What Causes Grubs in Lawns?

Grubs are the young of a wide variety of scarab beetles. The most destructive grubs on home lawns are the larvae of the Japanese beetles and the northern masked chafer, according to the Pennsylvania State University Extension. Other common grubs include the European chafer, green June beetle, Asiatic garden beetle, May and June beetles and Oriental beetle.

A number of factors increase lawn susceptibility to grubs. This includes the buildup of thatch in lawns. Dethatch lawns regularly and avoid thatch buildup of over 1/2 inch deep. The excessive use of chemicals in lawns harms the natural, beneficial insects present in the soil. This encourages the population of harmful pests like grubs to increase. Overuse of chemicals also affects the healthy growth of grass, and stressed lawns are more prone to pest infestations and infections. Use a mulching mower to recycle grass clippings and reduce thatch buildup.

Grubs are soft bodied and dirty white in color, with six pairs of legs and brown heads. Only the green June beetle grubs lack strong legs. The size of the pests varies with species and age but is generally a little over an inch long. The pests lie on their sides in a C-shape when disturbed. Grubs have distinct raster patterns on their abdominal undersides. These patterns are created with hair, spine and bare spots and help to distinguish between species. The adults of the larvae all differ distinctly on account of size, shape and coloring.

The adult beetles lay their eggs in the soil, and the hatching larvae or grubs immediately infest and start to feed on roots. Early symptoms include thinning and yellowing grass that eventually starts to die, creating brown, dead patches on lawn that continue to get larger. Infested lawn areas get soft and spongy as the nutrient and water-conducting ability of roots is affected. Lawns are prone to drought stress, and infested grass is easily pulled up. The insects start to attract secondary pests like armadillos, birds, moles and raccoons to the lawn that dig up the grass while looking for the grubs, causing heavy damage.

Introduce natural predators such as ground beetles, ants and parasitoids like Tiphia spp. in infested grass. Parasitic wasp larvae also feed on grubs. The commercial preparations of insect parasitic nematodes from the Steinernema and Heterorhabditis species are highly effective biological control strategies for grubs. Non-biological control options include the use of chemicals like trichlorfon, imidacloprid, halofenozide or thiamethoxam.