Calibration of sampling techniques and determination of sample size for the estimation of egg and larval populations of Helicoverpa spp. (Lepidoptera: Noctuidae) on irrigated soybean

Informed spray decisions require accurate assessments of the target pest's density. Currently, no advice is provided to farmers on the best method for sampling soybean for insect pests, although spray thresholds for Helicoverpa larvae are provided. This article describes the results of a trial designed to calibrate relative sampling techniques for Helicoverpa larvae; visual inspection of plants in situ in the field, beat cloth, sweep net and D‐vac sampling were compared with an absolute measure of population density. The absolute measure was derived from the bagging and removal of whole plants in the field, followed by subsequent examination in the laboratory. Analysis of the distribution of Helicoverpa larvae collected by the different samples was then used to calculate the number of samples required to determine whether the economic threshold had been reached to different levels of certainty and accuracy. Significant relationships were detected between all the relative sampling techniques and the absolute, suggesting that all could be used to estimate field populations. However, due to the high variance and therefore increased sample sizes required, or the length of time taken to collect samples, only beat‐cloth sampling appeared to offer a realistic method for farmers in the field. The results also suggest that the current best practice of sampling six locations per crop provides an adequate assessment of the field populations at the currently accepted threshold level of 6 larvae m−2. However, if the economic spray was reduced, the number of samples required to determine an accurate population estimate would increase dramatically.