Timing Of Fungal Insecticide Application To Avoid Solar Ultraviolet Irradiation Enhances Field Control Of Rice Planthoppers

The rice planthoppers belongs to the family Homoptera,and its wide distribution,high fecundity,and migratory nature make it one of the most important pests that seriously harm rice production.The control of rice planthoppers,which is highly resistant to almost all types of chemical insecticides,still relies heavily on chemical insecticides after decades of struggle.However,to prevent contamination of aquaculture with chemical residues,the use of chemical insecticides is strictly prohibited in the rice-aquaculture systems emerging in southern China,so there is a need to explore alternative methods for effective control of rice flies in highly profitable agricultural rice-fishery symbiotic systems.The common biocontrol fungi such as Beauveria bassiana and Metarhizium anisopliae can meet the green needs of pest control in rice-fishery systems,but their application in the field is subject to environmental factors and other constraints that can cause lagging insecticidal effects and unstable efficacy,making the evaluation of biocontrol agents’insecticidal activity in the field still challenging.In this study,we established a quantitative evaluation system for the insecticidal activity of biocontrol fungi indoors and in the field,and used representative Nilaparvata lugens as the target pest to determine and compare the spore viability of the strains of B.bassiana（ZJU435）and M.anisopliae（CQ421）,which have high virulence against N.lugens,at different temperatures,and compared the spore suspension emulsion products of the two strains in a field trial of rice planthoppers in Tongxiang,Zhejiang Province.Jiaxing Tongxiang,China,where field trials of rice lice control were conducted in rice fields,respectively.Based on which the effects of UV and high temperature effects in the field on the fungicidal activity of the fungus were evaluated using the spraying method at two different time points.The results of the trials were as follows:The indoor tests showed that both the conidial viability（average germination rate）of Bba ZJU435and Ma CQ421 strains were strongest at 25℃,with 99.1±0.6%and 89.7±1.1%germination rates,respectively,but both spore viability decreased linearly with increasing temperature.Both the conidial suspensions of B.bassiana and M.anisopliae at 1×10⁷spores/m L showed high insecticidal activity against N.lugens,Rhoplosiphum padi and Rhoplosiphum padi.The corrected mortality rates were 83.7±3.2%and 79.9±1.6%for the spray treatment group and 85.3±0.8%and 83.2±2.1%for the dip treatment group,respectively against N.lugens,on the 10^thday after the drug.Field efficacy of two fungal insecticides against N.lugens,here,two registered fungal insecticides,namely Bba ZJU435 and Ma CQ421,are reported as alternatives to fighting against the RPH population dominated by N.lugens in three 0.5-ha rice-shrimp rotation paddy fields（Trials 1–3）.During four-week field trials initiated from harsh weather of high temperatures and strong sunlight,the rice crop at the stages from tillering to flowering was effectively protected by fungal sprays at a 14-day interval.The sprays of either fungal insecticide after 5:00 pm（solar UV avoidance）suppressed the RPH population better than those before 10 am（solar UV exposure）.The ZJU435 and CQ421 sprays for UV avoidance versus UV exposure resulted in mean control efficacies of 60%and 56%versus 41%and 45%on day 7,77%and 78%versus 63%and 67%on day 14,84%and 82%versus 80%and 79%on day 21,and 84%and 81%versus 79%and 75%on day 28,respectively.These results indicate a feasibility of either fungal insecticide for RPH control in the rice-shrimp rotation or other rice-aquaculture systems and offer novel insight into a significance of solar UV-avoiding fungal application for improved pest control in sunny summer.