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I have been receiving a surprising number of questions on how to control slugs in greenhouses and high tunnels over the past few weeks. The cooler weather certainly favors them as well as the cloudy weather. Damage from snails and slugs can resemble that done by insect pests such as caterpillars and wireworms.
Slugs and snails are usually nocturnal so their damage is generally noticed before the pests are actually seen. Slugs and snails leave silvery slime trails on the ground and over the plants. Slugs and snails may consume several times their own body weight each night; damage can be serious within a short time.
Slugs are apparently not repelled by light, but are repelled by rising temperatures. As temperatures rise, slugs crawl down to their hiding places on the soil surface to rest and absorb water through their skin. As temperatures start to fall, slugs actively begin foraging.
Slugs are very sensitive to ambient temperature and can detect temperature changes as gradual as 2°F per hour. Slugs prefer to remain at 62 to 64°F although they lay eggs and develop normally (but slower) at lower temperatures. Development ceases below 41°F. Slugs can withstand slight freezing temperatures although their tendency to take shelter in cold weather protects them from freezing. Slugs try to escape from temperatures higher than 70°F. .
"Beer traps" (filling many shallow containers with beer and scattering them around) certainly work but need to be refilled often to be effective. You can pick them off but you will need to do it when it is cool and not too sunny since slugs are nocturnal. Iron phosphate (many trade names such as Sluggo) is labeled for use on many vegetable crops including greenhouse tomatoes and lettuce. It is highly effective. Another product, metaldehyde baits have been shown to attract slugs up to 3 feet away. The toxic effects of metaldehyde seem to be due to dehydration as metaldehyde elicits excessive mucus production. In any case, consult the label prior to intended use.
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