White grubs are the larval stage of scarab beetles and chafers. They feed on the roots of grasses and can be found in the top couple inches of soil. The body is creamy white with a reddish-brown head, and they have three pairs of legs on the thorax. They may reach nearly one inch long. Normally, they will be seen to curve their bodies into a distinctive C-shape. Most white grubs complete their development in one year. The adult stages of white grubs are scarab beetles. Common names for some of the species include June beetles, chafers, and dung beetles. Only a few species of white grubs’ damage turfgrass, and most feed on decaying plant material and animal manure, proving a necessary role in recycling nutrients of these materials. Native species of white grubs have long been common in Colorado, particularly in the TriRivers counties and in towns along the Arkansas and South Platte River valleys.
- White grubs are immature scarab beetles that injure turfgrasses by feeding on the roots.
- Heavy infestation of white grubs may kill grass or attract mammals, such as skunks, that damage grass when digging to feed on grubs.
- Insect parasitic nematodes are a biological control option for white grubs.
- Lawns that are adequately watered and in good condition can often tolerate much of the injury caused by these insects.
Larvae of three species of white grubs. Species from left to right are Japanese beetle, European chafer, and June bug. Image credit: David Cappaert, Bugwood.org
White grubs and their injury to creeping bentgrass. Image credit: Ward Upham, Kansas State, Bugwood.org
Adults of the masked chafer. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Japanese beetle larva. Image credit: David Cappaert, Bugwood.org
Adult and larva of June beetle Phyllophaga spp. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Injury to plants occurs as the white grubs feed on roots, which causes drought stress due to the root loss. In severe infestations turf roots are so severely pruned that areas of the lawn can be lifted or peeled back easily, as if it had been newly laid sod. Lawn areas can be killed by these injuries. The presence of white grubs in lawns is also attractive to raccoons and skunks, which will dig up lawns in search of grubs, often causing more damage than produced by the insects alone.
Lawns that are mowed at a higher level produce plants with larger root systems, which are better able to tolerate root pruning injuries. Watering practices can have variable effects. Irrigation that promotes deep root growth can allow lawns to better tolerate grub injuries. Maintaining good soil moisture in late summer and early autumn can help lawns recover from damage that has occurred. Conversely, lawns where soil moisture is kept high during the period when eggs are laid will tend to be more favorable to white grubs. Since the eggs and young grubs are sensitive to drying lawns that have some periodic drying will reduce their survival. This is particularly true for Japanese beetle.
White grubs can be killed by certain kinds of insect parasitic nematodes. These are tiny roundworms that develop within susceptible insects, eventually killing them. Several kinds of these nematodes are sold and those in the genus Heterorhabditis are effective for control of white grubs; Heterorhabditis bacteriophora is the primary nematode species used for white grub control. Grubs that are successfully attacked by these organisms turn a reddish-brown color and die within a few days. Insect parasitic nematodes will be unable to infect insects if soils are not warm enough (50° F minimum) for them to be active.
There are several insecticides that can provide very good control of white grubs when applied at the proper time. Since younger stages of grubs are much more effectively controlled than are large, older grubs, insecticides are best used in high concentration in the root zone at the same time eggs are hatching and young white grubs are present.
Insecticides applied to lawn areas can be a hazard to pollinating insects if there are dandelions, clovers or flowering plants mixed with the turfgrasses in the treated area. To reduce this risk lawns should be mowed to remove all blooms before applying the insecticides.
Hodgson, E. 2007. White grubs. Utah State University – Extension. Available https://digitalcommons.usu.edu/cgi/viewcontent.cgi?referer=&httpsredir=1&article=1821&context=extension_curall#:~:text=eggs%20in%20turfgrass.-,Eggs%20hatch%20into%20small%20white%20grubs%20that%20feed%20on%20small,they%20are%20almost%20fully%2Ddeveloped.
University of Nebraska-Lincoln. (n.d.). White Grub Management. University of Nebraska-Lincoln: Department of Entomology. Available https://entomology.unl.edu/turfent/documnts/wgmanagement