Spatiotemporal associations in beetle and virus count data

This paper analyzes two insect-related sets of agricultural field data, Bot h comprise spatially referenced count data sampled on a series of occasions . One concerns carabids (ground beetles) in cereals, the other the incidenc e and spread of an aphid-vectored virus disease of lupins. For both sets, t he major objective was to describe and quantify the stability through time of the spatial patterns found For each occasion: this was measured by the s patial association between successive samples. Traditional methods for anal yzing count data focus on properties of the frequency distribution of the c ounts and use little or none of the spatial information in the sample. We u sed methods that utilized all the spatial information and that, by conditio ning on the observed data, provided complementary inferences to the other m ethods. Our analyses are based on a class of methods termed spatial analysi s by distance indices (SADIE). There methods provide indices and formal ran domization tests, both fur the spatial pattern in a single population and f or the spatial association when the patterns of two populations are compare d, Our analyses showed considerable aggregation for both the beetles and th e infected lupin plants. Furthermore, both populations displayed positive a ssociation between successive samples that declined as the temporal lag inc reased. The beetles were affected greatly by the harvest of the cereal crop . The lupin infections showed maximal association for a 1-week lag despite the fact that the Intent period of the virus was a fortnight; it was inferr ed that the observed pattern of new infections was probably tracking the pa ttern of the aphid vectors 2 weeks previously.