Billbugs Are Here!

Here we are in the second week of July – a time when conditions start to become difficult for cool-season grasses. Heat, drought and wear during this time of the season may all contribute to declining turfgrass quality. However, one of the most commonly overlooked reasons for declining turfgrass quality during this difficult time of the season is damage caused by a group of insects known as billbugs.

These stem boring weevils (Figure 1) become active in the spring when soil temperatures rise into the mid 50’s. Adult billbugs migrate into turfgrass lawns during April and May by walking in from nearby overwintering sites. Adults begin feeding and mating, and eggs are deposited into the stems or tillers of turfgrass plants. This feeding and egg-laying activity does not pose much of a problem for cool-season turfgrass plants which are easily able to outgrow such minor damage during this cool, wet time of the year. After the eggs hatch, the tiny larvae feed inside turfgrass stems, eventually moving to the crown of the plant where feeding continues. Feeding on the crowns and roots eventually kills the plant causing small dead spots in the turf. This damage resembles that of dollar spot disease and can easily be misdiagnosed. As feeding continues and damage accumulates, the small dead spots coalesce into larger patches of dead turf as seen in figure 2. Again, because of other physiological stresses experienced by cool-season grasses during this hot, dry time of year, damage from billbugs is often misdiagnosed as disease, drought stress, heat stress or soil compaction.

The best way to positively diagnose billbug damage is by using a simple method called the tug-test. To perform the test, grasp several of the damaged tillers and pull straight upward. Turfgrass damaged by billbugs will break-off easily just at or below the soil surface. Examination of the bottom ends of the broken tillers will often reveal tattered or shredded ends and small amounts of fine, powdery sawdust-like material left behind by the larvae (Figure 3). These symptoms are diagnostic for billbug damage and no other insect or disease will cause these particular symptoms.

During July, billbug larvae can often be found in the soil beneath damaged turf.
Digging or coring the soil and carefully breaking it apart will usually reveal the larvae which may be accompanied by pupae and teneral adults that can be recognized by their unique, brick red color (Figure 4).

After the adults emerge from the soil, their exoskeletons harden and take-on the typical black, brown or gray coloration. Throughout the Midwest, these new adults will typically feed for a short period before they begin moving to overwintering sights where they will remain until the following spring. Under some circumstances, a partial second generation may occur. The larvae resulting from these eggs can sometimes be found in September, but they do not survive the winter in this part of the country.

Doug Richmond, Turfgrass Entomology