Maryland Grows

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Slugs are a common field crop and horticultural pest. Managing them is challenging because their damage is often confused with other pest damage, and pesticide options are few and expensive. Luckily, knowing what to look for and the growing practices that reduce slug damage helps reduce the problems with this pest.

What exactly are slugs?

Unlike most other plant-feeding pests found in fields or your garden, slugs are not insects. Instead, they are soft-bodied, legless mollusks that are covered in slimy mucus that they secrete and leave behind as a trail. Slugs dry out easily, so they prefer environments with lots of shade and moisture. Slugs feed on a wide variety of food sources, but when they eat plants they can cause a significant amount of damage making them the bane of many farmers and gardeners.

In our area, the most common slugs found in crop fields and gardens are:

• Gray garden slugs – about 2 inches long when fully grown. Ranging from cream-colored with irregular gray spots to dark brown with dark spots.
• Marsh slugs – Smaller, about 1 inch long. Tend to be dark.

Eggs of both species are small, clear, round, and gelatinous. They are usually laid in clusters under plant residues.

What kind of damage do they cause?

Properly identifying slug damage is the first step to managing their damage in the field/garden. Since slugs feed primarily at night, when the damaged plants are noticed and the field or garden searched, there is usually no culprit immediately identifiable. However, telltale signs of slugs include:

• Ragged holes, often between leaf veins (slugs scrape at plants with a file-like mouth part, rather than taking bites, so the holes are often rough)
• Nothing remaining of seedlings but leaf mid-ribs and stumps
• Crisp-edged, irregular holes in soft fruits such as strawberries or tomatoes
• Slime trails on plants, walls, rocks, or mulch

If all else fails, the slugs themselves may be found hiding under mulch or debris during the day.

In field crops like corn and soybeans, seedlings can tolerate a large amount of defoliation without losing yield. Yield loss usually occurs when the whole plant is killed, which rarely happens in corn, although it is more likely in soybeans. In fruits and vegetables, slugs often feed on the part of the plant we want to eat (strawberry fruit, lettuce and herb leaves) causing unsightly holes and making produce unmarketable or inedible.

How do I manage slugs?

As the weather warms up, slugs often stop being a problem because they cannot tolerate the heat and dry weather. In many cases, one can wait for plants to outgrow the damage. However, in particularly wet springs, management may be necessary. Below are some of these options.

Bait-treatments

Insecticides do not control slugs because they are not insects. Molluscicides (pesticides that target slugs and snails) can be used, although they are expensive and lose efficacy in wet weather. Molluscicides are formulated as baits, so slugs need to actively feed on them. They are best applied directly around the vulnerable plants in the late afternoon and when there is some soil moisture to encourage feeding. Iron phosphate-based products (e.g. Ferrox AQ® and Sluggo®) are options for home gardeners. Sluggo® is OMRI-approved and suitable for organic production.

When using pesticides, the label is the law. Make sure the product is registered in your state and crop(s) and follow all restrictions. A pesticide applicators license is required to use some formulations of these products, while others are available at home and garden stores.

Production practices

Certain growing practices can make conditions unfavorable for slug activity, and are often easier and less expensive than chemical controls. Because slugs do not like to dry out, practices that eliminate hiding places and damp conditions will make your field or garden less hospitable to slugs.

Prevention techniques for crop fields:

• Delay planting until soil temperatures are warm enough for rapid early growth
• Apply starter fertilizer to boost seedling growth and reduce the period of vulnerability
• Use some form of tillage when slugs are consistently a problem, or in years with high slug pressure

Prevention techniques for gardens include:

• Avoid using mulch where slug problems are common
• Avoid watering late in the day so the soil can fully dry by nightfall
• Remove or relocate slug hiding places like boards and pavers

Natural enemies

Slugs are prey for many different predators including arthropods, like some spiders and beetles, and vertebrates, like snakes and toads. These predators can be encouraged to make a home in your field/garden. Reducing insecticides where possible may support higher populations of slug predators (learn more about some natural enemies here).

Finally, in home gardens, more hands-on approaches like removing slugs and drowning them in soapy water can be worthwhile. Gardeners can go out at night with a flashlight and hand-pick slugs. You can also set out boards or shingles in the garden and check them each morning and drown any slugs found. You can also try trapping slugs: bury bowls in the ground so that they are flush with the soil and fill them with beer. The beer attracts slugs which drown when they try to drink. These should be emptied and replaced regularly, and you may need to place many in your garden to actually reduce damage.

In conclusion, if slug problems are suspected, you can take action by identifying the damage and deciding whether an intervention is needed. Using practices that will warm and dry the soil and boost plant growth is very effective in field crops. In gardens, removing slug shelters like mulch, pavers, and boards can really help. In both cases, avoid insecticides when possible to promote naturally-occurring biological control. As a last resort, baits may help with slug problems, but they need to be used carefully.

Praise-God Igwe is a research technician in the Hamby lab at the University of Maryland and a recent Ecology and Evolution graduate from the University of Maryland.

Maria Cramer is a PhD student in the Hamby lab at the University of Maryland who studies sustainable pest management in field corn.