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Where has all the wildlife gone? Blame the farmers dumping tonnes of poisons on our food crops, and then wonder why we are also getting ill! Not rocket science is it?

Von: Old Codger (oldcodger@anyoldwhere.net) [Profil]
Datum: 03.05.2008 20:25
Message-ID: <m9bp14l755p0p3etlnvnkdm4nof6mht3j3@4ax.com>
Newsgroup: sci.agriculture.poultry uk.rec.fishing.coarse uk.business.agriculture uk.rec.gardening uk.rec.birdwatching uk.environment.conservationtalk.politics.animals alt.animals.ethics.vegetarian
The Use of Molluscicides in UK Agriculture and their Effects on
Non-Target Organisms

The Malacological Society of London
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 0â46%,
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,

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.

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