Effect of Pesticides on Growth and Efficacy of Beauveria bassiana (Balsamo) Vuillemin for Controlling of Cassava Pink Mealybug

Abstract

The effect of chemical pesticides such as insecticides (thiamethoxam, amitraz), fungicides (mancozeb, benomyl) and herbicide (paraquat dichloride) on the growth of white mascardine fungus (Beauveria bassiana) and its efficiency for controlling of cassava pink mealybug (Phenacoccus manihoti) was carried out. The pesticides were mixed in medium. Spore suspensions of 1x10⁴ and 1x10⁵ spores/ml were spread on the medium. The spore survival after 1 and 3 days after spreading was evaluated. It was found that the highest survival was detected in control treatment (PDA without pesticides) with concentration of 1x10⁵ spores/ml at 1 day after spreading (444.33 cfu/plate) and significantly different (P<0.05) to others. Only the treatment of PDA+thiamethoxam, the fungus was able to grow and express viability of 185.33 cfu/plate For the concentration of 1x10⁴ spore/ml treated after 3 days, the maximum fungal growth was found in PDA+thiamethoxam medium (269.00cfu/plate) which was not significantly different to the following treatment (control treatment, 252.60 cfu/plate). Whereas, the growth of fungus on PDA+ amitraz (166.80 cfu/plate) and PDA+paraquat dichloride (145.00 cfu/plate) were similar and not significantly different. The effect of pesticides on mycelial growth was depended on age. It grew on PDA+thiamethoxam with colony diameter (Ø) of 2.72 (7 days) and 4.67 cm (14 days), which was not significantly different with control (2.72 and 5.32 cm, respectively). Only on PDA+benomyl, the fungus was unable to grow. When the spore of good survival fungi from all treatments were tested on the infection efficiency against the 3rd instar-adult cassava pink mealybug with conidial concentration of 1x10⁸ spore/ml, the result revealed that the fungal infectivity trended to increase by the time after inoculation. The fungal spore obtained from PDA+thiamethoxam had the most infection rate at all dates after inoculation
(17.78%, 43.33%, 46.67%, 46.67% at 7, 10, 14 and 21 days respectively), which were clearly higher than the spores derived from other treatments and control treatment (PDA) (9.44, 30.25, 30.25 and 33.28%, respectively). However, the infectivity from both treatments
(PDA+thiamethoxam, PDA) at all checking times were not significantly different. The result of this study indicates that the B. bassiana could be used in combination with the pesticides, especially thiamethoxam as well without adverse effect against cassava pink mealybug.