Time-concentration-mortality responses of carmine spider mite (Acari: Tetranychidae) females to three hypocrealean fungi as biocontrol agents

Ten isolates of Beauveria bassiana (Bb), Metarhizium anisopliae (Ma) and Paecilomyces fumosoroseus (Pfr) with different host insect origins were assayed for their biocontrol potential against the female adults of carmine spider mite Tetranychus cinnabarinus using a novel bioassay system, which was featured with paired fava bean leaves supported upright by the petioles individually inserted into rhizocaline-inclusive agar in caged bottles and provided a nearly natural habitant for free activity of the highly mobile mite adults. Each pair of the leaves harboring 15 females (⩽2-day-old) was exposed to a spray of each isolate at the concentration of 21–78 (low), 119–308 (medium) or 740–1946 (high) conidia/mm2 plus a blank control (sprayed with 0.02% Tween-80), maintained in a top-meshed cage at 25 °C and 12:12 L:D and observed daily for 9-day mortality records. Each of the bioassays was repeated 6–8 times. The three fungal concentrations resulted in the mite mortalities of 31.9–87.7% for B. bassiana, 24.2–80.3% for M. anisopliae and 19.4–77.7% for P. fumosoroseus, respectively. The LC50s of the tested isolates determined by the fitted time-concentration-mortality relationships declined over days after spray while their LT50s shortened as the concentration increased. The two isolates Bb734 and Bb2860 had the LC50s of 42–180 and 177–351 conidia/mm2 on days 7–9 and the LT50s of 5.8–6.9 and 3.6–8.4 days at 200–1500 conidia/mm2. These were followed by Ma456 with the LC50s of 179–925 conidia/mm2 on days 7–9 and the LT50s of 6.5–7.2 days at 750–1500 conidia/mm2. Other isolates had larger LC50s and longer LT50s, which represent intermediate or low virulence to the mite species. Thus, the mentioned three among the tested isolates are promising candidates for microbial control of the mite pest.