Chemical and biological control of slugs

Understanding how slug control options work, along with the use of monitoring and damage-risk thresholds, will help you target treatment effectively.

Due to relatively high costs, biological control of slugs is particularly suited to organic systems and high-value crops. It is also extremely useful in situations where it is difficult to target the slugs effectively with pellets, such as in mature lettuce crops.

A biological molluscicide, based on a nematode parasite of slugs (Phasmarhabditis hermaphrodita), is available. Unlike pellets, nematodes can target soil-dwelling slugs, as well as surface-active slugs. However, slugs may not feed or rest on substrates treated with nematodes.

The nematodes enter the slug's shell sack (mantle) and release bacteria from their guts. The nematodes feed on the multiplying bacteria. Eventually, the slug's mantle swells and bursts. Although it usually takes 7–21 days for an infected slug to die, it stops feeding soon after the nematode enters the body. When the slug dies, the nematodes feed on it until the food source is depleted. They then leave in search of more slugs.

The grey field slug is highly vulnerable to the nematode but larger species are only susceptible when they are young and small.

Nematodes can be applied in advance of expected damage, at sowing or any time during the crop’s lifetime. However, they are best applied in dull weather, in the evening and before rain. Success is dependent on wet conditions after application.

In ideal conditions, nematodes provide a reduction in damage for about six weeks after application. To get the best out of the product, it may need to be used soon after purchase (always follow the instructions carefully).

Natural enemies include ground beetles, rove beetles, parasitoids, birds, amphibians and hedgehogs, so providing habitats for these species may help to control slug populations.

The chemical content of bait/pellets affects their attractiveness to slugs and their durability. Too much active substance may deter slugs from feeding before receiving a lethal dose. Too little may also prevent slugs receiving a lethal dose, especially in larger slugs. There are no records of resistance to plant protection products authorised for slug control.

Ferric phosphate is particularly suitable for organic systems. Slugs quickly stop feeding, become less mobile and die within 3–6 days. As slugs often die underground, effectiveness of the treatment should be measured by the decrease of feeding damage in the crop.

Metaldehyde is a selective molluscicide and principally acts on slugs by inducing excessive secretion of mucus, leading to subsequent dehydration and death. At high temperatures (around 20°C), the activity of metaldehyde is optimised. At low temperatures, its toxic effect may be diminished. Metaldehyde does not harm predatory ground beetles.

This active ingredient has been a central component of the chemical control of slugs for decades. However, its detection in raw (untreated) water above the drinking water standard, along with environmental concerns, placed a great deal of scrutiny on this relatively cost-effective option. As a result, metaldehyde will be withdrawn and integrated control, using various techniques, will become increasingly important.

From 31 March 2021, no further supplies from manufacturers will be permitted, but distributors can still sell stocks and use can continue until 31 March 2022. From 1 April 2022, it will be illegal to sell and use metaldehyde products.

An industry-led voluntary approach, the Metaldehyde Stewardship Group (MSG) promotes and encourages best practice with metaldehyde slug pellets to minimise environmental impacts.

Enter your postcode into the Environment Agency’s online tool to find out if you farm in a Drinking Water Safeguard Zone.

• Use minimum active ingredient (a.i.) per hectare to avoid drainage and run-off losses
• Maximum application rate: 210 g metaldehyde a.i./ha*. For additional protection of water, BASIS-qualified suppliers or advisers may recommend rates reduced to 160 g a.i./ha or less*
• Maximum total dose from 1 August–31 December: 210 g metaldehyde a.i./ha*. For additional protection of water, BASIS-qualified suppliers or advisers may recommend rates reduced to 160 g a.i./ha or less*
• Maximum total dose rate (statutory limit): 700 g metaldehyde a.i./ha/ calendar year*
• No pellets to be allowed to fall within a minimum of 10 metres of any field boundary or watercourse
• Do not apply when heavy rain is forecast
• If drains are flowing, do not apply metaldehyde-based slug pellets

*from any combination of metaldehyde products.

a.i. = active ingredient (active substance)

Monitoring periods and damage thresholds are crop-specific. The damage-risk thresholds only apply where conditions are suitable for slug activity.