Turfgrass Insect Pests of Utah
There are a number of insects that can cause aesthetic and economic loss to turfgrass in Utah’s home lawns and recreational and athletic fields. White grubs and subterranean sod webworm (aka the cranberry girdler) are the most destructive, while billbugs and sod webworms are the most common.
When mature, white grub species range in length from 3/8 to 2 inches and form a C-shape when at rest. Grubs bear three obvious pairs of legs near their head end. Their life cycle length varies from several generations per year (black turfgrass Ataenius) to one per year (masked chafer) to one every three years (May/June beetle).
Brown patches in grass from grub feeding injury often aren’t apparent until late summer in an otherwise healthy turf. Heavily damaged turfgrass can feel spongy, pull up easily, and tear away from the chewed roots.
Economic thresholds for white grubs vary due to body size and feeding intensity: 3-5 per ft2 for May/June beetle, 8-10 per ft2 for masked chafer, and 30-50 ft2 for black turfgrass Ataenius. To determine density, cut a 6 x 6 inch square, peel the sod back, and count grubs in the soil and roots.
Biological control using beneficial nematodes and fungi can help maintain grub populations below economic thresholds; however, when damage is detected, insecticides may be necessary. Apply systemic insecticides (Acelepryn, Arena, and Merit) in early summer before eggs hatch to allow adequate time for plant uptake. Contact insecticides should be applied in mid summer through early fall before the grubs move deeper in the soil zone to spend the winter. Contact insecticides include broad-spectrum products such as Dylox and Sevin, and more selective or reduced-risk products such as Mach 2 and Concern.
Before applying insecticides, it is critical to reduce the thatch layer to no more than ½ inch deep or aerate the soil to enhance chemical penetration, and to apply ½ to ¾ inches of water after application to move materials into the root zone. Repeat irrigation every 4-5 days to continue chemical movement into the soil. Long-lasting clean-up of white grubs often requires several years of treatment. For more information, see the USU white grubs fact sheet.
Three to four species of billbugs occur in Utah. In the northern region, the Denver and bluegrass billbugs are common, and infest cool-season grasses. The Phoenix and hunting billbugs infest warm-season grasses, such as bermudagrass and zoysiagrass. The Phoenix billbug has been identified in the Moab area and the hunting billbug may occur in lower elevation areas of southwestern Utah.
Billbugs are weevils. Larvae look similar to white grubs, but are smaller (up to only ½ in long) and legless. They have white bodies and a brown head and resemble grains of puffed rice. Billbug larvae feed in the crown and upper root zone and damage is typically apparent in early and mid summer. Early detection is important to good control because young larvae are easier to kill than older larvae.
Infested turfgrass looks stressed with small brown patches. Grass blades can be easily pulled away from the crown in small tufts. Sawdust-like frass (insect poop) is often evident in the thatch. The treatment threshold is 1 larva per ft2. If necessary, insecticides should be applied in early to mid summer when damage is noted and the threshold is reached. The same insecticide products discussed for white grubs are effective for billbugs. For more information, see the USU billbug fact sheet.
SUBTERRANEAN SOD WEBWORM
SSW larvae are dirty-white to grey in color with an orange-brown head. Adults are buff-colored moths with brown and cream stripes (shown lower left). Moths fly just above the turfgrass at dusk/night, and are active in mid to late summer.
The treatment threshold is only 1-2 larvae per ft2. Turf/soil samples are required to identify larvae feeding in the crown and root zone. The same insecticides discussed for white grubs are also effective against the SSW. In addition, the bio-insecticide, Bacillus thuringiensis (Deliver), kills caterpillars when ingested, and pyrethroid insecticides (Scimitar, Talstar, Tempo) kill larvae and adults. For more information, see the USU cranberry girdler fact sheet .
Larvae feed on grass blades at night and retreat to the thatch during the day. Feeding damage begins as general turf thinning, but can intensify into small brown patches. Injury is evident during the summer and into the early fall. Sod webworm feeding is above ground and roots remain intact, unlike with the SSW. Small green-brown pellets of caterpillar frass can be seen in the thatch (shown above). To sample, larvae can be flushed from the thatch by pouring soapy water (1 Tbsp liquid dishwashing detergent in 1 gal of water) onto the turf. The treatment threshold and control options are the same as for SSW. For more information, see the USU sod webworm fact sheet.