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Identification & Distribution:

Phorodon humuli apterae are small to medium sized, whitish to pale yellowish green and relatively shiny. The abdomen is marked with three dark green longitudinal stripes (see first picture below). The head has a characteristic pair of elongate projections on the inside of the antennal tubercles (best seen on preserved specimens below). The siphunculi are pale with slightly dusky tips and are more than twice as long as the pale cauda. The body length of the Phorodon humuli adult aptera is 2.0-2.6 mm on plum, and 1.1-1.8 mm on hop.

 

The images below show a Phorodon humuli aptera, in alcohol, dorsal and ventral.

 

The clarified slide mounts below are of adult viviparous female Phorodon humuli : wingless, and winged.

 

Micrographs of clarified mounts  by permission of Roger Blackman, copyright AWP  all rights reserved.

For further assistance in identifying Phorodon humuli see INRA. 

The damson-hop aphid host alternates from blackthorn or plum (Prunaceae) to hops (Cannabinaceae). It does not cause leaf curling itself but often lives on plum leaves distorted by Brachycaudus helichrysi.  Migration of winged forms to hops takes place from late spring. There is a return migration to the winter hosts in September, where sexual forms are produced and eggs laid. Phorodon humuli is indigenous to Europe and neighbouring areas, and has been introduced to North America and New Zealand.

 

Biology & Ecology:

Life cycle

Phorodon humili overwinters on various species of Prunus, principally on blackthorn (Prunus spinosa), but also on damsons (Prunus institia) & plums (Prunus domestica). The eggs are laid in the axils of the buds in autumn. These hatch in spring to give wingless fundatrices.

These fundatrices produce large colonies of damson-hop aphids on the young Prunus leaves.

Despite the large numbers of aphids, they do not produce any galling or deformation of the leaves. However, other aphid species on blackthorn do produce galls, in particular Brachycaudus helichrysi, and damson-hop aphids often feed on the galled leaves as shown below.

Although Phorodon humuli usually host alternates, it can stay on its winter host Prunus throughout the summer, particularly on the sucker growth and on seedlings (see picture below).

After one or two generations Phorodon humuli start to produce alatae which migrate to hops when the flight threshold temperature of 13°C is exceeded.

Much of the research carried out on Phorodon humuli has focused on the migration to hop plants in early summer. Thomas et al (1983)   related the dates of the beginning and end of the spring migration of Phorodon humuli to the temperature, rainfall and sunshine for varying periods. The beginning of migration at both sites was associated with temperature in late March and early April, and periods of rainfall in mid-January and mid-April, while the end of migration was associated with temperature and sunshine in mid- and late June, and with mid-January rainfall. Regression  equations were used to predict the timing of the migration in 1980 and 1981, those for mean maximum temperature predicting most accurately both the beginning and end of migration. Using multiple regression  equations did not increase the prediction accuracy.

The image below shows an emigrant alate (= spring migrant) about to take off from blackthorn.

Worner et al (1995)   predicted the spring migration from historical records of host-plant flowering phenology and weather. Taimr & Kriz (1978)   and Taimr & Kriz (1978)   marked migrant alatae of Phorodon humuli with radiophosphorus and then released them. When wind speeds were low, their results indicated that the aphids had flown at low levels by stratiform drift. On days with ascending convection, the majority of the marked aphids were carried to greater heights and distances where no detections were possible. The results also suggested that, having flown to hop, the alatae do not make further long flights, but tend to settle and reproduce.

Campbell (1977)   studied the distribution of migrant Phorodon humuli in two hop gardens during June 1972. Plant size affected distribution with more aphids on larger plants. 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. The distribution of alatae was thought to reflect local patterns of wind shelter. Differences between plants declined in successive weeks presumably as local movement eliminated local differences. The influence of hop variety on migrant settling was also studied. Fewer migrants settled on the variety Tolhurst than on Northern Brewer, and Fuggle was intermediate. The observed aphid deposition rates in the two hop gardens were compared with hypothetical deposition rates calculated from numbers of Phorodon humuli caught in a nearby high level 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.

