Effect of weedy culture on population densities, spatial distributions and sampling procedures of Spodoptera exigua and Sesamia cretica (Lep., Noctuidae) in corn fields.

Spodoptera exigua Hübner and Sesamia cretica Led. (Lep., Noctuidae) are two important pests of corn. In this study, the effect of weed bands in the corn field, as weedy culture, on population density and damages of the pests were evaluated during two growing seasons (2016/2017). Cumulative insect days (CID) of each lepidopteran pest in weedy culture were compared with non-weedy culture. Results showed that CIDs of S. exigua and S. cretica in the non-weedy corn culture were significantly higher than the weedy corn culture. There was no significant difference between the total yield in the weedy and non-weedy cultures. Also, determinations of spatial distributions of the pests in the weedy and no-weedy treatments using Taylor's power law (TPL) and Iwao's patchiness (IP) showed that TPL provides a better fit for the data than IP and spatial distributions of both pests on both cultures were aggregative. Moreover, minor differences were observed between spatial distribution parameters of the pests in the weedy and non-weedy cultures. Green's model was used for developing a fixed-precision sequential sampling plan of the pests on the weedy and no-weedy treatments. Optimum sample sizes of S. exigua ranged from 532 to 5370 and 428 to 5296 plants and S. cretica varied from 297 to 2040 and 43 to 186 plants in the non-weedy and weedy cultures based on the desired precision level (0.25-0.1). Estimated stop lines of non-weedy and weedy cultures for S. exigua ranged from 0.000057 to 52.59 and 0.00029 to 58.87 and for S. cretica ranged from 1.59 to 111.5 and 2.09 to 98.03 larval cumulative numbers, respectively (0.25-0.1). The performance of the sampling plan was validated by resampling analysis using RVSP software. Results of the study can be used in the integrated pest management program of corn fields.

A Fixed-Precision Sequential Sampling Plan for the Potato Tuberworm Moth, Phthorimaea operculella Zeller (Lepidoptera: Gelechidae), on Potato Cultivars.

Phthorimaea operculella Zeller is an important pest of potato in Iran. Spatial distribution and fixed-precision sequential sampling for population estimation of the pest on two potato cultivars, Arinda® and Sante®, were studied in two separate potato fields during two growing seasons (2013-2014 and 2014-2015). Spatial distribution was investigated by Taylor's power law and Iwao's patchiness. Results showed that the spatial distribution of eggs and larvae was random. In contrast to Iwao's patchiness, Taylor's power law provided a highly significant relationship between variance and mean density. Therefore, fixed-precision sequential sampling plan was developed by Green's model at two precision levels of 0.25 and 0.1. The optimum sample size on Arinda® and Sante® cultivars at precision level of 0.25 ranged from 151 to 813 and 149 to 802 leaves, respectively. At 0.1 precision level, the sample sizes varied from 5083 to 1054 and 5100 to 1050 leaves for Arinda® and Sante® cultivars, respectively. Therefore, the optimum sample sizes for the cultivars, with different resistance levels, were not significantly different. According to the calculated stop lines, the sampling must be continued until cumulative number of eggs + larvae reached to 15-16 or 96-101 individuals at precision levels of 0.25 or 0.1, respectively. The performance of the sampling plan was validated by resampling analysis using resampling for validation of sampling plans software. The sampling plant provided in this study can be used to obtain a rapid estimate of the pest density with minimal effort.