In some regions, farmers are already confronted with bare patches of grass on their meadows in spring instead of lush green. This phenomenon was particularly common in parts of Lower Austria in 2018, and the plague has been known in Upper Austria since 2003.
The enemy of grassland and productive meadows is called grub – the larva of the cockchafer
Increasing global warming, combined with the extremely hot summer of 2018, meant that healthy grass stands were already under considerable heat stress. The roots fought for every drop of water. In addition, enormous pressure from underground grubs led to significant root damage. In 2018, Austria was said to have experienced the most massive root damage by grubs in years. Lighter grassland soils in sunny locations are particularly affected by grub infestation. There, infestation densities of up to 12 LU insect larvae per hectare (up to 700 grubs per m²) can occur. The consequences are massive defoliation of plant roots. Slow-growing, herb-dominated, legume-poor stands in sunny locations are particularly favored, as they achieve high soil temperatures where the grubs thrive. Short cutting/grazing heights, sparse, patchy turf, as well as short-cut, multi-cut meadows offer particularly attractive conditions for female cockchafers to lay their eggs in early summer.
Development cycle from grub to cockchafer
The development cycle of the cockchafer lasts four years. In its first year of flight, the overwintered cockchafer leaves the ground at the end of April and begins its mating flight. The cockchafer then lays its eggs in grassland soils, and after four to six weeks the larvae hatch and begin feeding. In autumn, the young larvae retreat to deeper soil layers (approx. 60 cm) where they overwinter. In the second year, also known as the main feeding year, as temperatures rise they return to the uppermost layers of the soil (approx. 10 cm below the ground). There they have reached a feeding paradise, as this soil layer has the best root penetration. They feed all day and cause the greatest damage to the grassland during this development phase. In the third year the maturation feeding takes place. During this time they continue to feed until the summer solstice, after which they pupate and hatch.
In the 4th year, the development cycle of the cockchafer is completed and the flight year of the next generation begins again and mating follows.
Damage to grassland and prevention:
If grassland is severely affected by grub infestation, the top layer of turf can be pulled back from the ground like a carpet. The plants dry out completely, leaving only bare brown-gray patches. In this case, the grassland is no longer productive, and meadow rehabilitation is necessary. The critical infestation level is 40 grubs/m2 of grassland; at this point, active control is recommended.
The use of fungal barley has proven effective in controlling heavily infested grasslands. This barley seed is laced with fungal spores, which kills the larvae upon direct contact with grubs. A minimum infestation of 75% of the grassland and repeated applications are recommended (at least two consecutive years with approximately 30 kg/ha of fungal barley; the cost of this application is approximately €800/ha). This method requires the fungal barley to be carved into the soil, which requires special slitting equipment.
Mechanical control of grubs is also recommended. This requires a suitable soil temperature (so that the grubs are already in the topsoil) and machinery that kills the larvae through impact and crushing. Rotary harrows, rotary cultivators, tine rotors, and rotary tillers are suitable. Ideally, this will bring the still-living grubs to the surface, where they are killed by UV radiation. Furthermore, two applications at intervals of one to three days are always recommended.
Renovation
After successful mechanical control, the meadow should be reseeded. During reseeding, particular attention should be paid to the use of high-quality seed to sustainably rebuild the grass stand. This will ensure the future yield of the area.
ATTENTION! -Problem of the next generation
If grub control is not initiated in a timely manner, there is a risk that the larvae will enter the second development phase – the feeding year – the following year. The feeding damage can increase significantly in the following year. However, if June beetle grubs were present in the meadows in increased numbers last year, they will hatch in 2019. The next feeding year in these regions is expected in 2020. However, based on the exponential growth of the population, an increase of up to 40 times the initial population can be expected.