Campbell & Ridout (2001)   investigated the pattern of colonisation of dwarf hops by damson-hop aphid migrating from Prunus spp. at six plant spacings and where some of the hops were replaced by oilseed rape (Brassica napus), a non-host of the aphid. The number of migrant aphids that accumulated on hop stems increased with increasing stem 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. This was felt to support the theory that flying aphids respond to olfactory stimuli associated with their hosts. Each year, the rate of increase in numbers of aphids settling on plots of hops declined curvilinearly with increasing stem density. Maximum colonisation by Phorodon humuli occurred at a stem density of five per metre row, a density similar to that used commercially by growers of dwarf hops.

Female Phorodon humuli are sensitive to photoperiod, and when the light period falls below 13.5 h/day they start to produce alatae rather than apterae. In September and October there is a return migration to the winter host, Prunus. These winged forms produce oviparae which are ready to mate when the males arrive from the hops. Taylor et al (1979)   showed that the migration of the sexual autumn migrants of Phorodon humuli in Britain originated from dense, isolated and persistent population 'patches' in hop gardens in two small areas in southern England. Patterns of aerial distribution were the same for both sources and for both males and gynoparous females. As expected, migration was not random, but directional orientation was negligible. Evidence suggests that boundary layer migration, stratiform drift and cumuliform high level migration were all used on different occasions.

Pheromones & kairomones

The role of pheromones and kairomones in the biology of Phorodon humuli has received more attention for this species of aphid than for many others. Campbell et al. (1990)  carried out behavioural studies using an olfactometer to demonstrate that oviparae of Phorodon humuli, release a pheromone to which males respond. It was found to be a nepetalactol. A synthetic sample comprising ca. 70% 1S-isomer and 30% 1R-isomer attracted highly significant numbers of males to water traps placed within and adjacent to a hop garden. Initial studies also indicated attraction of males in both the olfactometer and in the field by volatiles from the primary host. Campbell et al. (1993)  then demonstrated that spring migrants of the Phorodon humuli respond to semiochemicals released by spring migrants feeding on hop leaves. Gas chromatography showed the presence of three active components : methyl salicylate, (E)-2-hexenal and β-caryophyllene. These three compounds elicited responses from separate olfactory receptors on the antenna. In the olfactometer, both (E)-2-hexenal and β-caryophyllene gave positive responses from spring migrants, and a mixture of the two compounds in the natural ratio was more attractive than (E)-2-hexenal alone. Addition of methyl salicylate eliminated the response to the active binary mixture.

Lösel (1996a)  confirmed cis,cis-nepetalactol as an important component of the hop aphid sex pheromone. His trap catches also showed that returning migrant alatae (gynoparae) were attracted to pheromone traps. This behaviour suggested that under natural conditions the returning migrant alatae may use the sex-pheromone cue emitted by oviparae already established on suitable primary hosts to locate sites at which to deposit their own oviparous larvae. Hence the compound also serves as an aggregation pheromone. Host plant volatiles were not significantly attractive to Phorodon humuli males, but these volatiles did catch significantly more gynoparae than did controls. This greater sensitivity of gynoparae to primary host volatiles could improve the gynopara's success in locating suitable hosts on which to reproduce. For both gynoparae and males, combining sex pheromone vials with those containing primary host extracts increased the overall efficacy of the traps. The much larger increases for males suggested a synergistic interaction of the kairomone on the pheromone. Given the ubiquitous nature of Prunus host plants in the wild, it would be disadvantageous for males to respond behaviourally to winter host kairomones in the absence of supporting pheromonal cues indicating the presence of oviparous females.

Lösel (1996b)  conducted experiments employing yellow water-traps with vials releasing various chemicals during the spring migration of Phorodon humuli, with the aim of identifying substances which might be used in the field to deter landing on hop plants. Methyl salicylate and two isothiocyanates reduced trap catches of the aphid. During the spring of 1994 a slow release formulation of methyl salicylate with or without acid-rich hop resin sprayed on to hop plants did not reduce aphid infestations significantly. In autumn cis,cis-nepetalactol , the main component of the sex pheromone, increased trap catches of males and gynoparae equally. The effects of kairomones from an extract of the primary host, sex pheromone and a visual cue from yellow compared with clear water-traps were additive. Lösel discussed the prospects for developing a semiochemicals-based control strategy against Phorodon humuli.

