Distribution of damson‐hop aphid (Phorodon humuli) migrants on hops in relation to hop variety and wind shelter

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Abstract
SUMMARY: The distribution of migrant Phorodon humuli was studied in two hopgardens during June 1972. Migrants accumulating upon the young tissues near the bines' apices were counted. Plant size affected distribution, 100 % more migrants were found on 2‐bine than on single bine strings and there was a positive linear relation between bine height and log10 aphid numbers. Plants surrounding wire‐trellis support poles were more heavily infested than plants not at poles, and migrants were more abundant on leeward orientated than windward strings. It is argued that the distribution of alatae reflected local patterns of wind shelter. Differences between plants declined in successive weeks.The influence of hop variety on migrant settling was studied in one of the hop‐gardens. Fewer migrants settled on the variety Tolhurst than on Northern Brewer, and Fuggle was intermediate. Part, but not all, of the difference between varieties was explained by the larger average size of Northern Brewer plants.The observed aphid deposition rates in the two hop gardens were compared with hypothetical deposition rates calculated from numbers of P. humuli caught in a nearby 12‐2 m Rothamsted Insect Survey suction‐trap, and used to estimate flight durations. The estimates indicated that most infestation probably resulted from aphids within 1 h flying time of their primary host source.