In order to know what to do about grubs, you have to learn about them. In the Northeast, most grubs are Japanese beetles, although a few are masked chafers. Both beetles look similar and have a similar life cycle. Adult beetles of both types appear in June as beetles that fly around, munching on ornamental bushes and roses. They mate and then the females look for beautiful, irrigated lawns in which to lay eggs in mid-July. The eggs hatch in early August and the new grub worms begin eating the grass roots. In November, the grubs tunnel deep in the soil for the Winter. The grubs come back up to the surface in early May and eat a little longer. In early to mid-June, the grubs build an in-ground "cocoon" and transform into beetles that emerge from the soil and start the process over again.
Grubs in lawns do damage by cutting off grass plant roots. The grass plant responds by growing new roots, but in the meantime, the plant is more susceptible to moisture stress if it lacks rain. So, technically, grubs do not damage or kill a lawn; a lack of water does. Japanese and masked chafer beetles populations change every year and are very mobile. All lawns, everywhere, have some grubs every year.
Newly hatched grubs are big eaters! Grubs eat the most in the Fall and progressively less as they mature in the Spring. In fact, when grubs are nearly ready to change into beetles, they stop eating. "Mature" grubs are very hard to kill with insecticides and why bother trying because they are not eating anything at this stage in development!
Why you shouldn't reach for the pesticide
There are two distinctly different types of grub killers available. One type kills the grubs soon after hatching and another kills older grubs. Many people put on insecticides in May for grubs that will actually hatch in August, thinking they are going to kill the grubs they currently have. This is wrong. That type of insecticide only works on hatching eggs, not young grubs. Additionally, many people are applying insecticides in June thinking they will kill the grubs they have. This is wrong as well, because the grubs are too big and or are protected by their "cocoon". Here’s what you should do -
Grubs don't carry diseases; their harm to us is only economic. How many grubs does it take to do economic damage? A rough guess is 5 or more per square foot, and even that number is debatable based on when you make the count. If, in late August, you notice your lawn looks a little dry, then get down on the ground and grab the turf and pull it up. If it comes up easily, look for grubs. Consider applying an insecticide if you find 5 or more per square foot. If it is late May, and you are planting flowers and find 5 or more per square foot, don't apply an insecticide because they are done eating and you will not kill them anyway. Grubs tend to be a problem in the same place year after year. If you find and treat for grubs for several years, then consider applying the correct type of insecticide about July 1 in advance of egg laying as a preventative. You can assist your lawn in repairing grub damage by fertilizing regularly to provide the grass with nutrients to grow new roots.
Many people assume because we are professional grass producers, we use a lot of pesticides. In fact, we are proud to proclaim just the opposite. We believe in responsible pesticide usage; for us, that means treating our sod only when economic damage is likely. To learn more about grubs, visit this website: http://www.umassturf.org/publications/fact_sheets/insects.html