The Bavarian Diabrotica research programme

Until now in our crop area maize cultivation was far less endangered by pests than others. With the first occurrence of the Western corn rootworm in Bavaria and Baden-Württemberg in 2007 this has abruptly changed. Since then the worldwide most important maize pest regularly makes headlines. To control it effectively one has to know about its life cycle. This in particular applies to the feeding habits of the beetles and its larvae, which will force to rethink maize cultivation in some regions.

Suitability of biofuel crops as hosts for larvae of western corn rootworm, Diabrotica virgifera virgifera (in collaboration with BTL Bio-Test Labor GmbH Sagerheide)

The aim of the project was to investigate the possible importance of alternative biofuel crops, used for the application in biogas plants, as food plants for the larvae of Diabrotica virgifera virgifera. In a series of greenhouse experiments the host status and quality of 49 plant species and varieties for the larvae of Diabrotica were evaluated, including 16 forage grasses (e.g. a number of Lolium and Festuca varieties), six switch grass varieties (Panicum virgatum), 18 Sorghum species/varieties, Miscanthus x giganteus, five other Miscanthus genotypes and three broadleaf species. Most of the tested plants had a low or no suitability as host plants for Diabrotica. The tested species of Miscanthus, especially M. x giganteus, were good hosts for Diabrotica. The long crop cycle of Miscanthus rhizomes of up to 20 years would appear to provide an excellent and long lasting source of high numbers of Diabrotica adults, as continuous maize production does. However, this depends on the egg laying behaviour of the female beetles. Only if they will lay their eggs in established Miscanthus stands the life cycle of the beetle will be completed. So far, there is no evidence of the beetles laying their eggs in Miscanthus fields. In addition, it is considered that older plants loose their suitability as food source for the larvae.

Influence of different tilling methods on population dynamics of Diabrotica virgifera virgifera (in collaboration with Banat’s University of Agricultural Sciences and Veterinary Medicine, Timisoara, Romania)

The aim of this project was to investigate the influence of different tilling methods on population dynamics of Diabrotica virgifera virgifera. The tested tilling methods are ploughing in autumn, grubber in autumn and grubber in spring. The first results showed no significant difference between the three tilling methods. Further, no significant influence on Diabrotica infestation was observed.

Undersowings in maize: Influence on the mortality of the larvae of Diabrotica virgifera virgifera (in collaboration with Georg-August Universitaet Goettingen, Department of Crop Science, Section of Agricultural Entomology, Goettingen, Germany)

In this project the effect of different undersowings in maize on the development of the larvae of the western corn rootworm (Diabrotica vrigifera vrigifera) was investigated. We hypothesized that the roots of undersown plants, when mixed with the target roots of maize, would reduce the ability of larvae to localize their host plant roots. We used rye grass, Italian ryegrass, a mixture of Italian ryegrass and clover, white clover, yellow mustard, and sunflowers as undersowings. All undersowings, either grass or grass/clover mixtures, did not result in lower numbers of larvae in these treatments. Contrary, slightly, but not significantly, more larvae were extracted from these treatments. Undersowings had no effect on larval dry weight nor did the undersowings enhance or delay larval development. In conclusion, undersowings do not provide an additional or alternative control measure against the western corn rootworm.

Experiments on dispersion behavior and egg deposition of Diabrotica beetles after an early maize harvest (in collaboration with Austrian Agency for Health and Food Safety (AGES), Institute for Plant Health, Vienna, Austria)

In the presented experiments, migratory flights of Western Corn Rootworm adults in relation to different dates of maize harvest were investigated. An early harvest of maize, at a time when flight period of the pest is still in progress, may trigger migratory flights of adults out of the harvested plots into adjacent maize fields. If the migrating beetles are females, they may lay a considerable portion of their egg load into the invaded fields. This harvest-driven migration may therefore be disadvantageous in eradication or containment programmes against the quarantine pest insect. The abundance of active adults in the maize fields decreased steadily at the end of the flight period of the pest. Population levels dropped significantly after harvest in the stubble field. However, results show that an early start of the maize harvest has negligible impact on the fluctuations of Western Corn Rootworm populations in regions where the pest is established since years. Consequently, it is unlikely that early harvesting of maize impairs the efficacy of containment programmes in infested zones. Diabrotica females caught at the end of the flight period of the pest were still able to lay a considerable amount of eggs. In addition, these eggs proved to be viable and larvae emerged after diapause in the following spring. It is therefore clear that females are able to found a population in recently invaded areas right to the end of the flight period, independent from their age.

