Slug populations set to explode after wet spring

Farmers and growers are being advised to prepare for an increased slug threat later this summer after the wettest April on record created ideal conditions for slugs to reproduce.

According to David Glen, an independent slug specialist with Styloma Research and Consulting and a former lead researcher with the Long Ashton Research Station, the current weather conditions are perfect for slugs.

“April to May is an important peak of the breeding cycle and with conditions as they are slug activity will be very high so populations are primed to increase significantly,” he says.

Slugs represent a threat to all crops, but their potential to inflict economic damage is perhaps greatest in high value crops such as field vegetables. The Horticultural Development Council (HDC) estimates that slugs cost the sector £8m a year in lost output. While in arable crops a single slug can kill up to 50 wheat seeds in the first week after drilling.

The preference for winter-sown crops, the increased area of oilseed rape and the trend towards lighter cultivation techniques have all helped to swell slug populations over the last 20 years. 

David Glen warns growers not to be misled into thinking the dry conditions of the last three years will have reduced slug populations to safe levels.

“Slugs are extremely resilient creatures capable of surviving harsh environments before breeding rapidly when conditions suit. Growers should be assessing the level of slug activity in the run up to harvest and in stubble after harvest to determine the size of the threat to next year’s crop.

”The threshold for wheat crops is four slugs per trap in stubble, but in stubble before oilseed rape, research has shown that just one slug per trap is enough to signal the need for action. Conventional practice has been to lay traps after cultivations, but before emergence of the next crop though this can give a false impression of slug numbers, says David Glen.

“Research has shown that disturbing the soil profile disrupts slug behaviour to the extent that indications might suggest a low population burden when the opposite can be the reality.

”Cultivations are an important part of slug control though it will not remove the need for pellets in high risk situations.

“Those following a direct drilling approach run a greater risk of sustaining slug damage. Cloddy seedbeds and wet weather around the time of drilling increase the damage risk whatever the method of cultivation, so be prepared to apply pellets at or around the time of drilling, ideally just after the crop has been rolled,” he says.

“The potential for damage is so great that waiting for activity to become visible means it is too late,” he adds.

It’s also important to use a high quality pellet to get sufficient active ingredient in to the slug for it to be fatal. Metaldehyde is the most cost-effective means of control available and capable of delivering exceptional levels of control when applied in suitable conditions.

“To be effective, slugs must ingest sufficient bait otherwise they can recover. Some pellets, such as TDS from De Sangosse, contain an attractant and a feeding stimulant to ensure sufficient active is ingested and cause paralysis,” says Simon McMunn of manufacturer De Sangosse.

“Slugs are free-roaming creatures capable of moving up to five metres a night and able it to detect food from 60cm away, so it is important to achieve an even spread of pellets across the soil surface for adequate control.

”Care should also be taken to avoid applying pellets to cloddy soils before heavy rain as they will be washed down the soil profile and not found by the slugs.

A single application should not exceed 210g metaldehyde per ha though in certain cases an advisor may suggest a maximum of 160g metaldehyde per ha.  The maximum total dose is 700g of metaldehyde per calendar year.

“Metaldehyde is a valuable tool without which growers would have to use more expensive alternatives so it is of the upmost importance that users follow the best practice guidelines laid out by the Metaldehyde Stewardship Group (MSG) and don’t exceed the maximum stated dose,” says Simon McMunn. 

More information can found at www.desangosse.co.uk andwww.getpelletwise.co.uk while advice on trapping can be found at HGCA Topic Sheet No. 84 available at www.hgca.com