The Use of Molluscicides in UK Agriculture and their Effects on Non-Target Organisms
Slugs have been a major problem in agriculture for more than a century. Early methods of slug pest control consisted of dressings of various materials to control slugs, including salt, lime, caustic soda and powdered coke. However, these were found to be of little use, as the slugs often recovered due to the effectiveness of the substances being reduced by moisture. Current molluscicide baits consist of metaldehyde or either one of the two carbamate compounds, methiocarb and thiodicarb. Pellets are forumulated with feeding stimulants or attractants (wheat, barley or bran) and enhanced with specific materials such as proteins, dextrose and casein. Stabilisers, binders and fungicides are also included to prolong the efficacy of the products. These baits are toxic to molluscs, by their very nature, but they can also have effects on non-target or beneficial organisms.
Molluscicide usage
The use of molluscicides is widespread within the agricultural industry, with over 800,000 hectares of both agricultural and horticultural crops being treated and over 250 tonnes of active ingredient being applied annually. Metaldehyde accounts for 55%, methiocarb 40% and thiodicarb 5%. The use of molluscicides has increased nearly 70 fold from the early 1970s to the mid 1990s with agricultural crops accounting for 99% of usage. An estimated 4,800 tonnes of molluscicide pellets are applied every year at a cost of £10 million. Clearly then, slug damage is a continuing problem and a number of compounds that control molluscs (molluscicides) are available for use.
Surveys
have indicated that UK wheat growers consider slugs to be the pest
of greatest importance and an equal to major diseases and weeds.
This importance is borne out by the general increase in the amount
of molluscicide active product used and the area over which it is
used. The figure opposite shows the twelve year trend in molluscicide
usage. . The economic consequences resulting from slug damage will
be greater during periods of high grain prices when a crop is more
valuable. At such times it is possible to justify the expense of
molluscicides, when the potential return is much greater, compared
with when grain prices are lower and the crop is less valuable.
The weather also has an important affect on molluscicide usage.
During summers with particularly low rainfall, usage drops (1990-92
and 1996-98), as slugs are less of a threat to crops. However, the
inverse can be seen in wet seasons where usage is much higher due
to the greater risk of slug damage. The use of molluscicides is
likely to continue, given the perceived trend for wetter autumns
and winters and the increasing areas of land used for wheat and
oilseed rape.
Effects of metaldehyde
Whilst it has been demonstrated that metaldehyde baits have little effect on non-target carabid beetles, there is little doubt of the toxicity to vertebrates. In the presence of gastric hydrochloric acid metaldehyde is decomposed to acetaldehyde, which then accumulates to toxic levels in the blood. In mice, for example, metaldehyde administered orally at a dose of 1 g/kg produced convulsions and death within 2 hours. In early preparations, metaldehyde was incorporated into large pellets which were palatable to many domesticated animals, leading to instances of poisoning in dogs, cats, sheep and poultry. In one incident, a dairy herd was found to have been poisoned when a broken bag of pellets was left in a field. Furthermore, evidence has been found to show that dogs actively searched for bait, even after having been previously poisoned. Changes in the size of pellets and the use of repellents have, however, reduced this danger, exemplified by a dramatic decrease in instances of poisoning of dogs in California after pellets were made less attractive to animals.
Effects of methiocarb
Whilst metaldehyde has little effect on non-target organisms, methiocarb has been shown to be more detrimental. Earthworms are one of the beneficial organisms known to be at risk from methiocarb pellets (Symonds, 1975). Bieri et al. (1989) demonstrated in the laboratory that the spreading of methiocarb pellets on the soil surface resulted in a significantly higher mortality rate in Lumbricus terrestris L. than metaldehyde. However, the probability of an earthworm encountering a slug pellet in the field is thought to be considerably less than in a laboratory experiment. Without detailed investigations in the field it is difficult to assess the long-term impact of methiocarb pellets on earthworm populations.
Unlike metaldehyde, methiocarb is also an insecticide and has been found to cause high mortality of non-target carabid beetles, an important predator of slugs, and one of the most salient groups of invertebrate predators in the arable ecosystem. However, as molluscicides are applied in the autumn when wheat is at its most vulnerable, the impact of methiocarb pellets on carabid beetle populations might be lower in autumn as the insects are less active. Should the pellets be applied in the spring, the number of species affected would be greater. Concern has also been expressed as to the effects of molluscicides on small mammals. In particular, methiocarb has been shown to be palatable to the wood mouse, Apodemus sylvaticus L., which were therefore thought to be at risk of exposure. However, subsequent work carried out at the Boxworth Project (a five-year Government funded investigation into sustainable methods of farming) has shown that whilst applications of methiocarb pellets in the autumn had a dramatic short-term effect on wood mouse populations, immigration of juvenile mice from untreated areas led to a rapid recovery in numbers. It was suggested that applications at other times of the year, or in areas without adjacent woodland to allow recolonisation, might produce longer lasting effects . The harmful effects of methiocarb were greatly reduced when the pellet was drilled with the wheat seed. Work conducted in Australia on house mice (Mus domesticus) confirmed the findings from the Boxworth Project, in that it was concluded that methiocarb baits reduced mouse numbers by 0-46%, in contrast to strychnine-treated wheat (86-94% mortality). However, it was conceded that mortality from methiocarb baits could cause a threat to rare or endangered rodents.
Data involving the effects of ingested slugs that have been poisoned is sparse but it has been suggested that predators would need to consume large numbers of poisoned slugs to ingest a toxic dose. This is confirmed by Johnson et al. (1992), who found, albeit it in a limited study, that A. sylvaticus fed newly poisoned Deroceras reticulatum (Müller), survived for a minimum of five days, with the exception of one mortality. Of the two chemicals it is likely that only methiocarb could accumulate in the tissues of predators (South, 1992).
The damage to crops by slugs is a problem that has existed for many years, and although slug control has undoubtedly improved over the years with more advanced formulations, the use of molluscicides is likely to continue, as are the problems associated with them, described in this paper. In recent years, alternatives to molluscicides have been investigated, in particular, the use of natural predatory organisms such as nematodes and carabid beetles. In gardens and nurseries, copper barriers or copper-impregnated matting have been shown to be effective. The use of alternative food sources to reduce damage to the commercial crop has also shown the potential to reduce slug damage, without the use of molluscicides.
Andy Brooks and Mitch Crook,
Harper Adams University College, Newport, Shropshire, UK
Barker, G.M. (ed) (2002). Molluscs as crop pests. Wallingford: CABI International, pp468.
Bieri, M., Schweizer, H., Christensen, K. and Daniel, O. (1989). The effects of metaldehyde and methiocarb slug pellets on Lumbricus terrestris. In: BCPC Monograph 41. Slugs and Snails in World Agriculture (ed. Henderson, I.F.). pp. 237-244.
Johnson, I. P., Flowerdew, J. R. and Hare, R. (1992). Populations and diet of small rodents and shrews in relation to pesticide usage. In: Pesticides, Cereal Farming and the Environment. The Boxworth Project. (ed. Greig-Smith P. and Hardy T.). London: HMSO, pp. 144-156.
South, A. (1992). Terrestrial Slugs: Biology, Ecology and Control. London: Chapman & Hall. pp428.
Symonds, B. V. (1975). Evaluation of potential molluscicides for the control of the field slug, Agriolimax reticulatus (Müll). Plant Pathology 24, 1-9.
Fig. 1 Twelve year trend in molluscicide usage in U.K.
