Thanks for your question.

Beneficial nematodes like Heterorhabditis bacteriophora (Hb) and Steinernema feltiae (Sf) are primarily used for biological control in soil to target various soil-dwelling pests. This makes them useful in agriculture and gardening.

These nematodes are adapted to living in moist soil and may not thrive in aquatic environments. Their ability to survive and remain effective in water might be limited, though I could not find any scientific studies that contained data on nematode survival in aquatic systems.

The following should not be considered endorsement, but this is what appears on the product labels of two products having these nematodes:

***The beneficial nematodes we sell are parasitic to insect pests that typically have a developing (larval or pupal) stage of life in the soil. They have been known to parasitize above ground stages of adults, nymphs and larvae. They will not harm mammals, aquatic life, birds, reptiles or amphibians.

***They will not harm mammals, aquatic life, birds, reptiles or amphibians. After being applied to the soil, the nematodes locate pests and enter through various body openings or directly through the body wall. Once inside, the nematodes produce bacteria that is injected into the pest's blood.

If these nematodes enter a water ecosystem, an important factor needs to be considered.  They could potentially infect and kill aquatic insects. This could disrupt the aquatic ecosystem, especially if those insects play important roles in the food chain. Any product that you purchase should indicate the insects that can be killed by these nematodes. Closely examine such a list to see if any specific aquatic insects are mentioned.

In general, while these nematodes are considered safe and specific to their target pests in soil environments, introducing them to water ecosystems could have unintended consequences. It's best to avoid introducing them into aquatic environments to minimize any potential harm.

Bottom line: due to the number of different commercial sources of beneficial nematodes, each perhaps having different effects upon aquatic insects, I cannot provide you with a single answer to your question that would be applicable to all these commercial products and under all environmental situations. Moreover, I could not find any credible research studies that would support a simple answer to your question. You will need to look at individual products, read their labels regarding environmental impacts, perhaps even contact the commercial manufacturers for input, and determine an answer to your question that is most applicable to your situation.

Finally, I would suggest that you contact the following two agencies and pose your question to them. They would be in a better position to answer your inquiry than a forum such as ours whose purpose is to answer gardening questions. See:

https://www.epa.gov/mn/forms/contact-us-about-epa-minnesota

https://www.mda.state.mn.us/pesticide-fertilizer/agricultural-chemical-monitoring-assessment

Good luck. Thanks for consulting with us.