Slug pressure explosion as wet weather arrives

Recent rain is significantly increasing slug pressure and growers should treat susceptible crops with pellets where the pest is present.

The arrival of wet weather will be a blessing for some crops struggling in dry conditions and also help with the efficacy of pre-emergence residual herbicides.

However, in an autumn that has been dominated by the threat of flea beetles and virus-carrying aphids, slugs now threaten to come to the pest party.

Independent slug expert David Glen says that although conditions have been very dry and not conducive to slug movement, the recent rains will give them the ideal conditions to move up the soil profile and feed on young crops.

“We are in for an extended period of rain too and with temperatures still reasonably warm, it will certainly get the slugs going,” he adds.

During dry conditions slugs aren’t visible on the surface, making it extremely difficult to gauge numbers present in the soil.

Despite that, Prof Glen says that the dry spell has only lasted one month through September and explains that slugs can survive in the soil for around two months waiting for moisture.

“Growers shouldn’t be misled into thinking that numbers are low, as they may have been buried deep in the soil during the summer. They will be hungry and quickly become active when conditions suit.”

Action required

Newly drilled wheat crops will be most at risk, with barley and oat crops less susceptible to slug damage.

Where wheat crops have just emerged, risk is more immediate, so growers should monitor the seedlings for damage and treat with an application of pellets if slugs are grazing.

In crops yet to emerge, Prof Glen advocates using traps to identify risk, with the treatment threshold in winter wheat four slugs per trap.

“Oilseed rape crops that are struggling to grow away and still at a susceptible stage should also be checked and a further application of pellets may be required,” he says.

Growers now have the limited choice of two slug pellet actives – metaldehyde and ferric phosphate – with evidence suggesting that both are effective slug killers.

In areas identified as high risk for metaldehyde getting into ground water, growers should switch to ferric phosphate-based pellets, which have limited restrictions on use (see box).

“When using metaldehyde, follow the metaldehyde stewardship group (MSG) guidelines, but whichever active you choose, it is sensible to use a high quality pellet that lasts,” says Prof Glen.

Pasta-based pellets

Pellet manufacturer De Sangosse’s commercial manager Simon McMunn agrees, saying that research suggests that a high-grade durum wheat pellet is not only more persistent, but also more palatable for the slugs.

“Effective control is largely determined by the field persistence and palatability scores of a pellet.

“Wet-process pellets of about 2.5mm persist better than dry-process or mini-type pellets because they are more able to withstand moisture penetration. They are also more easily found and eaten by slugs and spread more evenly due to their uniform individual mass,” says Mr McMunn.

Prof Glen adds that quality should not be sacrificed in favour of more baiting points, although he warns against lowering rates, particularly where young slugs are active on the surface.

“More baiting points are valuable in that situation, particularly as growers have to use reduced rates these days anyway.

“The concentration of active ingredient on the pellets has also reduced, so applying anything other than the recommended rate is a risky business,” he adds.

Slug pellet guidelines

Ferric phosphate

  • No water buffer zones or harvest intervals required
  • Can be applied in wet conditions when drains are flowing
  • Maximum rate of 7kg/ha and total dose of 28kg/ha/crop


  • Statutory maximum of 700g/ha per calendar year for metaldehyde.

  • Metaldehyde Stewardship Group (MSG) has set an autumn restriction period from 1 August to 31 December when total doses are restricted to a maximum total application rate of 210g/ha of the active ingredient.

  • This may be reduced to 160g/ha or less upon the recommendation of a BASIS-qualified adviser.

See also: Alternative pellet provides flexible slug control