Spatial distribution of larvae Spodoptera frugiperda （J. E. Smith） in mountainous corn plants and field

【Objective】 To clarify the spatial distribution characteristics of Spodoptera frugiperda(J. E. Smith) larvae on mountainouscorn plants, and provide basis for the prediction, sampling investigation and control of the pest.【Method】 From June to October in 2021, the population density of S. frugiperda was investigatedin the mountainous corn planting area of Longqing Township, Shizong County, Qujing City, Yunnan Province, using the W type 5-point survey sampling method in 3 maize fields, the population density of different instar larvae in different growth stages and different parts of the corn plant was recorded. The spatial distribution pattern and sampling technology of larvae were analyzed by aggrega-tion index, Iwao regression analysis method and Taylor power law. Based on the results of spatial distribution, the theoretical sampling model of larval density and the sequential sampling technique based on the control index of larval density were further fitted.【Result】The composition of larvae of S. frugiperda at each instar was different in corn plants at different growth stages. No 1^st instar larvae were found on the male ear, the larvae of each instar could be found in different parts of corn. The population density of larvae S. frugiperda on corn plants followed the rank of heart leaf>cornsilk>stem/leaf> tassel. At corn seedling stage, jointing stage, big bell mouth stage and silking stage, the number proportion of young larvae of S. frugiperda was higher, which were 100.00%, 71.45%, 63.39% and 78.07% respectively. At male drawing stage, powdering stage, grain filling stage, milk mature stage and wax mature stage, the number proportion of old larvae of S. frugiperda was higher, which were 70.01%, 83.55%, 81.35%, 95.94% and 95.56% respectively.The numbers of insects on the heart leaf and female ear of corn plant were 37.73% and 34.49% respectively, of which the number of insects on 100 ears of female ear was the highest, up to 237.78 insect/100 ears. The young instar larvae of S. frugiperda were clustered in mountain corn fields;the old instar larvae were evenly distributed. The spatial distribution of all larva samples in mountain maize field showed aggregation distribution, and the aggregation degree was density dependent. The best theoretical sampling number models of young instar larvae, old instar larvae and all samples of larvae were:N = (\frac3.84D^2)(\frac3.2575m^2 + 0.2874), N = (\frac3.84D^2)(\frac1.008m - 0.0644) and N = (\frac3.84D^2)(\frac1.4718m + 0.3370). The calculation formulas of sequential sampling interval under the control indexes of samples of young instar larvae, old instar larvae and all samples of larvae were as follows:T_Iwao(n) =0.33n±1.96 √1.1063n, T_Iwao(n) =0.33n±1.96 √0.3257n and T_Iwao(n) =0.33n±1.96 √0.5224n.【Conclusion】The young instar larvae of S. frugiperda in Yunnan mountainous corn fields are clustered and the old instar larvae are evenly distributed. The larvae of S. frugiperda are clustered and distributed in the whole growth period. The optimal theoretical sampling number and sequential sampling interval of young and old larvae can be calculated by the fitted model.