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

Abstract

The pattern of colonisation of dwarf hops (Humulus lupulus) by damson‐hop aphid (Phorodon humuli (Schrank)) migrating from Prunus spp. was investigated 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 aphids that accumulated on hop stems (bines) increased with increasing bine size and density. The numbers of aphids that colonised hops interplanted with oilseed rape reflected the density of the hop plants only and not the overall plant density. As the physical size of the wind shadow within which flying aphids can manoeuvre and land was unimportant unless provided by a host plant, the finding supports the theory that flying aphids respond to olfactory stimuli associated with their hosts. Variation in bine height (as a measure of plant size) explained 29–93% of the variance in aphid counts during the 3 years' study and bine density 1–14%. Standardising the data as the numbers of aphids per metre of bine and taking a square‐root transformation 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 bine density of five per metre row, a density similar to that used commercially by growers of dwarf hops.

Effects of associational resistance and host density on woodland insect herbivores

“…Only young larvae of cinnabar moths responded positively to the density of Senecio jacobaea ; the incidence of all other herbivores was greatest at low or intermediate host densities (Kunin 1999). Numbers of colonizing aphids Phorodon humuli increased with size and density of the host plants Humulus lupulus , but numbers of colonists per metre of host stem decreased with host density (Campbell & Ridout 2001). Nine other monophagous species showed decreases in herbivore load as host density increased, a consistent pattern of ‘resource dilution’ (Otway, Hector & Lawton 2005).…”

Within-field distribution of the damson-hop aphid Phorodon humuli (Schrank) (Hemiptera: Aphididae) and natural enemies on hops in Spain

“…A knowledge of the distribution of naturally-occurring enemies is fundamental to an integrated pest management system and this has not been studied here either. Campbell (1977) found that various features in plantations of hops such us bine density, bine height, plant position, hill type, string orientation and hop variety influenced the patterns of colonisation by migrant P. humuli and concluded that most of these probably reflected variation in local patterns of wind shelter within which aphids could manoeuvre and land (Campbell & Ridout, 1999). In addition, edge effects tend to be greatest where there are windbreaks, which are frequent in hop gardens in the UK.…”

Within‐plant distribution of Phorodon humuli (Hemiptera: Aphididae) and natural enemies on hops with implications for sampling and management

“…On hops, P. humuli does not produce winged morphs capable of re‐infesting other hops (Campbell ), so the pattern and intensity of aphid infestation within hop plantations largely reflects the deposition, accumulation, and secondary flight behavior of aphids migrating from Prunus spp. (Campbell & Ridout ). Population development of this aphid has been studied in different countries.…”

“…Spatial and temporal coincidence of beneficial insects and aphids would suggest that an integration of natural enemies into aphid management is feasible. Campbell () found that various features in plantations of hops influence the patterns of colonization by migrant P. humuli and concluded that most of these probably reflected variations in local patterns of wind shelter within which aphids could manoeuvre and land (Campbell & Ridout ).…”