Invasive Slugs

Module 4 – Monitoring and control


Slug damage can be deceiving as there are other pests which cause similar damage. Often, it’s necessary to verify slugs are in the area. There are a few methods to look for slugs and to verify slug damage.

Refuge traps

One of the simplest types is a cut up asphalt shingle or old board placed in an area where slugs are thought to occur. These will hold moisture on dry days and provide cover. They need to be checked early in the morning. The number of slugs under a board is not representative of the total number of slugs in an area for a variety of reasons. But these will tell you if slugs are a possible pest.

Beer traps

Using slugs’ interest in yeasty things, a beer trap is simply a plastic cup, buried at dirt level and filled with a small quantity of beer. The slug will fall into the cup of beer and drown. Placing a small roof over the cup increases the effectiveness of the trap by providing cover, and keeping the rain out. This type of trap does bring slugs in, so it’s best not to place near desirable vegetation but rather on a boarder.


Slug activity in Alaska spikes in the fall as the moisture returns but temperatures are still mild. This is also the time of year when night returns. Heading out shortly after nightfall with a flashlight or headlamp is a good time to spot slugs as they wake up are migrate to new areas.


Control of slugs is often difficult, but falls into two categories, killing slugs around desirable vegetation, or preventing slugs from from getting to the desirable vegetation. There is no sure way to prevent slug damage, often control is a process of trial and error involving multiple means.


There are several bait based and contact molluscicides on the market. Each has advantages and disadvantages, and depending on the application, the environments and the desirable vegetation being protected results vary.

Metaldehyde – Metaldehyde disrupts the slugs ability to produce mucus, leading to dehydration. It is fairly quick, depending on the environment. Forms of the molluscicide legal in Alaska are all restricted use pesticides and all in bait form. They may be used in agriculture settings with crops and produce where the molluscicide will not contact food. There is no metaldehyde product legal for sale for the home owner. It is also toxic to vertebrates causing tremors and shakes and eventually disrupting neuro channels killing the animal. Poisoning of dogs and cats with this molluscicide are not uncommon where use is high. Application is generally early in the season prior to planting to reduce the slug load prior to seedling emergence.

Iron phosphate – Iron phosphate causes the slug to stop eating, eventually the slug will starve. It does this by binding iron from the slugs blood. This molluscicide is slow, taking 3-7 days to kill the animals. Iron phosphate can be found in organic and conventional pesticides, and products with this molluscicide can be found in many home improvement stores. Often it is produced in a bait form. Bait forms may have added chelates which can cause poisoning in vertebrates. Use is often by the home gardener in garden settings to reduce slugs found.

Ferric EDTA – Ferric EDTA functions similarly to iron phosphate but quicker, usually in 3 days. It binds iron in the slugs blood much stronger than iron phosphate. Products with ferric EDTA are usually in bait form, and recommended application rates are lower than iron phosphate. These molluscicides are less effective in moist and rainy conditions than iron phosphate because it is more soluble. Application is similar to iron phosphate with most application being by the home owner, and products with ferric EDTA are available in many home improvement stores.

Sprays – Slug and Snail sprays often contain a mixture of essential oils. It is unclear how these products kill slugs, and evidence for their efficacy is mixed. Essential oils are exempt from EPA regulation and collateral damage on other types of garden insects or pets is unknown. Use is often by home gardeners on ornamentals or small plots.

Hand picking – Because the slugs causing damage are often migrating in from a source population elsewhere, hand removal can be effective in keeping damage down. Inspections for slugs should be done very early in the morning, or late at night when slugs are most active. Costs on this type on management is low, and it’s applicable to a number of settings. This often is the prefered way of controlling slugs in a home garden setting as there is no risk of pesticide poisoning or collateral damage.


There are a variety of barrier options available, and effectiveness varies on the environmental situation and the design of the garden or field. No barrier is 100% effective for a variety of reasons.

Xeric – The most effective barriers use the slugs need for moisture. One of the most effective and versatile barrier is a xeric one, often gravel or a vegetation free area. Slug behaviour is variable, and some slugs will cross dry areas, but many will not. Installation of a xeric barrier should consider the ability to keep the area vegetation free, so an area which can be rototilled, or gravel which can be occasionally raked are more effective. Barriers which hold moisture should be avoided as these may facilitate slug movement. These include wood chips or wood planks or concrete pavers. Moisture reduces the effectiveness of these barriers, but they are low cost and versatile to a variety of situations. And have no collateral damage concerns.

Elevated bed and planters – Often isolating plants from the larger slug population will help reduce slug damage. Often planters are placed on patios or settings which also offer a xeric barrier. Elevated beds can be further enhanced through the use of copper bands. Results from copper bands are mixed, as it appears they slow slugs progres across the bands. This may be effective during periods of limited darkness, and slug activity. In very high value situations the copper band can be electrocuted with a 10-12v power source. This has shown to be effective in trials, but has limited application and high cost.

Slug rings – Collars or rings are plastic devices placed around the plant. They are often made of a slippery material, or can be coated with a slippery product like Fluron. This prevents the slugs from reaching the vegetation. These rings have limited application, and the lubricant often requires reapplication, and is expensive.

Continue to Module 5