ARS | Publication request: Removal of molluscicidal bait pellets by earthworms and its impact on control of the gray field slug, Derocerus reticulatum Mueller, in western Oregon grass seed fields.

Submitted to: Crop Protection Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 16, 2012
Publication Date: N/A

Interpretive Summary: Earthworms were the primary agents responsible for bait removal in field observations where they removed significant quantities of bait pellets within 3-5 days of application. This bait removal reduced the efficacy of attempts to reduce slug numbers in these production fields. Utilization of a granular 7.5% metaldehyde formulation, or a 25% liquid formulation of metaldehyde increased slug mortality and was more effective at preventing damage to perennial ryegrass seedlings. The granular and liquid formulations also signifcantly reduced egg fecundity, suggesting these treatments have utility in long range strategies to reduce slug populations in production fields. It is likely that baiting approaches implemented during periods in the fall before rainfalls increase surface activity of earthworms provide greater efficacy in controlling slug numbers relative to approaches utilized after autumn soil moisture encourages earthworm surface activity.

Technical Abstract: Slugs are common pests of grass seed fields in western Oregon and are currently controlled using bait pellets that often fail to give adequate protection. Here we demonstrate the loss of bait pellet products to earthworms and its adverse effects on controlling slugs. Three years of field and greenhouse studies were conducted at 15 locations with contrasting crop, soil type, residue level, and tillage management programs. Baits were isolated in sunken, open bottom arenas in the field to exclude removal by slugs, rodents, or birds. Forty hours of nighttime field observations and photographic documentation were collected to support the hypothesis. Greenhouse studies were conducted on the gray field slug, Derocerus reticulatum (Mueller)., in screen topped arenas to determine the effects on mortality, seedling survival, and egg fecundity in a bait depleting environment. Data showed that 15.8% of all bait pellet types were removed nightly by earthworms with a range of 4.53 to 6.58 days until 100% disappearence. Individual earthworms were observed in the field removing three baits per hour. Earthworms removed a 5% metaldehyde formulation significantly faster than either 4% metaldehyde or 1.0% iron phosphate pellet baits at all sites or years, possibly because of the smaller physical pellet size. Slug mortality was improved by 25%, seedling survival by 38%, with a reduction of nine eggs per surviving slugs, using a 7.5% granular metaldehyde formulation, in a depleting environment. A 25% liquid formulation applied to the seedlings improved slug mortality by 42%, seedling survival by 53%, and reduced the number of eggs per surviving slugs by three over the depleting environment. Earthworms presented no behavioral interest in the granular or liquid formulations, providing growers with an alternate option for control.