Protected herbs are very susceptible to damage by slugs and snails, due to soft foliage and the damp conditions that occur when regular irrigation is carried out. Soil-grown crops and those in pots or trays stood on the ground are particularly vulnerable. Damage is very obvious and causes plant losses.
Recognition and host plants
A survey of slug and snail species damaging hardy nursery stock crops, which included potted herbs if these were also grown on the nurseries surveyed, was completed as part of HDC and Defra-funded project HNS 105 / HH1944TFV. In the survey the slug and snail species most commonly found damaging protected herbs were the same as those damaging protected nursery stock, namely Deroceras panormitanum (the ‘chestnut slug’) and a small semi-aquatic snail, Oxyloma pfeifferi (often known by growers as the ‘water snail’). Herb species identified as being susceptible to slugs and snails during the survey included basil, mint, thyme and sage. However, many other herbs are also vulnerable.
D. panormitanum is a small, brownish-grey slug, up to 3.5 cm long, with a pale underside. The mantle (pale flap of skin behind the head) is usually paler than the rest of the body and the breathing hole on the right hand side of the mantle is surrounded by a noticeable pale ‘halo’ (Fig. 1). O. pfeifferi adult snails are 9-12 mm long. Shell colour varies from pale brown to almost black, often with darker markings in the paler forms. The shell is pointed at one end and has three spiralling whorls (Fig. 2). Other species of slug and snail can also occur.
Slugs and snails rasp away leaf tissue. The damage can be confused with that caused by other pests, e.g. caterpillars, but slime is usually present on leaves damaged by slugs and snails. Symptoms of slug damage include irregular leaf holes, shredding and rasping around the leaf margins (Fig. 3). Young tender plants can be severed at ground level and killed. Feeding by O. pfeifferi can produce small leaf holes and long strips of epidermal tissue may be removed from leaves, but the snail often feeds on algae and damaged or decaying leaves rather than healthy material. As the snails are active and visible during the day, unlike slugs which tend to be nocturnal, they may be mistakenly held responsible for slug damage. If present in large numbers on plants or pots, the snails may lead to quality problems as contaminants.
Sources of infestation and favourable conditions
Slugs and snails can be resident on many nurseries and can also be brought in on plants and liners. Mild, damp conditions favour their activity and development. The biology of D. panormitanum and O. pfeifferi was studied in HDC and Defra-funded project HNS 105 / HH1944TFV (Bennison, 2003).
The slug D. panormitanum is active all year round, with peaks of activity during spring and autumn. Egg-laying starts in March and many young slugs are present in April. D. panormitanum is adapted to the high temperatures and damp conditions present in glasshouses and tunnels, where egg-laying can continue through the summer, and two or three overlapping generations can develop each year. Like many other slug species, D. panormitanum is active mainly at night, hiding during the day beneath pots or trays or at the base of plants.
The snail O. pfeifferi hibernates in unheated structures between late September/early October and late February/early March. Hibernation sites include the sides of pots and polythene tunnel structures. Eggs are laid between late March and August, with overlapping generations occurring during this period. Wet conditions stimulate activity and egg-laying and favour survival. Unlike slugs, O. pfeifferi is more active during the day than at night and is easily found on plants and pots, where it can occur in large numbers.
Integrated Pest Management (protected herbs)
Critical times for reducing slug and snail populations on the nursery are spring and early autumn. Control measures in March/April will reduce egg laying and development of juveniles, and those in September will reduce numbers of over-wintering slugs and hibernating snails. However, control measures may also be needed throughout the spring to autumn period.
- Maintain good nursery hygiene to reduce shelter and food for slugs and snails, particularly in soil-grown crops or those in pots or trays stood on the ground. Control algae, mosses, weeds and liverworts as thoroughly and frequently as possible, and dispose of unmarketable plants and waste plant material promptly.
- Do not leave unwanted plants, pots or trays in glasshouses or tunnels as these can shelter slugs and snails.
- Avoid over-watering as wet conditions favour slug and snail activity and survival, and also encourage the growth of algae, mosses and liverworts which can be alternative food sources.
- Copper-treated ground-cover mattings (e.g. Tex-R mattings) used as root retardants and weed suppressants can repel slugs and snails and reduce their activity. These mattings can be used to stand pots or trays on, to help prevent infestation of ‘clean’ plants. The matting is most effective when used over a large area e.g. the entire floor of a tunnel, and needs to be kept clean and free from moss and algae.
