White grubs (soybean)
Authors: Buyung Hadi, Jeffrey Bradshaw, Ken Ostlie, and Phillip Glogoza
White grubs are the larvae of beetles from the family Scarabeidae. True white grubs are the larvae of Phyllophaga species, including P. congrua, P. implicita, P. anxia and P. rugosa, while the annual white grub is the larva of Cyclocephala lurida. Adult beetles of true white grubs and annual white grubs are called May or June beetles and masked chafers, respectively.
White grubs are white to cream-colored, C-shaped with brown sclerotized head and dark tail section. Fully grown larvae are about 1.5 in (38 mm) long. All white grubs have three larval instars or stages. True white grubs have scattered hairs and 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 locations but lack the parallel rows of stiff hairs.
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 grubs are much smaller, typically 7/16 -1/2 in. (11-13 mm). Some species are brown while others, such as the sandhill chafer, have a dull greenish metallic head and thorax similar to a Japanese beetle.
Life Cycle and Seasonal History
The true white grubs life cycle lasts for 3-4 years (termed semivoltine) in the Northern Plains while the annual white grubs produce one generation per year (termed univoltine). Apart from the time it takes for a generation to develop, the biology of the two white grubs are 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. The hatching larvae feed in 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). 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). Annual grubs overwinter as larvae in soil cells. In the spring, true white grubs larvae move back up and feed on nearby host plant roots throughout the summer. Annual grubs also return in the spring to the plant root zone close to the soil surface, however the extent of their feeding can be quite variable, and often non-economic. They pupate soon afterward.
Although their preferred hosts are grasses, true white grubs are known to attack soybean following sod. In western Minnesota and eastern North and eastern South Dakota, one species (P. implicita) attacks soybean and corn grown on continuously cropped, coarse textured soils, such as sands to loams, that are 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 grubs overwinter as larvae and then pupate in the spring. Adults emerge in late June to early July, mate, lay eggs and typically are gone from soybean fields by late July. Hatching larvae feed the remainder of the summer on suitable host plants. One of these annual grubs (Strigoderma arboricola (Fabricius)), known as the sandhill chafer, spring rose beetle or false Japanses beetle, is known for its preference for sandier soils.
Plant Injury and Damage
White grubs live underground and feed on plant roots. While larval damage is typically associated with grasses, white grubs can also feed on the smaller soybean roots. Feeding may cause stunting, nutritional deficiencies and stand loss. Though not specifically defined, expect soybean stands to tolerate more true grubs because of greater plant populations and their different root structure when compared to corn. 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 soybean 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.
No insecticides are registered for white grub control on soybean. Some cultural practices may help manage white grubs population.
Due to its long larval development, crop rotation does not provide adequate control of white grubs. Spring tillage may reduce white grubs population by killing white grubs near the surface if done after grubs return to root zone. Moldboard plowing turns over soil exposing grubs to birds. However, the only real management decision is whether or not to attempt replanting soybean.
Scouting and Thresholds
When stand loss in soybean is first noticed, its prudent to dig at the base of dying plants and plants adjacent to them. Look for the C-shaped grubs. Check roots for signs of feeding or other disease problems that may be causing stand loss. Also pay attention to whether May or June Beetles or their emergence holes are present. Note if there are signs of skunk feeding activity (holes dug by skunks to get grubs). Finally investigate the field and its history. Are there trees nearby that could serve as feeding and mating sites for the beetles? Was the field brought out of sod in the last two years? Is damage occurring in sandier areas of the field? These efforts should help you diagnose if stand loss is caused by white grub. There are no thresholds for insecticide application since no products are registered in soybean for white grubs.
Currently, no insecticides against white grubs are labeled in soybean. Soil insecticides registered in corn are not registered for white grub in soybean Similarly, seed treatments that are somewhat effective in corn, such as imidacloprid and clothianidin are not registered. Differences in seed germination (corn kernel stays in the ground, soybean seed comes out of the ground as cotyledons)would limit the effectiveness of seed treatments in preventing stand loss in soybean.
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 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.
Other Online Resources
For information regarding labels of chemical control options, please visit Agrian.com