Malacological Society of London

The British Crop Protection Council's 66th Symposium, in conjunction with the Association of Applied Biologists and The Malacological Society of London, held at the University of Kent at Canterbury, 24-26 September 1996.

Slug and snail control is not confined to a wet field of winter cereal or potatoes on an icy March morning: the problems include Helix aspersa in Californian citrus orchards, the golden apple snail, Pomacea canaliculata, (The Society's logo) which has become a pest in rice paddies in the Philippines, and Mediterranean "white snails" which clog harvesters and reduce the value of grain in Australia. 160 delegates from 16 countries, representing Universities, Research Institutes and Agrochemical Companies, took part, closely matching the number taking part in the first BCPC Symposium on the subject held in Guildford in 1989.

In organising the Symposium, Ian Henderson, of IACR-Rothamsted, resoundingly refuted mischievous suggestions that the pace of the research matched that of the animals studied. Compared to the 1989 conference, this one gave notable regard to environmental protection. It was clear that slugs and snails themselves had also responded to environmental fashions in farming - conservation tillage, set-aside, conservation strips and the ban on straw burning. There was an emphasis on semiochemicals, especially repellants, rather than molluscicides; on biological control by beetles, nematodes, flies, predatory snails or ducks; on cultural control with wheat cultivars of different susceptibility or fundamental work towards transgenic plants containing slug antifeedants, and the identification of slug enzyme systems as potential target. The last of the eight sessions was on integrated pest management, using chemical, biological and cultural techniques in a compatible and environmentally responsible manner. Yet despite perhaps £20m spent on research, Trevor Lewis, the former Director of IACR-Rothamsted, said in his summing up, two chemicals remained the backbone of control. Some promising ideas deserved a closer look. Progress had been pretty good, but patchy, and he urged all delegates to focus their future efforts.

Incautious use of biocontrols has been ecologically disastrous, and in the opening address, Bryan Clarke (Nottingham University) picked the subtitle of the identification and taxonomy session, "Know your enemy", to provide his first salvo. Many slugs and snails are not enemies, yet many are now endangered and we would be impoverished by their removal. The processes of speciation in Partula snails of The Society Islands, particularly Moorea, have provided insights into evolutionary processes that rank with those provided by Darwin's finches and the Hawaiian Drosophila spp. Yet they have been driven to extinction in the wild by a misconceived attempt at biocontrol of Achatina after it was introduced for food. Despite advice, the carnivorous snail Euglandina rosea was introduced and proceeded to devour the native Partula spp. without controlling Achatina. Captive-bred Partula have been re-introduced to the island, protected by electric fences, and although these sometimes fail, they are a gesture.

Starting at lunchtime on Tuesday and ending at lunchtime on Thursday, the symposium packed in eight 90 minute sessions plus a reception and poster session on the first evening and a conference dinner on the second. The poster session, organised by Andy Evans (SAC, Edinburgh) was held alongside the Trade Exhibition, with interesting displays from Bayer plc, BCPC Publications, Chiltern Farm Chemicals, De Sangosse UK, Microbio Ltd and Rhône-Poulenc Agriculture Ltd. A display of live slugs, Pomacea canaliculata and a multimedia key to slugs, was organised by Tony Cook (Ulster) and Les Noble (Aberdeen). The smooth running of the Symposium was due to careful planning sessions between BCPC, the Symposium chairman and session organisers. Detailed guidelines for presenting typescripts and slides, slide previewing facilities, and a ban on OHPs, all encouraged well thought-out presentations, and helped all sessions to run to time, with adequate time for questions, and only a single non-appearance. Simultaneous French translation was available, and the full proceedings were available on registration, thanks to the use of camera ready copy and efficient printing. Generous financial support from Bayer AG, Leverkusen, kept costs down, and the £40 registration fee for students was only £5 more than the cost of the proceedings.

The Proceedings of the 66th BCPC Symposium (xiv + 450 pp., ISBN 0 948404 96 5) are available at £35 incl p+p within the EU, £37 elsewhere, from BCPC Publication Sales, Bear Farm, Binfield, Bracknell, Berks RG42 5QU, UK (Tel: 0118 934 2727; Fax: 0118 934 1998).

