Maintaining a healthy lawn in Michigan requires vigilance against pests. Among the insects that inhabit the soil, the larvae of scarab beetles, commonly known as white grubs, are the most destructive pest to Michigan lawns. These voracious underground feeders attack the root system, causing widespread damage. This activity quickly turns a lush green expanse into dead, brown patches. Even well-maintained lawns, which can tolerate some insect activity, are susceptible to the devastating effects of a substantial grub population.
Identifying Michigan’s Primary Lawn Destroyer
White grubs are the larval stages of several beetle species common in Michigan, including the Japanese Beetle and the European Chafer. The feeding activity of these grubs severs the grass roots just below the soil surface, preventing the turf from absorbing necessary water and nutrients. This root destruction leads to the appearance of irregular brown patches that fail to green up even with proper watering.
A grub-infested lawn often feels spongy underfoot due to the loose soil and severed root structure. In severe cases, the grass can be rolled back like a piece of carpet because the roots are no longer anchoring it to the soil. Secondary damage is also an indicator, as animals such as skunks, raccoons, and birds will actively dig into the turf to feed on the grubs.
To definitively confirm an infestation, a one-square-foot section of turf should be carefully lifted and inspected. A healthy lawn can tolerate a population of up to five to seven grubs per square foot without showing noticeable damage. Finding ten or more C-shaped, gray-white larvae with brown heads in this small area suggests a population density high enough to warrant intervention.
Understanding the Pest’s Destructive Life Cycle
The destructive potential of the white grub is linked to its annual life cycle, which dictates the timing of its most aggressive feeding. Adult beetles emerge from the soil primarily in late June through July, after which they mate. The females then burrow into the soil to lay their eggs. These eggs hatch into tiny larvae in late summer, usually around late July and August, and immediately begin feeding on grass roots.
The grubs grow rapidly during late summer and fall, reaching their largest and most destructive stage, the third instar, by late September or October. This intense feeding period causes the majority of visible lawn damage as the grubs consume the root system to build energy reserves. When soil temperatures drop below 50°F, the grubs move deeper into the soil to overwinter, becoming inactive until spring.
In the spring (March or April), the grubs move back toward the surface to feed briefly before they pupate in late May or early June. This short spring feeding window can cause additional damage, though it is generally less extensive than the fall feeding. Management success depends on targeting the vulnerable, actively feeding grubs in late summer or early fall.
Non-Chemical Strategies for Prevention
Implementing certain cultural practices can make a lawn less appealing for egg-laying and increase the turf’s tolerance to low grub populations. Setting the mower blade to three inches or higher promotes a deep, robust root system better equipped to withstand minor root-feeding. Taller grass also shades the soil surface, making it less attractive to female beetles laying eggs.
Adjusting watering habits deters egg-laying activity, especially during the peak beetle flight period in July and August. Adult scarab beetles prefer to lay eggs in moist soil, so reducing irrigation during this time makes the lawn less hospitable. A deep but infrequent watering schedule encourages grass roots to grow deeper, away from the grubs feeding near the surface.
Improving soil health through core aeration and topdressing enhances the lawn’s overall resilience. Aeration reduces soil compaction, allowing for better water infiltration and deeper root growth. Healthy soil provides the nutrients needed for the grass to quickly recover from any minor root damage.
Applying Targeted Insect Controls
When grub populations exceed the tolerable limit, targeted control measures are available, categorized into preventative and curative treatments. Preventative insecticides are applied before the grubs hatch (late May to mid-July) and kill the larvae shortly after they emerge. Ingredients like imidacloprid or chlorantraniliprole offer season-long protection by being absorbed by the grass roots.
Curative treatments are applied later in the season (late August through September) after a damaging population has been confirmed. These products, often containing trichlorfon or carbaryl, have a shorter residual effect and are meant to kill the larger, actively feeding grubs. Thorough watering after application is necessary for both treatment types to move the chemical into the soil where the grubs are present.
For a less toxic approach, biological controls such as beneficial nematodes can be introduced into the soil. Species like Heterorhabditis bacteriophora are microscopic worms that parasitize the grubs, releasing bacteria that kill the pest. Nematodes are most effective when applied in late summer while the soil is warm and the grubs are small and feeding close to the surface.