Three-dimensional distribution and sampling indices of insects and mites in horizontally-stored wheat

Studies were conducted in three flat storerooms in Southern Greece, filled with approx. 45 tons of wheat each, stored in 1.5 m deep bulks, in order to examine the seasonal abundance of insect and mite species, in relation to their horizontal and vertical spatial distribution into the bulked grain. For this purpose, wheat samples were taken from June 2000 until March 2001, at 10-day intervals, by using a partitioned grain trier. The surface of the grain bulk was divided into three sampling zones, the central zone (the central part of the bulk), the edge zone (close to the walls) and the corners (the corners of the store rooms). The trier samples were examined separately for the upper, medial and lower 0.5 m of the bulk. Significant differences were recorded for wheat moisture content among zones (p<0.0001) and among depths (p=0.0012). On the contrary, no significant differences were noted for temperature among zones (p=0.2281) and depths (p=0.3049). In general, the moisture content was higher in the upper 0.5 in of the bulk, compared to the other two depths. Temperature decreased during the sampling period and this reduction was more vigorous from October and on. Eight insect species and 25 mite taxa were found during the sampling period. The most abundant insect species were Plodia interpunctella Hubner (Lepidoptera: Pyralidae), Rhyzopertha dominica (E) (Coleoptera: Bostrychidae) and Oryzaephilus surinamensis (L.) (Coleoptera: Silvanidae). The most abundant mite species were Acaropsis sollers Rohdendorf (Prostigmata: Cheyletidae), Tyrophagus putrescentiae (Schrank) (Astigmata: Acaridae) and Glycyphagus domesticus (De Geer) (Astigmata: Glycyphagidae). For the majority of the most abundant insect and mite species, the highest population densities were recorded during autumn. More individuals were found at the corners and the central zone, compared to the edge zone. In addition, significantly more individuals were found in the upper 0.5 m of the bulk (p<0.05), compared to the other two depths, with the exception of A. sollers which was equally distributed in the upper I m. All the aforementioned species of insects and mites showed an aggregated spatial pattern, as indicated by Taylor's Power Law estimates. The use of Taylor's parameters for the estimation of accuracies and sample sizes for the most abundant species indicated that these parameters were notably varied among species.