The role of molecular taxonomy was illustrated by David Rollinson (Natural History Museum) in work to identify the aquatic snails, Bulinus. Some of the 37 species of Bulinus in Africa are intermediate hosts for human blood flukes, and many species have flourished as a result of water development projects. In order to understand the spread of schistosomiasis, reliable identification is needed, so molecular techniques of DNA analysis and enzyme electrophoresis have been added to those of shell morphometry, chromosome number and soft part anatomy. Three techniques in particular have provided useful markers:

variation in ribosomal genes (rRNA), determined by restriction fragment length polymorphism (RFLP) analysis and polymerase chain reaction (PCR) RFLP of the internal transcribed spacer (ITS),
randomly amplified polymorphic DNA (RAPDs),
analysis of mitochondrial DNA.

Molecular techniques are simple and robust, and require minute quantities of tissue. DNA can often be extracted from museum material in ethanol. Good laboratory facilities are needed and the reagents are costly, but, apart from development work, the facilities should be available to most workers.

Kurt Jordaens presented electrophoretic studies from Antwerp and the Royal Belgian Institute on the land slug subgenus Carinarion (Arion fasciatus, A. circumscriptus and A. silvaticus). The species reproduce uniparentally, probably by self-fertilization rather than by parthenogenesis, but the occurence of spermatophores and heterozygotes suggests outcrosses can occur. Colour differences between species may reflect a fixation of different alleles resulting from selfing, rather than specific differences. Thierry Backeljau (Royal Belgian Institute) presented work from Belgium and Spain using protein polyacrylamide gel electrophoresis (PAGE) to identify Arion species. A. hortensis, A. distinctus and A. owenii are biological species; A. intermedius is a selfing species (although spermatophores occur in Spanish populations) and A. pascalianus is just a dark morph of this species. The species status of the A. fasciatus complex in NW Europe is doubtful - they may be self-fertile homozygous strains. Arion subfuscus may be more than one species, and several closely related species have been described recently from the Iberian peninsula. Heike Reise (Gorlitz Museum) presented PAGE evidence that Deroceras juranum and D. rodnae are genetically extremely similar and probably conspecific.

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Organiser: Jonathan Oakley (Agricultural Development and Advisory Service, Bridgets)

Molluscicide usage has increased 70-fold since the early 1970s, David Garthwaite (MAFF Central Science Laboratory) revealed: 4,800 tonnes (250 tonnes of active ingredient) are applied in Great Britain each year at a cost of nearly £10m. Metaldehyde accounts for 55% of the area treated, and Ignaz Heim, of Lonza, Basel, the principal manufacturer of metaldehyde, described the interactions in product development and new applications between manufacturers, research institutes, customers, registration authorities, the press and concerned groups. Spanish farmers spend £5m each year on molluscicides, and José Castillejo, Ines Seijas & Federico Villoch (Santiago) are to try denaturing slug eggs with tannins from the bark of trees. Peter Davis (Agriculture Western Australia) described a new copper silicate spray for use against Helix aspersa in vineyards and citrus orchards: trials indicate immobilisation and repellency rather than mortality.

Ron Hammond (Ohio) reviewed the impact of conservation tillage practices in midwest US on slug damage to maize and soybean. In conservation tillage, 30% or more of the ground is covered with crop residue: the practice reduces labour, machinery and fuel costs, reduces soil erosion and compaction, improves soil organic matter, tilth, moisture, and water infiltration, and provides resources for wildlife. Deroceras reticulatum, D. laeve, Arion subfuscus and A. fasciatus cause severe defoliation and stunting of maize, and stand reduction of soybean. Conventional tilling has been the main cultural method of control; carbamate molluscicides are not permitted and metaldehyde has given poor control. Before IPM techniques can be used, economic injury thresholds need to be determined. In Norway, D. reticulatum, the dominant agricultural slug,can obliterate spring barley , Arild Andersen (Norwegian Crop Research Institute) said. A. lusitanicus has spread dramatically since it first occurred in 1988, and Helix aspersa has become established locally after escaping from snail farms. In SW France, slug damage to sunflowers has been observed, but less often than expected (Yves Ballanger and L Champolivier, CETIOM). Tom Kennedy (Teagasc, Ireland) reported on treatment of slug damage to seedling beet, which is often accompanied by and confused with leatherjacket damage.

