Answer: I have not been able to find any beneficial nematodes available for slug control. However, let's consider your growing environment. Do you mulch your young plants? Do you mulch plants heavily? Mulch is a first rate hiding place for slugs during the day. To make their environment a bit less hospitable, don't mulch newly planted flowers/vegetables until the ground is warm and the plant has grown some. On larger perennials, such as hostas, don't mulch under the plants leaves which would create the perfect Bed & Breakfast for the slugs. Many hostas growers need to mulch to keep the moisture constant on their hostas. If its necessary, try to keep the mulch in a thinner layer and as far from the plant as you comfortably can. Sprinkle DE, sawdust, or crushed egg shells on the mulch frequently. Consider using a rough surfaced mulch such as gravel, peanut hulls, etc. which is inhospitable to slugs.
Several types of ground beetles are predators of slugs, so don't spray insecticide arbitrarily on the plants. Make sure that you know what insect you are spraying for. Encourage birds in the garden, they LOVE slugs and other harmful insects. So do toads!
Learn which plants are enjoyed less by slugs and replace those that are being attacked by slugs with the less popular (to the slugs!) varieties. You CAN win the battle against slugs.
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