Chemical Control of Terrestrial Slugs and Snails

F1j Cc''-/ Chemical Control of Terrestrial Slugs and Snails Station Bulletin 628 July 1977 Agricultural Experiment Station Oregon State University, Corvallis CONTENTS Introduction 1 What are Terrestrial Molluscs? 2 History of Slug Control Studies at OSU Description of Testing Methods. 8 Rearing of Molluscs for Tests. Candidate Chemicals for Screening Tests and Tabulated Results. 11 Contact versus Oral Route of Toxicants. 12 Studies on Combination Baits. 13 Carbaryl-metaldehyde. 13 Methiocarb-metaldehyde and UC-30045-metaldehyde (Laboratory tests).14 Methiocarb-metaldehyde (Field comparisons). 14 Tests and Observations on Non-economic or Exotic Species. 15 Age-Related Differential Susceptibility to Molluscicides. 17 Field and Greenhouse Control of Slugs. 18 AUTHOR: H. H. Crowell, professor of Entomology, Department of Entomology, Oregon State University, Corvallis, 97331. For clarification, trade names have been used i:n this report. This is not to imply endorsement of products named or cr1tcism of those not included. ABSTRACT Many terrestrial molluscs (slugs and snails) are economically important because of their feeding activities on ornamental plants and vegetable and field crops. Studies of their habits, biology, and chemical control have been conducted by the Oregon Agricultural Experiment Station since 1911. This report discusses the common species of Oregon, their origins, and identification (including a key to the common species of western Oregon). The early history of chemical slug and snail control is reviewed with emphasis on studies at OSU during the l95Os. Research on chemical control since 1967 is covered in detail, including methods of raising the animals, annotated tables of chemicals explored, and methods for testing candidate materials. Special molluscicidal studies are discussed, including the relative value of baits containing more than one toxicant, laboratory and field comparisons of some leading toxicants, and the differential susceptibility to poisons according to the species and age of the molluscs. CHEMICAL CONTROL OF TERRESTRIAL SLUGS AND SNAILS H.H. Crowell INTRODUCTION The economic importance of land slugs and snails and the problems associated with their control have been discussed by various workers (Lovett and Black, 1920; Mead, 1961; Judge, 1969; Barry, 1969; Getzin and Cole, 1964; Thompson, 1942; Hunter, 1969, and others). Most species of these terrestrial gastropods (Phylum Mollusca) are omnivorous feeders -- leaves, stems, bulbs, tubers, fungi, lichens, algae, and animal matter of all kinds. Their feeding activities on leafy vegetables, green beans, strawberries, potatoes, leguminous crops, and ornamental plants in the Pacific Northwest are of greatest concern. The Oregon Agricultural Experiment Station recognized the importance of slugs to truck crops and ornamental plantings in the higher rainfall portions of the state by authorizing studies on life history and control as early as 1911 (Lovett and Black, 1920). Work done through 1919 resulted in recommending baits of chopped lettuce or other leafy vegetable, poisoned with calcium arsenate, the only effective toxicant available at that time which lent itself to bait formulation. Almost 15 years later, another chemical control substance was discovered -- this time by accident -- which remained the principal ingredient of poison baits for about four decades. Formed by the polymerization of four molecules of acetaldehyde,metaldehydeis a unique compound, highly insoluble in water, which has a rapid and violent effect on slugs and snails. By either ingestion or external contact, the material causes paralysis of the animal and stimulates the numerous mucus glands to produce great quantities of viscid slime. This strange reaction (no other animals are known to be similarly affected) was discovered about 1934 in South Africa when "Meta-FueU', a canned, solid fuel used to heat small domestic appliances, was left exposed outdoors overnight. The accumulation of paralyzed slugs and snails was noted the next morning and word of this new material for control of of these pests soon appeared in local gardening papersand magazines (Gimingham and Newton, 1937). The relationship between "Meta-Fuel"and metaldehyde was recognized and recommendations soonappeared for the use of metaldehyde-bran baits for slug and snail controlin home gardens. From about 1937 until 1973, there was no effectivesingle chemical available for slug and snail control except metaldehyde. The observation was also made soon after the discoveryof the activity of metaldehyde that control of molluscs exposedunder protected moist conditions was not as successful as when desiccatingconditions prevailed (Jary and Austin, 1937). Considerable study has been made of thisaspect of