Spatial and temporal analysis of wheat bulb fly (Delia coarctata, Fallén) oviposition: consequences for pest population monitoring

The soil dwelling larvae of the wheat bulb fly (Delia coarctata, Falle´n) can cause damage to a number of cereal crops. Monitoring of the oviposition of this pest is important to allow forecasts of larval attack after the overwintering period, allowing suitable management practices to be implemented. However, little is known about the population dynamics of wheat bulb fly, both spatially and through time. Analysis of the spatial dependence of oviposition was conducted, allowing the construction of detailed contour or scale-sized dot maps of oviposition in the sampled fields. The population estimate obtained by the sample for estimate technique, used to test spatial dependence, was compared to the traditional wheat bulb fly sample for decision-making methodology. Further to this, the peak period of egg population density was established for Scotland. Spatial dependence was observed in 50 % of fields assessed, while field monitoring of egg population densities using the traditional single-line transect pattern were found to vary greatly from the mean density collected from a more intensive 50 by 50 m sampling grid. These findings suggest that the single-line transect sampling pattern is not robust enough when estimating the patchy egg distribution observed in the sampled fields. The peak period of egg population density, observed over three successive seasons, was found to be in late August, with population estimates taken prior to, or after this period showing a marked reduction in the observed maximum population. The results highlight the difficulty of obtaining accurate population density estimates for aggregated pest species and indicate that monitoring programmes, that have been implemented without thorough investigation into pest population dynamics, can lead to the execution of incorrect management strategies.