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Content archived on 2024-05-21

Threat to european maize production by invasive quarantine pest, western corn rootworm (diabrotica virgifera virgifera): a new sustainable crop management approach.


Key processes enabling the recently introduced quarintine pest of maize, the Western Corn Rootworm (WCR) to invade central parts of Europe and possibilities to control this pest species will be examined by seven partners. Each of the partners offers expertise in crop management, plant-insect interactions, natural enemy evaluation, risk management and biotechnological control methods. The development of a practical approach for the management of this new insect pest species is urgent, because of continual invasion and its potential establishment in Italy confirms theories that WCR will threaten maize production areas in other EU member countries in Central Europe in the immediate future. By combining work on the ecology of WCR with work on applied aspects of maize production we will be able to develop and establish a safe and environmental friendly crop management practice adapted to specific maize production systems in Europe.
The nutritional ecology of WCR has been evaluated in detail. Although this species is regarded as a specialist feeding on corn, several additional plants have been detected as additional food sources for adult and larval feeding. Especially grass species will enable WCR to persist in areas at low population densities even if corn is not cultivated. Founder populations may thus colonise new areas much faster than expected from calculations taking into account corn growing areas alone. Transgenic corn varieties, expressing the Bt-toxin Cry1Ab do not impact larval development compared to other European corn cultivars. However, differences in feeding efficiency in common European cultivars may be used as a starting point for partial resistance or tolerance breeding programs against WCR. Alighting and egg laying behaviour of WCR females is stimulated by a complex sequence of stimuli including specific metabolites found on the leave surface of corn plants. Growth stages of the plants influence the behaviour as well. However, we have not been able to understand the behavioural processes involved in detail.

The identification of the genetic background of WCR populations from different countries did not indicate that the WCR populations in Europe stem from populations which lay their eggs also in soybean fields. The simulation of the spreading rates of WCR populations, simulated for Austria, Belgium, France, Germany, Italy and Switzerland (assuming different first local infestation), demonstrated that containment measures will reduce them to a high extend. The potential risk of WCR establishment is very high in most parts of EU countries mentioned above. Non-optimal conditions, depending on the climatic conditions, have been found for the Northern parts of Germany, some areas in Belgium and the Netherlands. Within the EU corn is grown on about 8 mio ha. High risk areas for establishment and economic damage coincide with a high proportion of corn in the rotation and have been identified on an national scale. Pecuniary losses due to local introductions and ongoing spread without phytosanitary measures, and also for costs of eradication and containment measures have been calculated for several countries and scenarios. A cost benefit analysis, assuming a one year eradication program near airports (typically places where new WCR introductions have been found in recent years) would cost on average 185.000 €. However, pecuniary losses due to damaged corn would sum up to nearly 35 mio € 10 years after WCR establishment. A novel, non-sticky, high capacity funnel trap named VARs has been developed, with a virtually unlimited catch capacity compared to traps commonly used in ongoing monitoring programs. For first detection situations the PAL or PALs trap types are recommended. Strategies implemented in the Venice Airport region in Italy to eradicate the isolated WCR population found in 1998 proved to be successful. However, the costs for a three year testing period amounted to about 320.000 €. Up to now there is no indication for a host plant shift of WCR to other crops grown in the same area. Therefore, the prohibition of corn monoculture within the focus area is mandatory for any eradication measure. Experiments to establish the mating disruption technique have resulted in recommendations for the optimal media and formulation of the kairomonal lure. Disruption technique experiments have demonstrated gradual effects, but did not result in a reduced mating status of females. Therefore, this biotechnological approach may not yet be recommended. Field experiments in Southern Hungary demonstrated that WCR females may lay their eggs in non-corn crops. Economic threshold levels will be encountered at 7 years after initial infestation. Taking into account the more scattered corn cultivation and crop rotation the selection pressure for WCR populations to adapt to new host plants seems to be low in Europe.

However, if all corn plots will be rotated to non-corn crops, a high selection pressure will be posed upon WCR. We therefore recommend the use of refuges of continuous corn to preserve rotation as a means for managing WCR populations. The extensive life table studies, done for the first time for WCR, identified no indigenous natural enemies in Europe. The high generational mortality (99%) is compensated by a high fecundity. An additional 33% mortality would have been needed to achieve no population growth. A parasitoid species, found in Mexico, which seems to be specialised on WCR may account for this additional mortality. However, the DIABROTICA project period was to short to recommend the tachinid Celatoria compressa as a classical biological control agent to be released in Europe. Further testing is needed for a final decision concerning the release of the species in Europe.

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