White grubs and other lawn insects…

Published: June 1, 2011

White grubs are the primary insect problem many turf managers and homeowners face annually. Damage is usually noticed in late July to early August, however, damage can still be noticed late in September. Moist soils may extend grub feeding into the fall, but with cooling temperatures feeding may start to subside. The earliest symptoms' of white grub feeding on turfgrass roots are stunted, wilted patches of sod with gradual thinning and weakening of the stand. Damage may progress from sudden wilting of the grass, even with adequate moisture, to patches of dead grass. Small or large patches of dead or dying grass will have roots pruned so that sod can be pulled easily or rolled back like a loose carpet. Numerous C-shaped whitish larvae with a brown head will lay in the upper soil directly below the dead sod. Mammals, such as skunks, raccoons, and armadillos, can cause additional turfgrass damage foraging for grubs.

Adults are scarab beetles - May/June beetle, masked chafer, Japanese beetle, and green June beetle. These are the primary white grub species we see with the May/June beetle and masked chafers being the most common. Identification of white grub species can be made by: time of the year the grub is present, size of the grub and raster patterns on the abdomen of the grub.

May/June Beetles:
Damage is typical wilting and small dead patches of sod. These beetles have a 3-year life cycle. Adult beetles can be damaging to trees and ornamentals. White grubs should be treated during late July to early August to control any newly hatched larvae. However, during the second year of the grub's life cycle, treatments can be made from April through September if threshold levels are met.

Masked Chafer:
Turfgrass infested with this species exhibits the typical white grub damage. Wilting, irregular dead patches of turf are the symptoms. These beetles have a 1-year life cycle. Treat grubs about four weeks after the adult beetles start to emerge, when egg deposits begin to hatch in late July to early August.

Japanese Beetles:
These beetles are now considered to be state-wide in Missouri. Grubs feed on roots of turfgrasses and cause a wilting appearance and gradual thinning, however we generally do not see large amounts of turf damage specific to Japanese beetle grubs. Adult beetles can be damaging to about 400 host plants of both turf and ornamentals. Adult females will lay about 200 eggs per season, throughout the summer months. Therefore, we do not have a single egg laying time frame. Using a long-term residual product will work best to cover multiple egg laying episodes. Adult beetles can be treated at any time. If large numbers of adult beetles are noticed defoliating trees and shrubs, a preventative long-term residual product may be warranted; however it is difficult to predict damage on lawns since the adult beetles are so mobile. Controlling grubs may not protect landscape plants and because you see adult beetles in the landscape does not mean you need to treat your lawn.

Green June Beetles:
Feeding activity of these grubs rarely causes severe turf damage. Rather, the damage to a lawn generally is mechanical in nature. The grubs burrow in and out of the turf, which produces mounds. These beetles are attracted to soils with high organic materials. The decaying organic matter in the soil is the primary food for this grub. This white grub is large, 1 ½ inches in length.

Control of white grubs
The major factor influencing white grub density in turfgrass appears to be soil moisture; that is, in years with normal or above normal precipitation, grub populations tend to increase. This is because all white grub species require moist soil for their eggs to hatch. Young grubs are very susceptible to desiccation. Irrigated lawns and turfgrass areas become more susceptible due to soils remaining moist. This dependence on soil moisture by white grubs can be exploited as a type of cultural control option. In areas where turfgrasses can stand some moisture stress, do not water as much when adults are laying eggs and young grubs are present.

Cedar oil is a known deterrent of feeding by white grubs. Other organic products would include neem oil and garlic juice. More distributors and garden centers are now carrying a line of organic products for lawn care purposes.

In recent years, several strains of insect parasitic nematodes in the genera Steinernema and Heterorhabtitis have offered somewhat effective biological control of white grubs. For these beneficial organisms to be most effective in managing white grub populations, it is critical that the labeled application instructions are followed exactly (e.g., time of day, soil moisture, size of grub, rates).

Because damaging white grub populations tend to be sporadic from year to year, preventative chemical control applications are not really justifiable. But in areas where moderate to damaging levels of grubs have been perennial, preventative applications made in late May or June may be warranted. Some products that have extended activity are imidacloprid (e.g., Merit), and halofenozide (e.g., Mach 2).

Insecticides that have shorter residual periods (3 weeks or less) or must be ingested (preferably by small grubs) to be most effective are best used in a curative control program. The successful use of these materials depends to a large degree on the proper timing of the applications. These products must be applied shortly after egg hatch when the grubs are small and actively feeding. Remember, the smaller (younger) the grub, the easier it is to control. As a general rule, the recommended time to treat for grubs is about 4 weeks after the adult beetles start to emerge, the time when the eggs begin to hatch. For the Masked Chafer, this period is around late July to early August. Because emergence of May/June Beetle adults can last for several weeks, chemical treatment for May/June Beetle grubs is also recommended during late July to early August. Insecticides that appear to be effective as curative treatments include trichlorfon (e.g., Dylox), halofenozide (e.g., Mach 2), and carbaryl (e.g., Sevin).

