Cockchafers are members of the family of the scarab beetle (Scarabaeidae), which also include well-known insects such as the rose beetle, dung beetle and rhinoceros beetle. The German name of this family -- Blatthornkäfer -- is derived from the last antennae segments that carry “leaves” or lamellae and can fan out. The beetles grow up to 30 mm in length and appear in a variety of colours: while the majority have a dark head, dark pronotum and chestnut elytra, there are also very dark, completely reddish-brown and lightly coloured specimens, which were given different names by German children (Rauchfangkehrer/chimney sweep, Kaiser/emperor and Müller/miller). There are three different species of cockchafers in Central Europe: the common cockchafer (M. melolontha), the forest cockchafer (M. hippocastani) and the sporadically found, large cockchafer (Melolontha pectoralis), the names of which do not say much about their way of life. All three species look very similar and are also linked, so that they will be treated here, together. One difference is that forest cockchafers have a range that includes  Scandinavia and Siberia, while the common cockchafer is predominantly found in Central Europe.

The most typical trait seen in cockchafers is their periodical, mass flight. During this time, they can strip bare forest trees, grapevines, fruit and ornamental trees. It appears that in doing so, many cockchafers from one area gather at the edge of the forest at particularly prominent trees to mate – it almost seems to be some sort of “wedding market” for May bugs. The females return to the places they have spent their lives as larvae after mating to deposit their eggs. To do this, they have to dig up to 25 cm deep into the ground – not an easy task.

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.

Development Time

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.

Spread

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.