Scouting for Slugs

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Slugs primarily target soybeans and canola, but can also infest corn, forages and cereals.  They are hermaphrodites and have one generation per year but two populations each year, one maturing as adults in the spring and one as adults in the fall.  The eggs and adults survive during the winter, and the eggs hatch in the spring when the temperature reaches 5°C.  They can eat as much as 50% of their own body weight each day and travel up to 3 meters per day, and are most active during cool, wet periods in the spring and fall.

The larvae are 4 to 5 mm in length and look similar to the adults but are more bluish-purplish in colour.  Adults are 25 to 50 mm in length with a soft body, no legs and are brownish-grey to beige with or without markings.  Damage can be done at all stages of life.  The head has two pairs of tentacles, one of which holds the eyes.  They can be describes as snails without the shells and are covered in a slimy mucus to keep from dehydrating.

Damage can be noticed at the following levels:

·         Seeds

o    Chewed or hollowed

·         Seedlings

o    Partially or completely chewed

·         Leaves

o    In corn, strips are scraped off, causing the leaf to shred, resembling hail damage.  The growing point is rarely impacted.

o    In soybeans and other broadleaf plants, leaves are skeletonized and growing point can be killed.

o    Traces of slime trails on leaf surface.

·         Field

o    Gaps in plant stand

o    Uneven growth

Fall scouting can predict problem fields for the next spring since the same population of slugs are present in the fall and spring.  Scouting should be conducted from the end of April to the end of June and September to mid-October – at night or early in the morning.  Look for visual observation of the signs noted above, including traces of slime on the leaves.  You can also set up shelter traps and check for the presence of slugs under the traps every five or so days.

There are no economic thresholds available, but finding slugs under shelter traps can help guide preventative and cultural practices to reduce risk.  In spring prior to seeding, slug eggs and overwintered slugs can be found by looking under crop residue, especially on mild days soon after rain. Another approach to finding slugs is to place artificial shelters in the field, such as roofing shingles, old boards, wet cardboard, or anything that will create a dark, cool, moist environment. Several days after putting them out, slugs can be found under the shelters during the day. Once crops have emerged, slugs can be found by inspecting crops in the evening with a flashlight. With all of these methods, be sure to look closely as juvenile slugs can be very small.

Insecticides do not control slugs, preventative and cultural options are the best bet:

·         Plant early before eggs hatch and slugs become active

·         Ensure seed slots are closed

·         Tillage to eliminate residue and expose slugs to dehydration and predatation

Some bait products are avail able but are not very cost effective for an entire field but can effectively treat small hot spots.




Sources: Grain Farmers of Ontario. Guide to Early Season Field Crop Pests. (verified May/15). Penn State University Extension. 2015. Slugs as Pests of Field Crops. (verified May/15).
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