Oviposition and Hatching of Larvae
The depth reached depends strongly on the ground structure. Once in the ground, they deposit about 30 cream-coloured eggs of about 3 mm. It was found that the female beetles return repeatedly to their swarm trees following the deposition of their eggs to mate again and lay another batch of eggs. The typically shaped larvae hatch after about one month: the U-shaped, forward-bent abdomen is a standard characteristic, in addition to the head capsule and the segmented legs – such beetle larvae are referred to as “white grubs” or “chafer grubs” and sometimes as “rook worms”.
The young grubs first feed of fine fibre roots -- once they grow in size, they turn to the stronger roots of their hosts. During its life phase, the grub sheds its skins several ties, letting us differentiate between three larvae stages because of the grubs’ different sizes. The damage caused depends also on the age of the larvae, in addition to other factors, and are at the most important in the year after their flight. The grubs stay at various depths in the ground dependent on the time of the year: while they live in relatively shallow ground up to 20 cm in depth during the vegetation period, they spend the cold time of the year at depths of up to 60 cm to avoid low temperatures. The old grubs, which have reached a size of 4 cm by then, begin to build a small cave at a depth of approximately 40 cm in late summer before they fly, where they pupate. The finished beetle emerges three weeks later, but remains in the ground to overwinter and works its way to the surface once the ground temperatures have increased in the following spring, closing the cycle.
The entire development time lasts for several years and depends strongly on the ground temperature. Development takes about four years in cooler regions, such as in Northern Germany and in many Alpine valleys, while it is only three years in other areas. This results in mass flights every three or four years, depending on the region. These periods of mass flight vary from region to region. Thus, it is possible to develop a “cockchafer map” by marking regions with corresponding flights in different colours. This was already started by Zweigelt in Austria and continued by Faber and Cate, who recorded the annual flight of May bugs with the help of primary schools across Austria. The current distribution of flight areas is not fixed, but can change subject to climate conditions and ground temperatures. This has occurred repeatedly in the past and, thus, illustrates the historic development of climate change. It is said, for instance, that in a particular part of Switzerland a part of the cockchafer population of a region developed slower following a very cold year and that the mass flight year has moved back one year ever since.
Regions in which mass flight has to be expected based on the long-term cycles of cockchafers are marked in colour on geographical maps, such as the following. Red dots indicate mass flights that have been observed physically. Mass flight years that came earlier in 2009 in the districts of Kufstein (Tyrol), Deutschlandsberg (Styria) and Feldkirch (Vorarlberg) and late mass flight years in the area around Korneuburg, Stockerau and Tulln are noteworthy, for instance. The analysis of these irregularities could provide information on whether the climate change observed by meteorologists already affects animals.
Reporting Cockchafer Sightings
You are very welcome to share your own observations in relation to May bug flights with us. Click here for our online report form, in which you can see all current flight areas and download cartoons on the life of the grubs and the cockchafer beetle.