supply the beetles with essential protein when fed upon in conjunction with maize tissue like kernel or leaves. The no-choice experiments of the past (Ludwig and Hill, 1975) are not likely to explain the situation of adult D.v.virgiferain the field, where a mix of maize and non-maize diets is most often encountered (J. Moeser, unpublished data). Latest research suggests that adult D. v. virgifera profit greatly by using all pollen resources available, although Pavuk and Skinner (1994) concluded that weeds in maize fields have no influence on D. v. virgifera populations.
These studies from the US Corn Belt have only limited applicability for Europe because European agroecosystems may provide a different and a wider array of possible pollen sources because of the reduced land use intensity. The use of alternative pollen may enlarge the amount of time available to the females for oviposition, increase fecundity, increase longevity in areas even when maize is already harvested and facilitate spreading by providing food in areas without maize.
Rearing of D. v. virgifera
The ability to rear a species being studied is a requirement for certain types of work and a great advantage in most types of studies. Methods for rearing D.v.virgiferahave been available for about 40 years (George and Ortman, 1965) and several fairly comprehensive methods are described in papers (Howe and George, 1966; Branson et al., 1975; Jackson, 1985, 1986). Jackson (1986) is particularly useful because several different tech- niques applicable for differing reasons are presented. None of these tech- niques uses an artificial diet for rearing D. v. virgifera larvae. Since screening transgenic crops for production of proteins toxic to D. v. vir- gifera larvae requires an artificial diet, several seed companies have developed their own proprietary diets. Representatives from Monsanto have now published an artificial diet for D.v.virgiferalarvae (Pleau et al., 2002). However, maize is still superior to the diet in producing large numbers of viable adults, so all of the ingredients for an optimal diet are not known at this time.
Implications of the Results on Nutritional Ecology for Management
Europe. How far the invasion process in Europe is affected by the use of alternative hosts remains to be seen. The focus of European agriculture on monocot crops other than maize in Central Europe may result in a com- pletely new field of research if D. v. virgifera undertakes another host switch such as happened in the past. The potential threat posed by alter- native hosts to resistance management plans in the use of GM maize is an area that has received more attention since registration by the EPA. The past has shown that relying on a single counter-strategy led to resistance within a few decades. In the case of GM maize another valuable tool would then be lost quickly if applied without the necessary caution.
Finally, newly developed methods may contribute in screening for resist- ant maize varieties and deepen our understanding of this complex insect–plant interaction.
The patterns behind the invasion process are becoming more clear as our understanding of resource usage by D. v. virgifera increases. While alternative pollen sources are widely used by D. v. virgifera in Europe, recommendations are difficult with regard to any implications of IPM strategies. On the one hand, the removal of weeds from maize fields may lead to an early start in the migration flight of a larger number of females, thus increasing invasion pressure. On the other hand, a high abundance of alternative pollen sources may provide additional nitrogen sources, which will possibly lead to an increase in successful oviposition and female fitness. Whether landscape diversity, for example, has any signifi- cant impact on the invasion process, besides the amount of maize cover- age, is still unknown.
The high adaptability of D.v.virgiferawith regard to the use of alter- native host plants as larvae and adults shows that Europe may indeed provide new areas for occupation, not only spatially (range expansion) but also by expansion of its ecological niche.
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