Accuracies and sample sizes associated with estimating densities of adult beetles (Coleoptera) caught in probe traps in stored barley

The surface of barley stored in a metal bin was divided into four concentric strata, and adults of six insect species in the top 16 cm of grain were sampled using 60 unbaited, perforated, plastic probe traps. Traps were left in grain for 1 wk on six consecutive weeks between 20 August and 2 October 1986. Except for the lesser grain borer, Rhyzopertha dominica (F.), the greatest amount of variation (81-89% of the total) in trap catch of the remaining five species was found among traps within strata, followed by strata within weeks (8-19%), and weeks (0-8%). Fewer adults of all six species were captured in traps in the stratum closest to the bin wall compared with strata near the bin center. Variance-mean relationships indicated that adults of the sawtoothed grain beetle, Oryzaephilus surinamensis (L.), and R. dominica were randomly distributed among the probe traps. However, the distribution of adults of Cryptolestes spp.; hairy fungus beetle, Typhaea stercorea (L.); foreign grain beetle, Ahasverus advena (Waltl); and red flour beetle, Tribolium castaneum (Herbst), was aggregated. Except for Cryptolestes spp., accuracies (expressed as a proportion of mean trap catch) in estimating mean trap catches of the remaining species improved with an increase in trap catch and the number of traps used. For Cryptolestes spp., the accuracy of estimates decreased with an increase in mean trap catch but improved with an increase in the number of traps used. Except for Cryptolestes spp., the number of traps required for estimating mean trap catches of the other species within 25 or 50% of the true mean decreased exponentially with an increase in trap catch. For all six species, the number of traps required increased with an increase in the mean trap catch when the accuracy was expressed as a constant value independent of trap catch. An upper ceiling on the number of traps required for estimating mean trap catches was determined by expressing the accuracy as a constant proportion of mean trap catch and as a constant value independent of mean trap catch.