Slugs are often prevalent in heavy clays where winter beans are generally cultivated, thriving in the wet conditions that often prevail during crop establishment, especially in the cloddy seedbeds produced when crops are ploughed in.
Slugs are controlled via ingestion of various pelleted insecticides. Slug death depends on the amount of insecticide ingested. Thus the slugs must feed continuously on the pellet in order to maximise their effect. Some pellets contain attractants or sugars to increase palatability which will increase the amount of insecticide eaten by the slug.
The insecticides used to control slugs differ in their mode of action and tend to perform differently under varying environmental conditions. Comparative experiments are inconclusive as to the best product although in a series of 44 experiments, methiocarb was superior to metaldehyde in 59% of experiments, there being no discernable difference between the two products in 38% of experiments. Metaldehyde dehydrates the slugs and is generally more successful in dry conditions. However if the slugs are re wetted via rain after application, they may recover, rendering the pellets useless. Metaldehyde also causes jaw paralysis and this may reduce the slug’s ability to take in enough insecticide to kill it.
Carbamates such as methiocarb e.g. Draza pellets poison the slug via ingested poison. They are thus less sensitive to soil moisture and tend to work equally well in wet and drier conditions. However, they are also active on beneficial insects such as carabid beetles, which feed on slugs and other pests, such as aphids. Slug pellets are generally only active for about 8 days following even if the pellet is present for longer (Figure 1). They should therefore be applied when slug activity is apparent, i.e. in wet conditions.