Ash Sial, Department of Entomology, University of Georgia;
Renee Holland, UGA Extension, Bacon County
I have recently received reports of slug infestations in Georgia blueberries. A number of slug species including Limax spp., Arion spp., and Deroceras spp. are known to infest blueberries. Slugs are soft-bodied mollusks without a shell. They can be 0.25-10 inches long. Their body color can vary from yellow to green to brown to black, and some may have spots or colored patterns. They have stalked eyes that appear like insect antennae.
Slugs start reproducing at the age of three months and can reproduce year-round. Each slug has both male and female reproductive parts. During mating, two slugs cross-fertilize to reproduce. Slugs generally lay 3-40 small transparent eggs at one time and each slug can lay up to 400 per year. Eggs are laid in soil cracks and hatch in a few weeks during late spring and summer when weather is cool and moist. Newly hatched slugs travel through the soil and feed on roots. Slugs can climb up the blueberry bushes through the stem or through the branches when fruit laden branches bend down and contact the ground or vegetation between the rows. Slugs cause damage by feeding on foliage and berries. They can contaminate the harvested fruit and reduce fruit quality by leaving the slime trails on the fruit. They are most likely to be a problem in cool and wet summers.
If you suspect slug infestation in your blueberry field, start monitoring by looking for slime trails in early spring (March). Slug populations can also be monitored by placing small mounds of slug bait (or bait stations) near potential slug habitat (consistently wet/moist areas in the field) and checking periodically for activity. Definitive thresholds have not yet been developed to implement control strategies for slugs, and tolerance levels vary according to end product usage and processor. It is always a good practice to discuss your particular situation with your marketer to determine their tolerance.
Slugs (and snails) rarely cause economic damage in blueberries. However, if it is determined that treatment is necessary to control slugs, it would be best to use one of the baits registered for slug and snail control. A number of slug baits are registered for use in blueberries. The most widely used, and the most effective, are baits with metaldehyde. Baits containing iron phosphate are also available and have the added benefit of being approved for organic production; however, their efficacy is questionable. Baits can be applied broadcast or in a band, but must not come in contact with the fruit. Baiting prior to harvest is common if slugs are known to be present. Baiting after harvest in the fall helps reduce next year’s population by controlling the egg-laying adult slugs. Here are some of the baits available for slug control: Deadline M-P (Metaldehyde, 4%), Bug-N-Sluggo (Iron Phosphate + Spinosad), and Ferroxx (Sodium Ferrix EDTA).
Following preventative strategies can also be helpful for long-term management of slugs. 1) Trellising the blueberry plants to keep branches from touching the ground can reduce the number of slugs gaining access to the plant; 2) Mowing or complete elimination of the vegetation in the plant row and between the berry rows can reduce slug habitat; 3) Keeping crates and pallets away from damp soil and grass prevents migration of slugs into and under crates taken to the field before harvest which helps reduce the chance of contaminated fruit; 4) Geese can also be used as predators to control slug populations specifically in organic blueberry fields; and 5) Eliminating consistently wet/moist spots in the field with good drainage, reduces favorable habitat for slugs and prevents establishments of slug populations in the field.
1) Integrated Pest Management for Blueberries: Slugs. Washington State University, Whatcom County Extension. https://whatcom.wsu.edu/ipm/manual/blue/slugs.html (Date accessed: 27 Oct 2020)
2) A Field Guide to Identification of Insect Pests, Diseases, and other Disorders in Blueberries: Slugs and Snail. https://fieldguide.bcblueberry.com/slugs-snails/ (Date accessed: 27 Oct 2020)