14 September 2001

Dont just rely on pellets to rid your crops of slugs

By Louise Impey

THERE are bad years for slugs and terrible years. The good news is that this year is likely to fall into the bad category.

"Certainly therell be slugs around, especially in crops following oilseed rape," says Mike Wilson of the University of Aberdeen. "But in my opinion it hasnt been wet enough for the worst to happen."

Moisture is the critical factor. "Slugs have no rigid cuticle, so are totally dependent on water to move. Thats why heavier soils, which tend to retain water, have more slug problems."

&#42 Cultivations

The more soil is cultivated the better the slug control. "With ploughing, slugs either get crushed or are brought to the soil surface, where they are prone to predators or a lack of moisture. Conversely, minimum tillage regimes work in the slugs favour."

Unlike earthworms, slugs cant burrow. "So any measures which stop them getting to the crop are useful." That is why a fine, firm seed-bed free of clods is a good deterrent. Straw and trash help create spaces for slugs to move through the soil, so they need dealing with effectively.

Rolling is advised after seed-bed preparation and wheat should be drilled 4-5cm deep, making it more difficult for slugs to get to seed, suggests Dr Wilson.

&#42 Risk assessment

Slug numbers can be assessed by putting out traps before drilling. A tile laid in the field is adequate, although there are other techniques available, including specialist mats, says Dr Wilson.

"Put down 10 traps/field and assess them daily." A spoonful of slug pellets under each tile or trap will help. An average of one slug/trap is enough to reveal that the field may have a problem.

"Growers do seem to have an instinctive feel for problem areas. And bear in mind that any crop following oilseed rape is likely to be at risk."

&#42 Slug pellets

There is a choice of three active ingredients in commercial slug pellets – metaldehyde, methiocarb and thiodicarb. All give similar results in the field, especially since formulation processes have improved, believes Dr Wilson.

"Dont expect slug populations to be reduced by more than 50% where pellets are used. The important thing is to use them in combination with cultural and agronomic control methods."

Pellet quality is important because treatment will fail if the pellet is not palatable. "The slug has to ingest the required lethal dose. If pellets arent palatable, slugs stop eating and move away before this has occurred."

The pellet must also be firm. "If they break down too quickly, especially in wet conditions, they arent effective and just attract fungus."

Dry process pellets are preferred in early season when seed-beds are drier. Wet extruded pellets have a hard outer coating and are better for wet conditions because they dont disintegrate in the rain.

Broadcasting pellets onto the soil surface is Dr Wilsons preferred method of application. "If slugs have reached the seed it tends to be too late."

Splashy rainfall may cause pellets to wash away, so a split application can be sensible if the forecast is bad. "Its a case of weighing up the costs of traveling again versus the price of using a lower quality pellet."

&#42 The future

Forecasting should help growers in future, he suggests. "Theres a great deal of work being done on modeling populations, so bad years can be predicted accurately.

"Damage thresholds are also being devised – pellets are often used as an insurance policy, so it would be good to move away from this practice and target their use better."

Seed treatment and coating is another promising area. "These offer growers the opportunity to reduce the amount of active ingredient used by 20-50%, save the cost of a separate pass simply for slug control and prevent unintentional harm to other wildlife."

After the first year of HGCA-funded trials with oilseed rape, he is optimistic about the results. "Weve seen both economic and environmental benefits." &#42

SLUGCONTROLPLAN

&#8226 Agronomic + chemical control.

&#8226 Drill seed to 4-5cm depth.

&#8226 Create a fine, firm seed-bed and consolidate after drilling.

&#8226 Remove surface trash and reduce air spaces.

&#8226 Put out traps now and monitor daily.

&#8226 Broadcast pellets rather than admixing with seed.

&#8226 Seed coatings offer promise.