Effects of plant spacing and interplanting with oilseed rape on colonisation of dwarf hops by the damson-hop aphid, Phorodon humuli

The pattern of colonisation of dwarf hops (Humulus lupulus) by damson-hop a phid (Phorodon humuli (Schrank)) migrating from Prunus spp. was investigate d at six plant spacings and where some of the hops were replaced by oilseed rape (Brassica napus L.), a non-host of the aphid. The number of migrant a phids that accumulated on hop stems (bines) increased with increasing bine size and density. The numbers of aphids that colonised hops interplanted wi th oilseed rape reflected the density of the hop plants only and not the ov erall plant density. As the physical size of the wind shadow within which f lying aphids can manoeuvre and land was unimportant unless provided by a ho st plant, the finding supports the theory that flying aphids respond to olf actory stimuli associated with their hosts. Variation in bine height (as a measure of plant size) explained 29-93% of the variance in aphid counts dur ing the 3 years' study and bine density 1-14%. Standardising the data as th e numbers of aphids per metre of bine and taking a square-root transformati on of these standardised counts improved the precision of the analyses and, by stabilising variances, facilitated comparisons between years as growth became more vigorous as the plants matured. Each year, the rate of increase in numbers of aphids settling on plots of hops declined curvilinearly with increasing bine density. Maximum colonisation by P. humuli occurred at a b ine density of five per metre row, a density similar to that used commercia lly by growers of dwarf hops.