One of the most easily recognized insects in the home landscape is the white grub. Almost every gardener has seen white grub larvae in the soil, while installing new plants or tilling the vegetable garden. The term "white grub" actually encompasses the larval stage of several scarab beetles, the most common, and most damaging, being the June beetle or masked chafer, and the Japanese beetle. Less well-known are the May/June beetle, and green June beetle. All have a white grub larval stage that can cause damage to turfgrass. The grubs are off white, with six legs located just behind their reddish-brown head, and are usually found curled into a "C" shape in the soil.
Adult June beetles are stout bodied, oval-shaped insects, about 1/2 inch in length, and dark yellow to light brown in color. They are most active at night and, unlike other scarab beetles, do not feed on plants as adults. Japanese beetle adults are slightly smaller, only 3/8 inches in length, with a dark metallic green head and coppery-brown body. They also have 5 tufts of white hairs on the sides of their abdomen. Unlike June beetles, Japanese beetles do feed as adults, and can cause severe damage to a wide range of landscape plants. Roses, oaks and cherries are a few of their preferred plants and can suffered significant defoliation.
Lifecycle
Masked chafers and Japanese beetles have a 1-year lifecycle. Masked chafers are Nebraska native insects, so a few white grubs can often be found in almost every area of the landscape in spring, and do not warrant control. The theshold level for turfgrass damage by masked chafer larvae is 8-10 white grubs per square foot of lawn.
Masked chafers overwinter in the larval white grub stage in the soil, so are commonly found in the spring garden. However, these mature grubs do little damage and are difficult to kill ; so when control is necessary it is targeted at the next generation of insects. After digging in my garden last weekend, I found white grubs in the pupation stage, which means they are right on target to emerge as adults from late May through July. Watch for the adult beetles around yard lights at night early next month.
After mating, females tunnel a few inches into the soil and lay eggs, which hatch in about three weeks. Soil moisture is critical to egg development and eastern Nebraska's current dry conditions could lead to reduced populations this year if they continue. This year, the new generation of white grub larvae will begin hatching around mid to late June.
Damage
White grubs feed on turf and ornamental plant roots, and other organic matter in the soil. Newly established lawns, and low maintenance lawns usually have few problems with white grubs. Turf-type tall fescue lawns also have few problems. Kentucky bluegrass lawns that are maintained at a high level with frequent fertilizer and water applications most prone to attack. White grub infestations also tend to be localized to preferred locations in the landscape, such as a sunny, irrigated slope or turfgrass underneath a yard light, instead of being uniform. Spot applications of grub control products can be made to areas with a history of attack, and not applied to the entire yard if the homeowner prefers.
Damage from root-feeding white grub larvae usually is at it's worst in late July and early August if high insect numbers are present and not controlled. As insects feed on the turfgrass' roots, small patches of grass turn brown and die. Damage is most severe in hot, sunny locations. Initially damage may appear to be drought injury, or even a disease such as summer patch. But close inspection of affected areas will show that patches of turf can be pulled back easily, like a carpet, and numerous white grub larvae will be found.
Later in the season, September and October, birds and other types of wildlife can cause further damage to your lawn as they rip up turfgrass to find juicy, fat mature grubs.
Control
Due to the anticipation of a slightly earlier appearance of adult beetles this year, grub control products should also be applied slightly earlier than normal. Imidacloprid (Merit) and halofenozide (Mach 2) provide excellent grub control if applied at the right time, which this year is mid June to early July. Both products are very effective against young white grubs, and provide three months of residual control.
However, if grub control is needed in August or September for an infestation that went unnoticed earlier in the summer, carbaryl (Sevin) or dylox provide the best control due to their higher kill rate against mature white grubs.
Be sure to water-in grub control products after application for best control.
