University of Vermont Extension
Department of Plant and Soil Science
Summer News
Article
CONTROLLING SLUGS
Dr.
Leonard Perry, Extension Professor
University of Vermont
Got holes in your leaves of vegetables, flowers, and
perennials such as hostas? Then you may
well have slugs eating them. A slimy
trail on leaves is proof the chewing is from slugs and not other chewing
insects. There are several methods to
control slugs with little or no adverse effect on the environment.
The
first fact to know is that slugs are not insects, so insecticides generally
wont work on them. Slugs actually are
mollusks, related to oysters, clams, and shellfish. They are like snails, only without an outer
shell. There are over 32 species of
slugs in the U.S., with some
in the Pacific Northwest several inches
long! Both slugs and snails “eat” leaves
by rasping them with a horny file in their mouths.
Slugs
mainly feed at night, retreating by day into moist places—under leaves, boards,
flowerpots, mulch, or similar. These
areas, too, are where they lay their eggs.
Slugs develop and multiply most when both hot and moist, maturing in
about a year. They tend to go dormant when
hot and dry in summer, and of course over winter.
So
keeping the slug habitat in mind, a good method of control is to modify
these
ideal conditions. Keep moist leaves and
mulch out of beds where they are prone to congregate and eat
leaves. Pick off or “deleaf” lower leaves of plants
such as hostas, allowing more air circulation.
Remove plant refuse such as pea or bean vines as soon as you’ve
finished
the harvest.
Avoid watering in the evening so your soil surface
goes into the night dry. This alone can reduce slug damage by up to 80 percent.
There
are several other mechanical means to control slugs. Some place
boards or shingles in gardens that slugs can hide under, then
collect them from under these by day.
Slugs will congregate in a roll of wet newspaper you can then just
throw
away.
Some collect slugs in saucers of beer to which they
are attracted. Or, you can substitute
water containing a pinch of baker’s yeast, or an overturned melon
half. One recipe calls for a quart of warm water, a
packet of dry yeast, a pinch of sugar, and some molasses or
honey. If using a can sunk in the ground to hold
your liquid of choice, just make sure the rims are about a half inch
above
ground. Slugs can crawl over this, but
it prevents ground beetles from falling in.
Many of these beetles are beneficial in the garden, eating insects,
sometimes even slugs.
Create barriers.
Wire fly screen, as from windows, can be cut into strips four inches
wide and set around plants. This also can be tacked around coldframes. Slugs can be controlled as well by placing
copper strips around plants, set on edge like a fence, although with recent
rises in copper prices this might get rather expensive. The slugs don’t like to cross the copper.
Some have had success wrapping a copper wire gently around stems, as of
tomatoes. Just make sure the wire doesn’t constrict the stems as they
grow. Or cut the top and bottom off of
plastic bottles and place these around stems.
Sink them a couple inches in the ground.
There is even an electronic slug and snail “fence”.
An inexpensive
barrier slugs don’t like to cross is a thin band of wood ashes around
plants,
or coffee grounds. If you are near the
coast, or have access to seaweed, this too repels slugs. It is salty,
and they don’t like salt. When seaweed dries it becomes rough,
something else slugs don’t like. Diatomaceous earth is composed of
sharp,
jagged skeletal remains of microscopic creatures
which pierce soft-bodied pests causing them to
dehydrate. It can be mixed with water
and sprayed on plants too. You’ll have
to spread more of this around plants after a rain. Sharp sand, or broken egg shells last after
rains (although the shells can be blown about).
There are slug baits in various forms you can find in garden
and hardware stores. These often contain
the chemical metaldehyde. If using,
follow all label directions and precautions.
These baits may be less effective after rains, the time when slugs are
most active. Keep in mind these
pesticide baits often resemble pet food, so can poison pets, songbirds, slug
predators, and even children that might come in contact with them. As an
alternative, look for much safer garlic-based sprays.
If you are a night owl and want to go after slugs
directly, take a salt shaker with you to the garden along with flashlight. Sprinkling salt on them, as well as misting
them with an ammonia solution (70 to 80 percent mixed in water) will show
instant results. If this sounds too
evil, just remember this instant death is much quicker than that caused by most
pesticides.
You may need to do little of these control methods if
you promote their natural enemies.
Toads, snakes, birds, turtles, ducks, and even chickens will eat
slugs. Some of the birds known to eat
slugs include blackbirds, thrushes, robins, starlings, jays, seagulls, and
owls. If your main problem is on hostas,
try growing more slug-proof selections—those with puckered or thick blue
leaves. Or try a trap crop, a crop such
as lettuce that slugs prefer, interplanted among your other plants. These will attract the slugs that can then be
dealt with all in one place.
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