Lawn grubs are the larvae of various scarab beetles that cause significant turf damage. These soft-bodied pests live just beneath the soil surface, where they consume grass roots. An infestation quickly leads to large, unsightly patches of dead turf, compromising the health of the entire lawn. The most effective way to eliminate these destructive larvae involves using biological controls that target the grubs without harming the surrounding environment.
Identifying the Problem and Optimal Treatment Timing
The first indication of a grub problem is often the appearance of irregular, dead brown patches during late summer or early fall. These patches result from grubs severing the grass roots as they feed near the surface. To confirm an infestation, try pulling up the grass in a damaged area; if grubs are present, the turf will roll back easily like a loose carpet because the root system has been eaten away.
Another common sign is increased activity from local wildlife, such as skunks, raccoons, or birds, who feast on the grubs. Recognizing the grub life cycle is essential for effective natural treatment. Adult beetles lay eggs in the soil during early to mid-summer, and the resulting young larvae hatch and begin feeding from late summer through early fall.
This late summer to early fall period is the most vulnerable stage, making it the only optimal window for applying biological controls. During this time, the grubs are small, actively feeding close to the surface, and highly susceptible to treatment. Treating in the spring is often ineffective because the grubs are larger, have already caused their most significant damage, and are preparing to pupate deeper in the soil.
Applying Biological Grub Control Agents
Biological controls rely on naturally occurring organisms to manage pest populations, offering an alternative to synthetic chemicals. The two most effective options are beneficial nematodes and the bacterium known as milky spore. Each one requires specific application conditions to ensure its viability and effectiveness.
Beneficial nematodes, particularly the species Heterorhabditis bacteriophora, are microscopic roundworms that actively hunt for grubs in the soil. Once they locate a host, the nematodes enter the larva and release a symbiotic bacterium, typically of the genus Photorhabdus. This bacterium causes a fatal blood infection, killing the grub within a few days.
Application requires careful attention to the environment. They must be applied on an overcast day or in the early morning or evening to protect them from damaging ultraviolet light. The soil needs to be thoroughly moistened before and after application to allow the nematodes to swim and migrate through the soil profile effectively.
The ideal soil temperature for H. bacteriophora is between 19°C and 33°C (66°F and 91°F). Nematodes are typically purchased in a dormant state and must be refrigerated until they are ready to be mixed with water and applied using a watering can or sprayer. Once mixed, the solution must be used within two hours, as the organisms will settle and die without constant agitation and aeration.
The second biological control option is milky spore, which contains the bacterium Paenibacillus popilliae. This control agent is highly specific, targeting only the grubs of the Japanese beetle. When a susceptible grub ingests the spores while feeding on grass roots, the bacteria multiply within the grub’s body, causing a fatal “milky disease.”
The dead grub then decomposes, releasing billions of new spores back into the soil, creating a self-sustaining cycle. Milky spore is considered a long-term solution because it can take two to three years for the spore population to fully establish and reach effective levels in the soil. Once established, however, it can provide effective grub management for a decade or more.
Adjusting Lawn Care for Long-Term Prevention
Implementing lawn maintenance practices can significantly reduce the appeal of your turf to adult beetles looking to lay their eggs. A strong, healthy lawn is more resistant to damage, even if some grubs are present. Modifying watering and mowing habits are two preventative measures.
Adjusting your watering schedule is a primary preventative strategy. Adult beetles prefer to lay their eggs in soil that is moist near the surface. Switching to a deep, infrequent watering schedule creates an environment less conducive to egg survival. This practice also encourages the grass to develop deeper, more robust root systems that can tolerate minor grub feeding.
Another adjustment involves raising the cutting height of your lawn mower. Keeping the grass consistently taller, ideally between three and four inches, helps to shade the soil surface. This shading makes the turf less attractive to egg-laying female beetles, and the longer blades support a more extensive root network, increasing the grass’s resilience to damage.
Natural predators also play a role in long-term prevention. Birds, skunks, and raccoons actively seek out grubs as a food source, helping to keep the population in check. Accepting some minimal foraging damage from these beneficial animals is part of embracing a natural pest management approach.
Monitoring and Follow-Up Care
Assessing the success of biological controls requires patience, as the results are not as immediate as those from synthetic chemical applications. Instead of a sudden die-off, you should look for a gradual reduction in the appearance of new brown patches and a decrease in the activity of digging predators over several weeks. A successful treatment means fewer young grubs survive to adulthood.
After the grubs have been addressed, it is often necessary to repair the damaged areas of the lawn by overseeding or patching with new sod. Maintaining the preventative lawn care measures, such as deep watering and high mowing, is necessary to support the long-term health of the turf.
Beneficial nematodes may require re-application in subsequent years because they do not always establish permanent, self-sustaining populations in the soil. For lawns treated with milky spore, establishment may take up to three seasons to become fully effective.