In a session devoted to aspects of molluscan biology which might lead to novel treatments, Rita Triebskorn reviewed work at Tubingen and IACR-Rothamsted on the responses of slugs to molluscicides and heavy metal pollutants. X-ray studies show molluscicides remain in the foregut and crop, where light and electron microscopy reveal pathological damage. Autoradiography shows rapid absorption of radiolabelled molluscicides and transport to digestive gland, stomach and skin. Storage of pollutants in the digestive gland makes slugs useful indicators of environmental pollution. Mucus production and mucopolysaccharide synthesis in the skin, and levels of the stress protein hsp70 increase in slugs treated with molluscicides, heavy metals or PCP. John Pickett described electrophysiological studies of potential repellant seed-dressings carried out by colleagues at IACR-Rothamsted and Portsmouth: the olfactory epithelium of the optic tentacles responds to repellant or antifeedant semiochemicals extracted from scented geranium leaf. A related chemical, (+)fenchone, showed repellant activity and caused bursts of activity in the olfactory nerve, whereas the - enantiomer was almost inactive. Extracts of the beetle Pterostichus melanarius, a slug predator, produced a strong discharge, while extract of the phytophagous beetle Zabrus tenebriodes was ineffective. Volatiles from hemlock and chervil produced intense nervous discharges in the olfactory nerve of slugs, and also caused marked feeding reduction - the active components in chervil were (+)fenchone and 4-allylanisole: that in hemlock is a new discovery (Catherine Dodds, IACR-Rothamsted). Craig Kelly (Huntingdon Life Sciences Ltd.) et al. described work at Manchester on Deroceras reticulatum, aimed at using slow release pH-sensitive formulations to overcome the repellancy of molluscicides and increase the amount eaten. In the presence of food, the pH ranges from 5.7 in the crop to 7.9 in the late intestine. Interference with feeding could be a novel control tactic, and led Tim Wright et al.(Lancaster and CSL-MAFF) to investigate the pharmacology of the slug buccal mass: ACh, GTP and 5-HT stimulated contraction, FMRFamide was without effect. Karl Baumforth et al. (Wolverhampton) described several uniquely molluscan features of peroxide-generating oxidases, which may assist in the search for a molluscicide with little impact on non-molluscs. Matthew Bradley & Norman Runham's (Bangor) study of cellular pathology of copper, zinc and manganese in farmed Helix aspersa shows that copper ruptures epidermal cells of the foot, mantle and lung, causing blood seepage; zinc causes necrosis of digestive gland and disruption of spermatogenesis; manganese also transforms the basophil cells of the digestive gland. Fiona Graham et al. report that heavy infestation of H. aspersa lung by the mite Riccardoella limacum slows growth and delays maturation. Invertebrates possess several classes of antibody-like factors, and Emiko Furuta (Dokkyo Medical School) described the internal defense system of a slug, Incilaria, against invading microorganisms. Injected materials ranging from sheep blood cells to latex beads were phagocytosed or encapsulated by type I (macrophage-like) cells, and yeast particles injected into the haemolymph, increased the number of circulating blood cells within 1 h. Vindhya Mendis (Cardiff) described how specific monoclonal antibodies were developed against the eggs and young of D. reticulatum and Arion ater, as diagnostic probes to test the gut contents of potential natural slug-egg predators.

Not ploughing in trash, leaving cuttings and production of a 'green cover' all potentially improve the environment for slugs, and Andrew Bolton et al. (Leeds) studied the effects of different methods of rotational set-aside on slug numbers and dispersal. Slug numbers were initially highest on uncultivated plots, but after establishment of vegetative cover were highest in ryegrass plots. Dispersion was random. Geoff Baker (CSIRO, South Australia) reported on the population dynamics of the introduced mediterranean helicid Cernuella virgata. Aestivating snails in the barley heads or legume pods clog harvesting machinery or lead to rejection when contaminated grain is delivered to the silos. In fields managed as pasture or cereals in alternate years, snails were more abundant in spring, particularly in pastures and after a wet autumn, and the fecundity was inversely related to adult density. Jane Ward-Booth et al. (Christchurch College Canterbury) reported that malacofaunal diversity and abundance in farmland was higher in ditches than flat areas, but except on chalk, woods had fewer species than crops, and cropping, especially on sandy soils, has a significant effect on the numbers of species and individuals. Andy Young (Newcastle) concluded that a refuge trap consisting of a cheap hardboard square baited with layers mash poultry food recorded higher numbers of slugs than other traps tested. Mertxes Ortega, Jesus Txurruka & Altzua's (Bilbao) paper and poster described a shift from somatic tissue to ovotestis development in Arion ater: at first all excess resources are devoted to growth and then are switched to reproduction. Starch was the most important factor in food for growth, and growth of female genitalia is partially met from body wall reserves.

