The economic impact of adult D.v.virgiferafeeding is negligible with the exception of seed maize production (Culy et al., 1992). Here an increased number of adults that are silk feeding may lead to reduced ear filling and to seed shapes that cannot be used in modern planting equipment (C.R.
Edwards, personal communication).
Understanding the nutritional ecology of adult D. v. virgifera is important for several other reasons: the spreading and actual invasion process is carried out by the adult females, it’s the females that are responsible for the increase in population density and their nutritional status is therefore of utmost importance. D.v.virgiferais mainly a pollen feeder that also uses other above-ground plant organs of maize (Chiang,
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1973; Ludwig and Hill, 1975). Maize pollen and silk were shown to be the best for egg production in female D.v.virgifera(Elliot et al., 1990), but are only available during flowering, which is a relatively short time period.
Protein-rich food is scarce after this time. Limited food supply within maize fields may lead to dispersion flight or even to large-scale migration towards the end of the maize-growing season (see Chapter 6, this volume).
Movement at the maize field boundaries increases where females move frequently between maize fields and outlying pollen sources (J. Moeser, personal observation). Still, pollen feeding by D.v.virgiferais poorly doc- umented for the USA, while data for D.v. zeae(the Mexican corn root- worm) are available. Pollen from plants from 63 genera was found in the gut of feral D. v. zeae in Texas (Jones and Coppedge, 2000). While D. v.
zeaeis generally believed to turn its attention to alternative pollen sources after the flowering period of maize, D. v. virgifera was expected to prefer feeding on other maize tissues (Naranjo, 1991). The European invasion was in need of some more specific data on the food sources used by D.v.
virgifera, so Moeser and Vidal (2004c) performed gut content and pollen analysis with 1200 beetles from southern Hungary. He compared fields with high and low abundance of weeds to estimate the impact of alterna- tive pollen sources on D.v.virgiferafeeding behaviour. Moeser and Vidal (2004c) showed that D.v.virgiferafed on pollen from 73% of the flower- ing weeds or crops present in maize fields in southern Hungary (= 19 species). Ludwig and Hill (1975) mention only two weed species whose pollen was detected in D.v.virgiferaguts. Whether this is due to the single sample date, the small sample size or the agroecosystems with a decreased number of alternative pollen sources remains to be investigated. The high plasticity in the adult feeding behaviour was also clearly demonstrated by Spencer et al. (Chapter 6, this volume), where an increase in soybean feeding was shown after the flowering period of maize.
While McKone et al. (2001) demonstrated that Diabrotica barberi Smith and Lawrence fed on sunflower, but not D.v.virgifera,Moeser and Vidal (2004c) presented data which even suggest a certain preference in European D. v. virgifera for sunflower pollen. Similarly Mullin et al.
(1991) showed data indicating antifeedant attributes for sunflower and Solidago canadensisL. pollen, concluding that Asteraceaeare unsuitable pollen sources for D.v.virgifera. Moeser and Vidal (2004c) found pollen from five Asteraceaespecies in European D.v.virgifera, with a preference for Ambrosia artemisiifolia L. towards the later vegetation period. This preference was found in beetles which contained high numbers of pollen from this species but were sampled in weed-free fields not exhibiting this weed.
Three major factors were identified as influencing the nutritional ecology of adultD.v.virgifera (Moeser and Vidal, 2004c):
1. Time and changes in maize phenology: The changing phenology of maize and time dictates which food is provided within the maize field.
During the first weeks of adult life, maize provides enough high-quality
56 J. Moeser and B.E. Hibbard
food as pollen and silk are generally readily available at this time. Only after these food items became scarce did leaf and kernel feeding begin significantly. At this time, the number of individuals with empty guts also increased. At the same time, the use of alternative pollen sources increased as well. These pollen sources were located in and outside the maize fields. A similar increase in non-maize pollen over time was also noted for D.barberi(Cinereski and Chiang, 1968). The influence of chang- ing maize phenology with regard to soybean foliage feeding was investi- gated thoroughly by O’Neal et al. (2002) and Spencer et al. (Chapter 6, this volume).
2. Habitat and abundance of alternative pollen sources: The amount of alternative pollen resources within the maize field influences the choice of adult D.v.virgifera. During the first weeks, beetles from weedy fields fed to a certain extent on weed pollen, while beetles from non-weedy fields began to use this resource only at a later time, when maize was not an attractive food source, especially during the later vegetation period.
Weeds which provide pollen in large quantities during the whole vegeta- tion period and especially towards the end of Diabrotica adult life are considered extremely valuable as alternative pollen sources. Fields with flowering crops like sunflower close to maize fields supply the beetles with an additional pollen source. Whether weeds keep the beetles inside maize fields when maize is becoming unsuitable and thus impede spread- ing remains to be investigated. The amount of pollen of each plant species does not reflect the plant’s abundance in the fields. Ambrosia and sun- flower were clearly over-represented in the pollen analysis, while the abundance was minimal. A preference for late-flowering weeds (A.
artemisiifolia) and flowers that offer a large quantity of pollen and are largely visible (sunflower) has to be considered.
3.Sex of the beetles: Female D.v.virgiferause alternative pollen sources to a greater extent than male beetles. Females from weedy fields feed more on alternative pollen than females from non-weedy fields. Males from weedy fields also feed more on alternative pollen than males from non-weedy fields. Males use a larger number of alternative host plants but less pollen of alternative hosts is found on average in males.
Adult D.v.virgiferado not use specific alternative pollen sources accord- ing to their abundance. Pollen of weeds that were not present in weed- free fields (like A. artemisiifolia) was detected in large quantities in the guts of D. v. virgifera beetles sampled in these weed-free fields. Short excursions outside their natal field, as documented for D. barberi (Naranjo, 1991), or flights from fields containing a larger array of alterna- tive pollen sources are likely to explain these findings. The nutritional status of the adults influences oviposition, longevity, flight activity and migration behaviour (Naranjo, 1991).
Feeding assays with pollen of Asteraceaeled to the assumption that they are of lesser value for D. v. virgifera adult nutrition (Mullin et al., 1991). These no-choice experiments neglect the possibility that they may
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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.