Radar observations of moths migrating in June, 1997

These different sources of echoes can be quite clearly distinguished using the general properties of the echoes themselves, though a single radar image can not tell much in this respect. The diamond back moth (plutella xylostella) is a small moth, but suh a large insect that a single moth can be observed about 10 kilometre range from the radar. The radar measuring volume at that range is about 100 metres wide im every direction. Taking into account the approximately gaussian shape of weighting inside the volume if insects are flying more thjan 50 metres away from each other they are observed clearly as single targets.

The radar echoes on the 9th and the 10th of June, 1997 showed clearly separate targets, especially in the higher echo layers between 0.5 and 2 kilometres. Using this as a starting point it is possible to make a rough estimate on the amount of moths arriving in Finland in a certain migration period.

Starting points for calculations:

Mean distance between individual insects is 50 metres meaning that their number density in air C = 1/50/50/50 in a cubic metre

Effective thickness of the migration layer H = 500 metres (several layers)

Migration speed (mostly wind) U = 10 m/s

Width of migration L = 300 km

Total time span of migration T = 10 hours

Now in a time span T an air layer with thickness H moving with velocity U through a plane that is L wide corresponds to a certain volume of air. When the number density of the moths is taken into account the total number of migrated moths N is got using the following equation:

N = H*L*U*T*C

The values given previously produce a total number of 430 millions.

If this figure is devided by the area of the landing area, the mean number per an area unit is available. The landing area can be estimated to have been 400 km long, and as previously 300 km wide producing an area of 120000 square kilometres. This would give us 3500 moths/squarekm as a mean areal density. However, Southern Finland is largely covered by forests and lakes, and suitable areas for the moths might be of the order of 5% of the total area. This would mean about 70000 moths per squarekm i.e. about 700 moths per hectare - the usual unit of field area in Finland.

Actual levels of moth densities on some fields were more than ten times higher, but the mean figures might have been of the same order of magnitude as the very preliminary calculations using radar data would give. A more fundamental study of this migration is still waiting to be done.