Weevil Larvae

Weevil larvae or billbugs develop below ground in turfgrass and injure plants by feeding on their roots. Adults are beetles with a characteristic snout.  

Order: Coleoptera
Family: Curculionidae



Weevil larvae, or billbugs can develop in turf grasses. Adult weevils are gray to nearly black beetles, up to 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 0.3 to 0.5 inches long when full-grown.

Adult rocky mountain billbug

Whitney Cranshaw, Colorado State University, Bugwood.org
Adult rocky mountain billbug, Sphenophorus cicatristriatus.

Bluegrass billbug damage to front yard.

David Sheltar, The Ohio State University, Bugwood.org
Typical damage to a home lawn from the bluegrass billbug and drought. Image taken early July.


Third or fourth instar bluegrass billbug larva.

David Sheltar, The Ohio State University, Bugwood.org
Third or fourth instar bluegrass billbug larva.

Quick Facts

  • 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  



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.  

At least two species of weevil larvae infest lawns in Colorado: 

Bluegrass billbug, Sphenophorus parvulus

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, Sphenophorus cicastristriatus

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.  





Cultural controls

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. 

Biological controls

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. 

Chemical controls

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.  

CSU Extension Fact Sheet

Download or view the CSU Extension’s PDF fact sheet for your reference.