Control of slug damage in oilseed rape by seed treatment - development and field tests

Abstract

Slugs are serious pests of oilseed rape that eat rape seedlings immediately after emergence, thus reducing plant stand. Current slug control methods of broadcasting bait pellets on the soil surface often fail to give adequate protection. A previous HGCA-funded project (HGCA Project Report OS54) demonstrated that molluscicidal seed coatings are highly effective at protecting oilseed rape seedlings from slug damage under laboratory conditions. This project aimed:

To compare efficacy of the insecticide imidacloprid as a seed treatment with that of methiocarb and metaldehyde.

To determine efficacy of seed treatments against a broad range of slug pests.

To determine the duration of protection given by seed treatments.

To demonstrate efficacy of seed treatments in field and semi-field experiments.

To develop a quantitative analytical method to monitor metaldehyde uptake by seedlings.

Laboratory experiments were done to test the efficacy of metaldehyde, methiocarb and imidacloprid seed treatments to a range of slug species. Metaldehyde and methiocarb reduced slug damage to oilseed rape seedlings caused by representative species of the Limacidae (field slugs), Arionidae (round back slugs) and Milacidae (keeled slugs), the families that comprise all UK pest species. Conversely, imidacloprid did not protect seedlings from damage by any species. In outdoor pot experiments investigating the length of protection offered by seed treatments, both metaldehyde and methiocarb protected seedlings for up to 8 weeks after planting. By this time all plants had reached at least 6 true leaves, a stage beyond which oilseed rape is capable of outgrowing all slug damage. In semi-field trials, metaldehyde and methiocarb seed treatments protected seedlings better than the controls, but not as well as slug bait pellets. However, in the field trial using standard agricultural practice, slug bait pellets protected oilseed rape seedlings from slug damage, but none of the seed treatments did. In order to determine reasons for lack of efficacy in the field we have developed a method to quantify metaldehyde residues in plant material using gas chromatography-mass spectrometry.

Both metaldehyde and methiocarb showed promise as seed treatments to control slug damage in oilseed rape from a broad range of slug species. Further investigations are required to identify the constraints to efficacy of molluscicidal seed treatments in the field. Once these have been detected, new seed dressing formulations could be produced to overcome the constraints thus increasing the efficacy of our seed treatments. It must also be noted that while seed treatments can protect seedlings from slug damage, they act as repellents and are not necessarily fatal to slugs in this type of application. It is therefore suggested that molluscicidal seed treatments may play an important role as part of an integrated pest management system, together with slug bait pellets, reducing the number of bait pellet applications and giving seedlings vital protection at their most vulnerable stages.