The slug parasitic nematode, Phasmarhabditis hermaphrodita (‘Nemaslug’) was shown to kill both the slug D. panormitanum and the snail O. pfeifferi in HDC and Defra-funded project HNS 105 / HH1944TFV (Bennison, 2003). Nemaslug is applied as a drench to moist compost or soil. Once inside the slug or snail, the nematodes multiply and release lethal bacteria that kill their host (Figs 4 and 5). In laboratory tests in HNS 105, Nemaslug caused high slug and snail mortalities within 3-4 weeks of treatment (Bennison, 2003). In the same project, Nemaslug did not control slugs or snails on hardy nursery stock in two trials in glasshouses and tunnels on commercial nurseries. This was likely to have been due to very hot conditions shortly after application, which will have reduced nematode survival. Subsequent use of Nemaslug by growers has given more successful results. The optimum time for Nemaslug application is April or September, when slugs and snails are active, and compost or soil temperatures are likely to be within the optimum range of 5-25°C. As with other nematode products, Nemaslug needs to be applied carefully, following all supplier recommendations, e.g. the suspension of nematodes should be kept agitated during application, and the compost or soil kept moist so that the nematodes can survive and move within the substrate.
Monitoring within IPM
- Check plants regularly for signs of slug or snail damage.
- If using nematodes for control, infected slugs or snails should be showing symptoms of infection within 3-4 weeks after nematode application. Infected slugs often have a swollen mantle (pale flap of skin behind the head) and nematodes can often be seen on the body surface (Fig. 4). Infected snails quickly start to decompose, and nematodes can be seen inside the shell (Fig. 5). Once infected snails are fully decomposed, empty shells can be seen.
If a molluscicide is needed, it should be selected carefully, taking into account compatibility within IPM (see Section A – Principles of IPM) and harvest interval (see Table 2 on the homepage).
For full, specific information on safety of pesticides to individual biological control agents, consult your biological control supplier. General guidelines on the selection and compatibility of molluscicides within IPM are given below:
- Ferric phosphate products e.g. Ferramol Slug Killer and Sluggo) are approved for use on any edible crop. These products have low toxicity to non-target organisms such as ground beetles, rove beetles, birds and mammals (e.g. hedgehogs), which are all natural predators of slugs and snails. Thus these products should be safe to use within IPM.
- Various metaldehyde products (e.g. Allure, Gusto4) are approved for use on any protected edible or non-edible crop. Metaldehyde products pose risks for birds and mammals but should be safe to invertebrate biological control agents.
Various garlic products are available as biostimulants. Some growers claim that garlic products used as repeated foliar sprays give preventive control of slugs and snails, although garlic is not approved for use as a molluscicide. Research in project HNS 105 / HH1944TFV showed that garlic repelled slugs and snails in laboratory tests, but a single spray did not reduce numbers of slugs, snails or plant damage in a trial on commercial protected hardy nursery stock (Bennison, 2005).
- Full details for the use of biological control agents and compatibility of pesticides are available from biological control suppliers or consultants. Also see Section A – Principles of IPM.
- Pesticide approval information in this guideline is current at 29 April 2013.
- Regular changes occur in the approval status of pesticides arising from changes in pesticide legislation or from other reasons. For the most up to date information, please check with a professional supplier or the CRD website http://www.pesticides.gov.uk/. General enquiries on pesticides and detergents are now being handled by the Defra Helpline (as of April 2013): Tel: 08459 33 55 77.
- Always follow label recommendations or statutory conditions for use on Extension of Approvals for Minor Use (EAMUs).
- Always follow instructions for Pesticide Resistance Management guidelines given on the label or EAMU.
- Growers must hold a paper or electronic copy of the current EAMU before using any product under the EAMU arrangements. Any use of a pesticide with a EAMU is at grower’s own risk. Relevant EAMUs are sent to HDC members by HDC, or are available from CRD (see above) or from consultants.
- Use pesticides safely.
HDC Factsheet 07/02. Integrated control of slugs and snails.
Bennison, J. (2003). Hardy nursery stock: Integrated control of snails and slugs. Final report for HDC project HNS 105 (Defra project HH1944TFV).
Bennison, J. (2005). Hardy nursery stock: Integrated control of snails and slugs. Final report for HDC project HNS 105 b.