We have had a report of sod webworm damage to a golf practice green in northern Illinois. The sod webworm most likely to be present in bentgrass is the cranberry girdler, Chrysoteuchia topiaria. Most sod webworms, including the cranberry girdler, overwinter as larvae, so they can become a spring problem, as well as later in the season.
The cranberry girdler gets its name from feeding as a larva on the runners of cranberry plants, killing the plants. In Illinois, it is primarily a turfgrass pest, feeding on Kentucky bluegrass, bentgrass, and fine fescues. Larvae are slender, white caterpillars with tan heads. Unlike other sod webworm larvae, they do not have obvious brown spots. Mature larvae range from 5/8 to 3/4 inch long. Both mature and immature larvae overwinter, with the immature larvae resuming their feeding in the spring. They mature by late May to early June, when they pupate along with the overwintered mature larvae. In 2 to 4 weeks, adult moths emerge from the pupae, being present from late June to mid-August.
Adult moths are similar to other webworms, being slender, tan moths with elongated palpi associated with the mouth. These palpi are held out in front of the head like a snout, resulting in the family of moths containing sod webworms being called snout moths. Adult cranberry girdlers are more colorful than other sod webworms, with the wings being brown and cream striped, with wing tips that are orange-brown, containing three black spots, a silver V-shaped marking, and silver fringes. They are smaller than the common large sod webworm, having a wingspan of only 1/2 to 3/4 of an inch.
Mated female moths do not scatter their eggs while flying as other sod webworm moths, preferring to drop their eggs into the turf while sitting on a plant. Eggs hatch in about 10 days into caterpillars that bore into grass crowns, as well as feed on turf roots, stems, and leaves. In October, the larvae spin a tough silk hibernaculum in which they spend the winter.
Damage appears as small, circular spots of dead turf. Heavy infestations can cause large areas of turf dieback. Inspection of the thatch reveals the webbed tunnels of the larvae, lined with feces. Heavily infested areas attract large numbers of insectivorous birds, such as starlings, red-winged blackbirds, robins, and killdeer.
Control of cranberry girdler requires treatment more similar to grub control than typical sod webworm applications because of the grass-crown boring. Treatment is most effective in August and September, when more of the larvae are actively feeding. At this time of year, as much as 90% of the population may be present as prepupae, mature larvae that are no longer feeding. Treatment may be needed at this time to control damage, but reapplication may be needed later because most of the population was not controlled with the spring application. Apply trichlorfon (Dylox) or halofenozide (Mach 2), and water the application into the turf as when applied for white grubs. In the long run, timely summer applications of these insecticides for grub control should also provide control of cranberry girdler. However, the use of imidacloprid (Merit) for grub control may allow cranberry girdler to become a pest problem because imidacloprid does not control caterpillars. (Phil Nixon and Derek Settle)