Laboratory assessment of virulence of Cameroonian isolates of Beauveria bassiana and Metarhizium anisopliae against mirid bugs Sahlbergella singularis Haglund (Hemiptera: Miridae)

摘要

The brown cocoa mirid, Sahlbergella singularis (Hemiptera: Miridae), causes cocoa yield loss of about 30 % to 70 % in Cameroon. The pathogenicity of six indigenous isolates of Beauveria bassiana (BIITAC) and Metarhizium anisopliae (MIITAC) to the fourth and fifth nymphal stages of S. singularis was evaluated under laboratory conditions. Two methods of inoculation were tested at various conidial concentrations: (1) immersing the pod in 200 ml of the suspension for 3 min (ingestion method) and (2) immersing each insect in 1 ml of the suspension for 5 s (immersion method). Tween (R) 80 at 0.1 % (v/v) was used as a control. After 14 days from initial exposure to conidia, corrected mortality ranged from 35 to 100 % for immersion and from 16 to 94.3 % for ingestion. Mortalities due to fungi isolates were significantly different from that of their control (P < 0.05). The effect of immersion was more significant than that of ingestion, and mortalities increased with increasing spore concentration. Mycelial outgrowth and sporulation after seven days on some of the dead insects, kept on humidified filter paper in dark conditions, demonstrated that death was due to fungal infection (mycosis). Based on pathogenicity results, LC50, LT50 and LT90 fungal outgrowth, B. bassiana isolates BIITAC10.3.3, BIITAC6.2.2, MIITAC6.2.2 and the M. anisopliae isolate MIITAC11.3.4 could be selected for their virulence, and advanced to field trials for the development of microbial control.