Microsclerotial Granular Formulation of the Entomopathogenic Fungus Metarhizium brunneum and Its Combinations With Hydrogel and Imidacloprid Against the Annual Bluegrass Weevil (Coleoptera: Curculionidae)

We determined the potential of the entomopathogenic fungus Metarhizium brunneum Petch (Hypocreales: Clavicipitaceae) F52 strain, and of a microsclerotial formulation, for the control of the annual bluegrass weevil, Listronotus maculicollis Kirby, which is a major pest of golf course turf in eastern North America with widespread insecticide resistance. Under laboratory conditions in Petri dishes with moist sand, the microsclerotia (23–46 kg granules/ha) caused high rates of mortality (85–100%) and infection (67–80%) in annual bluegrass weevil adults, but these levels did not occur until after 9 d at constant 26°C and 12–15 d at 14 h at 23°C and 10 h at 17°C. Production of viable conidia was marginally higher at the higher temperature regime (7.3 vs. 5.2 × 109 per gram of granules). Application of microsclerotia did not provide significant control and infection of adults in pots with grass in the greenhouse. In field trials targeting spring generation larvae, microsclerotia application (50–100 kg granules/ha) was ineffective, and coapplication of hydrogel to stabilize soil moisture did not increase larval control. A liquid M. brunneum F52 conidial formulation (4.75–9.5 × 1013 colony forming units/ha) provided up to 51% control. Combinations of M. brunneum F52 with the neonicotinoid insecticide imidacloprid provided additive control with up to 70% control with the conidial formulation. Field efficacy was probably limited by suboptimal temperatures for the fungus, and future tests need to examine whether higher control rates can be achieved in applications targeting the summer generation larvae.

Effect of temperature on post-diapause reproductive development in Listronotus maculicollis (Coleoptera: curculionidae)

The annual bluegrass weevil Listronotus maculicollis requires chilling exposure to terminate reproductive diapause during overwintering, but the effects of temperature on its post-diapause development in spring remain unclear. To explore this effect, overwintering adults were transferred from cold conditions (6°C/4°C, L:D 10:14) to different warm-up temperatures at L:D 12:12. When weevils were transferred to 7, 14 and 21°C in December and late January, the sizes of male and female reproductive organs were significantly smaller at 7°C than at 14 and 21°C. When weevils were transferred to 7, 9, 11, 13 and 15°C in late January, higher temperatures facilitated the post-diapause development. In both sexes, the sizes of reproductive organs and developmental rate increased with temperature. Reproductive organs did not grow significantly at 7°C in males and at 7–9°C in females, at which the percentage of developing weevils remained low. The time required for 50% of individuals to resume development was 44, 18, 13 and 8 days at 9, 11, 13 and 15°C, respectively, in males and 19, 14 and 8 days at 11, 13 and 15°C, respectively, in females. The threshold temperature for post-diapause development was 7.8°C in males, based on which 61.7 degree-days coincided with 50% of individuals developing. Under field conditions, the percentage of male and female maturity and insemination rate were low until early March, but all reached 100% by late March.