 Because we were always taught throughout the years that spring and fall are the two times, that you do it twice, because that's when they're coming up from the frost line, and then that's when they're going back in the ground and breeding and eating your lawn. So the twice a year is the most vulnerable point that they are. The Japanese beetles are never more vulnerable than they are in their larval stage. And that's true for most insects. So getting them at those two stages is going to be your easiest route.