Evaluation Of Alternative Rice Planthopper Control By The Combined Action Of Metarhizium Anisopliae And Low-rate Buprofezin

The brown planthopper (BPH), Nilaparvata lugens, has occurred as a major rice pest more frequently in China in the past few years due to high resistance to common chemical insecticides. This study sought to search for fungal candidates for alternative microbial control of the rice pest, to which little attention has been paid in the past decades. The results are summarized below.Screening of various Metarhizium isolates for BPH control. Thirty-five isolates of Metarhizium anisopliae and M. flavoviride from different host insects worldwide were bioassayed triply for their lethal effects on third-instar BPH nymphs at 25℃and 14:10 L:D under the highly concentrated spray of～1000 conidia mm-2. On day 9 after spray, the pest mortalities attributed to mycosis were 6.5-64.2% and differed significantly among the tested isolates, showing little relation to the host origins of the fungal isolates. Only two BPH-derived M. anisopliae isolates from Philippines (Ma 456) and Indonesia (Ma 576) killed >50% of the sprayed nymphs. They were further bioassayed for the time-concentration-mortality (TCM) responses of the nymphs to the fungal sprays of 19-29, 118-164 and 978-1088 conidia mm-2 in bioassays repeated four times. The resultant data fit very well a TCM model. The LC50S of the two isolates were estimated as 731 and 1124 conidia mm-2 on day 7 after spray and dropped to 284 and 306 on day 10, respectively. The two selected isolates are potential biocontrol agents against BPH. This is the first report on the lethal effects of global Metarhizium isolates on BPH and the fungal candidates are selected for further research for integration into the paddy field.Fungal effects on BPH fecundity and longevity. To evaluate accurately the effect of fungal infection on BPH population, the selected M. anisopliae isolate (Ma 456) was sprayed on brachypterous adults at the high concentration of 1021 (967-1055) conidia ml-1. A blank control was sprayed only with 0.02% Tween-80. After spray, each pair of male and female (30 pairs included in both fungal treatment and the control) was transferred to a single tillering plant in meshed glass tube and maintained at 25℃and 14:10 L:D for oviposition. All paired adults were monitored daily for their death time and supplied with a renewed plant for their feeding and oviposition at the time of daily examination. The replaced plants harboring BPH eggs were maintained at the same regime and individually examined each day for the count of new nymphs hatched from the eggs. As a result, BPH adults sprayed with Ma 456 survived much shorter than those in the control, irrespective of females (9.6 versus 21.8 days) and males (11.1 versus 23.9 days). BPH females sprayed with unformulated Ma 456 conidia exhibited significantly lower fecundity (71% reduction) than those not infected and the hatch rate of their eggs dropped by 36%. Both effects contributed to 81.4% reduction in the reproduction potential of the infected females (111 versus 595 hatched eggs per female).Fungal/chemical interaction on BPH. To develop alternative BPH control strategy based on fungal/chemical interaction, an emulsifiable formulation of the selected M. anisopliae isolate (Ma 456) was interacted with the applied buprofezin rates of 0,10,20,30 and 40μg AI ml-1 on third-instar nymphs in bioassays 1-5 at 25℃and 14:10 L:D, respectively. Each bioassay included three fungal sprays (36-45,194-233 and 944-1084 conidia mm-2) and blank control (97-138 nymphs per treatment). As a result of the TCM modeling, the LC50 and associated 95% confidence limits of the fungal formulation against BPH nymphs dropped from 1426 (853-2381) conidia mm-2 on day 5 to 199 (145-273) on day 10 after spray. The LC50 estimates at the same time interval were greatly reduced by including a low rate of buprofezin into the fungal sprays, dropping from 998 (551-1808) to 92 (54-156) conidia mm-2 at 10μg AI ml-1, from 830 (506-1362) to 96 (64-146) conidia mm-2 at 20μg AI ml-1, from 245 (163-367) to 76 (44-130) conidia mm-2 at 30μg AI ml-1, and from 113 (70-181) to 27 (13-56) conidia mm-2 at 20μg AI ml-1, respectively. The chemical LC50S on days 2-4 were 1647,486 and 233μg AI ml-1. The chemical rates of 30-40μg AI ml-1 were found enhancing or accelerating significantly the fungal action on BPH. The applied chemical rates showed no significant effect on fungal growth and sproulation on mycosis-killed cadavers.Field efficacies of M. anisopliae formulations in BPH control. The emulsifiable formulations of Ma 456 and Ma 576 were separately applied alone or together with buprofezin at～20% of its labeled rate for BPH control in two single-season rice fields (trial 1 and trial 2) of Jinhua, Zhejiang, eastern China during a period from 17 August through 28 September 2008. Aqueous dilutions (769 L ha-1) of the two formulations at the application rate of 1.54×1013 conidia ha-1 or buproezin at the low rate of 30.8 g AI ha-1 were sprayed to plots in randomized blocks (13×3 m per plots,3 plots per treatment) with mean density of 11.1-14.4 hoppers per sample (two hills) in trail 1 and 10.6-15.1 hoppers per sample in trail 2, respectively. Based on the control efficacies of different treatments on all weekly sample occasions except the first, an overall mean efficacy from the pure fungal treatments was around 50% for both Ma 456 (61% in trial 1 and 65% in trial 2) and Ma 576 (47% in trial 1 and 54% in trial 2) during the 40-day period of hopper population increase. This efficacy is higher than that from the pure buprofezin treatment (39% in trial 1 and 32 in trial 2). Inclusion of the low chemical rate in the fungal sprays enhanced the overall efficacy to 81% and 85% in trial 1, and 78% and 80% in trial 2, respectively. Thus, the fungal/chemical interaction at the applied rates in both fields led to ca 25% net increase in overall efficacy compared to the fungal action alone. The buprofezin-inclusive fungal sprays at the rate of～1.5×1013 conidia ha-1 applied three times at 14-day interval provided excellent planthopper control for 40 days, yielding overall efficacy of ca. 80%, making it a promising alternative measure for rice planthopper control.In summary, the bioassay system developed for evaluating efficiently the virulence of fungal biocontrol agents against BPH was successfully applied to screening fungal candidates with high biocontrol potential. Interactive effects of M. anisopliae and buprofezin on BPH were elucidated in both laboratory and field. All the results would facilitate incorporation of M. anisopliae into planthopper pest management system.