Have been spraying with water/alcohol/soap mixture. Will nematodes help and when would be best to apply. The Hosta are ruined.
Thank you for contacting the Toronto Master Gardeners with your query.
Flea beetles can cause such damage, not only to your hostas but also to vegetables in the brassica family
Flea beetles are tiny black beetles that at the adult stage leave very small white holes in the leaves of your plant which seems to be what has happened to your hostas. The easiest way to determine whether you indeed have flea beetles on your plants is to place yellow sticky traps next to your plant. Flea beetles jump from plant to plant when you disturb the leaf which is why they are difficult to identify.
To discourage these beetles from setting up shop in your garden, make sure you clean up any debris in the fall and cultivate the soil carefully so as not to injure any plant roots. Do this again in the spring. And yes, beneficial nematodes will help you get rid of these destructive pest. The nematodes feed of the larvae so application in the spring is appropriate. Nematodes are available in your garden centre and are kept refrigerated until use. Follow the instructions carefully – moist soil is essential for this process to be effective.
The following web site offers excellent descriptions of how nematodes deal with your soil born insects such as the flea beetle and others.