The truth behind thunder flies
Thunder Flies
Mass flights of thrips are reputedly associated with thundery weather and therefore, in many parts of Europe these insects are called »thunderbugs«, »thunder flies« or »storm flies«.
Like many other animals, thrips are influenced in their locomotion and flight behaviour by various abiotic factors, i.e. light intensity, day length, wind, air pressure, but all above temperature and humidity. Their mass flights, in general, occur on warm and dry midsummer days, often associated with the ripening of crops. Triggering factors for the sudden increase of flight activity are consistent weather, temperatures of at least 20°C and a dew point between 5°C and 15°C. Under these conditions particularly cereal thrips (Limothrips) are known for swarming in enormous numbers. Of course, these climatic situations can implicate summer storms in a certain high probability and thus, the phenomena of thrips’ mass flights may appear coupled obviously with thundery weather.
However, findings so far suggest mainly aspects of atmospheric electricity that might affect thrips during thundery weather and influence their motion activity – but not in a way of providing more favourable conditions. Thunderstorms per se are not a meteorological trigger initiating the mass flights; rather, these weather situations induce landing of thrips that had flown before dispersed in a wide range of different air layers. As a result, at ground level the tiny insects appear then in vast numbers, suddenly attracting attention with their spectacular density of individuals.
Usually, the Earth is negatively charged and the atmosphere is positively charged, leading to a potential that increases with height. The rate of change – known as the potential gradient – may be more than 50kV/m in a thunderstorm (0.1kV/m during fine weather), resulting in a current of positive charges towards the Earth’s surface caused by the unipolar ions in the atmosphere. It has not been finally settled that thrips respond to changes in either the electric field or the density of air ions, but these factors are known to reduce the motion activity of different other insects. Drosophila (Diptera: Drosophilidae), for example, shows a significant decreasing motion activity upwards from a potential gradient of 6kV/m and remains completely inactive at 75kV/m. Smaller sized insects generally react more sensitive to changes in the electric field, some are known to stop totally their flight activity at a potential gradient of already 8kV/m. Thrips seem to recognize the electrical field strength with the sensoria on their antennal segments III and IV.
The mass occurrence of thrips is a quite annoying phenomena, particularly, if the tiny insects – sometimes attracted by coloured cloth – land on people. Trying to hide, they crawl under cloth and finally reach the skin. Sucking sweat thrips often penetrate the skin with their maxillary stylets. Sensitive people are known to react against these unintentional bites with itching symptoms or inflammations. Feeding by sucking blood has been recognized occasionally in a few species only (e.g. Karnyothrips flavipes).
But what happens to the thrips when the thunderstorm with its showers has arrived?
Aside from cold temperatures, rainfall is to be considered as the most harmful abiotic factor for thrips. Their behaviour to hide in small crevices (thigmotaxis) like leaf sheath or cracks in the bark of trees is a protective peculiarity during this adversary conditions as well as their hydrophobic integument, effecting that they – once washed away – remain at the surface of the water and are able to reach the ground again. Nevertheless, many studies show that strong rains in general deplete thrips populations and eliminate most specimens.
Thrips have not only been associated with thunderstorms. In Denmark, in the summer of 1853 high abundances of thrips appeared concurrent with high rates of Cholera infections. Thrips therefore were called »cholera flies«.