Please note: Below are management guidelines and are currently under review. As more information about slugs is known, additional recommendations will be added.
Seasonal Guide to Managing Slugs in the Pacific Northwest (Provisional)
Draft prepared by: A.J. Dreves, OSU-CSS Dept., May 2016
Population dynamics, life history, behavior and habits, species composition, and seasonal phenology of SLUGS are not well understood and rather limited in the Pacific Northwest. This has hindered the creation of effective, practical and economical management strategies.
An integrated approach should be considered to protect crops from slug damage. A combination of monitoring, cultural practices, biological control agents, and chemical controls are key. Effective management requires applying methods that coincide with different seasonal phases of the slug’s life cycle and behavior. It is important to target slugs when they are active above ground, before numbers get too high and extensive egg laying occurs.
Monitor early in the season and often. Slug populations can increase rapidly and crop damage may result. The more you inspect your field by looking and/or setting up monitoring stations, the more accurate you will be in understanding the magnitude and threat of slugs. A piece of plywood, blanket, or a shingle can be placed on the ground for ≈48 hours to allow slugs to hide underneath it and slugs counted the next morning. Monitor for slug presence before seeding a new crop. It is important to identify the species and age of the slug, as this relates to the seasonal emergence of each species and their feeding strategy (either surface active, burrowing species, or feed directly on germinating seeds in the furrows).
Threshold knowledge tells a grower the relationship between the number slugs found under a particular sampling unit over a period of time in establishing and established crops, and how it correlates to crop damage and economical loss. The decision to treat for slugs has not been necessarily based on a threshold count, hence needs further study. (e.g., estimated 4-6 slugs per blanket in PNW).
- Adults are present in the fall and a small number of eggs (however egg-laying may increase and some neonates will hatch if mild weather).
- Bait after the first rains arrive to achieve best control-the window to treat may be small.
- Slug activity is stimulated by increased precipitation and falling temperatures after dry summer months.
- Timing is critical when using baits; pay attention to the weather, during and after baiting.
- Avoid baiting during cold weather, heavy rain, and windy conditions.
- Target adults before they begin egg-laying, as this will decrease total number of eggs laid.
- Highest surface activity is observed in Oregon during October and November. The water table rises and stimulates slugs to move to the surface for feeding.
- Egg-laying is reduced after the fourth week of October, and little to no hatching until the following spring if the temperatures are cold and heavy rains arrive.
- Chances of slugs encountering bait pellets are increased when ground has less vegetation or is bare.
- Graze stubble with sheep in fall (or spring) to remove slug refuges and reduce slug populations by crushing with their hooves and inadvertently eating them while feeding on vegetation.
- Monitor for slugs after applying baits to see if you need to reapply.
- It has been suggested to treat (e.g., bait in strips) around the perimeter of a field to reduce numbers of slugs coming into a crop from the borders or perimeter with abundant ground cover.
- Slugs are less active over the winter period when night time temperatures dip to or below freezing or when too wet, so winter is typically not a good time to treat for slugs.
- When a freeze occurs, it takes 4-5 days before slugs become active again regardless of weather.
- Adults, eggs, and maybe some neonates (if winter is mild) overwinter under debris, soil clods, and in soil cracks, not too far down in the soil as the water table may be too high.
- A mix of winter adults, eggs, hatching neonates and growing juveniles are present in the spring
- Spring-timed controls are more difficult because most slug populations are in the juvenile stage-very mobile, highest in numbers, less interested in pelleted baits, competition with alternative foods, and spring rains can reduce effectiveness of baits
- However, if conditions are favorable and machinery can get into the field, a spring application (Mar & Apr) of bait can further help reduce numbers to avoid damage
- Adults emerge in the spring and proceed to lay the majority of their eggs
- Neonates hatch do not prefer to feed on baits, they are mainly algal and fungal feeders, but can feed on vegetation
- Smaller bait pellets will increase the chance of a slug feeding on them
- Consider applying applying a treatment prior to the next crop seeding, if slugs are present
- Slugs become inactive as temperatures increase, and heavy rains decline
- Slugs have the ability to aestivate (a state of summer dormancy) for several months if environmental conditions are not favorable. They will rest under residue, travel deep below the ground surface in soil cracks, and enter vole or earthworm burrows
- If slugs are evident, choose a date to treat them when dewpoint is high at ground level or after a good summer rain or irrigation event. Slugs can be active on dewy and overcast mornings
- Control weeds to decrease availability of food sources or use weeds as a trap crop and treat for slugs on the weedy areas
- Review control options to minimize risk of killing natural enemies in the field (e.g., ground beetles)
Other possible practices/thoughts to consider as treatments for slugs:
Before seeding field, consider baiting along fencelines or weedy areas to reduce numbers of slugs before seeding
Assess previous crop for slug levels before seeding new crop
Remove refuge or treat border vegetation as a ‘trap crop’, to prevent feeding on the main crop
Cultivation can kill slugs directly and expose slugs to predators, but also can provide a clean pathway for slug movement. However, soil can be mismanaged by too much cultivation causing soil degradation of soil structure, erosion, and loss.
Short shallow periods can reduce slug numbers
Unfortunately, increasing the organic matter in soil, minimum tillage, tiling, lack of burning, and retaining mulch or residue in field, ALL favor slugs.
Birds (ducks, geese, starlings), Insects (ground and rove beetles, Schomyzid flies), diseases (bacteria, fungus, virus and protozoans), and Parasites (nematodes) all provide control of slugs.
Enhance beetle presence by using reduced risk pesticides for targeting other pests like cutworms, aphids, symphylans, etc.
Purchase nematodes as a means of control [certain European countries only, not presently available in the US]