Susan Haynes et al.(Newcastle) showed that in rough grassland, all size classes of Deroceras reticulatum are present throughout the year. Brigitte Grimm (Austria) described transponder implant studies on movements of A. lusitanicus.

Trevor Martin's session covered the efficacy of methiocarb and metaldehyde pellets, and effects on non-target organisms. James Coupland ("Helix Consulting") found that 5-6% metaldehyde baits were most effective against the Mediterranean "white snails" which have become serious pests of crops and pastures in Australia, but even 1% effectively stopped snails feeding on carrot. Cochlicella acuta was very resistant compared to the other two species tested.Richard Meredith (Bayer plc) outlined various methods of assessing new formulations, illustrated by the development of a formulation containing a phagostimulant and Ifor Bowen, Samir Antoine (Cardiff) & Trevor Martin compared the efficacy of 1, 2 and 4% methiocarb formulated with synergists such as piperidine and veratrylamine. Craig Kelly (Huntingdon Life Sciences) reported different feeding strategies in A. distinctus and D. reticulatum; A. distinctus stopped feeding sooner on methiocarb baits, and possible reformulations were examined. Albert Ester et al. (Lelystad) showed that winter wheat seeds treated with methiocarb and metaldehyde consistently performed well, but other pesticides either gave insufficient slug control or were phytotoxic. Slug mortality occurs mostly in the first 5 days after applying pellets, but André Chabert (ACTA, Lyon) found that 5% metaldehyde remained effective after 12 days, although duration decreased with wet weather. Patrick Wellmann (Aachen) & Fred Heimbach (Bayer AG, Leverkusen) described new laboratory studies on the effects of methiocarb on Lumbricus terrestris. The Daniel-funnel test is appropriate for this surface-feeding earthworm, and 25 cm funnels were filled with artificial soil and given 0.7 g of dried, powdered cow manure. Over 21 days, weight gain, behaviour and activity were affected only at 10 x the normal application rate. Methiocarb pellets are toxic to carabid beetles which are active at the time of application, but Gordon Purvis (Dublin) showed that most carabid populations found in arable fields have a distinct dispersal phase which allows them to recover from the substantial mortality which they routinely suffer as larvae from soil cultivations. Populations of Bembidion obtusum lack this dispersal phase and are permanently depressed by methiocarb.

In Professor Bowen's session, the emphasis shifted to control of aquatic pests and the search for novel chemicals. Ed Taylor et al.(Cardiff) described new toxicity tests for potential freshwater pollutants which can be modified to screen chemicals against pest species. Plant extracts and other chemicals have been screened for use against Pomacid and Lymnaeid snails either as molluscicides or as growth inhibiters, activity arrestants, or ovicides (Pomacea lays its eggs out of water). Sub-lethal concentrations may prevent reproduction directly or by inhibiting feeding. Their poster (Julian Arthur et al.) examined plant extracts for relative toxicity to Pomacea canaliculata and Daphnia pulex. Felicitos Palis et al.

(Bayer Philippines Inc) described how Pomacea canaliculata, introduced into the Philippines as a potential protein source, had become a pest of lowland rice paddy fields. The snail is prolific, polyphagous, able to survive buried in soil for long periods, and has no natural enemies in the region. Niclosamide, already registered in the Philippines for control of schistosomiasis vectors, prevented rice seedling damage at 1 and 1.5 l ha-1, and left no detectable residues in rice (detection limit 0.03 mg kg-1). However, if fish are cultured in rice paddies, they must be introduced at least 3 days after niclosamine application. Illda Borlongan et al. (Southeast Asian Fisheries Development Center, Philippines) showed that brackishwater snails (Cerithium spp.), which are serious competitors for the algal food fed to milkfish, can be effectively controlled by metaldehyde at 30kg ha-1, without any effect on juvenile milkfish.

Alan Powell et al. (Cardiff) screened several phenolics, aromatics and monoterpenes for use as seed treatments of winter wheat, testing for molluscicidal and repellant effects and phytotoxicity. (-)Menthol and thymol were highly effective. Colin Young (Melbourne) described the use of "environmentally friendlier" metal chelates against H. aspersa, a severe pest in suburban gardens and horticulture in temperate parts of Australia, and Theba pisana and Cernuella virgata. In laboratory tests, Iron EDTA proved superior to metaldehyde at lower temperatures. Glenn Dawson et al. (IACR-Rothamsted) found several surfactants were very repellent to slugs in the laboratory but formed an ineffective barrier in wet field conditions.

