Weevil larvae or billbugs develop below ground in turfgrass and injure plants by feeding on their roots. At least two species of billbug are of concern in Colorado turfgrass. They are the bluegrass billbug (Sphenophorus parvulus) and the Rocky Mountain billbug (Stenophorus cicastristriatus). Adults are beetles with a characteristic snout and gray or black bodies, up to13 mm (0.5 inches) long and have a pronounced snout with chewing mouthparts. Adult weevils will “play dead” when disturbed, drawing in their legs and remaining motionless. Adults can cause chewing injury to turf as well, but their feeding causes minimal impact and is rarely noticeable. The larval stage is the primary damaging stage. Larvae are legless, generally white or cream, with a brown head capsule. They may reach 7.5-13 mm (0.3 to 0.5 inches) long when full-grown.
- The immatures of some weevils develop in turf grasses and injure plants by feeding on roots
- Adult weevils have a characteristic long snout with chewing mouthparts. Adult females cut small holes in stems of plants where eggs are laid
- Weevil larvae are best controlled when adults are present on the surface of the lawn in spring
Adult Rocky Mountain billbug, Sphenophorus cicatristriatus. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Typical damage to a home lawn in early July caused by drought and feeding of the bluegrass billbug. Image credit: David Sheltar, The Ohio State University, Bugwood.org
Third or fourth instar bluegrass billbug larva. Image credit: David Sheltar, The Ohio State University, Bugwood.org
Life history and habits
This weevil overwinters as an adult in protected areas, such as under debris near building foundations or at the interface of turf and sidewalk. Eggs are laid in late May, June and early July. Larvae develop over the course of several months and peak activity occurs in late June and July. When full grown, the larvae pupate a few inches deep in the soil. The adults emerge in two to three weeks, feed briefly and seek overwintering shelter. There is one generation per year.
Rocky Mountain billbug
This weevil is common throughout Colorado. This insect’s life cycle is more complicated than the bluegrass billbug. Some of the insects overwinter as adults, but most remain in the larval stage and feed throughout the spring. Egg-laying occurs throughout most of the growing season, peaking in June and July.
Young larvae feed within the crown area of the plant and kill it. The stems of infested plants are easily detached at the soil surface and the ends show evidence of ragged chewing. Larvae also produce a characteristic sawdust-like excrement around the base of the plants. Older larvae feed in the lower crown and plant root zone in a manner similar to other white grubs. Injury appears as wilting and occasional death of grass, often in small, scattered patches, and is most common on new lawns, particularly those established with sod.
Serious problems with weevil larvae can be largely avoided using resistant turf varieties. Varieties of Kentucky bluegrass that have fine stems and leaves and tougher plant tissues have increased resistance to larval feeding. Ryegrasses and fescues that contain endophytic fungi can have levels of weevil resistance. Cultivars that more aggressively spread can allow damaged areas of lawns to recover more quickly. Adequate fertilization and watering will also mask injury symptoms and help lawns repair damage.
A few species of insect parasitic nematodes (Steinernema carpocapsae, Heterorhabditis bacteriophora) are effective against both larvae and adult weevils and may be used as a biological control.
Insecticide treatments are most successful when applied in early May to kill adult insects prior to egg laying. Chemical control is more difficult when weevils are in the larval stage since insects are protected within the plant. Insecticides applied to lawn areas can be a hazard to pollinating insects if there are dandelions, clovers or flowering plants mixed with the turfgrasses in the treated area. To reduce this risk lawns should be mowed to remove all blooms before applying the insecticides.
Richmond, D. 2022. Managing Billbugs in Turfgrass. Purdue University – Extension. Available https://extension.entm.purdue.edu/publications/E-266/E-266.html
University of California. 2016. Billbugs. University of California – Agriculture and Natural Resources. Available https://ipm.ucanr.edu/agriculture/turfgrass/billbugs/
Wickwar, D., and R. Ramirez. 2022. Billbugs in Turfgrass. Utah State University – Extension. Available https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1860&context=extension_curall