Masked Chafer (White Grub)

Masked chafer larvae (grubs) are found in the soil from August through June.

• C-shaped beetle larvae
• Large, up to 1 inch (2.5 cm) long
• White with dark translucent dorsal stripes
• Brown head capsules and legs
• Bristles on the posterior end of the abdomen (raster)

Adult beetles:

• About 1/2 inch (1.25 cm) long
• Golden brown
• Hairy on the underside of the thorax
• Dark brown head with distinctive antennae, shaped like a club composed of several plates (lamellae)

Masked chafers complete one generation per year, overwintering as mature larvae. The larvae form earthen cells in the soil where they pupate in April. Adults emerge from mid-June to mid-July in the San Joaquin Valley.

Adult beetles hide in the soil during the day and fly around in search of mates during the first 2 hours after dusk. Adults of both sexes are highly attracted to black-light traps.

Masked chafers are in the insect family Scarabaeidae, and like others in this family, are commonly called scarab beetles.

Masked chafer grubs are found throughout California but are most likely to reach damaging levels in the San Joaquin Valley. They feed on roots, resulting in plants that have the appearance of drought stress, even where there is sufficient irrigation. For mature plants it may take several years for grub numbers to build up to a damaging level. However, root feeding on one-year-old plants can cause plants to desiccate and die within a few months of being planted.

Most damage usually takes place in late summer or early fall when second- and third-instar larvae are actively feeding and summer temperatures are at their peak.

Severe stunting or desiccation and death of new plants can occur any time new blueberry fields are being planted next to existing fields. Older fields can have large numbers of grubs, but tolerate grub feeding, and so show relatively little or no damage. Adult beetles from these existing fields are very attracted to fresh organic matter that is commonly incorporated into new fields prior to planting. Adult beetles fly to this fresh compost and lay eggs in it even before blueberries are planted. Then, as larvae grow, they find the new plants, feed on the new roots, and can cause extensive plant damage or death, particularly if plants lack vigor due to inappropriate cultural practices.

In most California locations, masked chafers do not reach levels requiring an insecticide treatment. Where damage does occur, control grubs with imidacloprid or entomopathogenic nematodes through the drip irrigation system.

Naturally-occurring biological control in blueberry has not been documented. However, applications of a commercially available entomopathogenic nematode, Heterorhabditis bacteriophora, can effectively control masked chafers. Steinernema carpocapsae nematodes are not effective.

Maintain overall plant health with optimum soil and water pH management and pruning practices.

Monitoring is based on the time of year and stage of the pest. Because treatment thresholds do not exist, monitor to determine if grubs are present and to evaluate the effectiveness of management programs.

1. From August to May monitor for grubs or pupae by digging in the root zone.
2. In the spring, apply entomopathogenic nematodes through the drip irrigation system. Apply when soil temperatures are at least 60°F to ensure that the nematodes are active and allow as much time as possible for them to infect the larvae before adult beetles emerge in June and July.
3. During June and July monitor for adults using a black light or by looking for adult emergence holes from pupal cells on the side of the berm.
4. In early August, apply entomopathogenic nematodes or imidacloprid when grubs are small.

Irrigate before and after applying nematodes or imidacloprid to ensure good soil moisture. Store nematodes properly before use as directed, and apply during the coolest time of day in hot areas.