Profile

May beetles are only common in certain areas of Austria. The typical mass occurrence of the beetles in strong flight years is striking. May beetles feed on leaves of trees and shrubs. The larvae feed underground on the roots of their host plants.

Biology

May beetles belong to the leaf-horned beetle family (Scarabaeidae), which also includes the rose chafer, dung beetle or rhinoceros beetle. Three different species of cockchafer occur in Central Europe: the more common field cockchafer(Melolontha melolontha), the rarer forest cockchafer(Melolontha hippocastani) and the only sporadically occurring Melolontha pectoralis. All three species look very similar and their ranges partially overlap, so they are treated together here. The forest cockchafer is also found in Scandinavia and Siberia, while the field cockchafer has its distribution center in Central Europe.

Cockchafer beetles are the characteristic larval form for the entire group of leaf horn beetles as well as the stag beetle relatives. Other common larvae referred to as "grubs" are those of the June beetle (Amphimallon solstitiale), which become especially damaging in lawns, and the grubs of rose beetles (Cetoniinae) that live in compost piles, but are usually not damaging. In decaying logs, on the other hand, are the particularly large grubs of stag beetles (Lucanus cervus) and rhinoceros beetles (Oryctes nasicornis). The latter forms are not pests or are even under nature protection and are to be spared at all costs.

Mass occurrence

The most striking characteristic of the cockchafer is its annual mass occurrence. During this period, forest trees, vines, fruit trees or ornamental trees can be completely eaten bare. The swarming during the evening hours at forest edges or prominent single trees is conspicuous. Here the beetles carry out a maturing feeding and mate. After mating, the mated females return to the site where they spent their larval life to lay their eggs. To do this, they must first burrow into the soil to a depth of about 25 cm.

Development period

The total development takes several years and is strongly dependent on the prevailing soil temperatures. In cool areas, development takes four years, while in warmer areas it is completed after only three years. As a result, depending on the area, strong flight can be expected every three or four years. This predominance of certain flight years varies by area.

Egg laying and hatching of the larvae

The female cockchafer lays about 30 cream-colored eggs, which are about 3 mm in size. The soil depth reached during egg-laying is highly dependent on soil structure. Female beetles return to their swarm trees after laying eggs to mate again and lay eggs. After about one month, the grubs hatch.

The young grubs initially live on finer fibrous roots. As they grow larger, they turn more and more to the stronger roots of their host plants. In the course of their lives, they molt several times, so that three larval stages of different sizes can be distinguished. Damage depends, among other factors, on the age of the beetle larvae and is therefore most significant in the year following flight. Depending on the season, grubs reside at different soil depths: while they live in shallow soil layers up to 20 cm deep during the growing season, they spend the cold season up to 60 cm deep to avoid low temperatures. In late summer before the year of flight, the old grubs, which now already measure 4 cm, create a small cavity at a depth of about 40 cm in the soil, where they can pupate undisturbed. Three weeks later, the finished beetle hatches, but remains in the soil, hibernates here and only moves to the surface when soil temperatures rise in the following spring.

Host plants

The adult cockchafer feeds on a wide variety of deciduous trees and shrubs such as: Oak, maple, beech, stone fruit, walnut, larch, vine and hazelnut.

The larvae feed on the roots of various fruit trees, forest trees, vine, clover, horn clover, dandelion and other meadow herbs and grasses.

Distribution

Already about 100 years ago, the first large-scale survey on the flight of the cockchafer was started in Austria. At that time, attempts were made to predict the mass flight in order to be able to take countermeasures in time. Today, moreover, the connection with changed climatic conditions is the focus of interest. Due to the regular development cycle of the cockchafer, it is possible to predict quite well in which regions of Austria an occurrence of cockchafer is to be expected.

In geographical maps, those areas are marked in color where flight of cockchafer is to be expected due to long-term cycles of cockchafer development. Red dots indicate actually observed strong flight. Depending on the average temperature, May beetles take three or four years to develop from egg to finished beetle. In the wake of particularly warm or cool years, however, the beetles may hatch a year earlier or later than anticipated in some areas. This new flight periodicity is then maintained in the future until climatic conditions change again. In this way, cockchafer beetles are a good indicator of changes in climate patterns.

Prevention and control

• Manual collection of the beetles.
• Tilling the soil destroys some of the grubs. This method is only effective as long as the grubs do not yet linger in deeper soil layers for overwintering.
• Covering the soil of sensitive crops with beetle-proof nets to prevent beetles hatching in the crop from reaching their swarming sites or mated females returning from there to lay their eggs.
• Incorporating Melocont mushroom barley into the soil to infect the grubs with the insect pathogenic fungus Beauveria brongniartii. The method is very suitable for wetter areas, such as alpine valleys. The remedy should be applied several times starting in the year before planting.
• Treatment of the gathering places of cockchafer with the plant protection product NeemAzal-T/S (see list of plant protection products approved in Austria).

Specialized information

Reporting May Beetle Sightings

You are invited to share your own May beetle flight observations with us using our reporting form.

By filling out the online form below, you will help us record flight abundance for each area - the date of observation is less important. This will allow us to refine the existing May beetle flight map and document any changes.

Downloads

Below you will find several comics by biologist and children's author Dr. Michaela Stolz about the life of cockchafer and their grubs, especially for children.

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