Of 41 grasses and 27 broadleaf species evaluated in early studies, larvae survived in Petri dishes for 10 days on only 18 grass species, giving insight into ‘grasses only’ as larval hosts (Branson and Ortman, 1967a,
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1970). Developmental information was absent and many maize field weeds and native prairie grass species were not evaluated.
Recently, Oyediran et al. (2004) evaluated a series of 21 prairie grass species thought to be among those dominant in the western Great Plains in the 1860s for D.v.virgiferagrowth and development, along with maize and sorghum as controls. Twenty pots of each species were planted. Five weeks after planting, pots were infested with 20 neonate D. v. virgifera larvae using a moistened camel’s-hair paintbrush. At 5, 10, 15 and 20 days after infestation, four pots of each species were placed in Tullgren funnels for larval recovery. The remaining four pots of each species were used for adult emergence. The percentage of larvae recovered, larval head capsule width and adult emergence varied significantly between the grass species. The percentage of larvae recovered from western couch grass, Pascopyrum smithii (Rydb.), pubescent couch grass, Elytrigia intermedia (Host), and side-oats grama, Bouteloua curtipendula (Michx.), were not significantly different from the percentage of larvae recovered from maize. The number of adults produced by pubescent couch grass was not significantly different from the number produced from maize. The average dry weight and head capsule width of adults produced from maize were not significantly differ- ent from the head capsule widths and dry weights of those adults from any grass species. Overall, adults were produced from 14 of the 23 species eval- uated. In summary, several of the couch grasses were quite good hosts for D.v.virgifera, perhaps even their ancestral host. The dogma that D.v.vir- giferafollowed maize up from Mexico may be correct, but hosts other than maize were maintaining the D.v.virgiferapopulation in western Kansas in the 1860s and a number of the species evaluated are capable of producing D.v.virgiferaadults today (though not documented in the field).
Although D. v. virgifera has not yet been documented as being pro- duced naturally from hosts other than maize, its subspecies, the Mexican corn rootworm, Diabrotica v. zea Krysan and Smith, has. Mexican corn rootworm beetles were collected in emergence traps placed over a mixture of four grass species: Brachiaria plantaginea (Link), Eleusine indica (L.), Eragrostis indica (Hornem.) and Digitaria ciliaris (Retz.) (Branson et al., 1982b). Larvae and pupae of the Mexican corn rootworm were also collected from B.plantaginea and Panicum hallii (Vasey) and the sedge Cyperus macrocephalus (Liebm.) in Mexico (Branson et al., 1982b). Other examples suggest that this was not an isolated incident. In Sutton County, Texas, Mexican corn rootworm adults were found at a density of two beetles per plant in a maize field in its third year of pro- duction (the second generation of potential beetle production from maize) despite more than 250 km of isolation from any other maize production area (Krysan and Smith, 1987). The establishment of an economically damaging population (Witkowski et al., 1986) in the second possible year of adult production, despite isolation of more than 250 km, strongly sug- gested that this population of the Mexican corn rootworm was main- tained on hosts other than maize in the area.
To evaluate the food conversion efficiency for alternative host plants,
48 J. Moeser and B.E. Hibbard
Moeser and Vidal (2004b) applied the same technique as for maize roots described above. Larval weight gain, the amount of ingested food and the ECI were measured for several monocot and dicot weeds and crops which are common in or near European maize fields. The same quantity of food as in the ECI experiments with maize roots was used. It was shown that larval growth and performance were comparable to maize root feeding when they were given sufficient root mass of alternative hosts (Fig. 3.1).
There were significant differences between the different host plants and varieties (analysis of variance (ANOVA): F9.276= 3.5; P= 0.001).
These differences were more pronounced within the varieties and within the alternative host plants than between maize varieties and alter- native hosts. There were no significant differences between the most suit- able alternative hosts (Setaria verticillata(L.), Setaria glauca (Poiret.) and Panicum miliaceum (L.)) and the most suitable maize varieties (OSSK 617, Greenfields and Panama), as well as between the worst host plants from both categories (weeds: Cynodon dactylon (L.) and Sorghum halepense (L.); maize: DK440 and Marano). These findings suggest that root morphology and root mass of suitable alternative hosts are more important for larval development than anticipated before. Providing insufficient food for the larvae in earlier studies (Branson and Ortman,
Nutritional Ecology of Larvae and Adults 49
Fig. 3.1. Food conversion efficiency of different alternative hosts (left) and different maize varieties (right). Same letters above bars indicate no significant differences between host plants (ANOVA, Bonferroni adjustment). These data are an excerpt from a bigger data set from Moeser (2003).
8 6 4 2 0 –2 –4 –6 –8 –10 –12 –14
ECI Index C. dactylon S. halepense S. verticilata S. glauca P. miliaceum DK 440 Marano OSSK 617 Greenfields PAN
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1967a, 1970) may have led to the assumption that alternative host plants are in general less suitable than maize. On several host species that were not suitable for larval development, a considerable amount of feeding was still recorded. This gives the first evidence that antifeedant substances are not responsible for this incompatible insect–plant interaction (no antibiosis). It is more likely that the presence or absence of certain compounds of regular plant biochemistry render these host plants unsuitable for D.v.virgifera.