· by Michael Blair, Nashville's Weed and Feed
Late summer into fall, a lawn in Middle Tennessee can develop irregular brown patches that look like drought stress, except that watering does nothing and, when you grab a handful of the grass, it lifts off the ground like a piece of carpet, with no roots holding it. Underneath are white, C-shaped larvae the size of a fingernail. Those are grubs, the larvae of Japanese beetles, June beetles, masked chafers and a few others, and they have eaten the roots. The lawn cannot take up water because it has nothing to take it up with.
Which grubs, and when they do the damage
Japanese beetles and masked chafers lay eggs in the lawn in early summer, and the eggs hatch into small larvae that feed on roots through late summer and into fall, which is when the damage shows. They go deep for the winter, come back up to feed briefly in spring, pupate and emerge as beetles to start again. The heavy feeding, and the visible damage, is August through October. Spring damage from the same generation is usually lighter.
Some June beetle species have a multi-year cycle, with larger grubs that feed for more than one season. The white grubs all look alike to most people; a county extension office or a lawn care company can identify them if it matters for timing, but the treatment approach is similar.
Confirming it is grubs
Cut a square foot of sod a few inches deep at the edge of a brown patch, where the damage is spreading, and fold it back. Count the grubs in the root zone. A few per square foot is normal in any lawn and does no harm; a healthy lawn outgrows that level of feeding. A number in the high single digits or more per square foot in a lawn that is showing damage is a population worth treating. Check several spots, because grubs cluster, often in the sunniest, best-irrigated parts of the lawn, since that is where the beetles preferred to lay.
The other sign is the animals. Raccoons, skunks, armadillos and crows tear up a grubby lawn at night and in the early morning to get at them, and a lawn that looks like something has been rooting through it usually has a grub population worth looking for. Moles feed on grubs too, though a mole lawn is not necessarily a grub lawn.
- Grass lifts away with no roots: grubs have eaten them
- Sod cut shows many grubs per square foot: treat
- A few grubs per square foot and no damage: leave it
- Animals digging at night: look for grubs
Treating, and the timing that matters
There are two kinds of grub products and they are not interchangeable. Preventive products, applied in late spring to early summer before or around egg hatch, stop the young grubs before they cause damage, and they are the right tool for a lawn that had grubs last year. Curative products kill grubs that are already feeding, and they work best on small grubs in late summer; on the large, deep grubs of October and on the spring grubs they do much less. A curative applied in spring to grubs that are about to pupate is mostly wasted.
Both need to be watered in, because the grubs are in the soil and the product has to reach them. Follow the label; a treatment applied to dry soil and left sits on the thatch. Whoever applies it commercially in Tennessee needs the applicator license for it. Biological options, milky spore for Japanese beetle grubs and beneficial nematodes for several species, work slowly and are best as part of a longer program rather than a fix for a lawn that is already peeling.
Repairing the lawn
Grass with no roots does not recover. Once the grubs are controlled, the dead areas are raked out and reseeded, or sodded if they are large, and September and October is the right time for fescue here anyway. Keep the new seed watered and the lawn fed through the fall so it roots deeply before winter. Warm-season bermuda and zoysia often fill back in from the runners around the damage the following summer if the area is not too large.
A lawn that is otherwise healthy and deeply rooted tolerates a moderate grub population without showing it. Watering deeply and less often, mowing high and feeding at the right times builds the root mass that makes the difference between a lawn that shrugs off a few grubs and one that peels up.