Suitability of winter wheat and volunteer cereals, respectively, as host plant for the Western Corn Rootworm (in collaboration with Austrian Agency for Health and Food Safety (AGES), Institute for Plant Health, Vienna, Austria)

The aim of the presented experiments was to investigate the possibility of Diabrotica virgifera virgifera development on winter wheat under field conditions. Several cereals – including wheat - have already been proved to be suitable as host plants of the Western Corn Rootworm in laboratory experiments. Under field conditions, however, additional factors such as crop phenology may impede successful juvenile development of the pest insect. Early winter wheat varieties, for example, may mature - and consequently loose root vigour – at a time when larvae are still foraging in the soil. Late varieties, on the other hand, may be more suitable for pest development. Last but not least, corn rootworm larvae may not be able to complete development before maturation of the regular crop, but some may survive until volunteer wheat plants appear after harvest. This small portion of the larval population may then be able to complete development in the already harvested winter wheat fields. The results show clearly that an epidemic population development as known from continuous corn is impossible. One reason for this is a lack of synchrony between the phenological development of the juvenile pest stages and the host plants. Crop rotation therefore remains to be a very effective pest management tool for containment measures in already infested regions, even if winter wheat follows maize in the crop rotation system.

Experiments with emergence cages on host plant specificity, population dynamics and the development of a control threshold (in collaboration with Banat’s University of Agricultural Sciences and Veterinary Medicine, Timisoara, Romania; Versuchsreferat Steiermark, AGRO DS Austria, Chamber of agriculture Styria, Austria)

The aim of this experiment is the development of an appropriate strategy to combat the maize pest Diabrotica virgifera virgifera.

Therefore, the following methods are used:

  • Investigation of population dynamics and damage potential of the Western Corn Rootworm
  • Evaluation of the utility of different crop plants for the population development of the pest, aiming at the containment of spread by adequate crop rotation
  • Development of a containment and damage threshold

Suitable method for extracting eggs of Diabrotica virgifera virgifera from soil samples (in collaboration with BTL Bio-Test Labor GmbH Sagerheide)

In order to forecast the damage caused by and control the pest Diabrotica virgifera virgifera, it is important to have more detailed information on its egg laying behaviour (number and where they are laid). The objective of this project was to develop a method for extracting eggs of D. v. virgifera from soil samples. To determine the number of eggs laid by females of D. v. virgifera in fields a soil washing apparatus was constructed and tested. The reliability of the soil washing apparatus was tested by using soil samples containing known numbers of eggs of D. v. virgifera obtained from a culture of this species reared in-house by BTL since 2003. This apparatus is to be used to obtain qualitative and quantitative data on the abundance and distribution of Diabrotica eggs in the field. The number of eggs may be used to forecast expected future abundance of D. v. virgifera. In addition, this information will be used to address a number of questions related to this species egg laying behaviour and also for modelling the population dynamics of this pest.

Studies on the migration and oviposition behavior of Diabrotica virgifera virgifera in non-maize crops to improve advice and guidelines for crop rotation in Bavaria (in collaboration with CABI, Ungarn)

Females of the Western Corn Rootworm lay their eggs partially in neighbouring non-maize fields. If maize is grown on these fields the following year, D. virgifera finds the optimal host plant. Thus, Diabrotica can lead to yield decreases, while its population increases. Experiences in Eastern Europe show that in areas with a small structured agriculture, which can be found frequently in southern Germany, the amount of eggs placed in neighbouring non-maize fields increases noticeable. This experiment should show if different non-maize crops provide a different attraction for oviposition. Therefore, potatoes, peas, wheat, sugar beet, soy bean, oil seed rape and sorghum were tested. The first results show that the beetles fly into neighbouring fields. The most attractive crop was non-infested maize. In addition, the experiment showed that the females do lay eggs in neighbouring non-maize fields. Overall, the majority of beetles stays in its natal field, non-infested maize comes second. According to the first results of this experiment, all in Bavaria cultivated crops can be rotated with maize.