Northern Plains Integrated Pest Management GuideIPM

Authors: Buyung Hadi, Robert Wright, Janet Knodel, Phillip Glogoza, Mark Boetel, Phillip E. Sloderbeck, and R. Jeff Whitworth

White grubs are the larvae of scarab beetles from the genera Phyllophaga, Cyclocephala and Popilia. True white grubs are the larvae of Phyllophaga species, while the annual white grubs are the larvae of Cyclocephala species. Adult beetles of true white grubs are called May or June beetles.

Identification

White grubs are white to cream-colored, C-shaped with brown sclerotized head and shiny transparent tail section. Fully grown larvae are about 1.5 in (38 mm) long if straightened out. All white grubs have three larval development stages (instars).

True white grubs are distinguishable from other white grubs by two conspicuous parallel rows of stiff hairs on the tail end that are zipper-like in appearance. Annual white grubs possess the scattered hairs on the same location but lack the parallel rows of stiff hairs. The white grubs of Popilia beetles possess two rows of spines in inverted "V" pattern on the underside of the abdominal segment.

Adult May or June beetles are brownish, heavy bodied beetles that are about 1 in (25 mm) long. The beetles are attracted to lights after dark and the sound of their impact and buzzing on the screens of open windows on summer evenings is memorable. In contrast, the adults of the annual white grubs (beetles from genus Cyclocephala) are much smaller, typically 7/16 -1/2 in. (11-13 mm). Some species are brown while others have a dull greenish metallic head and thorax similar to Japanese beetle. The adult Japanese beetle has metallic green or greenish head and thorax, shiny copper brown wing covers and diagnostic tufts of white hair near the end tip of the abdomen.

True white grubs

June beetle

''Cyclocephala'' beetle

Japanese beetle

Life Cycle and Seasonal History

The true white grub life cycle lasts for 3-4 years (termed semivoltine) in the Northern Plains while the annual and Japanese beetle white grubs produce one generation per year (termed univoltine). Apart from the time it takes for a generation to develop, the biology of the three white grubs are quite similar.

Adults emerge from late spring to early summer and fly to nearby trees. The May or June beetles feed on the leaves of deciduous trees, particularly willow, cottonwood, poplar, oak, hickory or walnut. Later, mated female beetles drop or glide to the ground and deposit eggs in the soil. May or June beetles seem to prefer grass sod as egg laying sites. The eggs hatch in a few weeks and the larvae feed on the roots of nearby host plants just a few inches below the soil line. As winter approaches and soil temperatures drop, the true white grubs burrow deep into the ground below the frost line (7-48 in.) and overwinter as larvae in earthen cells (first 2-3 years of true grubs). Most of white grub damage occurs in the second year of the true white grub life cycle when these grubs move up to feed voraciously on plant roots. During the true grub's final winter, they overwinter as adults in the earthen cell where they pupated in late summer or fall (6-24 in. soil depth). In the spring, true white grubs larvae move back up and feed on nearby host plant roots throughout the summer.

Although their preferred hosts are grasses, true white grubs are known to attack corn planted following sod. In western Minnesota and eastern North and South Dakota, one species (P. implicita) attacks corn grown on continuously cropped, coarse textured soils, such as sands to loam soils, that are located near cottonwoods, poplars and willows. Grub infestations are greatly influenced by soil texture and they are not expected to be a problem in poorly drained, clay soils.

Annual and Japanese beetle grubs overwinter as larvae in soil cells. In the spring, these white grubs move to feed on plant roots close to the soil surface, however the extent of their feeding can be quite variable, and often non-economic. They pupate soon afterward. Adults emerge in late June to early July, mate and lay eggs. Hatching larvae feed the remainder of the summer on suitable host plants.

Plant Injury and Damage

White grubs live underground and feed on plant roots. While larval damage is typically associated with grasses, white grubs also feed on corn roots. Feeding may cause stunting, nutritional deficiencies and stand loss. Corn with severely pruned roots appear stunted and easy to uproot. The larvae, particularly older grubs, can feed aggressively and easily move from root to root within the row. In corn-soybean rotations, stand loss is usually more severe in corn than in soybean. Following tillage of grass pasture or CRP, damage may be worse the second year when degradation of grass roots has progressed far enough that white grubs only find corn roots to eat. Overall, serious economic damage due to white grubs on soybean is a rare occurrence and typically occurs when large populations of grubs are present.

Management Approaches

Cultural Methods

Due to its long larval development, crop rotation does not provide adequate control of white grubs. Spring tillage may reduce white grub populations by killing white grubs near the surface if tilled after grubs return to root zone. Moldboard plowing turns over soil exposing grubs to birds.

Scouting and Thresholds

Sampling in late summer or early fall provides better estimates to white grub populations than spring sampling. Sampling should be conducted by taking 1 ft2 (0.09 m2) of soil samples, about 8 inches or 0.2 m deep. At least thirty samples per field should be taken. The sampling sites are randomly selected about 45 yards (41 m) parallel to the shelter belt on the field edges. If less than 40% of the samples contain one or more white grubs, control may be required on parts of the field that are within 20 yards (18.2 m) from the tree line. If 40-60% of the samples contain one or more white grubs, control is needed on parts of the field within 45-65 yards (41-59.4 m) from the tree line. If more than 60% of the samples were found with one or more white grubs, control is needed on parts of the field within 90 yards (82.3 m) from the tree line. Chemical control options include seed treatment or soil insecticide applied at planting the following spring.

Biological Control

White grubs are attacked by a variety of natural enemies. Insect-attacking (entomopathogenic) nematodes, such as Steinernema or Heterorhabditis spp., occur in nearly all soils. Bacterial diseases, such as milky spore Bacillus popillae, also infect white grubs. These nematode and bacterial disease are available commercially for gardens, lawns, and gulf courses, but are cost-prohibitive for field crops. Various beneficial insects, including ground beetles, ants and parasitoid wasps add to mortality. Finally vertebrate predators including skunks, various small mammals, and birds will feed on white grub larvae.