The Distribution and Abundance of Slugs on an Arable plot in Northumberland

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Abstract
The distribution and abundance of Agriolimax reticulatus (Mull.), Arion hortensis Fer. and Milax budapestensis (Hazay) were investigated in a study based on routine sampling from an arable plot in Northumberland. Vertical distribution in the soil varied throughout the year, slugs of all three spp. being deeper underground during frosty and dry weather. There were also differences between species, A. reticulatus being found nearer the surface than A.hortensis and the latter nearer than M. budapestensis. Horizontal distribution of slugs on the plot was fairly uniform but M. budapestensis was slower than the other 2 sp. at recolonizing an area in which the population had been depleted. All three sp. exhibited aggregation, especially just after hatching. There was a general reduction in the density of slug generations shortly after breeding, i.c. senescence is short. Decreases in the numbers of immature A. reticulatus and A. hortensis occurred during the winter (probably due to the effects of frost) and of young M. budapestensis during autumn (possibly due to a local shortage of food). Estimates of the "capacity for increase" of A. hortensis and M. budapestensis were obtained from field cultures. A. hortensis has a higher capacity than M. budapestensis, but neither of these species is able to reproduce so quickly as A. reticulatus. It was concluded that the most important factor to influence the density of populations was the length of generation interval. It was noted during the study that the density of the population was affected by the severity of the previous winter, which regulated the maturation time of preceding generations.

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