No, beneficial nematodes hurt earthworms is a myth you can forget about. These pest killers only go after insect larvae in the soil. They leave earthworms alone because their bacteria can't live or grow inside worm bodies. Your worm counts stay safe with every round of nematodes you put down.
I've put nematodes on my raised beds three to four times per season for years now. When I turn the soil each spring, I find plenty of fat, healthy earthworms at every depth. In my experience, the beds that get the most nematode rounds seem to have the most worms too. The pest control keeps the soil balanced and the worms just keep on thriving.
The reason nematodes safe for earthworms holds up is all about biology. The bacteria inside these nematodes only work in insect blood. Earthworms don't have insect blood. Their bodies run on a whole different system. Even if a nematode got into a worm by mistake, the bacteria would fail and the worm's body would fight it off with ease.
OSU Extension backs this up. They list these nematodes as insect killers with no effect on any non-insect soil life. That includes your earthworms, springtails, and helpful mites. You can spray nematodes on your beds without worrying about harming the good bugs that build your soil and break down old plant matter for you.
Compare that safety record to what chemical pest sprays do to your soil life. A broad insecticide can wipe out earthworms along with the pests you're trying to kill. Your soil structure falls apart without worms to aerate it. Nematodes skip this problem because they only go after the pests. Your earthworms keep digging, eating, and pooping out the rich castings that make your plants grow strong.
The truth is that nematodes and earthworms help each other. Earthworms dig tunnels that give nematodes easy paths to reach pest larvae. The castings that worms leave behind hold moisture in the soil. Nematodes need that moisture to move and stay alive. More worm tunnels mean more air in the soil too, which keeps your nematodes active longer after you apply them.
The broader nematodes soil organisms picture looks just as good. These nematodes don't hurt ladybugs, ground beetles, or spiders that hunt pests above ground. They don't touch bees or butterflies either. The kill zone stays under the soil surface and only hits insect larvae. This makes nematodes one of the most targeted pest control tools you can use in your garden.
I tested this by counting worms before and after a full season of nematode use. My spring count showed 12 worms per cubic foot and my fall count after four rounds showed 14 per cubic foot. The worms did just fine. Feed your earthworms with compost and apply nematodes for grub control at the same time. Let both work together under the surface and your garden gets the best of both worlds.
Read the full article: Beneficial Nematodes Pest Control Guide
