Slugs and Snails: Constant Nemeses of Ornamentals

January 28, 2003 - 13:42

They're not one of the most common pests, but they're pests in their own right. Find out how to prevent and eliminate slugs and snails.

Slippery! Slimy! Creepy things, yuck! This pretty much
describes the general opinion of what a slug or snail is. These pests fall
under the general responsibility of entomologists when it comes to who has to
deal with calls and questions concerning their control, but they are far from
being an insect. In fact, they are not even in the same phylum.

Slugs and snails belong to the phylum Mollusca, which puts
them in the same grouping as oysters, octopods, crustaceans and clams. They are
in the subgroup Gastropoda, which includes animals with a head, a ventral
muscular foot attached to the abdomen and a shell. In fact, most of the
seashells you collected walking on the beach during your last vacation were
members of Gastropoda. No, slugs do not have a visible shell, though they
actually have a much-reduced internal shell. Snails have an obvious external
shell that is large enough to hold the entire body. The bodies of both groups
are soft, unsegmented and yes, slimy. The head has two pairs of tentacles: one pair
is short, located on the front of the head and used for touch and smell; the
other pair of tentacles is longer, located on top of the head, with an eye at
the distal end of each tentacle. Slugs and snails are hermaphrodites,
possessing both male and female organs. Even though they can be
self-fertilized, they usually mate with another slug or snail and may use both
male and female organs during mating. In some situations, they may be male or
female during part of their life, in addition to being a hermaphrodite.

There are many more species of land snails than slugs, and
most do not cause noticeable damage to ornamental plants. However, the many
that do are among the most damaging pests in the greenhouse, garden and
landscape. The slugs and snails that cause most of the damage are introduced
species that are not native to the area. They are usually gregarious and build
up large populations in a localized area, potentially causing severe damage.
Even though slugs and snails are not insects, they are very similar in their
biology and behavior.

Many species of slugs and snails are economic pests of
ornamental plants. The most noteworthy snail species is the brown garden snail
(Helis aspersa), which was initially introduced into California from France in
the 1850s. It was brought into the United States to be used as food but escaped
into the environment and has been a major pest since then. There are several
species of snails that are common in moist areas, but most of the damage is
from their presence on plants that are being shipped to market. Some snails are
predators and feed on other snails, while others feed primarily on algae and
other vegetative material common in wet areas. In much of the Southeast, slugs
are the most damaging group. Some of the important species are brown slug,
garden slug, gray garden slug, greenhouse slug, Lehmannia slug and spotted
garden slug.

A slug/snail life

Slugs and snails move by sliding along on a muscular foot.
Wave-like movements on the contact portion of the muscular foot on the mucus
layer secreted by a gland behind the mouth and smaller glands on the foot
propel them along. The protection of the slippery mucous allows them to move
over very rough surfaces. This foot is very sensitive to particles and
chemicals, and it is a common target to many of the home remedies used against
snails and slugs. Once the animal has passed, the mucus dries leaving the
telltale shiny slime trail that is characteristic of these pests and is the key
to identifying that they are the villain that chewed holes in your plants the
night before.

Snails and slugs are most active at night, but they can be
out during the day if it is overcast and especially if it is foggy or rainy. When
the sun is out, they seek a hiding place out of the dryness and heat. As the
temperature increases, they seek shelter in a cooler area to protect themselves
from desiccation during the day. They can commonly be found under boards,
leaves, rocks and any other dark cool place that is protected from the sun.
Some species will even burrow into the soft earth where they will rest and
feed. Eggs are laid in these protected areas, reducing the chance of predation
by their natural enemies. They are active spring through fall and hibernate in
protected areas during the winter. In warmer climates and greenhouses, they can
be active all year.

The damage caused by slugs and snails is a result of chewing
mouthparts. The mouthparts consist of numerous strong teethlets that allow them
to remove chunks of plant material. On plants, they chew irregular holes with
smooth edges very similar to some caterpillars, making it difficult to
distinguish the culprit. With slugs and snails, there is that famous slime
trail that can be used to determine which pest caused the damage. They prefer
succulent plant material, so damage is most common on seedlings and herbaceous
plants. They can clip seedlings off by eating through the stem and then eating
the small leaves. Soft, ripe fruit is also attractive to slugs and snails, and
they are very damaging to strawberries, tomatoes and other soft fruit that is
close to the ground. Some species will actually burrow into soft earth and feed
on the succulent roots of herbaceous plants.

Control

Control is a major problem in all habitats. There are many
things that can be done to reduce the potential of a problem occurring. A major
source of infestation in greenhouses is the movement of plant material. Growers
need to inspect plant material as it arrives at the greenhouse for the presence
of pests that can establish themselves and become a major problem. It is a lot
easier to prevent bringing in a pest than to control it once it is in the
greenhouse. Conversely, shippers need to make sure their plant material is
clean before it is shipped out to growers or consumers.

