Just as the weather starts to back off and the moisture falls from the sky, your lawn may not be out of danger. Grubs are coming, and they can be a problem.
"There are a lot more lawns treated for grubs than there are lawns that have grub problems," said Chuck Otte, Geary County, K-State Research and Extension agent. "Flocks of birds probing the soil in your lawn or skunks digging in your yard or moles tearing up your yard are indications of a possible grub problem. The best way to know if you have grubs in your lawn is to inspect areas of your lawn where the grass turns up brown and dead from now through late September.
"If the cause of these dead spots is grubs, then you can grab the brown grass and literally peel the turf up, exposing cut roots and white grubs underneath. If the brown grass is caused by some other culprit, like brown patch disease in fescue lawns, the leaf blades will simply pull off at the ground level."
Two beetles are mostly responsible for grubs in the Midwest. The May beetles/June bugs are the ones attracted to light in late spring and early summer. Having lots of these light brown to nearly black beetles near your porch or yard lights may be an indicator of required action on your part.
May beetles are on a three-year cycle. The first year, the beetle lays it eggs and the grub begins its growth, overwintering deep in the soil. The second year, the grubs are much larger and move up to the root zone to feed. The third year, they again surface - but as adults only wanting to pupate and emerge as a beetle. The real damage is done in their second year, as they near the surface in June through early September. These beetles are not all on the same three-year cycle, so you may have some of each age.
The second grub is the result of the masked chafer beetle. They are on a one-year cycle. The eggs are deposited in the ground in June. They grow quickly, and the grubs feed heavily during August and September. These are the ones that cause the most damage.
Populations of three or more grubs per square foot may be enough to justify treatment, and populations of eight to 10 grubs per square foot usually cause severe damage and warrant corrective action. Treatment can be either for prevention or rescue.
Preventative treatments need to be applied starting in late July through August. The application is dependent on the chemical used and the amount of thatch present. The chemical must penetrate to the root zone to be effective. You also have the choice of treating the whole lawn or just areas with a history of problems.
If the chemical is in liquid form, use a hose end sprayer and 15 to 20 gallons of mix per 1,000 square feet. A three-gallon pressure sprayer will not deliver the volume of chemical needed. It is best to apply after a nice rain or a manual quarter-inch watering. If you are using granular chemicals, a gravity spreader - versus a broadcast spreader - is best for uniform distribution of the particles. These granules are applied to dry grass and watered after the application. In all cases, follow the manufacturers' directions rigorously.
Rescue treatments requires a wait-and-see attitude. If there are indications of damage by grubs and their existence is verified, then spot treatment may be warranted. Some turf damage may occur while waiting, but grass can be reseeded easily after the grubs are gone. This method calls for chemical application only when and where needed, avoiding the expense and contamination of broad applications.
Any application should be followed by several one-inch waterings over a two- to four-week period. Monitor any run-off. The goal is to soak the chemicals into the root zone. Children and pets must be prevented from entering the treated areas until the post irrigations have dried completely. Take the time to clean your equipment and store the remaining chemicals properly.