Answer: I would suspect slugs are to blame. They are nocturnal so you won't see them at work, but you might find the slimy trails they sometimes leave behind. Of the options below, I would suggest the DE or Sluggo plus a few evening forays with a flashlight for the quickest results.
There are several approaches to controlling slugs. You can try disturbing the soil and mulch layer in very early spring to dislodge and expose the overwintering pests and with luck interrupt the population. You can also sprinkle diatomaceous earth (horticulture grade not the swimming pool stuff) around the plants (reapply after rain). This will kill the slugs and if applied early in the season, should also interrupt the population. Slugs are attracted to moisture, so if you water, water early in the day rather than in the evening. There are also some products on the market that are effective at killing slugs including the methaldehyde baits (take care to read and follow instructions and by all means make sure pets and children can not access it) and a newer product called Sluggo which is less toxic to people and animals. Some gardeners have good success with beer baited traps as well. Last but not least, handpicking at night (drop them into soapy water) and encouraging predators such as toads, snakes and birds can also be very effective in the long run. Usually, a combination of some or all of the above done fairly regularly early in the season will bring the slug population down to a tolerable level. I hope this helps!
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