Beneficial nematodes do not kill earthworms. The entomopathogenic nematode species sold for garden pest control, primarily Steinernema and Heterorhabditis varieties, target insects and their larvae. Earthworms are not suitable hosts for these nematodes, and studies examining their interactions have found no impact on earthworm populations even when the two organisms share the same soil.
Why Earthworms Are Not at Risk
Entomopathogenic nematodes work by entering an insect’s body through natural openings, then releasing symbiotic bacteria that kill the host within 24 to 48 hours. This system is highly specialized. The bacteria and the nematode have co-evolved to exploit insect biology, and earthworms simply don’t provide the right internal environment for this process to succeed.
Researchers have directly tested what happens when entomopathogenic nematodes encounter earthworms in soil. Multiple studies using different nematode species and different earthworm species found that while nematode development was sometimes observed, there was no measurable impact on earthworm populations. The earthworms survived and continued to function normally in the soil.
Nematodes Actually Hitchhike on Earthworms
Rather than harming earthworms, beneficial nematodes appear to use them as a transportation system. Research published in the Journal of Nematology found that Steinernema carpocapsae, one of the most commonly sold species for grub and pest control, dispersed significantly farther through soil when earthworms were present. In soil column experiments, nematodes placed on the surface moved to deeper layers more effectively when either of two earthworm species (Lumbricus terrestris or Aporrectodea trapezoides) was in the column compared to when earthworms were absent.
The nematodes were found both on the exterior and inside the digestive tracts of the earthworms, suggesting they hitch rides as earthworms tunnel through the soil. This is a meaningful benefit: entomopathogenic nematodes are poor travelers on their own, particularly Steinernema carpocapsae, which tends to stay near the soil surface. Earthworm activity helps distribute them to where soil-dwelling pests actually live. So not only are earthworms unharmed, they actively improve how well your nematode application works.
What Beneficial Nematodes Actually Target
The species you’ll find at garden centers are designed for specific pest groups. Steinernema carpocapsae is effective against surface-dwelling pests like cutworms, sod webworms, and flea larvae. Steinernema feltiae targets fungus gnats and some fly species. Heterorhabditis bacteriophora goes after white grubs, Japanese beetle larvae, and weevils that live deeper in the soil.
Each species has a preferred hunting strategy. Some ambush prey near the soil surface, while others actively cruise through the soil searching for hosts. None of these strategies involve earthworms as targets. The nematodes detect insects by sensing carbon dioxide, warmth, and other chemical cues specific to their prey. Earthworms simply don’t register as food.
Applying Nematodes in Earthworm-Rich Soil
If your garden soil is full of earthworms, that’s actually an ideal environment for a nematode application. The earthworm tunnels create pathways through the soil that improve moisture retention and aeration, both of which help nematodes survive and move after you apply them. The hitchhiking effect means your nematodes will reach pest larvae faster and more efficiently than they would in compacted, earthworm-free soil.
When applying nematodes, water the soil before and after application, and apply in the evening or on a cloudy day since UV light kills nematodes quickly. The moist, organic-rich conditions that earthworms thrive in are the same conditions that give beneficial nematodes the best chance of establishing and finding their insect targets. You can treat your lawn or garden beds without any concern about collateral damage to your earthworm population.