Pest insect immigration warning by an atmospheric dispersion model, weather radars and traps

In an experimental set-up in and around Helsinki, Finland (60� N, 25� E), we have detected pest insect immigration using weather radars and insect traps in the field. This study was part of a project to develop a system to give warning of a possible arrival of long-range migrant insect pests. Bird-cherry aphid, Rhopalosiphum padi, and diamondback moth, Plutella xylostella, were found on the ground following migrations in warm airstreams at the end of May 2007. This migration episode was successfully forecast by the meteorologists in the project team. For the summer 2008, we developed a pest insect immigration alarm system based on SILAM, a Finnish Meteorological Institute atmospheric dispersion model. The first important pest insect immigration occurred in late June, bringing bird-cherry aphids. Our alarm system correctly produced a warning of this immigration. We studied the migration path in the observed events in 2007 and 2008 with the help of the atmospheric dispersion model. Weather radars frequently showed rain echo over the area, but there was also a lot of echoes originating from the migrating insects. Using the polarimetric weather radar in Helsinki, we could differentiate insects from other sources of echoes. Insects were common in layers below 1 km, and were observed up to height of about 2.5 km. Using Doppler weather radars we were able to observe the speed and direction of the migration. The experiment showed that an atmospheric dispersion model is an effective tool for predicting the movement of airborne migrants. The alarm system would work still better, if the sources of the immigrants were known in more detail. In addition, the very simple modelling of airborne migration should be refined. Weather radars, and especially polarimetric systems, are able to detect insect migrations and reveal details of the phenomenon not obtainable by other means. J. Appl. Entomol.