Grubs are the larvae of insects such as Japanese beetles, European chafers, and Oriental beetles. They are usually creamy white and C-shaped.
Grubs live underground and feed on grass roots. When infestations are severe, turf will look wilted and won't revive even when watered. Damage usually shows up in August and into September when cool-season grasses are stressed.
Birds and small mammals may also damage turf while feeding on grubs. Just because there is animal damage doesn't mean that there are enough grubs to treat. Animals often come back to the same spots looking for grubs even though their numbers may be low.
Late July through September is a good time to scout for grubs to see if there are enough to justify treatment:
- Peel back about 1 square foot of turf.
- Count the number of grubs.
- Sample a few areas about 20 yards apart.
- Note where animals are feeding and damaging turf and sample those areas.
- Replace sod and water.
Treatment may be justified above the following threshold levels:
- Japanese beetle grubs: More than 10 grubs per square foot.
- European chafer: 5 to 10 grubs per square foot.
- Oriental beetle: 8 grubs per square foot.
Adults of these species lay eggs in late June through July. The grubs hatch and go through a series of stages called instars. The young instars feed at the soil/thatch border. Treating for grubs in spring as they near maturity is often not effective. Pesticide, if needed, should be applied while the grubs are still young and vulnerable. Sometime in August is usually best.