Application of molluscicidal nematodes to slug shelters: A novel approach to economic biological control of slugs

Slugs are a serious pest in floriculture, horticulture, and agriculture in many parts of the world, and Deroceras reticulatum is generally the most destructive species. A molluscicidal nematode, Phasmarhabditis hermaphrodita, has potential for the biological control of pest slugs, but application rates of 3.0 X 10(9) infective juveniles (IJs)/ha are usually required for effective plant protection, rendering applications very expensive. The nematodes are usually applied as an overall application to the surface of soil to provide uniform coverage of the entire area. However, slugs possess a well-developed homing behavior with an ability to locate homing sites or shelters from over a meter away. As slugs are nocturnal foragers and spend significant time in the homing sites during the day, we hypothesized that the application of nematodes to the homing sites may be as effective as the overall application to the entire area. This hypothesis was tested in a newly designed bioassay arena consisting of 90 X 68 X 36-cm-deep plastic tubs mounted with copper slug barriers. Each tub contained muck soil to a depth of 15 cm, four Impatiens or Hosta plants, and 12 adult D. reticulatum slugs. The nematode, P. hermaphrodita, was applied at 0.3 X 10(6) or 0.6 X 10(6) IJs/m(2) either to the entire area or only under a 30 by 30-cm roofing shingle that served as an artificial slug shelter. A standard chemical molluscicide, metaldehyde, and an untreated control treatment were included for comparison. Although metaldehyde caused more rapid slug mortality than the nematodes, the nematodes provided plant protection (i.e., number of leaves damaged and leaf area eaten) equal or better than metaldehyde. Overall, the nematode application rate of 0.6 X 10(6) IJs/m(2) applied only under the shingles provided equally or more effective plant protection than the application of nematodes to the entire area at 0.3 X 10(6) IJs/m(2). This represents a 63% reduction in the total number of nematodes required to treat the same area (i.e., 1.1 X 10(9) IJs/ha vs 3.0 X 10(9) IJs/ha). The additional presence of recycled nematodes provided 96-100% control of the reintroduced slugs. Even the low rate (0.3 X 10(6) IJs/m(2)) applied only under shingles was as effective as the high rate (0.6 X 10(6) IJs/m(2)) applied as an overall treatment to the entire area. We conclude that the application of P. hermaphrodita to slug shelters may provide more economical control than the overall application to the entire area. (C) 2001 Academic Press.