White Grubs
Phyllophaga spp., Scarabaeidae, COLEOPTERA


DESCRIPTION

Adult - Known as May beetles, the shiny reddish-brown to black adults are 19 to 26 mm long.

Egg - A dull pearly white when first deposited, the oval to spherical egg turns dark just before hatching. It may be 1.5 to 3 mm in diameter. Small masses of 15 to 20 eggs occur in cells in the soil.

Larva - The young grub is creamy white and about 5 mm long. The grub is about 26 mm long, and the mature grub about 30 mm long. The C-shaped grub has a distinct brown head; a shiny, smooth body; and three pairs of legs just behind the head. Two rows of hairs on the underside of the last segment distinguish May beetle grubs from similar grubs.

Pupa - Approximately the same size as the adult, the pupa may be creamy white, pale yellow, or dark brown.


BIOLOGY

Distribution - More than 200 species of white grubs are found throughout North America. During the summer, these beetles are often seen flying around lights at night. Populations of grubs tend to be highest in older plantings of sod or soils high in decomposing organic matter. In North Carolina, they occur in all counties but are most numerous from the Piedmont to the coast.

Host Plants - White grubs attack the roots of many cultivated crops as well as pasture, field grasses, and nursery plants. Corn, sorghum, soybean, strawberry, potato, barley, oat, wheat, rye, bean, turnip, and tobacco are a few of their common hosts. Adult beetles are strongly attracted to fragrant flowers and ripe fruits.

Damage - White grubs are among the most destructive soil insects in North America. In addition to severing roots and stems of potatoes, white grubs feed on tubers, leaving large shallow circular holes in them. The infested plants often do not show symptoms on above-ground plant parts. As a result, considerable damage may be done before the grub problem is discovered. In heavy infestations, the soil may become soft and fluffy due to grub movement.

Life History - In spring, overwintering adults emerge from the ground at dusk, feed on the leaves of trees, and mate during the night. At dawn, they return to the ground where females lay 15 to 20 pearly white eggs in cells several centimeters below the surface. Eggs hatch in 3 to 4 weeks. Newly hatched grubs feed on plant roots throughout the summer and complete 1/3 of their development before fall. These grubs burrow below the frostline (to a depth of 1.5 meters) and hibernate.

The following spring, May beetle grubs return near the soil surface and resume feeding. They continue to feed and grow throughout the growing season and overwinter again the second year. The grubs become fully grown by late spring of the third year. At this time, they dig cells in the soil and pupate. Pupae become adults by late summer but the beetles do not leave the ground. New May beetles overwinter in their earthen cells and emerge the following spring to feed and mate. White grubs complete one generation every 3 years.


CONTROL

Several cultural practices help reduce white grub populations. Late summer or early fall plowing may expose larvae, pupae, or even adults to predaceous birds. Crop rotation, however, is the most effective control method. Corn and potatoes should be rotated with resistant or less susceptible crops like clovers or other legumes. These crops should never follow grasses in a rotation, especially in years following a heavy beetle flight.

Chemical control is rarely necessary for white grubs in vegetable crops. Should a serious infestation develop, consult the current North Carolina Agricultural Chemicals Manual.

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