New Slug Research

Farmers are again struggling this spring to get crops planted. Depending upon the amount and distribution of rain, some farmers may be almost done while other farmers are struggling to get their first field planted. In late April, there was a nice warm stretch of weather for a few days with some relatively dry weather. Some farmers got a lot planted. Others waited. Some fields of corn and soybeans are up but with the cool weather, some are struggling to grow.

For slugs; cool, wet weather with slow growing crops are the best conditions for a “slug fest smorgasbord"! Slugs like wet cool conditions and they love to eat plants under stress. Most plants, especially corn, can out grow slug damage with good weather. Soybeans are the most susceptible to slug damage. Once you see slugs starting to eat newly emerging soybeans, it is too late because once the cotyledon (first new emerging leaves from the seed) is damaged, the plant dies. A lot of fields have either weeds still growing or cover crops growing which is a natural refuge for slugs populations and slug damage.

University researchers from around the Atlantic Coast, Midwest, and the Northwest Pacific are studying ways to reduce slug damage. One big conclusion is that pre-plant broadcast applications of insecticides may actually increase slug damage. Virginia and Pennsylvania researchers have shown that early insecticides tend to kill off all the beneficial slug predators while having no effect on the slugs. Known predators to slugs include ground beetles, spiders, firefly larvae, and harvestmen known as daddy long legs (not a spider, a small scorpion).

In the moist Atlantic states of Virgina and Pennsylvania, about 20-25% of the soybean fields and 5-10% of corn fields experience slug damage yearly. Some Extension specialist in the Atlantic states say that 95% of the time an insecticide is not needed on soybeans. The biggest bang, almost a 50% decline in slug damage occurred on corn when broadcast insecticide was eliminated. Corn insecticide is mostly applied in the row (in furrow).

Baits are expensive, generally only work for 2-3 days, and generally degrade quickly with rain. Common baits include Deadline M-Ps, Deadline GT, Slugger Ultra 4.0, Slug Fest or Ferrox. The rate is generally around 10 pounds but read the label. There may be water quality concerns if the product washes into local streams, creeks, and rivers. Each product has advantages and disadvantages. Slugs do not all feed at the same time. If they do not get a lethal dose, they will not touch the bait again, so baits, like most methods, do not give 100% control.

All University researchers agree there is no magic bullet to totally eliminating slug damage. It takes several combined strategies to fight slugs. Some farmers plant early so that if slugs damage the crop, they can replant. Others try to kill off all vegetation early then plant late hoping to starve the slugs. Unfortunately, slugs may consume the dying vegetation and they can wait for a new crop to grow.

Generally, warm sunny weather and fast plant growing conditions help plants survive slug damage. Drying the soil or crop residue out with a rotary how has been shown to dry out the soil and desiccate slug eggs. The use of residue managers also helps move moist residue away from the growing plant and dries out the soil. Slugs do not like to move across bare soil or dry soil.

Another option seldom talked about method is the use of gypsum (calcium sulfate), both as a fertilizer and as a slug deterrent. Spring broadcasting 100-200# of gypsum across a field offers three advantages. First, slugs do not like consuming calcium. It disrupts theirs digestive system. Second, slugs can not consume sulfur. They get gas and literally blow up. Third, calcium sulfate applied 4-6 weeks before soybeans bloom increases pod formation and pod set. For a farmer, keeping the soybean population intact from slugs and then adding yield may be a great advantage. The most difficult problem is getting equipment set properly to spread 100-200# of calcium sulfate across the field. When lime (calcium carbonate) or gypsum (calcium sulfate) are applied, common rates are 1-2 tons per acre.

Another break through being studied is the use of parasitic nematodes. In the Netherlands, they use a product called “Nemaslug” which contains a biological slug nematode parasite. Unfortunately, it is not labeled for the USA. Researchers are looking for a USA parasite that does the same thing. Finally, tillage by plowing, discing, and vertical tillage also reduces slug populations, but again, does not eliminate slugs. Keeping slugs in check requires several management methods combined together.