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Identification & Distribution:The Aphis pomi aptera (below first) is bright apple green or yellow green and is not wax-powdered. The abdominal dorsum is pale and usually entirely membranous, although rarely a small sclerite or short bar may occur on the spine of tergite 5. The fused last two rostral segments are more than 120 μm in length and marginal tubercles are present on abdominal tergites 2-4 (the latter two characters distinguish Aphis pomi from the very similar invasive Aphis spiraecola). The siphunculi and cauda are conspicuously blackish. The cauda has 10-19 hairs (rarely less than 13). The body length of an adult aptera is 1.2-2.2 mm.
Aphis pomi alates (above second) have a black thorax. The alate abdomen is green, usually with 3 pairs of weakly pigmented black lateral circular spots on the anterior abdominal segments, and a semicircular spot in front of and behind each siphunculus. The pictures below are micrographs of an aptera in alcohol - dorsal and ventral view.
The green apple aphid does not host alternate. It feeds in dense colonies on the young shoots and undersides of leaves of apple (Malus spp.) and related plants including pear (Pyrus), hawthorn (Crataegus), Sorbus and Cotoneaster, causing leaf curl. Colonies are often attended by ants. Sexual forms occur in autumn and after mating the females lay sometimes large egg masses on the twigs. It is generally common and is distributed throughout Europe, north Africa, Asia eastwards to India and Pakistan, and North America.
Biology & Ecology:
The eggs of the green apple aphid hatch somewhat later in spring than those of Rhopalosiphum oxyacanthae and Dysaphis plantaginea, the other aphid species commonly found on apple. Both those species move to secondary hosts in late spring and summer respectively. Aphis pomi remains on apple where colonies develop on the young shoots. Winged green apple aphids start appearing in early June. These winged migrants move to other apple trees where large infestations build up in June and July.
Some work has been done on the effect of chemicals in apple leaves on the probing behaviour of different aphid species. Klingauf (1971) found that the glucoside phlorizin promotes colonization by Aphis pomi. However, Montgomery & Arn (1974) report that it should not be regarded as a feeding stimulant. They found it was neutral as a probing stimulus to Aphis pomi, an apple feeder, but was a probing deterrent to the non-apple feeding aphids. It was also an ingestion deterrent to all three species. Apple can only be utilized as a host by Aphis pomi since it feeds in the phloem, which appears not to contain phlorizin.
Several species of aphids coexist with Aphis pomi on apple for all or part of the year including the closely related Aphis spiraecola. Foottit et al. (2009) deal with the characters distinguishing Aphis pomi from Aphis spiraeola. Other species living with Aphis pomi include the rosy apple aphid (Dysaphis plantaginea), the apple-grass aphid (Rhopalosiphum oxyacanthae) and the woolly apple aphid (Eriosoma lanigerum). Coexistence between species can be very intimate as in the picture below where an Aphis pomi fundatrix is living in the midst of a Dysaphis plantaginea colony.
Similarly the picture below shows several mature Aphis pomi amongst young nymphs of Eriosoma lanigerum.
Minarro et al. (2010) looked at the role of ants in structuring the aphid community in apple orchards. Ants were found to establish a mutualistic relationship with the aphids Dysaphis plantaginea and Aphis spp. but not with Eriosoma lanigerum. In observational studies in the field, Dysaphis plantaginea performed better in the presence of ants while no effect was observed in Aphis spp. However, in a manipulative experiment, populations of Aphis spp. performed better in the presence of ants while no differences were observed for Dysaphis plantaginea. Populations of Eriosoma lanigerum were reduced in the presence of ants.
Ants also had a significant negative effect on the abundance of natural enemies, which could partially explain the benefits to the tended aphids. However, while ants did not provide a benefit to Aphis spp. when it was reared alone, in the presence of other species ant attendance increased Aphis abundance by 256% and simultaneously reduced Eriosoma lanigerum abundance by 63%. Therefore, ants benefited Aphis by reducing competition with other aphid species, explaining the benefit of ant attendance. Considering all the aphid species together, ants had a net positive effect on aphid abundance, which was consequently considered harmful for the plant.
