Will Bti harm beneficial nematodes? Timing of Neem oil use and beneficial nematodes

Asked April 12, 2016, 2:57 PM EDT

I am currently fighting a fungus gnat infestation in my 100+ vegetable seedlings and dozens of house plants. I have ordered S.feltiae nematodes for treatment of new soil; have treated existing seedlings and every plant in the house with reconstituted mosquito dunks (active ingredient is Bt-israliensis or Bt H-14) in the watering can and am spraying with pyrethrin to catch the escapees; posting sticky cards to get a count. With time, wine (pour moi) and muttered curse words, I hope to have these buggers knocked down soon. After I introduce the 'todes to the new soil, will they be harmed by watering with Bti- like is there a residual bacillus carryover from larvae that are dying to the 'todes that eat them or if the larvae are dead, will the Bti attack the 'todes? Obviously, if there are no live larvae, the 'todes will starve to death and the Bti will be ineffective against the dead gnat larvae. Since I don't have a microscope or loupe, I can't tell if there are any live larvae in a soil sample so I'm using "absence of adults" as my best determination that anything is working. I have read up that I should not use my Neem oil spray on soil with beneficial nematodes. I am going to get a shipment of S.carpocapsae for the garden and the fruit trees grounds when it warms up enough to put in my seedlings. How long should I wait to spray any plant with Neem oil?

Cass County Minnesota

1 Response

No, Bacillus will not harm beneficial nematodes. It only targets lepidoptera larvae (caterpillars). Neem oil should also not affect the nematodes.

One way to test for fungus gnat larvae is to put a slice of potato on the soil surface (if there is room).

You'll want to reduce your watering to control the gnats, too.

For any other questions, please contact your local county extension office in Cass County. http://www3.extension.umn.edu/county/cass