- Scientific Name: Sphenophorus venatus vestitus Chittenden. [Phylum Arthropoda: Class Insecta: Order Coleoptera: Family Curculionidae]
- Common Names: hunting billbug, zoysiagrass billbug
- Climatic Zone: subtropical and warm temporate
- Geographic Distribution: This pest is found world wide where bermudagrasses and/or zoysiagrasses have been introduced. Damaging populations have been reported from North America, Japan, Spain, and Northeastern Africa.
- Damaging Stage: larvae
- Hosts: Primarily attacks bermudagrass and zoysiagrass, but it may be found in bahiagrass, centipedegrass and St. Augustinegrass and Kentucky bluegrass. Also attacks field crops such as corn, sugarcane and wheat.
- Damage Symptoms: Patches of turf turn yellow and then brown or straw colored. General infestations look similar to the results of summer drought or diseases. In bermudagrass, the damage is often mistaken for spring deadspot or delayed spring green-up. Because billbug larvae damage roots, the turf reacts more rapidly to dry conditions.
- Description of Stages:
Eggs: The elongate, bean-shaped white eggs are inserted into stems of the preferred grasses.
Larvae: First instars are about 1.5mm long and have the typical weevil larval form. Mature larvae have tan to brown head capsules with black mandibles. Full grown larvae are 7 to 10mm long.
Pupae: The pupae are first cream colored changing to a reddish brown just before the adult emerges. The pupae have the distinct snout of the adult weevil.
Adults: The adults are generally larger and more robust than the bluegrass billbug. Adults range from 6 to 11mm and often have a coating of soil which adheres to the body surface. Clean specimens have numerous visible punctures on the pronotum with a distinct Y-shaped, smooth, raised area behind the head. This area is enclosed by a shiny parenthesis-like mark on each side.
- Life Cycle and Habits:
Little is known about the biology of this pest. In the northern part of its range, this pest overwinters as dormant adults in the soil or weedy areas adjacent to managed turf. In southern states the adults may be found walking and feeding all year whenever temperatures are high enough for activity. Generally eggs are inserted into grass stems or leaf sheaths over a long period from late spring into mid-summer. The eggs take 3 to 10 days to hatch and the young larvae mine down the inner surface of the leaf sheaths and bore into stems. As the larvae increase in size they drop into the ground and feed on roots and stolons. Severed stolons are often broken into one to two-inch segments with one end distinctly hollowed out. Mature larvae pupate after 3 to 5 weeks of feeding. The pupae take 3 to 7 days to mature. Because of the extended period of oviposition, larvae may be found almost any time of the season and larvae apparently continue to feed during warm spells of winter months in semitropical zones.
- Control Approaches:
Cultural Control - Masking of Damage - Damage from light to moderate billbug infestations can be overcome with adequate irrigation and fertilization. This is especially important for bermudagrass and zoysia culture. Use fertility programs that extend growth in the fall and promote early spring greenup.
Natural Control - Fungal Diseases - Billbug adults and larvae are susceptible to the insect killing fungus, Beauveria bassiana. However, this fungus usually does not attack significant numbers of billbugs during dry conditions.
Biological Control - Parasitic Nematodes - Field tests of products containing insect parasitic nematodes such as Steinernema carpocapsae have shown they can be effective against billbug larvae and adults. In the hot southern zones, S. riobravos may be a better choice. Careful attention to label details which include avoiding direct exposure of the nematodes to sunlight, post treatment irrigation and keeping the soil moist are keys to success.
Chemical Control - Insecticides -
Targeted Treatments. Billbug larvae first feed inside the grass stems and stolons before they move to the soil. Once they exit the plant, they are susceptible to contact insecticides but treatment must be applied while the larvae are near the surface. They may move deeper into the soil in periods of summer drought and heat. Treatments applied after this movement must be irrigated thoroughly. Long residual, systemic insecticides applied in May through June kill young larvae as they begin to feed in the stems.
Curative Treatments. Once significant damage is noticed, especially in the spring, most of the billbug larvae have finished development and are beginning to pupate. These larvae are difficult to reach and insecticide treatments will require thorough irrigation to achieve any results. Damage detected in July or August can often be controlled if thatch layers are less than 1/2 inch, are moist, and thorough irrigation immediately follows the application.
Preventive Treatments. Long residual insecticides applied in late May through June have been effective in eliminating adult billbugs as they feed and lay eggs. Applications of long residual chloronicytinyl or diacylhydrazine insecticides in mid-May into mid-June have been effective in eliminating billbug adults and/or larvae, as well as, provide sufficient residual activity to control annual white grub populations in July and August.
There are several methods to monitor billbug adult activity. The simplest is to place small plastic cups (pit fall traps) inside holes made by using a 4-1/4 inch cup cutter. These can be placed along the turf margin, near flower beds so that they are out of the way. Adults falling into the traps can be easily counted by inspecting them two to three times a week.
Another common method is to watch driveways and sidewalks for migrating adults. This works well on warm, sunny days in September and October but may miss the first Spring activity period.
Early detection of early summer larvae is difficult because they are very small. But, by mid-July, the larger larvae can be found by using a cup changer to pull turf and soil cores. By dividing the core in several directions, any billbug larvae present will be exposed.
Hunting billbug damage is most easily diagnosed in the early spring, before spring greenup. Areas that appear to have the upright, dormant stems lying over, as if they were beaten down by traffic, should be closely inspected. By taking you hand and brushing it briskly back and forth, look for pieces of stolons that surface. Carefully inspect the ends of these stolons and if they are hollow in the ends or have concave cavities, billbugs are likely the problem. At this time, digging in the underlying soil may expose larvae or pupae.
Reliable thresholds have been difficult to establish because actively growing southern grasses can overcome considerable billbug feeding. In southern turf, the most significant damage occurs when larvae are still feeding as bermudagrass or zoysia goes into winter dormancy. Populations of 10 to 15 larvae per ft2 are probably necessary to cause damage to growing turf and half this number may damage or kill dormant turf.