Empirical results demonstrate that uncertainty about costs and requirements for environmental compliance is an important determinant of dairy producers… investment behavior. Ex ante forecasting of how uncertainty and irreversibility are likely to affect producers… responsiveness to agricultural technologies has implications for the design of environmental policies. Simulation modeling methods are described. The empirical analysis focuses on Texas producers… propensity to adopt free‐stall dairy housing. Free‐stall investments offer advantages for both productivity‐augmentation and pollution abatement, yet uncertainty and irreversibility are obstacles to adoption. Implications of this ex ante paradigm for policy design and implementation are discussed.
Thirty Vicia species and 28 V. faba cultivars were evaluated for Aphis fabae resistance by measuring rates of aphid population increase (rm) on the host plants. A subset of these species (22) and cultivars (five) were evaluated for resistance to Acyrthosiphon pisum and Megoura viciae. For the same subset of 22 Vicia species and all three aphid species, positive correlations were observed between the concentration and numbers of leaf and stem non-protein amino acids and the level of aphid resistance. Correlations were also observed between trichome density, on some organs of the host, and both nymphal survival and development time. Aphid resistance, together with these possible physical and chemical resistance mechanisms, showed a pattern which was described in relation to the taxonomy of the genus Vicia and the degree of domestication of the host. The least advanced Vicia species were most resistant whilst the more specialised species and those most closely related to V. faba and V. faba itself were most susceptible. Within each taxonomic group (subgenus or section), the most domesticated species were least resistant. The merits of utilising the resistance found in some V. faba cultivars and in other wild Vicia species were considered. 0 1984 Association of Applied Biologists 548 J. HOLT A N D N . B I R C H crop plants including Vicia faba L. (faba bean) and vetches, because a degree of aphid resistance is found in some existing V.faba cultivars, and because V. narbonensis L. (a species closely related to V. faba in the section Faba in which the unknown progenitor of V. faba is placed) has been shown to be relatively resistant to the aphid Aphis fabae (Scop.) (Bond & Lowe, 1975). METHODSVicia species were selected to maximise the number of subgeneric sections (Kupicha, 1976) represented and to include several species from the larger sections. Vicia faba cultivars were selected to represent the three varieties major, equina and minor and groups within these varieties, according to the classification of V.faba cultivars by Higgins, Evans & Reed (1981). Table 1 lists the Vicia species and V.faba cultivars in this study.The resistance of Vicia species and V. faba cultivars to three aphid species Aphis fabae, Acyrthosiphon pisum (Harr.) and Megoura viciae (Buckt.) was investigated by using a simple bioassay technique, similar to that described by Birch & Wratten (1984). Bioassays were performed in Perspex boxes in an insect rearing room similar in design to that described by Scopes, Randall & Biggerstaff (1975). The temperature was 20°C, range 1°C and the light intensity was 75 wE/m2 at foliage level with a photo-period of 16 h. Five plants of each of five cultivars/species were arranged in a randomised block design in each box. Constant experimental conditions were maintained over the period 1979 to 198 1, in which the bioassays were performed. Plants were grown in 10 cm-diameter pots. Viciafaba cultivars were used for bioassays when 4-6 wk old (8 to 10 nodes). Aphid resistance at this plant age corresponded well w...
S U M M A R Y Field caged populations of Metopolophiurn dirhodum were established on winter wheat (cv. Norman) during the late spring and summer of 1983. An early (G. S. 30-41), middle (G. S. 41-65) and late treatment (G. S. 65-77) were compared. Yield losses were 5, 8 and 27% respectively, although aphid indices were similar in the three treatments.For other field cage populations, all established at G. S. 38 and allowed to develop freely, a significant negative relationship existed between aphid index and yield.
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