What are these white grubs (and what do they eat)? I wondered as I dug into the mulch pile. My spade kept unearthing the big, fat, white grubs. What on earth were they, why were they in my mulch pile, and should I kill them before they eat my plants?
What Are White Grubs?
If you guessed that my white grubs were the larval stage of the dreaded Japanese beetle, you are right.
The grub stage of the Japanese beetle, (Popillia japonica), overwinters in soil and in some cases, mulch. It’s a common and serious garden pest in Virginia.
What Do Grubs Eat?
Grubs chew the roots of plants, including lawn grass, and destroy it. Brown patches in the lawn are common signs of grub infestations, but as you’ve seen, Japanese beetles aren’t particular about where they lay their eggs.
The mulch pile probably made it easy for the female to burrow in and lay her eggs. As I used the mulch and found grubs, I killed them or left them on the fence for the hungry robins and bluebirds.
Preventing Grub Damage
To prevent Japanese beetle infestations, it’s important to combat them at each stage of their lifecycle: egg, grub, and adult (beetle).
Grubs turn into a pupa in June, then emerge as adults in late June and July to feed on the leaves of garden plants, mate, and lay their eggs. Eggs require moisture to hatch, take about 8-9 days to hatch. The immature grubs begin feeding until cold weather in the fall forces them deeper underground. They burrow down four to eight inches in the soil and wait for the warmer weather to resume feeding. Droughts during August reduce the likelihood of the eggs hatching, so some dry weather in August may be helpful…even if you end up watering your tomatoes by hand (as I do when the drought hits!).
Entomophagous nematodes can be used to control the white grub stage of Japanese beetles. Use preparations containing Heterorhabditis spp applied in mid-August. Irrigate with about 1/4 inch of water both before and after the application of the nematodes.
Traps also work but can lure more beetles into the yard. They work by excreting a pheromone to attract the adult beetles, who then fall into a trap bag and cannot escape. The bag is then discarded at the end of the mating season. Be sure to use traps at least 30 feet away from the plants you wish to protect to lure the adult beetles away from them.
Jeanne Grunert is a certified Virginia Master Gardener and the author of several gardening books. Her garden articles, photographs, and interviews have been featured in The Herb Companion, Virginia Gardener, and Cultivate, the magazine of the National Farm Bureau. She is the founder of The Christian Herbalists group and a popular local lecturer on culinary herbs and herbs for health, raised bed gardening, and horticulture therapy.