Masked Chafers (White Grubs)

Masked chafers are large C-shaped beetle larvae that feed on roots of turfgrass plants. These grubs are white, up to 1 inch (2.5 cm) in length, with dark translucent dorsal stripes, brown head capsules and legs, and a characteristic pattern of bristles on the underside of the posterior end of the abdomen (the raster). Masked chafers have a scattering of bristles, while less commonly encountered May or June beetles have two parallel rows of bristles. Masked chafers are larger than black turfgrass ataenius grubs and have a slight constriction at the forward portion of the abdomen, which helps distinguish them. Adult beetles are golden brown, hairy on the underside of the thorax, and have a darker brown head. Cyclocephala hirta is common throughout California; C. pasadenae is found in southern California. Both species complete one generation per year overwintering as mature larvae, which form earthen cells in soil where they pupate. Adult males are attracted to lights at night, mostly from mid-June through July.

All turfgrass species are susceptible to masked chafer damage. Damage is usually more serious on ryegrass and bluegrass turfgrasses, whereas fescues are somewhat less affected. Warm-season grasses tend to be the most tolerant of grub feeding.

Masked chafer grubs feed on roots, resulting in irregular dead patches. Symptoms resemble drought stress and persist even where there is sufficient irrigation. Grub activity can cause the ground to feel spongy; extensive root feeding sometimes allows the turf to be rolled back like a carpet. Most damage usually takes place in late summer or early fall. Digging by vertebrate predators, such as crows, raccoons, skunks, and coyotes, is a common indication of high grub populations. Damage becomes most apparent in late summer or fall.

For turfgrass infested with masked chafers, biological and cultural controls may help reduce their numbers. If monitoring indicates a need, treatment may be warranted. Commercially available parasitic nematodes are among the treatment options.

Tiphiid wasps are common parasites of masked chafers, but may not consistently be effective in reducing grub populations below damage thresholds. Milky spore (Paenibacillus spp.) organisms have been detected infecting masked chafers in California, but the milky spore pathogens infecting the larvae do not include the one that controls Japanese beetles and are not commercially available. Heterorhabditis bacteriophora are commercially available pathogenic nematodes that can effectively control masked chafers. Steinernema carpocapsae nematodes are not effective.

Establishing warm-season grasses may reduce white grub damage. Although not a reliable control method, thorough spike-aeration of turf also kills significant portions of white grub populations when they are feeding close to the soil surface.

Carefully dig around roots of grass to detect white grubs. If the infestation is heavy, the turf may be loose and easy to roll back like a carpet. For more information on monitoring, see MONITORING AND TREATING INSECTS AND MITES. Threshold levels on golf greens are low; if you detect more than one grub per square foot on greens, treatment should be undertaken. In other turfgrass settings, up to six grubs per square foot can usually be tolerated.

Proper timing for insecticide treatments for white grubs is difficult. The most effective insecticides (imidacloprid, thiomethoxam) are most effective when applied preventively when adults are laying eggs and before damage is seen in summer. These treatments are usually effective but only a small percentage of turf in California is infested with white grubs in any year and requires treatment. Alternatively treatments can be applied when damaging levels of grubs are found later in the summer. Nematode treatments can be effective at this time. Other materials applied to control larvae include carbaryl.

Current chemical control options are most effective against early instar larvae (less than 0.5 inch long). Grubs may take up to 10 days to die following contact with an insecticide, so wait at least this long to evaluate insecticide efficacy. Adult activity generally occurs during the period from mid-June to July. Because applied insecticides bind to the leaf blades and thatch, remove thatch before and irrigate immediately following application to obtain good results.