Campbell. et al. (2003)  caught gynoparous female and male damson-hop aphids in the field by water traps that were releasing the sex pheromone of this species, cis-cis-nepetalactol. No behavioural response was shown to another stereoisomer of nepetalactone which is the major sex pheromone component of other aphid species such as Megoura viciae,  even though olfactory cells were found on the third antennal segment of Phorodon humuli that responded strongly to this compound.

The damson-hop aphid (Phorodon humuli) and the bird cherry-oat aphid (Rhopalosiphum padi ) migrate at the same time of year and colonize closely related Prunus species as primary hosts, but utilize different stereoisomers of nepetalactol as sex pheromones. Pope (2007)  investigated interactions between these sex pheromones and benzaldehyde and methyl salicylate, plant volatiles common to primary hosts of both species. Female autumn migrants (gynoparae) and males of these two species were caught in the field with water traps baited with their respective sex pheromones. Rhopalosiphum padi gynoparae and males also responded positively to benzaldehyde. Release of either benzaldehyde or methyl salicylate with the conspecific sex pheromone increased catches of both species of aphid. However, releasing both plant volatiles with the sex pheromone of Rhopalosiphum padi increased catches of gynoparae and males, but reduced those with the sex pheromone of Phorodon humuli. These results support the hypothesis that specific plant volatiles synergize responses of autumn migrating aphids to their sex pheromone. Because these interactions are species-specific, they may be important in allowing males to discriminate between conspecific sexual females (oviparae) and those of other aphid species.

Natural enemies

On winter host

There have been few studies on the natural enemies of Phorodon humuli on its primary host Prunus. We have on several occasions found syrphid larvae feeding on colonies of Phorodon humuli on blackthorn (Prunus spinosa) in East Sussex (see image below). Both of these appear to be larvae of the syrphid Epistrophe eligans.

 

The adult hoverflies are characteristically found in spring, visiting blackthorn flowers to feed, and then ovipositing on the leaves near colonies of Phorodon humuli, Hyalopterus pruni  and Brachycaudus helichrysi  feeding on the young leaves.

On summer host

The only evidence of predation we have so far found on hops is the egg of a syrphid predator (see picture below).

The paucity of predators we have found is partly a reflection of the small number of hop plants we have examined, and partly because most plants we have examined have been sprayed with insecticide.

Copland (1979)  studied the aphid parasitoids in hop gardens receiving three different pesticide regimes. Aphidophagous species accounted for 50% of the total Hymenoptera caught in each site, using water traps, and comprised equal numbers of primary parasitoids and hyperparasitoids. There was no significant increase in the proportion of aphid parasitoids captured when aphid populations were allowed to rise. However, there was a significant decrease in all Hymenoptera when weed control was employed. Parasitoids bred from Phorodon humuli comprised three species of Aphidiinae, which are primary parasitoids, and five species of hyperparasitoid. From mummies which completed development, 48% hyperparasitism was recorded. However, 34% of mummies failed to emerge, most likely killed by predation from Anthocoridae or unsuccessful attack by hyperparasitoids. It was concluded that native Aphidiinae do not appear to offer a significant contribution towards the integrated control of the hop aphid.

Wright & James (2001)  looked at the parasitoids and hyperparasitoids attacking Phorodon humuli in Canada. Lysiphlebus testaceipes was the most abundant primary parasitoid, and Praon unicum was second in abundance. Other primary parasitoids were Aphelinidae, Aphidius ervi , Diaeretiella rapae, and Praon occidentale. Hyperparasitoids were in the genera Alloxysta (Charipidae), Asaphes and Pachyneuron (Pteromalidae), and Dendrocerus (Megaspilidae). Their study concluded that the primary parasitoids had potential as biological control agents.

