Concerned that neem oil may harm earthworms? Many people are when they first start using neem oil for pest control.
But I have good news for you:Neem products definitely don't hurt earthworms.
In fact, neem seems to be good for earthworms!
Neem products are used to improve soils in India and other countries where neem grows. The neem product used as soil conditioner is called neem cake.
Neem cake is the solid residue that is left after neem seed kernels have been crushed to extract neem oil.
Now, all parts of the neem tree contain the active ingredients, though in varying concentrations. That means, even if neem cake has been thoroughly de-oiled, it still contains all the active substances in the dry matter that's left.
And that dry matter improves soils, increases the number of beneficial organisms and helps protect plants from soil born pests and diseases. (It also increases soil fertility and prevents nitrification, the loss of nitrogen, but that's a different topic.)
Farmers know from experience that neem cake stimulates earthworm activity. Earthworms do great in soils treated with neem cake.
Researchers have also done some studies to get hard numbers and facts regarding neem and earthworms.
Here is what they found:
Either neem leaves or neem seed kernels were ground and then added to the potting soil in a greenhouse. Voila, the number of earthworms in the soil increased by 25% (Rössner and Zebitz, 1987).
In a similar field trial various neem products were added to the soil. There was a very short lived repellency effect. The worms did not move into the soil as fast as they moved into untreated soil. But once the worms entered the soil they grew a lot better. The number of earthworms was the same as in untreated soil, but the weight of the worms in the neem treated soil was higher (Rössner and Zebitz, 1987).
Researchers even vermicomposted neem itself. The worms loved the neem, fed on it voraciously, and as a result grew a lot faster and multiplied more than if fed with mango leaves (Gajalakshmi and Abbasi, 2004).
Use as much neem as you like, in whichever way you like. Neem can only do your earthworms good!