What is Causing Those Holes in Your Plants?

In Pennsylvania, there are estimated to be 124 different species of land snails and slugs from 28 families, and all but twelve of these are native species. These slimy pests chew irregular-shaped holes in plant leaves, making them unsightly and completely consuming seedlings or small plants. Members of the phylum Mollusca, these pests are mollusks, like clams and oysters, not insects. Soft-bodied and legless, they move by gliding on a muscular foot that secretes a slimy mucus, facilitating their movement and leaving a silvery trail in their wake. They are typically brown or gray, and similar in structure, except snails have soft shells and slugs do not. They have two pairs of feelers on their heads, one carrying their eyes, the other used for smelling.

While they are hermaphrodites and have both male and female sex organs, they usually require another individual to fertilize the eggs they all carry in their bodies. Once fertilized, they can lay up to 300 eggs a year that will hatch in around thirty days and grow from ½  to over 2 inches in length. They can live for several years, burrowing as deep as 3 feet in the soil and hibernating through winter. Slugs and snails will begin feeding in late spring and continue through frost on both living and decaying plant material. They do prefer some plants over others. Basil, beans, cabbage, dahlia, delphinium, hosta, lettuce, marigolds, strawberries, and many other vegetable plants are favorites. They tend to avoid plants with highly scented foliage such as lavender, rosemary, and sage, as well as other common annuals and perennials, such as impatiens, nasturtium, astilbe, bleeding heart, sedum, hydrangea, columbine, milkweed, creeping and woodland phlox, evening primrose, and spiderwort. Growing plants that they avoid can help control their population.

They absorb water directly through their skin and can drink from puddles, but they have little protection against water loss. During the day, they will seek cool, damp locations to protect themselves from dehydration, so you typically will not see them feeding during the heat of the day. Piles of boards, pavers, and flat stones, excessive mulch, and thick ground cover all encourage snails and slugs by providing shelter. By eliminating these, you can create a less hospitable environment for them. Water your plants carefully at the base of the plant in the morning rather than evening.

There are other steps you can take to reduce their number. Handpicking can be effective with a small population. Once removed from your plants, drop them into a bucket of soapy water. You can trap them by placing boards or damp newspaper on the ground in the evening, then kill any you find hiding under them the next morning. Another way to trap and kill them is to bury a container level with the surface of the ground and fill it with beer or a yeast mixture of one teaspoon of yeast dissolved in three ounces of warm water. The scent will attract them, and they will fall in and drown. There are pesticide baits available, but these should be used as a last resort. Diatomaceous earth, a fine powder made from ground fossilized skeletons of ancient aquatic algae, can be used to create a barrier around your plants. As the slugs and snails crawl through the powder, it scratches their bodies and causes them to dry out.  This is not generally a good deterrent as it needs to be reapplied after rain. Sprinkling them with salt will kill them almost instantly, but this is not recommended in your yard or garden as it can cause injury to your plants.

Slugs and snails can be an ugly nuisance or a devastating garden pest. Making your garden less appealing is the first step toward managing their population. Embrace other visitors to your garden that will feast on them, such as ground beetles, firefly larvae, toads, snakes, and songbirds.