OTHER SOIL ANIMALS - The Soil - page 113

lichens from rocks in a manner very like limpets, their marine relatives. The glass
snails Vitrina pellucida, Oxychilus draparnaudi and O. cella-rius are carnivorous,
partly on enchytraeid worms and small earthworms among litter and moss. The Testa-
cella slugs mentioned earlier are also predatory. They can reach 100-120mm when
fully extended, and can tackle quite large earthworms such as Lumbricus terrestris ,
though they will also tackle centipedes and other slugs.
A few species of slugs give the whole group a bad name among farmers and
gardeners. These are not the really big slugs such as the great black slug Arion ater.
This may look imposing when extended to its full 150mm but it rarely frequents ar-
able land. Rather it is its smaller relatives in the Arion distinctus group that attack
various root crops, causing damage that makes them unsaleable if not inedible. They
can be quite discerning in their preferences for different potato varieties if given the
choice ( Table 6 ).
T ABLE 6
Ranking of potato varieties according to their susceptibility to attack by slugs. (From
N.W.Runham & P.J.Hunter 1970).
The most common British slug, the grey field slug Deroceras reticulatum , is an
important pest of winter wheat because it hollows out the newly sown grain and also
grazes on the young shoots both below and above the soil surface. In the late 1960s,
slug damage was held responsible for the loss of 25,000 acres of wheat (10,000 ha)
in an average year. Crops of barley and oats are sometimes affected, and even newly
sown leys. Damage by slugs, and other soil pests, becomes increasingly important as
modern farming techniques reduce sowing rates for crops like sugar beet. Formerly,
many more seeds were sown than were needed and the excess seedlings were hoed
away. With monosperm seed it is now possible to sow only what is needed to grow to
full size so there are fewer to spare for rook or slug.
The keeled Milax slugs are very largely subterranean and feed almost entirely
on roots. They can burrow through compacted soil by ingesting it, like Aporrectodea
earthworms. M. budapestensis is a serious pest of root crops but M. rusticus , a hand-
some pink species with black spots, lives in woodland. It was first discovered in Bri-
tain in 1986 in a wood in Kent.