If slugs and/or snails are already present, there are still
measures that can be taken to prevent or reduce damage. Eliminate, as much as
possible, the areas where slugs hide during the day. Anything that is sitting
on the ground is a possible resting place for these slimy pests, such as
boards, boxes, stones, debris, weeds, plants in pots that have runners on the
ground or any other items that provide shelter. Reducing hiding places
decreases slug and snail survival. Of course, the potted plants that are set on
the ground will also provide harborage for these mollusks. Placing pots on
raised benches will reduce the numbers of slugs and snails that are present on
the pots and plants. Keeping things dry will also decrease the survival and
reproduction of slugs and snails and discourage them from staying in the area;
consequently, they may move out of the greenhouse to find a better habitat.

Bait or traps

The reduction of hiding places decreases the numbers of
slugs and snails, but if you are going to use bait or traps for control, hiding
places can be used as a location for baits. Slugs and snails will be attracted
to these areas, increasing the probability of them eating the bait. Traps can
be constructed of boards with runners on each side to allow room for entry, or
old pieces of wet carpet might be used. Traps are then checked regularly, and
the slugs are removed and destroyed. Some people recommend the use of beer,
cooked cabbage, dog food, grape juice, veggie mix, etc., to attract slugs and
snails to an area. These methods have been reported with various claims of
success. Some studies have even looked at different brands of beer and whether
it is best used fresh or stale. You will need to monitor and refresh baits at
regular intervals. The use of a food source under a trap board should increase
the numbers of these pests collected.

Traps can also be sunken into the ground and coated with
soap or grease to trap slugs and snails. Once they enter the slippery-sided jar
or plastic container, they cannot climb back out. The addition of your favorite
bait should increase the catch. In landscape situations when you are trying to
protect a small area, barriers can be placed around the area to reduce movement
into it. Some of the barriers used are coffee grounds, copper strips,
diatomaceous earth, horseradish roots, lava rock, lint from the dryer, nut
shells, rosemary sprigs, salt, sand, sandpaper, the spiny fruit of sweet gum
trees, wood ashes and many more home remedies. These do not have much
practicality when trying to protect a large area of plants. There are many
natural predators of slugs and snails, including insects such as ground
beetles, snakes, frogs and toads, turtles, lizards and other snails. One of the
major groups of predators is birds. Many birds feed on slugs and snails, and I
have visited greenhouses where quail are kept inside the greenhouses to feed on
slugs, snails and other ground pests.

Chemical control

Chemical control is the common method used when protecting
ornamental plants. There are very few pesticides registered for use against
Á slugs and snails, so the common method of application is spreading
baits. The key to control is the attractiveness of the bait. To obtain
mortality, the bait must attract the mollusk and stimulate it to consume the
bait containing the pesticide. If the bait is old, stale or rancid, it will be
less likely to succeed. The most common bait on the market is metaldehyde, and
there are different formulations available. The attractiveness of the bait is
the most important factor; good bait can attract slugs and snails from a meter
or farther. Metaldehyde does not kill slugs and snails by poisoning them, but
kills them by stimulating the mucous-producing cells to over produce, so they
die of desiccation. If they do not consume enough bait, they will recover.

Another bait is methiocarb, marketed as Mesural. Care must
be taken to make sure these baits are not deployed in piles where a pet would
be likely to consume the bait or where children could come in contact with it.
The placement of baits in traps, as mentioned above, reduces the hazard of the
wrong animal coming in contact with it. A fairly new bait, iron phosphate
(Sluggo or Escar-Go) is less toxic to mammals, reducing the concern for human
or animal safety. It causes slugs to stop feeding and will take a longer period
of time for them to die.

Baits are most effective under moist conditions, so consider
the locations where baits are placed. The placement of baits in traps or in
areas where slugs and snails are more likely to hide or feed will increase the
control with less bait. Sunlight will break down some of the chemicals, and
some bait breaks down faster than others. Mesural, carbamate (Sevin) and copper
sulfate can be applied as a spray of the foliage or ground. These chemicals
kill by poisoning the slimy pest.

Slugs and snails are very difficult to manage once they are
well-established. Most of the damage to ornamentals is a result of slug
feeding, but some snails, especially the brown garden snail, are just as
damaging. There are several university and industry Web sites that address
these pests if more information on the biology and management of slugs and
snails is needed.

About The Author

Ron Oetting is research and extension entomologist at the University of Georgia-Griffin. He may be reached by phone at (770) 412-4714 or E-mail at roettin@gaes.griffin.peachnet.edu.

Leave A Comment

  • Web page addresses and e-mail addresses turn into links automatically.
  • Allowed HTML tags: <a> <em> <strong> <cite> <code> <ul> <ol> <li> <dl> <dt> <dd>
  • Lines and paragraphs break automatically.

More information about formatting options

By submitting this form, you accept the Mollom privacy policy.
Email Subscriptions