In large numbers, grub worms can do a lot of damage to a lawn in a short amount of time. The symptoms of a grub worm, or white grub, infestation are similar to many other problems, including a simple lack of water. It takes some investigation to confirm that lawn damage was caused by grub worms.
Grub worms, also called white grubs, are the larva of June beetles. They are deposited as eggs in the lawn and when they emerge begin feeding on the roots of the grass and other nearby plants. Grub worms are small and white with c-shaped bodies and six legs. Just before entering the pupal stage, the grubs eat heavily and do the most damage to lawns. This typically occurs in July and early August.
Brown spots on the lawn are the only obvious symptom of a grub worm infestation, however, such spots can also indicate many other problems. To confirm that the damage is caused by grub worms, pull up the damaged grass near the edge. If it comes up easily and has no roots, grub worms are likely to blame. Look along the edge of the remaining healthy grass for grubs. They will often be found around the edges of the brown spots underneath the turf.
Once grub worms are in the lawn and are causing damage, the best treatment is an application of insecticide. Choose one that contains permethrin, pyrethrins or imidacloprid. Water the lawn before applying the insecticide. Apply according to the manufacturer's instructions, and then water the lawn again to help spread the insecticide into the roots and soil where the grub worms will come in contact with it.
Allowing the lawn to go dormant and introducing a natural predator are the most effective methods of preventing grub worm infestations. Restricting water to the grass in July and allowing it to dry out and go dormant will make it an unattractive place for June beetles to lay their eggs. It may also kill any larvae or eggs that are already in the lawn. Nematodes can also be used to control grub worms if allowing the lawn to go dormant is not an attractive option. Introduce Heterorhabditis bacteriophora nematodes, which are available at garden supply stores and nurseries, to hunt down and kill the grubs.
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