Spatial distribution pattern of Spodoptera exigua (Lepidoptera: Noctuidae) on sugar beet and advantage of site-specific spraying in the pest management

Spodoptera exigua (Hübner) is a destructive insect pest of sugar beet globally. In this study, we utilized some geostatistical techniques to locate sample points and determine spatial distribution pattern of the pest. Pheromone traps were used to predict accurate time of oviposition and visual sampling was used to determine larval density. In order to predict the population density at non-sampled locations, Inverse Distance Weighting (IDW) and Ordinary Kriging (OK) techniques were used. The densities of different developmental stages of S. exigua were estimated weekly during August and September, 2016. The degree dependence values of S. exigua eggs in majority of dataset were larger than 76%, indicating their clumped distribution and strong spatial autocorrelation. Broadcast spraying was used when the density of the larvae in tracts overpassed the economic threshold. Also, we applied site-specific spraying to grid cells when larval densities were above the economic threshold. In most cases, larval mortality was not significantly different between broadcast spraying and site-specific spraying methods. Comparing two interpolation methods indicated that the data calculated for density of larvae and eggs of S. exigua were fitted better with OK model than IDW one. The findings of this research would be useful to develop sampling programs and to control S. exigua in Iran.