Aggregation and habitat use by Lucilia blowflies (Diptera: Calliphoridae) in pasture

The spatial distribution of blowflies of the genus Lucilia within fields in south west England was examined in 1999 and 2000. Blowflies are economically important agents of sheep myiasis in the UK and understanding local aggregation is an essential step in the development of appropriate sampling and fly control regimes. Fifty, 20 x 20 cm, non-odour-baited, sticky traps were used to catch flies, at randomized, 10 x 10 m grid co-ordinates in fields of permanent pasture. Clear aggregations were evident in all Lucilia distributions. All values of the sigma2:mean ratio were greater than 1. The catches were shown to be highly aggregated using Morisita's index of aggregation. Generalized linear modelling of binary presence/absence catch data was used to relate aggregation to microclimate and habitat. Deletion testing was used to identify significant terms in the models. In general, Lucilia blowflies were predominantly caught around the edges, in warmer and more humid areas of the field. The relationship between microhabitat and the distribution of Lucilia collected in 1999 was used as predictive model to explain the catches made in two fields in 2000. This gave a highly significant fit in one field (P = 0.001) and a relationship which approached significance in the second (P = 0.08). However, these regressions suggest that the relationships between abundance and microhabitat are complex and that 'hot spots' of blowfly catches were not necessarily found in the most extreme microclimate conditions. Nevertheless, microhabitat features do give a relatively good guide to presence or absence of Lucilia in the trap catches, thereby providing important information about the most appropriate location of traps to maximize and standardize sampling and control regimes.