From the July 2017 issue of GCM magazine:
Billbug species composition and life cycles on Missouri zoysiagrass fairways
Bluegrass and hunting billbugs have become a pest of zoysiagrass fairways in the upper transition zone, but they can be controlled if superintendents have a thorough understanding of the billbugs’ life cycle and times of peak activity.
Figure 1. Billbug damage on Meyer zoysiagrass turf (left) in October 2016. After the turf was removed, four billbug larvae were found actively feeding inside the PVC grid (right).
Photos by Michael Patterson
Michael E. Patterson Xi Xiong, Ph.D., and Bruce A. Barrett, Ph.D.
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This research was funded in part by GCSAA through the EIFG.
In the upper transition zone centralized around Missouri, Meyer zoysiagrass (Zoysia japonica Steud.) occupies more than 90% of golf course fairways. This predominance is attributed to its superior cold tolerance when compared with other warm-season turf species, such as bermudagrass (Cynodon species), and its status as being relatively pest-free. In recent years, however, billbugs (Sphenophorus species) — especially hunting billbug (S. venatus vestitus Chittenden) — have emerged as a problem that, if left untreated, seriously damages zoysiagrass fairways (Figure 1). Without proper diagnosis, damage caused by billbugs is often confused with other biotic and abiotic stresses, such as white grub, drought, winterkill or even fungal diseases.
Billbugs are small-sized weevils. Adults measure 0.3 inch (0.76 cm) long or less, and, like other weevils, have a long proboscis. The larvae are legless and feed within the stems of the turfgrass plants after hatching from eggs. When they become larger, they also feed underground on plant crowns and roots. Adults typically come to the turf surface in the evening hours, primarily between midnight and 4 a.m. (4). These habits probably explain why billbug infestations are often overlooked: They spend most of their life cycle either inside turfgrass plants or below ground.
Figure 2. A bluegrass billbug (Sphenophorus parvulus, left) and a hunting billbug (S. venatus, right) collected in Columbia, Mo. The pronotum (area behind the head) of the bluegrass billbug is coarsely punctated (has numerous tiny holes), and the hunting billbug has a punctated pronotum with a smooth, non-punctated, Y-shaped median area.
Photos by Robert Sites, Enns Entomology Museum, University of Missouri
In the United States, at least nine species of billbugs attack turfgrass plants (8). Bluegrass billbug (S. parvulus Gyllenhal) is considered the most widely distributed of these species and has received the most attention from turfgrass researchers. As their name indicates, the primary host plant for bluegrass billbugs is thought to be Kentucky bluegrass (Poa pratensis L.), although other turfgrass and non-turf species can be attacked as well (6). The hunting billbug, which can be found from Florida to New Jersey, is the species that damages warm-season grasses, including zoysiagrasses (7). Accurately identifying a billbug to the species level is almost impossible at the larval stage without examining its genetic makeup. However, at the adult stage, billbugs can be diagnosed to species primarily based on the markings on the pronotum and elytra (Figure 2).
Despite our belief that bluegrass billbug mainly attacks Kentucky bluegrass and hunting billbug prefers warm-season turf, research studies have found complex populations that often feed on diverse turf species. For example, a study conducted in North Carolina reported abundant (>20,000) hunting billbug adults from mixed Kentucky bluegrass/bermudagrass (C. dactylon (L.) Pers.)/zoysiagrass turf, while only one bluegrass billbug adult was found from the same sites over two years of sampling (1). This trend was also reported in Florida, where more than 15,000 hunting billbug adults were collected over two years of sampling from four golf course bermudagrass roughs, while only two bluegrass adults were found during the same period (5). In Kansas, natural infestation of bluegrass billbug on zoysiagrass turf is responsible for up to 38% of plot damage (3).
Figure 3. Pitfall traps installed on zoysiagrass fairways of Columbia Country Club (top) and Country Club of Missouri (bottom) in April 2015.
Photos by Brett Loman
The complex nature of billbug composition is suspected to be influenced by geographic location and environmental conditions, which result in significant variations in microclimate conditions and in plant species that have adapted to and become established in different areas. Similarly, these variations also affect the biology of the billbugs. For example, bluegrass billbug is generally thought to go through one generation per year, although it has been reported that all developmental stages — including egg, larva, pupa and adult — can exist simultaneously during most of the year (8). Hunting billbug was thought to share a similar life cycle with bluegrass billbug, but research has shown that it likely exhibits one generation per year in northern states, and possibly two generations in Georgia and North Carolina (1,5). Such knowledge is essential for developing a management plan — for example, for determining the best insecticide application timing that targets larval and/or adult populations. Therefore, the objective of this research was to evaluate the billbug composition and biology on Missouri zoysiagrass turf.
Materials and methods
Beginning in spring 2015, the seventh fairway at Columbia Country Club (CCC) and the 17th fairway at Country Club of Missouri (CCMO), both in Columbia, Mo., were monitored for billbug adult activity by using pitfall traps (Figure 3). Both fairways were Meyer zoysiagrass maintained under typical fairway conditions.
Twenty pitfall traps were installed just below the soil surface and spaced 10 feet (3 meters) apart in two offset rows that were also 10 feet apart. A total of 40 pitfall traps were installed on each fairway. During the experimental periods in 2015 and 2016, the two fairways were maintained as usual, except that insecticides were not applied within 20 feet (6.1 meters) of the pitfall traps.
