Most fall problems with white grubs are due to root-feeding by Japanese beetles and/or masked chafer grubs. Severe damage results in irregular patches of rootless dead grass that can be pulled up like pieces of loose rug. In contrast, activity of “the other white grub” – the green June beetle is marked by piles of crumbly soil around approximately 5/8-inch diameter holes in the ground. The grass stand in the affected area may be very thin with mostly clover or other deep-rooted plants present but the root system is sound.
Burrows of green June beetle grubs are usually open and extend 12 inches or so beneath the surface. The beetles tunnel through the soil as they feed on decaying organic matter. They come to the surface at night to empty their digestive tracts, leaving deposits similar to fine soil found around ant mounds. These large grubs also can be forced to abandon their homes following heavy rains.
June beetle grubs usually become inactive as the soil temperature drops but they can be intermittently active during warm periods. The larvae will pupate in underground cells during May and June an emerge as adults about late July. There is one generation each year.
Active grubs can disrupt re-seeding efforts
Attempts to re-seed thin stands can be thwarted by June beetle grubs. Soil churning by these larvae can uproot newly germinated seedlings, greatly reducing stands. However, it is best not to apply a “rescue” insecticide treatment at this time because large numbers of green June beetle grubs can come to the surface to die. Subsequent decay can create a smelly mess and can pose a threat to birds and other animals that may consume them. It may be best to delay seeding operations to spring. A preventive grub treatment can be applied during May or early June to prevent damage in late summer.
What about the “regular” white grubs?
Dr. Dan Potter, UK turf and horticulture research entomologist, and his crew have been evaluating their white grub research plots. Notable numbers of Japanese beetle and masked chafer larvae have been found in untreated plots. However, no visible damage has been seen due the above normal rainfall over the last four to six weeks. Rains or irrigation during late summer and early fall generally increases survival of Japanese beetle larvae and can mean a healthy population in 2019.
By Lee Townsend, Extension Entomologist