Grubs And What You Need To Know.
Grubs are dirty white, soft bodied, and robust with a brown head and six well-developed legs, with exception of green June beetle grubs, which do not have well-developed legs. When the turf is lifted to expose the grubs, they usually will be lying on their sides in a C-shaped position (Fig. 2).
The size of a white grub varies with the species and its age. Full-grown third-instar Japanese beetles and northern masked chafer grubs average slightly over one inch in length. White grub species can be distinguished by examining the grub’s raster pattern. The raster is a grouping of definitely arranged hairs, spines, and bare spaces on the underside of the last abdominal segment in front of the anus. Japanese beetle grubs have a V-shaped raster pattern, while northern masked chafer grubs have a nondescript “random” raster pattern (Fig. 3). Adult beetles differ considerably incolor markings, shape, and size. Japanese beetle adults are brilliant metallic green, 3/8 to 1/2 inch long, bearing coppery brown wing covers, with five lateral spots with white hairs on each side of the abdomen, and short gray hairs covering the underside of the insect.
They were first observed in the United States in 1916 in southern New Jersey. Northern masked chafer adults (Fig. 4) are chestnut brown, covered with fine hairs, resemble small June beetles, and are roughly 1/2 inch long. Likewise, the adults have dark chocolate brown heads that shade to a light brown (masked appearance). Northern masked chafer is native to the United States.
Figure 2. White grubs in soil
Figure 3. Japanese beetle grub raster (left) Northern masked chafer grub raster (right)
Images courtesy of Ohio State Cooperative Extension Service
Figure 4. Masked chafer beetles with eggs
General Life History
With the exception of the common May or June beetle, which has a three-year life cycle, the life history of the beetles mentioned above is completed in 12 months (Fig. 5). The adult beetle lays its eggs in the ground during the summer. As soon as the grubs hatch, they start feeding on the roots until cold weather drives them two to eight inches deeper into the soil where they overwinter. When warm weather arrives in the spring, the grubs move up from the lower soil regions and resume feeding near the surface until they become mature and pupate (Fig. 6) from May through early-June. Adult northern masked chafers, which are active at night, and Japanese beetles can emerge from mid-June through mid-July. Later emergence can occur and is dependent on local weather conditions.
Figure 5. Japanese beetle life cycle; Image created for Paul Heller.
Image 6. Japanese beetle pupae.
Heavy white grubinfestations can destroy grass roots, causing the affected area to become spongy, which allows the sod to be rolled back like a piece of carpet. Evidence of grub damage, including patches of dead or dying turf, are visible during spring (April and May) and late summer and fall (September and October) A good indication of a grubinfestation is the presence of skunks, crows, or moles feeding on turf. However, remember that moles also feed on earthworms or insects living on shallow tree roots. The symptoms of a grubinfestation are not always obvious. Survey and map the area in question to be sure an infestation is present. Using a garden spade, remove one square foot of turf and soil (two to four inches deep) at several locations.
Examine the soil, thatch (excess grass cuttings and debris), and turf thoroughly for grubs. When the examination is complete, lay the sod back in place and pack it in firmly. Watering in the areas previously sampled is advantageous to preventing the area from drying out and dying.
Surveying can be done anytime the grubs (with a one-year life cycle) are near the surface. However, to obtain maximum results from any curative insecticide treatment, the home lawn area should be surveyed during August through early September. A wide array of variables can influence the severity of damage from white grubs. It is difficult to establish a set number of grubs per square foot that can be used to determine whether a curative control measure is needed. However, in most cases, five to ten grubs per square foot often is used as the threshold for curative treatment depending on which grub species is present. Thus, each situation must be evaluated and judged separately.
Adult northern masked chafers can be monitored by using a black light trap, while Japanese beetle adult flight activity can be observed by using a sex pheromone floral lure trap.ntent here