Submitted to: Entomologia Experimentalis et Applicata
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/31/2007
Publication Date: 7/1/2007
Citation: Toepfer, S., Ellsbury, M.M., Eschen, R., Kuhlmann, U. 2007. Spatial clustering of Diabrotica virgifera virgifera and Agriotes ustulatus in small-scale maize fields without topographic relief drift. Entomologia Experimentalis et Applicata. Oxford, UK:Blackwell Publishing Ltd. 124(1):61-75. Interpretive Summary: Western corn rootworms were accidentally introduced into eastern Europe over a decade ago and have spread over much of that area. Most maize fields of the region are much smaller than their North Americian counterparts and thus the question arises as to how this may affect the movement and spread of this pest and how rootworms interact with agronomic and soil factors on smaller scales typical of east European agriculture. This study was done to help identify soil and agronomic factors that may influence the distribution of western corn rootworms and a common wireworm pest species that often occur in the same fields. The results of the studies showed that the two pests have differnent ecological requirements and that both pests show clustered distributions in fields that are ostensibly flat with little apparent within-field variability. Weeds, soil type, and soil moisture may play a role in determining spatial distribution of these pests.
Technical Abstract: The soil living larvae of Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) and Agriotes ustulatus Schaller (Elateridae) can cause economic damage to maize roots. This study investigated the spatial clustering of both pests in four small-scale maize fields in southern Hungary, where no clustering had been expected due to the lack of topographic relief drifts and soil structuring. Between 2000 and 2002, numbers of D. v. virgifera larvae and adults, and of A. ustulatus larvae were determined at four randomly chosen geo-referenced maize plants in each of 24 plots / field. Soil moisture, soil bulk density, and vegetational characteristics were assessed. Moran’s I test for spatial autocorrelations, semivariogram analyses, and interpolated mapping revealed that D. v. virgifera larvae and adults were spatially clustered in 67 % and 50% of the fields or years. Larvae of A. ustulatus were clustered in 75 % of cases. Soil moisture was clustered in two of three fields, whereas soil bulk density was not spatially structured. Vegetational cover, weed species numbers, weed density, density of each of 27 weed species, maize density, and distribution of flowering maize were spatially clustered. Diabrotia v. virgifera larval distributions were negatively correlated with higher soil moisture, and with increasing weed density, in particular with high densities of Cirsium arvense (L.). Adult distributions of D. v. virgifera were correlated with its larval distribution and with the adult distribution of the previous year. The density distributions of male and female adults were not correlated. Male and female D. v. virgifera were correlated with the density distribution of flowering maize. Female density was additionally correlated with higher soil moisture and Poaceae density, e.g. with Sorghum halepense (L.) Pers. No relation was found between the larvae of A. ustulatus and D. v. virgifera. Agriotes ustulatus were correlated with less vegetational cover. Conclusively, male and female D. v. virgifera adults, larvae of D. v. virgifera, and larvae of A. ustulatus all have different ecological requirements and will display spatial clustering even within ostensibly homogeneous habitats of flat small-scale maize fields. Pest monitoring must account for this clustering.