Pherocon AM Trapping and the Whole Plant Count Method - A Comparison of Two Sampling Techniques to Estimate the WCR Adult Densities in Central Europe

Pherocon AM (PhAM) trapping and visual counting are two commonly used methods for identifying maize fields in the USA that contain a sufficient number of adult western corn rootworms (WCR), Diabrotica virgifera virgifera LeConte to cause economic root damage by larvae the following year. Therefore, the estimation of adult WCR population plays an important role in management decisions (rotation or treatment). The goals of this study were (1) to compare adult WCR data obtained by PhAM trapping to whole plant count data in different maize developmental stages, (2) to determine the period in which the data obtained by both methods show the highest correlation, (3) to determine the regression curve between capture of WCR on PhAM as an independent variable and number of WCR observed by visual plant count, and (4) to determine if the whole plant count method can serve as a substitution for PhAM trapping under European conditions. Three samplings were conducted in 2006 in 38 fields in Croatia and one sampling was conducted in 22 fields in Serbia. Sampling in Croatia was conducted when maize plants were in the R 65-69, R 73-77 and R 79-85 developmental stages according to BBCH identification keys of maize. Sampling in Serbia was conducted when maize plants reached R 69-71 developmental stages. Three to six PhAM traps were placed in each 0.5 to 4 ha maize field seven days prior to sampling. Visual counts were conducted on the same day as trap inspections. Adult WCR densities in the PhAM traps were correlated to adult densities on whole plants. Statistical analyses comparing the results of PhAM trapping with the visual whole plant count method showed that there is a significant medium to strong correlation between them. Correlation coefficients varied depending on the data used in the analysis. When sampling was conducted in the first half of August (maize developmental stages R 69-77), the correlation coefficients were the highest (r((R) (69-71)) = 0.8677 and r((R 73-77)) = 0.706). Coefficients of determination were the highest as well [r((R 69-71))(2) = 0.753; r((R 73-77))(2) = 0.4984]. Therefore, the most appropriate period for sampling is during the first half of August. At that time, the majority of the maize plants were between the R 69 and R 77 developmental stages. According to the results, 0.5-1 visually determined adult/plant corresponds to 40 adults/trap/week. This shows that both methods are equally useful for estimating the population densities of WCR in the Central European region.