Slugs in gardens: Their numbers, activities and distribution. Part 2

Section 8. Observations of slugs feeding in gardens indicate that very little of their food consists of plant material grown for human consumption or pleasure. In places where one crop only is grown, e.g. potato patches in the autumn, the damage would be higher owing to the absence of alternative food. Section 9. Mating of Agriolimax reticulatus and Arion subfuscus takes place out in the open on the surface of the ground after dark, the peaks being soon after the peaks of seasonal activity; the former species also mates to some extent throughout the year, whereas in the latter species mating is restricted to July-October. Arion hortensis and ater usually require some shelter, e.g. dead leaves, under which to mate, while it is presumed that the Milax species usually mate underground or under cover. Section 10. By weighing the slugs species by species en masse as collected and then calculating the weight per 100 individuals, regular changes in weight throughout the year have been found. This method has been found to be as satisfactory for assessing seasonal changes in weight as the half-hour method of collecting slugs is for measuring seasonal changes in activity numbers. The slugs are heaviest soon after the greatest numbers are found active and at the time when the peak of mating occurs. Section 11. The distribution of the species varies from garden to garden. Arion hortensis and Milax gracilis are most abundant in the gardens at the bottom of an old river bed slope and decrease steadily until their lowest numbers occur half-way up it. Milax sowerbyi has a ridge of abundance across the slope. Arion subfuscus is almost completely absent on the east side of the valley. Two particular gardens form the focal point of abundance of Limax maximus, and Arion ater is more abundant in gardens of recent origin in close proximity to coarse grass areas than in old well-established gardens. These distributions have persisted month by month throughout 2 years. Section 12. There is some evidence that each species has its own regular curve of nightly activity, providing of course weather conditions are suitable. In the summer activity appears to be more closely adjusted to the time of sunset than in the winter. Immature specimens of Arion hortensis preponderated in steadily decreasing numbers in twilight collections made from May to December. Section 13. All species are fully active at about 40 degrees F., but some activity continues until almost freezing point; below this there is no activity. Some species, e.g. Milax gracilis, resume activity after cold spells later than others, e.g. Agriolimax reticulatus, perhaps owing to their deeper penetration of the soil. Lack of surface moisture seems to be one of the factors limiting activity. Slug activity in rainless periods is reduced more at some periods of the year than at others. Rainless periods in the spring have less effect on limiting activity than summer droughts. This is probably due to the different water contents of the soil at these seasons. Activity is reduced while heavy rain is actually falling and also in heavy wind. The optimum conditions for slug activity may be summarized thus: a warm still night with plenty of surface moisture either in the shape of recent rain or dew.