By Christian Krupke and John Obermeyer
- Reports of slug damage received from NC Indiana, due to cool/wet spring.
- Crop damage and stand losses are most severe when slugs enter open seed slots.
- Control options are limited and difficult to implement. Slugs are bad news!
Slugs are soft-bodied, legless, slimy, and grayish or mottled gastropods – relatives of snails, clams, and other aquatic animals. Their length, depending on species, can reach up to 4 inches, but is usually 1/2 to 1-1/2 inches long. Build-up of slug populations is greatest in no-till systems and weedy fields, because the optimum conditions for slug survival (wet soils, lots of residue) are most likely to occur under these conditions. Juvenile slugs, which are present now, will continue to increase in size, as will their appetite. Fortunately, their feeding on crops is due to slow down soon, as sun shines and temperatures increase, along with quicker growth of the crop out of the danger period.
Classic slug damage to 4-leaf corn.
Both corn and soybeans can be significantly damaged by this nocturnal pest. Their mouthparts cause a scraping type of damage, where the top layer of leaf tissue is removed. On corn, slugs feed on the surface tissue of leaves resulting in narrow, irregular, linear tracks or scars of various lengths. Severe feeding can result in split or tattered leaves that resembles hail damage. Soybean damage is not as predominant on the foliage, but rather on the hypocotyl and cotyledons. Given good growing conditions, plants usually outgrow slug damage once the crop is up. Most damage and stand losses by slugs occur when fields are too wet to plant and seed slots are not properly closed. In this situation, slugs can be found feeding on the seedlings within the slot, day or night. That is really a worst case scenario, and pretty uncommon. But once the growing point of corn or soybeans is injured, plant recovery is unlikely.
Control of slugs is difficult, if not impossible. Disruption of their environment, i.e., tillage, is typically not an option, especially on long-term no-till or highly erodible land. A metaldehyde-pelleted bait is labeled and available for use. However, spreading the pellets evenly over the field or damaged areas is another matter; a commercial mechanical dispenser is one possibility. Field trials at Ohio State have shown good results when the pellets are evenly distributed. With the significant cost and difficulty of application, consider these baits only as a last resort to protect crop stands in high slug populated areas. Remember that time is on your side – as the season advances, the risks of slug damage decrease with increasing temps and crop growth.
Slug damage to soybean hypocotyls.
Where replanting is necessary from slug damage, one should strongly consider lightly tilling the area first, or at least a zone tillage for the seed row. This should help dry the area and break-up and bury crop residue. This will discourage further slug activity. Granular, liquid, and seed-applied insecticides are ineffective against slugs, as they are able to “over-slime” them, not a technical term. Bt corn has no effect on slugs. Home remedies, such as spraying plants at night with liquid fertilizer (high salt concentration), have proved futile and are obviously impractical for most large-scale plantings.