Campbell & Cone (2001)  studied the population development of Phorodon humuli on insecticide-free field-grown hops (Humulus lupulus L.) in 1992. The influence of predators was assessed using large sleeve cages installed after aphid immigration ended. Aphid immigration numbers increased geometrically throughout July in cages that excluded predators but increased more slowly and then fell in cages with access by predators. Aphid numbers remained much lower on uncaged plants. Hop cone yields were 97, 381, and 598 grams per plant from exclusion-caged, open-caged and uncaged plants, respectively. The main predators were Coccinellidae, Chrysopidae, Hemerobiidae, Anthocoridae, Geocoridae, and Nabidae. Predatory Diptera were scarce, as were hymenopterous parasitoids.

 

Other aphids on same host:

Primary host

Blackman & Eastop list 12 species of aphid  as feeding on blackthorn (Prunus spinosa), 10 species of aphid  as feeding on damson (Prunus initia) and 25 species of aphid  as feeding on plum (Prunus domestica) worldwide, and provide formal identification keys.

Of those aphid species, Baker (2015)  lists 16 as occurring in Britain on one or more of the primary hosts: Aphis craccivora,  Aphis fabae,  Aphis gossypii  Aphis pomi,  Aphis spiraecola, Brachycaudus amygdalinus, Brachycaudus cardui,  Brachycaudus helichrysi,  Brachycaudus persicae,  Brachycaudus prunifex, Hyalopterus pruni,  Macrosiphum euphorbiae,  Myzus lythri, Myzus ornatus,  Myzus persicae,  Nearctaphis bakeri, Phorodon humuli, Rhopalosiphum nymphaeae,  Rhopalosiphum padi  and Rhopalosiphum rufiabdominale.

Secondary host

Blackman & Eastop list 11 species of aphid  as feeding on hop (Humulus lupulus) worldwide, and provide formal identification keys.

Of those aphid species, Baker (2015)  lists 8 as occurring in Britain on hops: Aphis fabae,  Aphis gossypii  Aphis nasturtii,  Aulacorthum solani,  Macrosiphum euphorbiae,  Myzus persicae,  Phorodon humuli and Rhopalosiphoninus staphyleae.

 

Damage and control

Hops are dioecious, perennial, climbing plants that are grown on a 6 m trellis. They grow best where day length increases rapidly in the spring, and do not grow well where days are less than 14 h long. The hop cones (female flowers) contain pollen-like, lupulin glands. The commercial value of hops lies in the lupulin glands, which contain resins and oils. The resins are used for bittering beer, and the essential oils contribute to beer flavour. Hops also help preserve beer from spoilage. Heavy infestations of the plum-hop aphid reduce plant vigour and may induce defoliation. Even light infestations of harvested hop cones can reduce their value. The aphid also transmits three plant viruses: Hop mosaic carla virus, hop split leaf blotch virus and Hopline pattern virus, which reduce yield.

Chemical control

The Bayer Expert Guide  recommends routine insecticide treatment every year, at the very least at the beginning of aphid flight. This recommendation for the prophylactic use of insecticides appears to be generally followed by farmers. This has resulted in some resistance to organophosphate, carabamate and pyrethroid insecticides.

Muir (1979)  collected samples of Phorodon humuli annually from 1966 to 1976 from five commercial hop gardens in Kent and from other hop gardens where problems in control occurred. A susceptible stock was obtained from wild hop growing in northern England in 1969. The samples were cultured in isolation on potted hops and bioassayed against insecticides in common use. The level of resistance to demeton-S-methyl was about 10 times in 1966 after 10 years use, and more than doubled from 1968-1974 apparently due to the spread of a more resistant type. There was a further increase to about 50 times in 1975-1976. There was also resistance of 20 to 30 times to omethoate, 2 times to methidathion and 4 times to methomyl. There was no clear change in response to endosulfan. Weichel & Nauen (2003)  reported moderate resistance to pyrethroids in hop gardens in Bavaria, Germany, the largest hop growing area of the world.

