When the brown patch in the lawn appears in springtime, it may be a sign that grubs have moved in. If you can take a handful of grass and lift the turf easily from the soil, the roots have been destroyed; look closer at the revealed soil. Just underneath the lifted turf will be c-shaped, repulsive off-white grubs that make most gardeners recoil -- not only for the unpleasant surprise, but because grubs now mean voracious beetles later. Harsh pesticides are effective but can be dangerous. Thankfully, these squirming white larvae are extremely vulnerable to Heterorhabditis bacteriophora, a tiny parasitic roundworm related to earthworms and commonly sold as Hb or under the trade name 'Ladies in Red.'
Determine how serious the infestation is by using a shovel to lift up a section of lawn, preferably at the edge of the browning area, and counting the grubs. Even severely damaged lawns can appear green as long as the water is plentiful; once the rain or irrigation stops, the lawn will quickly wilt and die. Less than five grubs per square foot should not damage a healthy lawn.
Select an evening in early spring, before the beetles pupate, or an evening in summer or fall, after the beetle eggs have hatched, to spray. Time the application to provide the nematodes with enough host grubs. Nematodes will be damaged by direct sunlight within minutes and should never be sprayed on sunny days. If it is not possible to spray in the evening, wait for an overcast day. Additionally, the temperature should be no hotter than 90 degrees Fahrenheit and no cooler than 50 degrees Fahrenheit.
Water the lawn deeply. This brings the grubs closer to the surface, where the nematodes will be spread. The damp grass is also vital to keeping the nematodes from drying out.
Prepare the nematodes. Parasitic nematodes are alive and should be handled with care; never expose them to sunlight or extreme temperatures. Granulated nematodes are often sold off the shelf but are not as effective as those that are sold live. Live nematodes are usually packaged on a damp sponge or even with a grub host and can be ordered online. Follow the package directions and pour the solution into the hose-end sprayer.
Spray the lawn well with the Hb solution. The nematodes are harmless to plants and animals and so no special protection is needed. Water the lawn again to soak the Hb into the soil where the grubs reside. As a bonus, the nematodes often overwinter alongside the grubs, becoming active again the next spring.