Chemical applications can be rendered useless if the material has not been thoroughly watered-in (0.5-inch). The water not only moves the chemical down to the thatch layer (the final destination for most of the chemical), but it will often stimulate grub movement upward in the soil, closer to the insecticide. However, if the thatch layer is 0.75 inch to 1 inch thick, grubs may not come into contact with lethal doses of the insecticide. It may be necessary to remove some of the thatch before a chemical application.

To determine if a chemical treatment is necessary, a sampling of the grub population is necessary. To do this, cut a 1 square foot piece of sod in several areas of the lawn, pull it back, count the number of grubs, and inspect their rastral patterns to identify the species. Replace the sod squares back on the soil. If you have on average more than 10 Masked Chafer grubs or more than 5 May/June beetle grubs per square foot, then a chemical treatment is recommended. Remember, it is not unusual to have more than one species of white grub infesting the same lawn.

Other lawn insects…

Larvae are marked with green, orange or tan colored stripes. Their head has a Y-shaped marking (white) that serves as a key identifying characteristic. Larvae feed aggressively when small. Adult moths are mainly active at night with females laying 3 to 5 clusters of 1000 eggs each. Several generations can occur per season with new generations developing every 23 to 28 days.

Control of fall armyworm
Early scouting is best when thinning or yellowing of the lawn begins. A soap drench of liquid dishwashing soap (a few tablespoons) in 4 to 5 gallons of water poured over an area will bring larvae to the surface. One larva per square yard will cause sufficient damage to treat. Liquid applications of insecticides (Sevin and Mach2) are best when applied to foliage and left to dry. Avoid irrigation and mowing. Spot treatment on an "as needed basis" is acceptable.

Sod Webworm:
Sod webworm larvae are buff-colored with pairs of brown spots on each body segment. Adult moths resemble slender slivers of wood about ¾ inches in length. Females drop eggs over turf in flight and lay several 100 eggs. Life cycles are complete in 6 to 10 weeks; therefore 2 to 3 generations can occur per season.

Look for chewed leaf blades, as this insect does not consume the entire plant as the fall armyworm does. Larvae can be found in retreats that are silken-lined with dead grass or frass. Entries are pencil-sized opening in the surface of the soil. The soap test can also be used to determine a threshold count. Threshold levels of 10 to 15 larvae per square yard can cause economic damage and justifies treatment.

Control of sod webworm
Several of the natural pyrethroids as well as Sevin and Mach2 are effective in control. Liquid applications of insecticides are best when applied in mid-afternoon to foliage and left to dry. Avoid irrigation and mowing.

Hunting Billbug:
Eggs of this insect are deposited in leaf sheaths or in feeding punctures on stems. Heavy feeding by larvae produces brown frass piles near the crown of the plant. Egg laying usually occurs in late spring to early summer, and only one generation occurs per year. Larvae are of the white grub type, but very small (3/16"). Adults are about ¼" in length, grayish, and have a pointed snout for piercing and feeding. Adults usually play "dead" for a short period of time when disturbed. Turfgrasses (mainly zoysiagrass) begins to turn yellow then dead brown as liquid flow through plant stems is disrupted. A "tug" test can be performed to see if stems have been fed upon. Pull the affected stems upward. If the stem breaks easily at the crown and is hollowed out with frass present, then billbugs are the likely suspect.

Control of hunting billbug
Pitfall traps (metal cans insert into the soil with the top edge flush with the soil surface, then filled half way with water) can be used to monitor numbers of adult or simply count adults as they move across sidewalks and driveways.

Several of the natural pyrethroids as well as some combination products (Allectus and Aloft) are effective in control. Steinernema carpocapsae nematodes have also proven effective as a natural control.

Chinch Bug:
Chinch bugs are a sucking type insect that removes plant juices from leaf sheaths and stems. These insects are about 3/8 inch long with white wings that have triangular black markings. Females lay 20 eggs per day for 2 to 3 weeks and 1 to 2 generations can occur per season. Damage tends to occur in shaded areas first and most damage in Missouri has occurred in zoysiagrass.

Control of chinch bug
Pitfall traps can be used to monitor chinch bug numbers. Usually 20 to 25 nymphs per square foot justify treatment. If cool, wet springs exist; avoid insecticide treatments; since these conditions reduce numbers. Several of the natural pyrethroids as well as Sevin are effective in control of chinch bug.

All insecticide information is presented with the understanding that no endorsement of named products is intended, nor criticism implied of similar products that are not mentioned.

Before using any insecticide please read the label carefully for directions on application procedures, appropriate rate, first aid, storage and disposal. Make sure that the insecticide is properly registered for the intended use.