Weihrauch & Moreth (2005)  investigated anecdotal observations that the two hop cultivars Hallertauer Magnum and Spalter Select are very different in their susceptibility to Phorodon humuli. Spring migration and initial population development of Phorodon humuli were monitored on the two cultivars in an experimental hop garden over two years. Numbers of migrant aphids on Spalter Select were significantly lower, comprising 18.8 and 30.2% as compared to Hallertauer Magnum. Population development of apterous aphids on these two cultivars differed significantly, with more aphids on Hallertauer Magnum. It was concluded that Spalter Select is repellent to Phorodon humuli and, compared to Hallertauer Magnum, is possibly nutritionally less suitable for the aphid.

Biological control

Hartfield et al. (2001)  used two types of sex pheromone-emitting trap for testing an isolate of the pathogen Verticillium lecanii. Their efficiencies for capturing gynoparae and males of Phorodon humuli were compared. Gynoparae rarely entered either type of pathogen dissemination trap. Males only entered traps that released the sex pheromone of Phorodon humuli. Out of a sample of 16 live aphids removed from a trap dispensing Verticillium lecanii, 9 died from infection by the pathogen and 5 of the 16 initiated colonies of the fungus after they walked on a sterile agar plate for five minutes. None of the 15 aphids collected from traps without Verticillium lecanii became infected or initiated colonies of the fungus.

Trouve et al. (1997)  looked at the possibility of using the coccinellid Harmonia axyridis to control the damson-hop aphid in a dwarf-hop garden in northern France. Second and third instar larvae of Harmonia axyridis were released at different stages of the aphid population increase. The best control was obtained when larvae were released early, and the aphid population was approximately 20 per leaf. In this case, the average number of aphid per leaf did not exceed the insecticide treatment threshold of 80 aphids per leaf. Indigenous predators, especially Adalia bipunctata complemented the effect of Harmonia axyridis. The picture below shows Harmonia axyridis predating another species of aphid, the bird cherry-oat aphid (Rhopalosiphum padi ).

Solarska et al. (2004)  assessed the braconid Aphidius colemani and the cecidomyiid Aphidoletes aphidimyza for biological control of the damson-hop aphid. The efficacy of Aphidius colemani ranged from 5% to 65%. It was found sufficient to control damson-hop aphid in the period before flowering, but it was not sufficient later. The efficacy of Aphidoletes aphidimyza ranged from 50% to above 90%, and it was sufficient in one of the examined vegetation seasons. High air temperature and lack of rainfall reduced efficacy of both species, but especially that of Aphidius colemani.

Integrated Control

Aveling (1981a)  looked at the role of Anthocoris species in the integrated control of Phorodon humuli on hops. After the aphicidal effects of an early-season soil drench of mephosfolan had declined, natural enemies controlled the aphids for the remainder of the season. Anthocorid bugs, particularly Anthocoris nemoralis, were the most abundant predators. In each year a rapid decline in aphid numbers occurred in mid- to late-July, coinciding with the peak numbers of fourth and fifth instar larvae and adults, the most voracious anthocorid stages. Aphids in the cones remained under control for the rest of the season in 1974 and 1975, and increased in 1976 but damaging numbers did not develop. When predators were excluded by caging mephosfolan-treated stems, high aphid densities developed on the leaves, and the cones were heavily infested. Plants not treated with an insecticide were almost completely defoliated by late-July. Heavily infested "missed stems", due to uneven uptake of mephosfolan, attracted large numbers of anthocorids, which later dispersed into the surrounding plants.

Aveling (1981b)  assessed the toxicity of soil drenches of the systemic insecticide mephosfolan to anthocorid predators. The mortality of Anthocoris nemorum and Anthocoris nemoralis eggs in the leaf tissues increased with the dosage of mephosfolan while Anthocoris confusus eggs, which were laid in the petioles and veins, appeared to be unaffected at the dosages used. Mortality of Anthocoris nemorum eggs, which were laid mainly in the leaf margins, was higher than in Anthocoris nemoralis eggs, which were laid mainly in the interveinal areas. It is suggested that mephosfolan becomes unevenly distributed within the leaves, being most concentrated in the leaf margins and least concentrated in the veins and petioles. Oviposition rates, larval mortality and duration of larval development were unaffected when specimens of each species were reared from eclosion to maturity on a diet of either mephosfolan-killed or heat-killed Phorodon humuli.

