Measuring Slug Activity and Density

Slugs are mainly active above ground at night, whenever the relative humidity in their immediate environment is high, the optimum temperature ranges between 40 - 70°F, averaging 60°F, with moisture ample but not flooding, and the wind is negligible, less than 5 MPH. In seedling fields, slugs may feed in the furrow and destroy the growing point of the seedling plant.

Monitoring techniques for estimating relative slug activity as well as absolute slug density are described below to determine how many slugs are populating fields. Estimating crop damage in establishing and established grasses are discussed below. It is important to be able to distinguish the differences between damage caused by slugs and damage caused by other pests. There can be variability in slug numbers and crop damage in a field, so the more locations you monitor in a field, the more accurate determination of slug population numbers and damage level.

 Pit Bait Trap Sampling for slugs in furrow cup in ground


Visual Inspection of Field

Visual inspection of the field by Dr. Glenn Fisher, Emeritus, CSS dept   Clare Sullivan, OSU south county field agent, sampling in early morning, removing and counting slugs  Clare Sullivan, OSU south county field agent, sampling for slugs  

  • Assess presence of slugs before harvesting, after removing a previous crop, and during the inter-crop (fallow) period before next crop is planted. 
  • Visual night sampling can be accomplished by counting slugs on plants or the soil surface while illuminating the sampling area with a flashlight.
  • Inspect plants visually at dusk-dark on a humid, calm night, with a flashlight. Monitor over a number of nights.
  • It is recommended to examine 1-3 linear feet (0.3 to 1 m) of a crop row in at least 5-10 different areas of the field.
  • Check young seedlings and soil surface, approx. 9 inches (22.9 cm) on each side of the row (or dependent on crop row spacing), until the entirety of the a linear foot row has been evaluated.
  • Turn over clods and refuge and inspect closely to determine if slugs are present. Slugs in the soil may be difficult to find, since it is common for them to curl up or hide in holes in the ground.
  • Record the number of slugs found for each sample area.
  • Repeat this procedure in the other areas of the field. 
  • Keep a record of the results of each sample taken.

Relative Methods for Assessing Slug Activity Levels in the Field

Top of a slug blanket Bottom of a slug blanket secure slug blanket from wind

Examples of ‘slug shelters’ used to record relative slug activity, include: 18 x 18-inch slug mats/blankets, 12 x 12-inch ceramic tiles, white-rolled roofing material or shingles (12 x 12-inch), small flat plywood boards, bricks, 20 x 20-inch squares of hardboard covered with polystyrene, inverted black garden pots secured in ground, open or closed refuge stations with/without bait pellets in arena.

  • Sometimes relative measurements can be biased towards adult slug activity by not detecting as many juveniles and neonates; so may under-estimate slug populations.
  • Larger sizes of ‘shelters’ are not necessary, are difficult to carry in the field, and not always possible to deploy without destroying an area of the crop. Use a shelter size that fits between rows and/or over established crowns. A grass crown can be trimmed so shelter fits over top and should be secured.


  1. Clear an area from vegetation/foliage bigger than the size of each shelter.
  2. Place one to three shelters per research plot out in late afternoon or early evening. To get an idea of slug population levels and need for treatment,  one to six shelters can be placed in various locations in a field, depending on field size.
  3. Secure shelter, so it doesn’t blow away and is snug against the soil surface. Leave in the field for 1-3 days.
  4. Shelters should be inspected in the morning, before shelters are exposed to direct sun after placement, unless cloudy, overcast and cool all day.
  5. Carefully lift up shelter and visually count slugs under shelter and around soil surface to estimate activity on the ground surface.
  6. Remove slugs that are present on underside of shelter and on surface. Place slugs in a ventilated container to transport back to lab.
  7. Choose a subset of slugs from container to identify percentages of each species found.
  8. Weigh slugs to understand age structure and classify slug age by weight, which helps in the decision-making process for treatment .
  9. Shelters/stations should be re-randomized to different areas in plot weekly.


Shelters Used in Various Research Studies

  1. Commercial mats/blankets: Mats/blankets are composed of three layers with the top being metallic silver for maximum light reflection and the bottom consisting of black perforated plastic to absorb moisture. Between these layers is an insulating fabric enclosed to hold moisture within the mat.  Mats are 18 x 18-inch (50 x 50 cm) and can be cut to smaller sizes. If conditions are dry, blankets should be soaked in a bucket of water then secured to ground surface. Blankets should be re-soaked at each visit, if dry.

  2. Refuge traps (baited with slug pellets or unbaited with a cover over the top): Place a small wood board of any size on ground that has been cleared of vegetable. Mark with a flag to locate it. These traps should be examined the following morning (early) before traps are exposed to direct sun after placement. Bait (7-10 pellets) can be placed under a refuge trap (6 x 6-inch) or underneath mats/blankets, to poison the slugs and reduce the chances of slugs leaving the trap.

  3. Standard uncolored Marley tiles (10.5 x 6 x ½ inch or 26.7 x 15.2 x 1.3cm): Tiles can be arranged on a 5 x 4 grid placed 34 yds apart (big study area) (South 1964).

  4. Open bait stations (12 x 12-inch or any size)- Clear area from vegetation and add 3-6 bait pellets. A bait station consists of four pellets of either cereal bran-based bait with 2% metaldehyde plus 4% carbaryl or a standard metaldehyde. Keep the bait the same in the bait stations and always use the same amount of the bait in the bait stations (Fisher 2007).

