Screening And Formulation Of A Beauveria Bassiana Candidate Strain With High Virulence To Spodoptera Litura Larvae

Spodoptera litura (Lepidoptera:Noctuidae) is a foliage-devouring pest threatening to various crops. Long-term reliance on chemical control has caused its high-level resistance to common chemical insecticides, making it necessary to search for alternative control measures. This study sought to develop a mycoinsecticide for alternative control of S. litura. The results are summarized below.Screening of a Beauveria bassiana candidate strain. Twelve Beauveria bassiana strains derived from diffetent host insects worldwild were bioassayed for their lethal effects on second-instar larva of S. litura at 25℃and 12:12 L:D under the concentrated spray of 2500 (2132-2807) conidia mm-2. The corrected mean mortalities ranged from 33.9 to 82.4%, differing significantly among the tested strains. Bb2860 was selected as the most virulent strain for formulation development due to its maximal and rapid lethal effect on the pest species..Storage of dry conidia and improved oil formulation. A dry preparation of Bb2860 conidia and an oil formulation prepared by suspending the dry conidia in liquid paraffin were stored for 18 months at 4℃and 25℃, respectively. During storage, conidial viability and their tolerance to thermal stress and UV-B irradiation were monthly examined. As a result, conidial viability declined with the storage time and differed significantly between the two temperature treatments. Up to 85% of the conidia in the dry preparation were viable after 12-month storage at 4℃whereas no viability was detected in the same preparation stored for the same period at 25℃. In contrast,66% of the conidia in the oil formulation remained viable after 12-month storage at 25℃. Apart from changes in viability, dry conidia stored at 25℃became less tolerant to the thermal tolerance at 45℃and the UV-B irradiation of 312 nm weighted wavelength than those conidia stored at 4℃. Moreover, 2-20% vaseline was tested as stabilizer for for slowing down conidial deposition inthe oil formulation during storage. The increase of the oil viscosity by adding 10% vaseline was found improving significantly the oil formulation with 8% emulsifier (AEO-3), resulting in a uniform aqueous dilution of 1.2x 107 conidia ml-1 for qualified field spray. Co-action of fungal and chemical agents on S. litura larvae. Bb2860 dry conidia and oil formulation were bioassayed alone or in conjunction with a low application rate of chlorfluazuron against the second-instar larva of S. litura. Bioassay 1 included the fungal sprays of 1 ml spore suspension at the concentrations of 1x108, 11×107, and x106 dry (unformulated) conidia ml-1) in 0.02% Tween 80, which was used as control. Bioassays 2 and 3 included the same fungal sprays of 1 ml aqueous dilutions of the emulsifier-inclusive oil formulation with and without 10% vaseline, respectively. Bioassay 4 consisted of the treatments of the labeled application rate of chlorfluazuron (i.e.,1000x aqueous dilution of 700 ml 5% emulsion ha-1) and 20%(140 ml 5% emulsion) and 10%(70 ml 5% emulsion) of the labeled rate, respectively. Bioassay 5 combined the three fungal treatments of Bioassay 3 with 10% of the labeled application rate of chlorfluazuron in Bioassay 4. All sprays were porformed under antomatic spray tower and sprayed larvae were reared at 25℃and 12:12 L:D for 7 days. Modeling analyses of the resultant time-concentration-mortality data sets indicated that the mortality trends differed significantly among the five bioassays (F4,28=17.1, P<0.01) and also from one to another concentration treatment (F2,28=115.6, P<0.01). Based on fitted median lethal responses (LC50 and LT50), the oil formulation in Bioassay 2 showed higher virulence to the larvae than the unformulated dry conidia in Bioassay 1 but was less virulent than the vaseline-inclusive formulation in Bioassay 3. The low application rate of chlorfluazuron accelerated significantly the lethal effect of the fungal formulation on the pest species.Field trials of optimized emulaifiable formulation against S. litura. The optimized emulsifible formulation of Bb2860 conidia was applied alone or together with 10% of the labeled application rate or chlorfluazuron for field efficacies against S. litura on cauliflower in Maanshan city, Anhui province. Two trials were performed in the same cauliflower field during September of 2009 (Trail 1) and 2010 (Trail 2), respectively. Each trial included five treatments:Ⅰ) fungal spray at the rate of 1.5-1.8 x1013 conidia ha-1;Ⅱ) chlorfluazuron spray at 10% of the labeled rate (75 ml 5% emulsion);Ⅲ) chlorfluazuron spray at the labeled application rate (750 ml 5% emulsion);Ⅳ) combined spray of 2) and 3) for fungal-chemical interaction; andⅤ) water spray as blank control. The fungal formulation and the chemical insecticide were sprayed in randomized treatments of four replicate plots (15 m2 each) at the same aqueous dilution volume of 750 L ha-1. The pest population consisted of second and third instars at the averaged initial density of 13.3 larvae per 5 plants at the median growth stage. In Trial 1 with a single spray of each agent, treatment IV resulted in the relative efficacy of 91.1% on day 9 after spray, followed by 89.7%,77.6% and 49% in treatments III, I and II and, respectively. In Trial 2 with repeated spray of each agent, the relative efficacy of 99.6% was achieved in treatment IV on 19 and was not different from 100% control in treatment III while treatments I and II resulted in the same efficacy of 77%. The results from the two field trails indicate that the fungal formulation provided a significant control of more than 70%. The combined application of the fungal rate and 10% of the labeled chemical rate provided consistently desired control of over 90%, which was similar to what was observed in the chemical treatment at the labeled rate.Conclusively, the improved emulsifiable formulation based on the selected candidate strain of B. bassiana was an effective mycoinsecticide against S. litura. A combined application of this formulation with the low application rate of chlorfluazuron is of great potential for use in alternative control of the pest species.