Cranham (1982)  reviewed recent work on integrated control of damson-hop aphid on English hops. Current control methods rest heavily on the systemic organophosphate mephosfolan applied as a soil drench. After the persistent aphidical action of an early-season (May) soil drench of mephosfolan has declined, insect predators, particularly Anthocoris nemoralis and Anthocoris nemorum, can usually contribute greatly to control for the remainder of the season, especially within the hop cones. Means of suppressing aphids in July/August with minimum harm to predators are essential for a commercially feasible system of integrated control. The authors discussed possible methods of achieving this.

Campbell & Cone (1999)  assessed the consumption of Damson-hop Aphids by larvae of two species of coccinellids, Coccinella transversoguttata and Hippodamia convergens. Larvae of Hippodamia convergens consumed an average of 318 adult damson-hop aphids during their development at 20°C. Female larvae of Coccinella transversoguttata ate 413 adult Phorodon humuli, and males ate 357. This difference in the consumption of prey occurred only in their fourth larval instar, and was reflected in a corresponding size dimorphism between female and male larvae at pupation.

Lorenzana et al. (2010)  investigated population development of the Phorodon humuli and its natural enemies in hop cones. They also investigated if the alpha-acid content of the crop was affected by aphid populations. The price of hops in Spain is given per kilogram of dry product and according to their content of alpha-acids. Aphids were first observed within hop cones at the beginning of August and numbers rose sharply from mid- to late August onwards. Aphid infestation of cones reflected the numbers observed on foliage in July, with a greater population halfway up the stems than at the top. The alpha-acid content of dried hops was unaffected by the aphid population on leaves, but aphid contamination reduced the economic value of the crop because of arbitrary commercial criteria related to the presence of aphids in cones (If any aphids are noted inside cones there is a penalty of up to 10% of the dry weight). The hop aphid's main natural enemies within cones were anthocorids, all belonging to the genus Orius. The anthocorid population in cones grew in tandem with the aphid population, suggesting that these predators are worthy of consideration in an integrated management approach

Lorenzana et al. (2013)  carried out a field trial in order to analyse the population development of Phorodon humuli and its natural enemies in Spain, as well as to determine the most effective integrated program of insecticide treatments. The basic population development pattern of Phorodon humuli was similar in the three years: the population peaked between mid to late June, and then decreased in late June/early July, rising again and reaching another peak in mid-July, after which it began to decline, rising once more in late August; this last rise is characteristic of Spain and has not been recorded in the rest of Europe. The hop aphid's main natural enemy found on the leaves was Coccinella septempunctata. The most effective program of insecticide (imidacloprid) treatments consisted of an initial treatment in June and a second treatment in the second half of July or at the beginning of August. However, a single treatment in June would be sufficient when in this last period the maximum daily temperatures were higher than 27°C for at least 15 days, thus avoiding the harmful effects of imidacloprid on predators.

Acknowledgements

Our particular thanks to Roger Blackman for images of his clarified slide mounts.

We have made provisional identifications from high resolution photos of living specimens, along with host plant identity. In the great majority of cases, identifications have been confirmed by microscopic examination of preserved specimens. We have used the keys and species accounts of Blackman & Eastop (1994)  and Blackman & Eastop (2006)  supplemented with Blackman (1974) , Stroyan (1977) , Stroyan (1984) , Blackman & Eastop (1984) , Heie (1980-1995) , Dixon & Thieme (2007)  and Blackman (2010) . We fully acknowledge these authors as the source for the (summarized) taxonomic information we have presented. Any errors in identification or information are ours alone, and we would be very grateful for any corrections. For assistance on the terms used for aphid morphology we suggest the figure  provided by Blackman & Eastop (2006).

Useful weblinks 

References

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  •  Aveling, C. (1981). Activity of mephosfolan on anthocorid predators of Phorodon humuli. Annals of Applied Biology 97(2), 155-164. Abstract 

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