metaldehyde intoxication and the conclusions havegenerally been that, although metaldehyde is actually 'toxic" to land molluscs,the principal effect is the stimulation of the mucous* glands with thesubsequent loss of moisture to the animals through excessive sliming(Thomas, 1948; Craig and Vincent, 1952). Thus, under moist conditions, many animals arespared death through dehydration and are able to throw off the toxiceffects of the material in a few days. It is this ability of the molluscs tosurvive, metaldehyde intoxication under certain conditions that hasled, principally, to the search for new toxicants. The fact that toxicants considerably more active against land molluscs thanmetaldehyde do exist will be brought out in this report. WHAT ARE TERRESTRIAL SLUGS AND SNAILS? Slugs and snails are closely related molluscs,snails bearing the obvious distinction of an external shell large enoughto enclose the entire animal.' The ability to withdraw the soft bodyparts into a rigid Various proprietary slug/snail baits have containedin addition to metaldehyde, other toxicants, such as calcium arsenate,sodium fluosilicate, dieldrin or carbaryl. These materials were added either to enhancethe toxicity of metaldehyde or for their effects against otherpests such as cutworms, root weevils, sowbugs, and earwigs. * Theword "mucus" is a noun, referring to the slippery,viscous secretion from the "mucous" (adjective) glands. -2 shell is an advantage which allows the snail to survive under rather severe conditions of drought and heat. The large requirement for lime (CaCO3) for construction of the shell, on the other hand, tends to restrict snails to areas where the soil is rich in calcium. The land molluscs we refer to as "slugs" are usually without an external shell, and represent three families (on the West Coast) which evolved independently from three groups of snail ancestors (Janus, 1965). Besides economy in calcium, slugs have the advantage of being able toalter their form so as to slip through narrow places and to burrow into the ground in search of food or shelter (Morton, 1960). Lack of a protecting shell, however, leaves them vulnerable to desiccation if trapped in the open during the day. The moist climate west of the Cascade Mountains, plus the infrequent sources of calcium (lime), make much of the Pacific Northwest agood habitat for slugs and an inferior one for snails. With the exception of the slug genus Prophysaon (and probably the little marsh or gray field slug, Agriolimax laevis), all the land molluscs of economic importance as pests of crops and home gardens in the U.S. are introduced species, mostly from northern Europe. The most commonly destructive species is the gray garden slug, Agriolimax reticulatus (Muller) (referred to in most U.S. literature as Deroceras reticulatum), primarily because of its appetite for fresh vegetation and its ability to reproduce during almost any month of the year. There are estimated to be about 725 species of land snails and 40 species of slugs now recognized in the 48 contiguous states; of these, 44 species of snail and 16 species of slug are not native (Burch 1960; Getz and Chichester, 1971). (See Appendix I for key to common slug species of Oregon.) Only a few slug and snail species have been available in the field or have been laboratory reared for the chemical screening and other tests mentioned in this report. Common names for most of them have been approved and published by the Entomological Society of America (ESA), since these molluscs are often of concern to entomologists because of their econonic importance. Unfortunately, most of the common names are based more on long usage than on rationality. Many are cumbersome, descriptively inaccurate, or conflict with common names for the same species as used in other English-speaking countries. Since most of the species dealt with here have 3 more than one common name in general use, listed below is the one approved by the ESA, plus several others in use elsewhere. Species names of molluscs Common name Other common names used in the tests reported approved by in use on here the ESA Agriol imax reticulatus Gray Garden Slug Reticulated Slug; Mtiller Gray Field Slug (referred to in most U.S. Netted Slug literature as Deroceras Milky Slug reticulatum). Agriolimax laevis (Miller) Gray Field Slug Marsh Slug (referred to in most U.S. Brown Slug literature as Deroceras Smooth Slug laeve). Anon ater ater (L.) * European Black Large Black Slug Slug Black Slug Anon ater rufus (L.) * Large Red Slug Limax flavus (L.) Tawny Garden Slug Tawny Slug Cellar Slug Yellow Slug Limax maximus (L.) Spotted Garden Great Slug Slug Giant Garden SLug Great Gray Slug Prophysaon andersoni Reticulated Slug Netted Slug (J. G. Cooper) British Garden Slug Helix

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