White Grub Life-Cycle
Most turfgrass-feeding white grubs in Texas, such as the June beetle and southern masked chafer, require one year to complete their life cycle (a two-year cycle is suspected in a portion of the grub populations in north Texas). The May beetle, Phyllophaga congrua, requires two years to develop. For simplicity, the following discussion will be limited to species with one-year life cycles.
Once a year, in late spring or summer, adult beetles emerge from the soil to mate. Mated females then return to the soil to lay eggs. Within about two weeks the eggs hatch into small white grubs that feed on grass roots. The pupa, or intermediate stage between the larva and the adult, occurs the following spring and is the last immature phase of the insect's development cycle. Adults subsequently emerge from the pupal stage when environmental conditions are favorable in early- to late-summer. Most damage from white grubs occurs during mid-summer to early fall when the larger larvae are actively feeding.
The adult stage of the various white grub species produces heavy-bodied beetles, 1/2 to 5/8 inch long, brown, with long, spindly legs (Figure 2). The June beetle and southern masked chafer emerge from the soil and fly at night, usually after a significant rainfall or irrigation. Flight periods may last for several weeks, during which time mating and egg-laying occur. During flights, large numbers of adult beetles, primarily males, may be attracted to lighted windows or other lights at night. Females, being less active fliers, usually are less common around lighted areas than are males. For this reason, turning off outdoor lights during adult flight periods may not substantially reduce subsequent white grub damage. Heavy white grub infestations often can be found in areas with little or no outdoor lighting.
After mating, female beetles dig 2 to 5 inches into the soil to lay eggs. Each female can lay up to 30 to 40 eggs, which hatch in approximately two weeks.
White grub larvae are creamy white and C-shaped, with three pairs of legs (Figure 3). After hatching, the white grub passes through three larval life-stages, or instars. These instars are similar in appearance, except for their size. First- and second-instars each require about three weeks to develop to the next life-stage. The third-instar actively feeds until cool weather arrives. Third-instar larvae are responsible for most turfgrass damage due to their large size (1/2 to 1 inch long) and voracious appetites. Feeding by large numbers of third-instar white grubs can quickly destroy turfgrass root systems, preventing efficient uptake of food and water. Damaged turf does not grow vigorously and is extremely susceptible to drying out, especially in hot weather.
Figure 3. Turfgrass-infesting white grub larvae feeding on grass roots. Grubs are most damaging when they reach a length of 1/2 to 1 inch.
When cool weather arrives, white grubs become dormant until the following spring. During this dormant period, white grubs do little or no feeding and cause little damage. Occasionally, white grubs will be found in turfgrass areas that fail to green up in the spring; however, the damage is primarily the result of feeding that occurred the previous fall. Spring and winter treatments for white grubs with one-year life cycles generally are ineffective in preventing turf damage.
The pupal stage follows the third-instar and is the life stage during which the white grub transforms, or metamorphoses, into an adult beetle. The pupal stage does not consume food and does not consume food and does not move through the soil. This life stage occurs during the spring and lasts approximately three weeks. Pupae can be found in small earthen cells 3 to 6 inches below the soil surface. White grub treatments applied during the pupal life stage are both ineffective and unnecessary.
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