Matti Leskinen 1998-01-28 (2002-02-04)

Aphid mass migrations into Finland in 1988

In the spring of 1988 a project was started to find out how radar echoes could be used in insect migration studies. It was a pure coincidence that exceptional mass migrations of aphids into Finland started at the same time. Radar echo intensity on May 26th shows how the sea-breeze front, and organized convection northwards of it in the afternoon was brilliantly observed. Based on insect trap catches the echoes were mainly caused by birch aphids (Euceraphis betulae) - most of which had migrated into Finland on previous days with southerly winds.

The most fundamental work about the migrations and radar echoes in the spring of 1988 was done by Marko Nieminen as his M.Sc thesis (in 1992) - since then he has joined the research group on metapopulations. Following results are based on his M.Sc. thesis, and some further studies. (Article: Nieminen, M., M. Leskinen ja J. Helenius, 2000: Doppler radar detection of exceptional mass-migration of aphids into Finland. Int J Biometeorology, vol 44, number 4, 172-181.)

Birch aphids on the 18th of May, 1988

Superimposed on the map are air parcel trajectories calculated by Kalle Eerola in the Finnish Meteorological Institute (FMI) using his three-dimensional model to the atmospheric data analyzed by the Finnish Limited Area Model. The trajectories were calculated 48 hours backwards in time from May,18th at 15 o'clock local summer time. Every 6 hours are designated by circles on the trajectory lines. The end point pressure levels at about Helsinki were 97, 95, 93, 91, and 89 kPa (the lightest colour is for 97 and the darkest is 89 kPa), and these levels covered heights from the surface to about one kilometre.

Insect traps catched mainly birch aphids (Euceraphis betulae). The effective temperature sum (reference temperature +5°C) was calculated using the weather stations indicated by red full circles. The isochrones on the map indicate the date at which the calculated value got over 58°Cd that is the mean value found for silver birch (Betula pendula) leaf unfolding. Studies in Denmark about birch aphid development would require unfolding at about the 24th of April (24.IV). On the other hand about 15°Cd lower value for leaf unfolding that could be used based on some other studies on birches, would mean about 2-3 days earlier dates for the isochrones. The 48 hour trajectories have start points in the area that would have according to the approximate development times produced winged birch aphid adults, too.

The weather radar showed clear air echoes at noon reaching the heights of about 1.5 km to the east of Helsinki, and about 1.0 km to the west. This difference was probably related to weather situation having a precipitation area approaching from the west already covering the west coast of Finland. On the other hand the thickness of the echo layer may have been affected by the Estonian land areas that had convection.

The mean motion according to the Doppler weather radar measurements was at its maximum about 17 m/s from SSW. In the calculated trajectories the maximum speed was only 13 m/s, and in the soundings 15 m/s; in Visby at 00 UTC and in Leningrad (St. Petersburg) at 12 UTC. The radar derived speed was significantly higher than what the wind analysis for the trajectory model could produce, and soundings had slightly lower speeds, too. This was not caused by any collective orientation of insects having added some flight speed to wind speed, since that would have been observed through azimuthal variations in the echo intensity. This kind of situation was observed later on May 30th radar echo intensity.

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