The traps were monitored weekly during the two growing seasons. Billbug specimens collected from the traps were transferred to the laboratory, where they were counted and identified to species. Billbug counts included total counts, counts according to species, season-long cumulative counts, and counts made at each sampling during the two growing seasons. No statistical analysis was performed, because data collected reflected population dynamics at the two sites rather than the influence of predetermined variables (treatments). Every month during the experiment, a soil probe with a 4-inch diameter was used to collect 10 soil cores at random from the monitored area of each fairway. The collected soil cores were examined in the laboratory for larvae and/or pupae. A minimal number of larvae/pupae specimens were collected during this experiment, similar to previously documented results in the literature. No meaningful data could be derived from the larval populations, and therefore no larval data are presented in this report.
Figure 4. Cumulative count of bluegrass billbug and hunting billbug adults retrieved from the 40 pitfall traps at Columbia Country Club and Country Club of Missouri in 2015 and 2016.
Results and discussion
The number of billbug adults recovered from the pitfall traps varied significantly between the two sites. In 2015, a total of 876 billbugs were recovered at CCC, which was more than five times the number found at CCMO (Figure 4). In 2016, a total of 158 billbugs were found at CCMO, which was comparable to the amount found the previous year. At CCC, however, a total 1,077 billbugs were found in 2016, indicating a 22% population increase over 2015. It is worth noting that the areas installed with pitfall traps accounted for only 0.11% of the total area where insecticide applications were withdrawn. It is therefore reasonable to speculate that real billbug populations present at both fairways were much higher than indicated by the numbers trapped. The variation between the two sites is likely a reflection of multiple factors, including whether the roughs — 20 feet away from the monitored areas on both fairways — were regularly treated with insecticides.
On the two fairways, hunting billbug appeared to be the dominant species (Figure 4). At CCC, bluegrass billbugs collected accounted for 13.7% and 9% of the population in 2015 and 2016, respectively. At CCMO, however, we found only six bluegrass billbugs in 2015 and none in 2016. Previous research demonstrated that both billbug species can survive on a wide range of turfgrass species, including tall fescue (Festuca arundinacea Schreb.) and Kentucky bluegrass, the primary grass species for roughs, and zoysiagrass, the species for fairways at the two sites (3,6). However, why such a discrepancy in population composition was found at the two fairways, even though the turfgrass species at both sites were comparable, is unclear.
Despite the difference in population size and composition, activity of billbug adults at the two fairways showed a certain level of similarity (Figures 5 and 6). At CCC, adult activity in 2015 showed two clear peaks, with one from late April to the beginning of May, and another mainly in August (Figure 5). In 2016, despite the increased population primarily due to the elevated hunting billbug populations, the onset of active periods for billbug adults almost coincided with that of the previous year, both in spring and fall. In 2016, spring billbug activity did not show a clear peak as it had in 2015. This change might be related to the decline in temperature around May 2, 2016, when the average temperature dropped to 51 F, compared with 59 F during the same period in 2015 (data not shown). Previous research has found that billbug species are strongly influenced by air temperatures, and adult activity is particularly inhibited when the air temperature drops below 68 F (2). Compared with hunting billbugs, bluegrass billbug adults showed elevated activity mainly in spring at CCC, indicating that their life cycle has only one generation per year (8).
Figure 5. Count of hunting billbug and bluegrass billbug collected from a total of 40 pitfall traps installed on the seventh fairway of Columbia Country Club, Columbia, Mo., in 2015 and 2016.
At CCMO, billbug adults appeared to be active during the same periods exhibited at CCC (Figure 6). In spring 2016, a relatively smaller number of billbugs were caught on May 7, likely because of the sudden temperature decline that had occurred around May 2 (data not shown). Peak activity at CCMO in fall 2015 was less defined; the factors that may have contributed to such a phenomenon remain unknown.
Collectively, our data suggest that hunting billbug is the dominant species on zoysiagrass fairways in Missouri, although the size and composition of the billbug populations can vary substantially from site to site, regardless of proximity. Hunting billbugs in this area appear to complete one and a half to two life cycles per year and likely overwinter as large-sized larvae or adults, respectively.
The life cycle of hunting billbug varies geographically. It has been reported that hunting billbugs complete one life cycle per year in northern states (8), two overlapping life cycles in North Carolina (1), and up to six generations in Florida (4). In North Carolina, the peak active times were April to June and August to October (4). In Missouri, the peaks are from late April to mid-May and late July to the end of August. A thorough understanding of the billbugs’ life cycle and peak active times is essential for determining insecticide application timing for maximum efficacy. Results from this experiment suggest that two insecticide applications may be necessary in areas where heavy billbug infestation occurs, with one in late April/early May, and another in late July/early August in Missouri and surrounding areas.
Figure 6. Count of hunting billbugs and bluegrass billbugs collected from a total of 40 pitfall traps installed on the 17th fairway of Country Club of Missouri, Columbia, Mo., in 2015 and 2016.
The authors would like to thank GCSAA and the Missouri Valley GCSA for funding this project. We would also like to thank superintendents Joseph Herzog at Columbia Country Club and Ryan Sears at Country Club of Missouri, both located in Columbia, Mo., for graciously providing the experimental sites.
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Michael E. Patterson is a graduate research assistant, Xi Xiong, Ph.D., is an associate professor of turfgrass science, and Bruce A. Barrett, Ph.D., is a professor of entomology in the Division of Plant Sciences, University of Missouri, Columbia.