Common name: Slugs
Scientific name: There are many species of slugs. The gray garden slug (Deroceras reticulatum), which is non-native, is one species that is commonly found in many states and in most Canadian provinces.
Hosts: Slugs can attack a wide range of perennial and annual plants. Slugs are not insects. They are mollusks and are closely related to oysters and clams. Slugs are similar to snails, but do not have a shell. They are a common plant pest in moist locations and thrive during wet weather.
Slugs can be destructive to many plants in the landscape including annuals, perennials, bulbs, ground covers, fruits, and vegetables. They prefer succulent foliage found on seedlings and herbaceous plants, and fruit lying on the ground. Slugs damage plants by chewing them. Damage from slugs appears as irregularly shaped holes or tattered edges on leaves. Small seedlings may be eaten completely. Slugs can chew deep holes in ripe and overripe fruits and vegetables.
Slugs can live for more than one year. Some may live for as little as two years, while others may live up to six years. Eggs are laid in spring and fall and so slugs can overwinter in the egg, immature, or adult stage. Overwintering slugs will live in a variety of protected sites like burrows in the soil, under rocks and logs and even under loose bark.
Eggs are round, colorless, gelatinous spheres. They hatch in spring when temperatures exceed 40 degrees F. The young slugs begin feeding immediately. Overwintering immatures and adults also become active at this temperature.
Immature slugs resemble adults, but are smaller in size. Slugs have two pairs of tentacles on the front end of their body. The longer, upper tentacles have eyes on their tips. The shorter tentacles near the ground are used for feeling and smelling. The largest body part is the foot, which runs the entire length of the body. The gray garden slug is a very common species across North America. It is about 3/4 inch long, but may grow to 1½ inches. Besides gray, they may be white, yellow, lavender, purple, or black with brown specks and mottled areas. Other species will vary in size and color.
Slugs secrete a slimy mucus to help them move about. They need moisture to create this mucus, so they are highly dependent on soil moisture. When mucus trails dry up on leaves they are sometimes visible as shiny lines. Slugs most often feed at night when humidity is high. To determine if slugs are the cause of damage, check plants at night with a flashlight. Slugs hide under mulch, in debris, and groundcovers during the day.
Cultural management:
A combination of strategies may be necessary to manage slugs. They can be handpicked and placed in a jar with soapy water. Temporary traps, consisting of rolled, wet newspaper and flat boards placed near damaged plants, provide sheltered hiding sites during the day. Check under the boards and in the newspapers in the morning. Slugs in these traps can then be collected and destroyed. Stale beer can be placed in a small saucer that can be sunk into the soil in an infested area. Slugs are attracted to the yeast in the beer, fall into the pan and drown. The traps should be emptied and refilled with beer occasionally.
Habitat modification may be effective in reducing populations. Hiding places, such as excessive mulch piles, weeds, and other organic debris should be eliminated. Vegetable plants should be staked to keep fruit off the ground. Avoid watering late in the day because the moist conditions that are created will be conducive to slug movement. Using drip irrigation to water directly onto soil under plants may help reduce slug activity.
Thin strips of copper bands placed around the bases of larger plants can repel slugs. The copper will give them a slight electric shock when it comes in contact with the mucus on their bodies.
Biological management:
Toads, firefly larvae and ground beetles are natural slug predators, but they may not exist in sufficient numbers to control a serious slug problem.
Chemical management:
Insecticides are ineffective against slugs because they are not insects. Registered commercial baits and granules are available for slug management.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. Use pesticides safely and wisely; read and follow label directions. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.
For more information, contact The Morton Arboretum Plant Clinic (630-719-2424) or plantclinic@mortonarb.org).