Hosta Pests: Snails and Slugs




Several species of slugs and snails are troublesome throughout north America in field crops, home gardens, floricultural plantings and greenhouses. The gray garden slug, Deroceras reticulatum, the gray field slug, Deroceras laeve, and Arion fasciatus, are the most common species of slugs, while the banded wood snail, Cepaea nemoralis, is the most common species of snail. Sightings of the spotted garden slug or giant slug, Limax maximus, have also been reported in places as north as Ontario, Canada. These are important pests in the home vegetable and flower garden every year. In the field, the gray garden slug has proven to be the most destructive of these, causing severe damage to crops including corn, grains, clovers, vegetables and small fruits.

Slugs and snails are molluscs, as are oysters and clams. They are similar in structure except that the snail is protected by a hard, calcareous shell that makes it less susceptible than slugs to dry conditions and sun exposure. When adverse living conditions confront snails, they react by sealing the opening of the shell with a mucus sheet, known as the operculum, which soon hardens to a leathery texture. The snails can then become dormant and have the ability to remain in this state for up to 4 years.

Slugs range in size from 0.5 to 20cm (1/4 - 7 inches) in length, depending on species and age, with colour variations of dark brown and black to light gray. The banded wood snail has a shell which is a conspicuous, pale yellow with longitudinal chocolate-brown stripes, and is approximately 2.5cm (1 inch) in diameter.

Slugs and snails are legless creatures that glide along on a path of mucus. This mucus dries out and can be seen in the daytime as a shiny trail over leaves, fruit and soil. The detection of these "slime trails" may be the only way of determining their presence, as slugs and snails generally feed at night. Occasionally they come out of their hiding places and feed in the evening or on overcast days. For this reason, many growers do not attribute the apparent injury to slugs and snails, as the pests themselves are not visible. When trails and damage are observed, the slugs and snails can often be found on the ground near the injured plants, hiding under decaying plant debris, stones, clods of soil, or logs.

Slugs and snails feed on the lower leaves of many plants especially in the areas between the veins. Immature slugs and snails damage plants by rasping away the surface tissue, while adults eat holes through the leaves, nip off tender shoots or cause complete destruction of seedlings. Damage to the leaves, along with wind, often causes leaves to shred or in the case of grass and corn, to split lengthwise.

Litter heaps, compost piles, drain pipes, greenhouses, well walls and uncultivated areas with dense plant growth, provide ideal sites in which the gray garden slug, gray field slug and snails are capable of overwintering in all developmental stages.

IN THE HOME GARDEN

Slugs and snails feed on plants such as HOSTA, petunia, zinnia, salvia, lily-of-the-valley, bean, the fruit of tomato and strawberries, lettuce, Brussels sprouts, cabbage, pepper and many others.

Snails and Slugs are more active / troublesome:

  • 1. During prolonged periods of wet, overcast weather.
  • 2. In crops with a dense protective canopy.
  • 3. In gardens that are heavily mulched and/or surrounded by tall grass and weeds.
  • 4. In areas containing debris such as decaying leaves, poles, boards and logs.

Partial control of slugs and snails can be obtained by adjusting the above situations where possible. If damage persists, chemical control may be warranted. It is important to note that a combination of control methods is much more effective in reducing population levels.

Control Methods

Sanitation - Sanitation can be highly beneficial in controlling populations of slugs and snails. It involves the removal of all materials that could provide daytime hiding places and ideal egg laying sites, such as plant debris, dense plant growth, rocks, boards, and logs. This is especially important in shaded areas near trees and buildings. The use of mulches should be avoided in any of these sites. Tall or densely growing plants may need to be thinned to allow for more air movement and light penetration and therefore, drier environmental conditions.

Barriers - Slugs and snails avoid crawling over any dry abrasive material such as gravel, sharp sand, wood ashes, diatomaceous earth, or lime. The increased production of mucus required to free themselves of these materials exhausts them and soon causes death. A 30-45cm (12-18 inch) band of any one of these inert materials spread along borders or between rows of plants would be a beneficial repellent.

Fly screening approximately 10cm (4 inches) wide will also provide an effective barrier against slugs and snails. This screening should be partially imbedded in the soil for support to a depth of 5cm (2 inches) and completely surround the plant. When using the screening on a cold frame, it should be tacked across the top of the frame leaving the cut edges of the screen sharp.

For potted plants, placing the pots on boards or some other means of support over a pan of water can reduce slug and snail damage.

Wrapping plant stems with cotton batting will also deter these molluscs; however, this is labour intensive and, depending on the plant, not always practical.

Traps - Traps are an effective method of control in small areas. Boards, bark, or a similar material at least 15cm (6 inches) square can be placed in the garden close to those plants which are susceptible to slug and snail attack. These traps should be checked each morning so the slugs and snails can be removed from under them and destroyed. Although handpicking is labour intensive, one hour of doing so will provide a noticeable reduction in populations.

Stale beer placed in a container, such as a pie pan, to a depth of about 2.5cm (1 inch) is very attractive to slugs and snails. These shallow containers can be sunk into the soil so that the top edge is at ground level and placed 3 m (10 ft.) apart throughout the garden. The slugs and snails will crawl into this dish or pan and drown. These traps should be put out early in the evening when feeding activity begins and emptied regularly as slugs and snails accumulate.

Natural Predators - Slugs and snails have natural enemies such as toads, snakes, several species of ground beetles and their larvae, wild birds and ducks, with the toad being the most important. It is highly beneficial to encourage such predators to reside in your garden to maintain a natural balance.

Chemical Control - Molluscicides such as metaldehyde or methiocarb are available in bait form for use in the control of slugs and snails. Baits are sold under several trade names in garden supply and hardware stores. Metaldehyde is registered for use in vegetable crops, while Methiocarb can be used on ornamentals and lawns only. Follow label direction to ensure safety and efficacy of each product.

WARNING REGARDING PETS AND CHILDREN

These products are harmful if swallowed. Domestic animals such as dogs and cats as well as birds, are attracted to metaldehyde baits, as are curious children. Because poisoning may occur, it is important to keep them away from treated areas. Scatter bait thinly; never place in heaps or piles unless covered.

The gray garden slug, Deroceras reticulatum, is capable of causing serious damage to corn, especially in May and June before the plants become well established. Although corn and beans are most susceptible, broccoli, cabbage and other vegetables can be severely damaged throughout the season with heavier losses occurring in the fall.

Damage occurs most commonly in fields that were in sod the previous year, particularly those that were not spring ploughed until later in the season, and in fields with crop debris on the soil surface. Slug damage is also more common after a period of cool, wet weather or in wet areas of the field. Control

Molluscicides registered for use on field crops are too expensive for commercial use. Management techniques must then be considered; however, even with the best management practices, there may be some problems with slugs in certain years. To help minimize losses:

  • 1. Avoid planting corn, beans or other susceptible crops on poorly drained fields if there has been a problem in that field previously.
  • 2. Avoid planting susceptible crops following sod. If corn or beans are to be planted following sod, plough as early as possible in the fall.
  • 3. Use conventional rather than minimum tillage on wet fields or fields where slugs have previously been troublesome. Minimum tillage leaves the crop refuse of the preceding year on the soil surface, which provides an ideal hiding place and egg laying site for slugs.
  • 4. Remove or trim tall, dense stands of weeds along fences and ditches.

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