They can kill an insect larva in 24-48 hours. About the only issue is getting them into the soil alive. They need moist soil to move around.

Beneficial nematodes work by giving a deadly disease to the insects. The disease happens to be the food supply for the nematodes and it grows on insects. When the nematodes enter the insect larvae's body, the disease starts to grow and kills the host. The nematode lays eggs inside the hose and leaves. When the eggs hatch, the dead insect is engulfed in the disease microbes and the nematodes have a ready food supply. Then the babies emerge from the insect covered in the disease microbes and seek out more hosts.

Too bad you didn't catch them in the act earlier.