Heim, I. U.

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Section v''Sectionname: Fol iage &xSeed InsectsBASICS OF THE MOLLUSCICIDE A.I. METALDEHYDEI. U. HeimTechnical Product Management META®Lonza Inc.17-17 Route 208Fair Lawn New Jersey 07410201/794-2490iheim(g).lonza-us.comMetaldehyde has been used inmolluscicide formulations for almost 70 years. Recent studieshave now revealed the basics ofits mode ofaction that explain why metaldehyde has a favorableenvironmental profile inrelation to alternative pesticides for use against slugs orsnails.Mode of actionMetaldehyde poisoning is characterized by excess mucus secretion. This is the first symptom ofpoisoning. Physiologically, this is a detoxifying reaction bythe animal in order to flush thetoxicant out of itsbody. On the histological level, metaldehyde damages membranes andorganells ofthe mucus producing cells found in the foot, digestive tract and dermal layers thatallow locomotion, digestion and protection (Triebskorn et al, 1998).The consequences are manifold: The excess mucus production weakens the slug while depletingenergy reserves. Feeding ceases rapidly and the slug is immobilized. The damage onthe mucuscells is irreversible and lethal. Even ambient water (dew orrain) cannot reverse the effect asmetaldehyde does not cause dehydration but complete destruction ofthe mucus producing tissue.Dehydration occurs as a secondary effect when the slugis immobilized andunable to seekshelter in order to hide from sunlight or drying winds.This specific mode ofaction on the mucus producing cells explains the low effect on non-targetanimals such as carabid beetles (Samsoe-Petersen et al, 1992) that are predator on slugs andeggs and earthworms Bieri et al, 1989). Metaldehyde is rapidly broken down into acetaldehydethat enters into the Krebs cycle providing metabolic energy. This explains why metaldehydepoisoned slugs do not pose arisk to small mammals or birds that feed on them occasionally. Inthe soil metaldehyde is fully decomposed by microorganisms to C02 and H20.The observation ofthe first symptoms ofpoisoning (excess mucus secretion) has led to thehypothesis that metaldehyde does not kill directly but anesthetizes and leads to dehydration and,therefore, is characterized bylack ofefficacy under wet conditions. Triebskorn etal. and earlier143studies havedisproved this hypothesis. In addition, the use ofmetaldehyde to control aquaticsnails proves its molluscicidal ability evenin immersed conditions (Litsinger andEstano, 1993).The specific actionon themucus producing cells together witha total decomposition to C02andH20 result in the favorable environmentalprofile ofmetaldehyde as a molluscicide.Relevant factors for a good product efficacySince it has been shownthat metaldehyde is toxic to slugsor snails in principleandjust not anirritating agent it is important to identifyfurther factors determining the efficacyof a treatment.The active substance is not being applied on the target pest directly but via a bait. First ofall thebait preserves the active ingredient and secondit provides a vector to the target pest animal. Thepart in this system crucial for a good efficacy is the carrier material used in the bait. It shouldmatch the usual diet of the slug.Since slugs forage mainlyon livingplantmaterial an organic carriermaterial of high nutritionalcontentshoulddominate the composition of the bait. Further, humanand animal repellents,preservatives and agents stabilizingthe pellet should not reduce the palatabilityof theformulation to the slug and snail. Recovery of slugs observed afterbaiting is due to the ingestionof a sub-lethal dose of the active ingredient. This happens independently from the type ofactiveingredientand is likely in the case of baits of poor quality that contain cheap carrier material oflow palatability to slugs or snails.Slugs and snails forage on a randombasis and are able to locate a food sourcewithin only verylimited distances of2 to 3 inches (Teyke, 1995). It is crucial to avoid a failurewhile treating toprovide enough baiting points to allow the slugs to hit a bait rapidly. A bait density of 4 to 5 persq ft is suggested to be the minimum.A lower density will reduce efficacy over proportionally.On the other hand re-applying the full rate after 2 weeks enhances efficacy over time whilstdoubling the rate for one application will not give a better plant protection.References:BIERI M, SCHWEIZER H, CHRISTENSEN K, DANIEL O (1989) The effect ofmetaldehyde and methiocarb slugpellets on Lumbricus terrestris L. Slugs and snails in world agriculture BCPC Monograph 41:237-44LITSINGER JA, ESTANO DB (1993) Management of the golden apple snail Pomacea canaliculata L. inrice. Crop Protection 12:363-70SAMSOE-PETERSEN L, BIERIM, BUCHSW (1992) Interpretationof laboratorymeasured effects of slug pelletson soil dwelling invertebrates. Aspects ofApplied Biology 31:87-96T. TEYKE (1995) Food-attraction conditioning in the snail,Helixpomatia. Journal ofComp Physiol A 177:409-414TRIEBSKORN R, CHRISTENSEN K and HEIMIU (1998) Effects of orally and dermally applied metaldehyde onmucus cells of slugs (Deroceras reticulatum) depending on temperature and duration of exposure. Journal ofMolluscan Studies 64:467-87\kk

Basics of the Molluscicide A. I. Metaldehyde

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Basics of the Molluscicide A. I. Metaldehyde

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