  5. Foam square pad-traps 25 x 25 cm (Hommay et al 2003): Pad traps are 13 mm thick made of polyvinyl chloride. Spread traps 3 m apart in a plot. Soil surface should be cleared of vegetation and watered over a 50cm square area. Counts can be made under pad and 12.5cm strip around traps to estimate activity on the ground surface.

  6. Defined area traps (Glen et al. 2006), fenced beer traps and fenced slug mats: A plastic ring 25 cm high and 55 cm in diameter (0.25 m²) can be inserted 10 cm deep into the soil. The soil surface inside the ring contains 10 g slug pellets and the ring covered with black, light-impervious fabric.

  7. One square meter areas: Set up monitoring areas in the right corner at the front of all test plots after sowing, and treat with 40 g Metarex. Check regularly 2x week), early morning or at night to see how much bait is still visible and record numbers of slugs present.


 Absolute Measurement of Slug Density Per Sample

 Soil Sampler for Cold Water Extraction   


Cold Water Extraction Process, also called Gradual/Rapid Flooding of Soil/Grass Samples (South 1992)


The Cold Water Extraction Process requires time (at least 5 days), labor, space and patience to count actual number of slugs in a soil-grass sample.

  1. Remove soil-grass sample (8 -12 square inch or round sod-grass turve) at a depth of 4-6 inches (10-15 cm).
  2. Place sample in a plastic watertight container covered with a slug-proof fine-screened lid and transport to a cool place for flooding with water. The containers with samples should be kept out of direct sunshine at all stages of processing and brought back to the lab.
  3. Add slices of carrot/lettuce approximately 0.5 – 1.0 cm thick to the surface of the sample in each container to attract slugs to feed and rest. Seal container. 
  4. Add approximately 1 inch of water to the bottom of the container. Containers should be placed in the dark overnight.
  5. Examine each container for 3 minutes, after 24 hours, for slugs that had come up from the soil surface, taking particular care to look closely for small neonate and juvenile slugs. Remove slugs, count and weigh, and record species.
  6. The location where each slug was found can be recorded in the following categories: • Container lid • Container sides, above water level • Container sides, below water level • On lettuce/carrot slices • On soil surface• Inside cereal stalk or piece of straw • On soil below water level
  7. More water (another 1 inch) is added on each occasion to the base of the container, after a count. Slugs present in the sample should continue to come to the surface by flooding the sample from below to drive slugs to the surface. Remove, count, record, identify and weigh a subsample.
  8. Samples are flooded over a period of 3-5 days by adding 1 inch more of water to each container each morning.
  9. For the final stage of raising the water level to the soil surface, water is added carefully so that the water just reached the surface of the samplewith only the highest points showing about ¼-inch above the water surface.
  10. After the sample has been fully flooded the samples were left for a further day and then a final examination was made for slugs.
  11. Classify the size of each slug by weighing them. Record weights. The number of slugs present was totaled after 3-5 day submersion of a soil/grass sample.


Assessing Levels of Other Field Organisms

Pitfall trap for collecting beetles and billbugs

Pitfall traps can be used to assess the presence of ground beetles. Beetles with fall into the funnel attached to a jar below with alcohol.


Mustard Expellant/Extraction of Slugs (Gavin et al. 2005)

  1. A bottomless five-gallon bucket is driven into the soil with a mallet to a depth of 4 inches.  

  2. Prepare a dry mustard solution by adding 1 level Tablespoon of dried mustard to 1 gallon of water to create a mustard liquid solution. You can buy dry mustard powder in bulk. Prepare in advance.

  3. Stir constantly with a spatula or whisk until solution is free from lumps.

  4. Pour mustard solution into bottomless bucket over the soil at a depth of 1 inch.

  5. A second dose of mustard solution might need to be added to bucket. Then cover tightly with a lid.

  6. Total extraction time is approximately 30 minutes.

  7. Replicate buckets by placing in several areas of the field.

  8. Slugs, earthworms, and other invertebrates will leave the soil and can be counted on soil surface.


Collection of Ground Beetles Present in Field: (Symondson et al. 1996)

  1. Pitfall traps can be constructed by digging a small pit that fits a small plastic margarine or yogurt container flush to the surface of the ground. Punch a small hole in bottom of cup for drainage, if rain gets in.

  2. A funnel can be added that fits inside the container to reduce escapees. Large square hardware cloth can be added over cup to keep big animals out. 

  3. Place a roof, board or cover over the top of the container to provide darkness for the organisms captured and keep rain out.

  4. Check for contents twice weekly.


Extraction of Earthworms from Soil (Gavin et al. 2005; Steiner et al. 2006)

Researchers studying earthworms have employed chemical expellants to drive worms to the ground surface for counting. It is rather quick.

  1. Prepare mustard solution approximately 4 hours prior to sampling in the field (or alternatively, the day before sampling).


Bait Pellet




Put out slug bait in late afternoon and return early the next morning to check for slugs or evidence of slime trails in a cleared vegetative area. Put several bait stations in the field. Scratch a small area (12 x 6 inches) of the soil surface (making it easier to see small slugs), and drop four to six pellets of bait on it.





Wood covering bait




You can cover the bait with a scrap of wood, a shingle, or a tile. This prevents other creatures from disturbing the bait, and the cover helps to keep slugs sickened by the bait from moving away.