Flies, carabid beetles and nematodes all show potential as biocontrol agents against snails and slugs. James Coupland, searching the western Mediterranean home ground of Theba pisana, Cernuella virgata, Cochlicella acuta and C. barbara for agents to control these introduced pests of pasture and grain in Australia, discovered several sarcophagid and sciomyzid flies which showed potential. Salticella fasciata (Sciomyzidae) lays eggs in the umbilicus of Theba but was ineffective in killing its host, and is probably a saprophage rather than a parasite. Most sciomyzids occupy scrub or forest habitats; only Pherbellia cinerella is a pasture species, but like other sciomyzids it has a wide host range and is known to kill endemic Australian snails. Many sarcophagids are saprophagous, but Sarcophaga uncicurva and S. balanina parasitise Theba (and C. virgata to a lesser extent); they attack aestivating snails on plants, and produce several generations in a season. In their native habitat they are heavily hyperparasitised, but released from their hyperparasites they might increase rapidly. The sarcophagids are undergoing host-specificity screening in Australia.

Bill Symondson (Cardiff) has developed monoclonal antibodies (MCAs) to detect slug remains in guts of predators from the field. Slug predators may also control other pests, and carabids such as Pterostichus melanarius may switch between prey species depending on prey density: These polyphagous predators may lay in wait ready to prevent pest resurgence. MCAs are being raised to aphids, to study switching between prey. Kevin Ayre & Gordon Port (Newcastle) analysed gut contents of carabids from oilseed rape and winter wheat using ELISA. 13 of the 19 species analysed (and 26% of all individuals) contained slug tissue, including 37% of all Nebria brevicollis. The nematode Phasmarhabditis hermaphrodita, commercially released in 1994 to control slugs, was described by David Glen (IACR-Long Ashton, Bristol). The nematode can infect and kill all slug and several snail species, but larger species are only susceptible when young. Infective larvae applied to soil at 3 x 109 ha-1 rapidly reduced slug feeding and slugs avoid feeding or resting on treated areas. Field studies suggest that non-target species in field margins and hedgerows are not at risk, because there is almost no lateral movement in the soil. The nematodes are applied inundatively, and have no long term effect on slug populations, so they will not threaten wildlife feeding on slugs. It remains expensive, and after home gardens, its use will extend to high value crops (e.g. brussels sprouts and strawberries). Bernhard Speiser & M Andermatt (Switzerland) showed that Phasmarhabditis is a promising alternative to chemical molluscicides, but the conditions needed for successful use remain undetermined.

Nick Sakovich (California) described control of Helix aspersa in Californian citrus orchards where it can cause defoliation of young trees and damage to fruit. Pruning lower branches and placing a copper foil or paint barrier around the trunk prevent snails climbing into the trees. The predatory snail Rumina decollata, native to parts of California, can be introduced 60 days after a chemical check, but takes 4-6 years to control the snails. Orchard owners collaborate to introduce ducks for half an hour each day to eat the snails. Thomas Frank (Bern) investigated the spread of slugs into fields of oilseed rape and winter wheat from adjacent strips sown with ca 25 species of wildflower species which are known to encourage beneficial arthropods. Slug damage was much higher near the wildflower strips in rape but not in winter wheat. The difference might be due to adult Arion lusitanicus, which were active when rape was susceptible to damage but had died out by the time that wheat was at risk. Richard Cook et al.(Manchester) showed that palatable common weeds such as dandelion could, by acting as an alternative food, reduce slug damage to seeds and seedlings of winter wheat. Although less effective than metaldehyde treatment, the weeds might exert a longer lasting control over damage. Tony Walker (IACR-Long Ashton, Bristol) posed the intriguing possibility that the future use of transgenic plants which could express cysteine proteinase inhibitors might prove an effective control of slugs, since he has shown that the proteolytic activity of the slug digestive gland was almost totally inhibited by cysteine proteinase inhibitors. Andy Evans & Alison Spaull (SAC, Edinburgh) found differences in susceptibility of winter wheat cultivars to grain hollowing which were related to sugar content in ungerminated seed and release of sugars and other solutes during germination. Albert Ester & P Geelen (Netherlands) showed that in S. Holland, where sugar beet is direct drilled into a mulch to reduce erosion, slug damage may result: use of nematodes and methiocarb improved plant survival.

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