Biology and monitoring of slugs and snails on hardy nursery stock

Learn when the main species of slug and snail are active, when they breed, how to monitor for their presence and what the potential sources of infestation can be.

This page gives details of the biology of the main slug species occurring on UK hardy nursery stock, Deroceras invadens.

In project HNS 105, D. invadens was found to be active all year round, with peaks of activity during spring and autumn. Large adult slugs died in late winter/early spring. Egg laying started in March and large numbers of juveniles were present in April. However, the start of egg laying and slug growth rates were very variable, so generations are likely to overlap.

Deroceras invadens thrives in the warm temperatures and damp conditions present in glasshouses and polythene tunnels, with overhead irrigation favouring egg production and population growth. This species can survive temperatures as high as 40°C under protection. Temperatures above 12°C are needed for egg laying, with 15–20°C being the optimum. However, once eggs are laid, they can develop and hatch at temperatures above 5.4°C – so eggs laid in the autumn can hatch when temperatures rise in the spring. Under favourable conditions egg laying can continue all summer. The life cycle (from egg, to first egg laying by the next generation adults) takes 40 weeks at 15°C and 12 weeks at 20°C, so under protection, two or three generations may develop each year. Although D. invadens can also occur on outdoor HNS, activity may be low in very dry summers and cold winters.

Monitoring guidelines for slugs:

• Like other slug species, D. invadens tends to be mainly active between dusk and dawn, hiding during the day beneath pots and trays and at the base of plant stems. This tendency for nocturnal activity means that slug numbers are often under-estimated. However, in damp, overcast conditions they can be active and visible during the day
• When monitoring for slugs, check plants for damage and slime trails and look under pots and trays, as well as on the plants and growing media
• In arable crops, slug activity is monitored using refuge traps such as tiles or plant pot saucers placed over a handful of bait such as chicken layers' mash or cereal grain-based food (not slug pellets) – see AHDB web pages on how to monitor slug populations in field crops. However, this method has not been tested for D. invadens in hardy nursery stock

This page gives details of the biology of the main snail species occurring on UK hardy nursery stock (HNS), Oxyloma elegans.

In project HNS 105, O. elegans was found to hibernate between September/October and February/March on the sides of pots, on plants and on the structure of polythene tunnels. All hibernating snails were juveniles (less than 8 mm long). Over the winter, up to 50% of the snails died. Snail activity began again in February and most were active by late March. Adult O. elegans laid eggs between late March and August, with mixed ages and overlapping generations during this period. Damp or wet conditions stimulated both activity and egg laying, aiding survival. The life cycle from egg to egg was 14 weeks at 20–22°C. In addition to feeding on plants, O. elegans also feed on algae on the surface of growing media, matting and the structure and cladding of polythene tunnels, and on liverworts and decaying leaves.

Monitoring guidelines for snails:

• Monitoring for elegans is easier than looking for slugs, as unlike D. invadens. O. elegans is mainly active during the day and, even when resting, the snails are often visible on the surface of plants, growing media or pots
• Start monitoring for snails and damage from March onwards, when O. elegans activity will be starting after hibernation. Continue monitoring until September when they start to hibernate

Author(s) – Jude Bennison and David Talbot, ADAS Horticulture.

Original author(s) – Jude Bennison, ADAS Horticulture.

Webpage content correct as of March 2021.