Young larvae will remove the green layer of a leaf leaving behind a clear material. This is known as window feeding. As the larvae mature, they make holes in the leaf and eventually may consume all of the leaf but the major veins.
Since these insects are not caterpillars (larvae of moths or butterflies), BT, found in Dipel and Thuricide will not be an effective treatment. However, a strong jet of water will dislodge the slugs and make it difficult for them to return to the plant. Other effective treatments include insecticidal soap, horticultural oils, spinosad (Natural Guard Spinosad, Monterey Garden Insect Spray or Captain Jack’s Deadbug Brew) and permethrin (Eight Vegetable, Fruit and Flower Concentrate; Hi-Yield Lawn, Garden, Pet & Livestock Insect Spray). (Ward Upham)