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In a screen of seedlings of a worldwide range of 47 cultivars of Triticum (mainly T. aestivurn) the concentration of the hydroxamic acid DIMBOA ranged between 1 and 8 mmol/kg fresh wt. In a bioassay in which alatae of the aphid Sitobiorz avenue were released among replicated test seedlings, there were highly significant correlations between aphid 'preference' and DIMBOA levels in the seedlings. The value of these results in work leading to the production of aphidresistant cultivars is discussed.

Samples of the grain aphid Sitobion avenae (F.) and the rose-grain aphid Metopolophium dirhodum (Walker) were collected in late March from wheat fields and adjacent road-side grasses at a number of locations in southern England. Unparasitized aphids were DNA fingerprinted using the multilocus (GATA)4 probe. Over all locations, the fingerprints of individual S. avenae caught in wheat had lower overall average distances of band migration (ADBM) and shared a higher proportion of bands, than fingerprints of individuals caught in adjacent road-side grasses. The ADBM of fingerprints of S. avenae collected on road-side grasses altered significantly with geographical location, while the ADBM of fingerprints of S. avenae caught on wheat did not. A comparison of the fingerprints of individual M. dirhodum caught in wheat and neighbouring road-side grasses did not reveal any genetic differentiation. Fingerprints of M. dirhodum that were caught in the same host type did however, show significant variation in ADBM between different locations. With both S. avenae and M. dirhodum, spatial autocorrelation revealed that locations that were close together were no more likely to have individuals with similar ADBM than locations that were far apart. Our results suggest that (i) particular clones of S. avenae prefer to colonize wheat, and/or that (ii) particular clones of S. avenae perform better on wheat than other clones. It is unclear why M. dirhodum did not show any genetic structuring according to host type, but this species appears to engage in sexual reproduction much more frequently than S. avenae in southern England.(ABSTRACT TRUNCATED AT 250 WORDS)

RAPD‐PCR was used to determine the genetic variation of Metopolophium dirhodum collected in a winter wheat field and in a nearby 2.5‐m‐high suction trap at Lincoln, New Zealand. Over three collection dates, five distinct genotypes were identified, using two primers (OPK16 and OPC09) independently. There was a significant temporal effect on the ratio of genotypes in populations collected in the field. There was no significant spatial aggregation or association of these genotypes in the field. Two of the genotypes present in the field were also detected in the suction trap sample. Using a higher resolution method of RAPD‐PCR (with the Stoffel fragment of Taq polymerase), a total of 124 genotypes were distinguished from 142 individuals collected from Scotland and New Zealand. The Jaccard similarity index (S) was used to measure similarity between individual aphids within and between populations from both hemispheres. All populations were very diverse (S < 0.33). However, at similar crop growth stages, M. dirhodum was significantly more diverse in Scotland than in New Zealand. The results are discussed in relation to the value of monitoring aphid flights for pest forecasting, and in terms of the most appropriate RAPD‐PCR techniques.

Grain aphids (Sitobion avenae (F.)) were collected from forty-four wheat ears in a Hampshire field at three times during the growing season. On each occasion, individual aphids were profiled using the multilocus (GATA)4 probe. During the full head emergence and full anthesis growth stages of wheat, each ear generally supported a genetically distinguishable aphid colony which consisted of genetically indistinguishable individuals (putative clones). This information strongly suggests that individual ears were colonized by single immigration events. By the late milky ripe stage, most ears supported two or more such clones. The total number of clones declined and the spatial separation of identical clones increased markedly over the duration of the study, which strongly suggests that secondary spread rather than increased immigration was responsible for the increased clonal diversity of ears. In addition, the profiles of individual S. avenae became more alike as the season progressed and samples became dominated by particular clones indicating either differential survival or reproduction among clones.