True White Grubs In Pastures And Rangeland

By Adam J. Varenhorst

Over the course of the winter and spring, many reports came in of grub damage to pastures and rangeland. Many species of grubs feed on grass roots, which may result in reduced grass stands as the root systems are destroyed and the grass is killed. After visiting pastures with infested areas we determined that the damage to grass stands is primarily due to true white grubs. However, there are likely more than 40 species of true white grubs present in South Dakota, all of which are capable of causing damage to pasture or rangeland.

Lifecycle & Behavior

Adult true white grubs are referred to as May or June beetles, and are often considered nuisance pests because of their large size and affinity for lights near homes during spring and early summer. True white grubs (Phyllophaga spp.) have a three-year lifecycle. In the first year, female beetles lay eggs from mid- to late summer. Once these eggs hatch, the emerging true white grub larvae feed on root tissue before overwintering in the soil. In the spring of the second year true white grub larvae move toward the soil surface as soil temperatures increase, where they begin feeding on root tissue. In the fall, larvae will move deeper into the soil profile to overwinter. In the spring of year three, true white grub larvae once again move towards the soil surface to feed on grass roots. At the end of the third year true white grub larvae will move down into the soil profile and pupate. Adult beetles emerge during the following spring and summer.

During the first year of their lifecycle, true white grub larvae do not often cause noticeable feeding injury because of their small size. Slight injury is occasionally observed in the second year of their lifecycle as the true white grubs are larger and capable of consuming more root tissue. The grass in these slightly injured areas or patches typically turns light brown. Severe injury to root tissue occurs during year three as the true white grub larvae are much larger. Injured areas turn brown, and the grass may be easily lifted in “mats” from the soil. Secondary damage to grass also occurs during years two and three as animals hunting for grubs dig through pasture or rangeland. Burrowing rodents and other small mammals that prey on white grubs can cause forage loss and can create large holes or burrows in pastures.

So far in 2016, we have observed mainly adult May and June beetles, and only a single year three true white grub larvae. We will continue sampling pastures and rangeland to monitor the situation and work on developing a management solution.

True White Grub Profile

True white grub larvae are almost always C-shaped when observed in the soil (Figure 1a). The larvae have a brown head capsule and three pairs of light brown to cream colored legs. As their name implies, the majority of the true white grub body is a white to cream color. The end of the body is typically darker in color, and at the base of the abdomen are small hairs. True white grubs have a distinct pattern of these hairs, which is referred to as a “zipper” (Figure 1b).

Figure 1. A true white grub larva in the typical C-shape (a) and the “zipper” pattern of hairs on the base of the abdomen (b). Photo by Adam Varenhorst.

Adult True White Grub Profile (May or June beetle)

Adult true white grubs are referred to as May or June beetles. These beetles are typically fairly large and vary in color from light to dark brown (Figure 2). Adult beetles have light brown legs and a tan layer of hairs on their undersides. The adults are most frequently observed at night and are attracted to lights.

Figure 2. May beetle. Photo courtesy of Adam Varenhorst.