only occasional trap-baiting eliminates any that sneak in. The slug corpses in the greenhouse don't seem to be devoured by the other critters, so I still feel comfortable using it there [9]".

5. The feasibility of the "Duck method" was challenged by Colin Hamilton, Publications Director of Orchids Australia. His main concerns were how to keep the ducks between the boundaries of your greenhouse and what to do with their "messy deposits" around the place[17]. According to Max Redman, one way to deal with preventing the ducks flying away from the area is by clipping their wings or having them in an enclosed orchid house. The most difficult part is to train them to do their job under the benches and not in the paths: "Apart from that, snails and slugs are gone, weeds are being kept down under the benches, and the eggs taste wonderful [22]".

Discussion

Going through all the answers it is very difficult to pick one as the best solution. All have their pros and cons. Commercial baits (granules/pellets) or liquid products -like Deadline - containing metaldehyde received the highest scores in effectiveness by some OLDers. The dark side of this approach is the risk of toxicity by accidental ingestion in domestic animals and, likely, an environmental concern too, when these products are used in large, open areas. When they are applied inside greenhouses and in a controlled way - like spraying Deadline on small pieces of paper, on top of the compost in potted orchids, or placing it inside a specially designed trap- the potential for risks is greatly reduced.

For those reluctant to use chemical products, a combination of two different techniques used periodically seems to offer a good level of protection -in example, the beer method and DE-. Still, there is an additional option open: the use of "biological weapons". In these battles, one species is against another species. In this scenario, our hopes are that the good guys, which in theory will swiftly eliminate the bad guys, will not develop themselves as future "enemies". However, except for the idea of using ducks as a slug control measure -very efficient although not necessarily a "clean" method-, other biological control approaches deserve a careful and very attentive look. Predatory snails could be an elegant way to reduce the slug population, but there is some concern about how safe they could be for the environment.

Conclusion

Scott Strain, who started the "slug attack" thread in OLD, pointed out that "my greenhouse now is slug free". He attributed this success to the use of Deadline, a liquid product containing metaldehyde. The product was put in the clay pots (a circle around plants) where slug infestation was suspected. Metaldehyde in granules presented a few inconvenients: "I managed to kill a few of my orchids as well as the slugs because it would clog up the pot in the lava rock and drown the roots. Sprinkling the granules on the bench didn't do much since the slugs were living in the pots down among the lava rocks[23]".

Remarks

These personal solutions to the slug problem deserve being considered in their own class. I left to the readers the jugdement regarding efficacy and practicallity:

Bob Dudley's crushing technique: "I have always had problems with slugs in my garden and I have tried the beer trick many times with little success. Then, one day I discovered the trick. You need some rubber gloves. First, you consume the beer. Several bottles of beer, in fact. After consuming the beer, you put the gloves on, hunt down the little beasts and crush them between your fingers! [7]

Troy Meyers' skewering: "Outdoors, to be kinder to our wildlife, I have been using a labor intensive, fairly disgusting method to battle the slugs. In my forest, there is one native slug (commonly called a "banana slug") that doesn't do much damage to plants; it feeds mostly on detritus. The other slugs, all imports, vastly outnumber the native slug. The imports have a "nasty" habit of eating each other, regardless of species. Generally, this cannibalism (not necessarily the right word in all cases) only occurs when a slug has been maimed. But, as soon as a slug is in distress, others troop over and devour it. This behaviour is probably really an excellent survival tactic because the slug-victim has spent hours and days collecting its food, and all that can be had in a few minutes by the eaters. Unfortunately for the slugs, it is an opportunity for me to wreak mass destruction. If I wound one slug (avoiding the natives), its distressed scent becomes a powerful attractant and, at least, five show up soon to eat it. I then skewer them also, and another twenty show up. The skewering continues until I can't stand it any more. The battle is never won by me, but the reduction in plant damage does seems to be noticeable[9]".

Max Redman's "big foot" solution: "I have been following the writings on the problem with slugs and snails with some interest and also, I must admit, a little laughter at times. They are nasty little things and no matter what you try there will be some that will normally escape. A size ten boot will work wonders if you can find them and I really liked Troy's method of shish kebab sticks [15]".

References

Orchid ListDigest:

[1] Daffy Duck v1047 #14700;
[2] Sal Cherch, v1050 # 14751;
[3] Jessica v1051 # 14774;
[4] ElegantBee, v1054 #14829;
[5] Charles Sowle Price, v1052 #14785;
[6] PrimaD22, v1057 # 14890;
[7] Bob Dudley, v1052 #14791;
[8] Eric Muehlbauer, v1053 # 14810;
[9] Troy C. Meyers, v1057 # 14889;
[10] Willis Dair, v1053, # 14810 footnote;
[11] Alanrr, v1057 #14882;
[12] Harry Tolen, v 1056 # 14867;
[13] Kay, v1053 # 14814;
[14] Assana Fard, v1057 #14886;
[15] Max Redman, v1061, # 14956;
[16] Manfred Schmucker, v1065, # 15013;
[17] Colin Hamilton, v1064, #15009;
[18] J.L. Spitzer, v1056, #14877.

Private email:

[19] 08/29/'97 Manfred Schmucker;
[20] 08/31/'97, 09/01/'97 Eric Muehlbauer;
[21] 08/27-28/'97 Manfred Schmucker;
[22] 08/27/97 Max Redman;
[23] 09/04/97 Scott Strain;
[24] 09/08/'97 J.L. Spitzer.

Other sources:

[25] Rodale's Chemical-Free Yard and Garden, 1991.