Effects of Selected Insecticides on Zoysiagrass Fairways Infested with Billbugs.

Zoysiagrass (Zoysia japonica Steud.) is the dominant grass species on golf course fairways in Missouri and surrounding states. In recent years, billbug (Sphenophorus spp.), especially hunting billbug (S. venatus vestitus Chittenden), damage on zoysiagrass is becoming an emerging problem. Left untreated, billbug activity causes substantial injury and can lead to a total kill of zoysiagrass in affected areas. Among the multiple billbug species that attack turfgrass plants in the United States, bluegrass billbug (S. parvulus Gyllenhal) and hunting billbug are the common species found on zoysiagrass fairways in this region. Successful insecticide program often depends on proper application timing, especially when multiple species with different biology co-exist.  Therefore, the objective of this field experiment was to evaluate effects of various insecticides and their combinations for single or sequential applications. The first application timing occurred in May, a typical timing for control of white grubs in this region. The sequential application was carried out one month later to simulate situations where initial application failed to control billbugs and additional applications are required. Treatments included insecticides that target either adult, larvae, or both adult and larvae populations. There were a total of 15 insecticide treatments in addition to one nontreated plot. Treatments were arranged in a split-plot design with three replications where insecticide was whole-plot variable and single or sequential applications was sub-plot variable. Pitfall traps were installed in the center of each sub-plot measuring 1.5 × 1.5 m with a 3.0 m buffer between each whole-plot. In addition to weekly billbugs collection and identifications, turf quality and normalized difference vegetation index (NDVI) were determined weekly throughout this two-year experiment. Our current results showed that the plot area was dominated by hunting billbugs with two peaks of adult activities in spring and early fall. Single or sequential applications did not significantly affect billbug counts and/or turf performance. Throughout the two-year experiment, most insecticides evaluated resulted in better turf qualities compared to control, especially plots treated with deltamethrin + thiamethoxam which showed consistently higher NDVI in both years. Interestingly, the combination of deltamethrin + thiamethoxam also resulted in significantly higher amount of hunting billbugs, approximately three times greater than the untreated control during the second season. This experiment is still ongoing; more details will be discussed in the presentation.