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

Slugs are common pests of grass seed fields in western Oregon and the current focus of repeated, and often unsuccessful, efforts by growers to control them using molluscicides. Here we document rapid loss of molluscicidal bait pellets to earthworms and the resulting adverse effects on slug control. Three years of field studies were conducted at 17 locations with contrasting crops, soil types, residue levels, and tillage management programs. Baits were isolated in covered, 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 that earthworms were removing slug bait before it had the opportunity to kill slugs. 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. Field data showed that an average of 17% of all bait pellet types were removed nightly by earthworms, with a range of 5.1e6.4 days until 100% disappearance. Individual earthworms in the field were observed removing up to three bait pellets per hour. Earthworms removed a 5% metaldehyde formulation significantly faster than either 4% metaldehyde or 1% iron phosphate pellet baits, possibly because of the smaller physical pellet size. Seedling survival in the greenhouse 65% was achieved by the highest rate of 4% metaldehyde bait pellets, the two highest rates of 7.5% metaldehyde granules, and the two highest rates of 25% metaldehyde liquid formulation. Earthworms showed no behavioral interest in the granular or liquid formulations, providing growers with attractive alternatives to the ineffectual pelleted baits currently in widespread use. Published by Elsevier Ltd.