Spatial pattern of the Rio Cuarto corn disease vector, Delphacodes kuscheli Fennah (Hom., Delphacidae), in oat fields in Argentina and design of sampling plans

The spatial pattern of the Rio Cuarto Corn Disease vector, Delphacodes kusc heli (Hom., Delphacidae), was analysed in oat fields within the endemic are a of the disease, during the growing seasons 1993 and 1994. The spatial pat tern was analysed by fitting the probabilistic models Poisson and negative binomial and estimation of single-date and overall aggregation indices. The population of the different stage classes, sex: and wing forms showed a si gnificant trend to aggregation as the negative binomial model fitted the ob served frequency distributions in more than 78% of the cases (sampling date s) while the Poisson model fitted well in only 28% of cases or less. Single -date aggregation index, C-A, ranged from 0.3 to 0.84. Overall (whole seaso n) aggregation index, C-A*, estimated through the Bliss and Owen's regressi on method, ranged from 0.18 (female adults) to 1.08 (nymphs I-II), indicati ng a moderate degree of aggregation compared with other planthopper species . There were no significant relationships between aggregation and populatio n density. The minimum number of sampling units and critical lines for sequ ential sampling plans were calculated based on the estimation of C-A* for t he precision levels (D) 0.1, 0.2 and 0.3. Even low degrees of aggregation, like that of adults, demand much more sampling effort than randomly distrib uted populations, particularly at high densities. General implications and limitations of the proposed sampling plans for monitoring the vector popula tion abundance are discussed.