Efficacy of a metaldehyde-based seed treatment for slug control in winter wheat and winter oilseed rape

Abstract

Slugs are major pests of oilseed rape and wheat with most serious damage occurring just after sowing and seedling emergence. Current chemical control measures (bait pellets) often do not give adequate protection to plants, and pose an environmental hazard. Two HGCA-funded projects at the University of Aberdeen have demonstrated that metaldehyde seed coatings are highly effective at protecting oilseed rape seedlings from slug damage under laboratory conditions. However, in the second project, preliminary field experiments showed that protection was less than in the laboratory and was short-lived. If the factors responsible for limiting efficacy of molluscicidal seed treatments in the field can be identified, it may be possible to use modern seed-treatment chemistry to overcome these constraints. Factors are likely to include loss of seed coat caused by the irrigation of plots, volatilisation of active ingredients, reduced uptake at low temperatures and/or microbial breakdown of active ingredients.

We tested 5 environmental factors likely to cause this reduction in efficacy using an analytical method not previously published that analyses metaldehyde directly without prior depolymerisation. Metaldehyde residues in harvested shoots were analysed by GC-FID and showed metaldehyde seed treatments were broken down by soil micro-organisms and washed off seeds by rainfall. Neither wind, nor simulated rain on seedlings significantly reduced metaldehyde residues.

New seed treatment formulations developed by our industrial partner, Luxan (UK), were tested in 3 mini-plot trials on oilseed rape and wheat. In 2 of the mini-plot trials the industry-produced metaldehyde seed treatment protected seeds and seedlings, as well as bait pellets. When plots containing plants with the metaldehyde seed dressing received a half dose of bait pellets 5 days after sowing, protection from slugs was greater than that offered to control plants with a full dose of pellets applied at the time of sowing. In the third mini-plot trial, only treatments with slug bait pellets successfully reduced slug damage. It is likely the reduction in protection offered by the metaldehyde seed dressing was due to 2 unusually high rainfall events the 2nd and 3rd day after sowing. Precipitation for this month was 3 times the average for the time of year when the trial was conducted.

Previous HGCA-funded projects focussed on seed treatments to oilseed rape. The work reported here was extended to include wheat, which is severely damaged by slugs in the UK. Metaldehyde seed-dressings significantly reduced slug damage to wheat seedlings at all doses tested. However, no significant differences were found between doses tested.