In woodland Aphis pomi also occurs on hawthorn (Crataegus) where it is often tended by wood ants (Formica). The two pictures below show southern wood ants (Formica rufa) tending Aphis pomi on hawthorn (Crataegus). In our experience Formica ants provide a better level of protection to aphids than Lasius niger ants, at least against predators, and we have found some very large colonies of Aphis pomi in these situations.
Another host of Aphis pomi is rowan (Sorbus aucuparia). We have only found it on this host occasionally, and generally later in the summer. The colony (see picture below) was again attended by Formica rufa.
Most of the colonies we have found have been ant attended, which may explain why we have found rather few instances of predators attacking Aphis pomi. Adams & Prokopy (1980) found that Aphidoletes aphidimyza was the most abundant summer predator of Aphis pomi in an untreated apple orchard in western Massachusetts. The cecidomyiid larvae (see picture below) were responsible for a high level of mortality among the aphid population and a dramatic decline in infestation. The cecidomyiids overwintered in the orchard but were not active until mid-June. Consequently, early season aphid damage occurred before the appearance of the predators. Carroll & Hoyt (1984) found that early spring control depended on aphidiid parasitoids which killed but failed to complete development in Aphis pomi. There was then a month-long 'predator gap' which allowed rapid aphid colony in June, only slowed by generalised ground predators. Later populations were reduced to a varying degree by a wide range of predators.
Aphidoletes aphidimyza was also the most numerous predator of Aphis pomi over two years in a commercial apple orchard in Nova Scotia (Stewart and Walde (1997) ). This predator was found in the earliest colonies sampled each season and was present throughout the growing season. In one year with relatively high aphid densities a single peak in aphid and predator density was seen in mid-July, just prior to maximum predation rate. The predator's numerical response resulted in delayed density dependence in predation rate. In the year of low aphid densities, predator and prey dynamics appeared uncoupled, and other predators such as mirids may have been more important. Aphidoletes aphidimyza tended to aggregate oviposition in high-density aphid colonies, but this aggregation resulted in predation that was only very weakly spatially density dependent.
In an interesting series of papers, Braun & Flückiger (1984) looked at an infestation of hawthorn (Crataegus) with the green apple aphid (Aphis pomi) growing by a motorway near Basle in Switzerland. The motorway plants were much more heavily infested than plants some distance away. The researchers concluded that a reduced efficiency of natural enemies at the motorway might be an important factor contributing to the increased infestation, but other factors such as changes in host plant biochemistry also played a role.
Damage and control
Leaves carrying colonies may roll and curl, but will not discolor. The aphid occasionally feeds on immature apples, which then become malformed. Heavy infestation, especially of young trees, can lead to stunting, and in extreme cases to the shedding of leaves: thus considerable damage can occur in nurseries, in which plants at the susceptible age may become permanently deformed. Older trees suffer considerably less.
Various studies have been carried out assess whether chemical control of Aphis pomi is worthwhile. Oatman & Legner (1961) conducted a study of Aphis pomi on young Cortland and Red Delicious apple trees under a treated (artificial) and control (seminatural) environment. The population was highest in the control and was twice as high on Cortland as on Red Delicious. Alates were most numerous the first of July and were primarily responsible for the build-up of aphids following the second aphicide application in the treated section. Intraspecific competition was considered to be the primary biotic factor limiting the aphid population. The increase in terminal growth on treated trees was significantly greater than that on the control trees. Hamilton et al. (1984) investigated the impact of spray timing on the level of apple aphid infestation, and the effect of infestation on apple yield for two apple varieties. The level of infestation was only related to yield for one of the two apple varieties. Most of the current economic thresholds were found to be unsatisfactory.
Hull & Grimm (1983) evaluated various sampling schemes for estimating the population size of the apple aphid, Aphis pomi on an apple tree. The method of counting the number of aphids on the most infested leaf provided the most accurate assessment of aphid density, but it required the most effort. Counting only the number of infested leaves per shoot resulted in a decrease in accuracy, but also a 4- to 8-fold decrease in the time required to sample. Reliable estimates of the aphid population over an entire tree can be made from sampling only those areas of the tree reachable from the ground.
Traditional biological control (release of predators) has been attempted. Hagley (1989) released eggs of the chrysopid, Chrysoperla cornea at a rate of about 335,000 eggs per hectare. This significantly reduced the numbers of the green apple aphid on dwarf apple trees. There have been several attempts to enhance the number of natural enemies by habitat manipulation. Haley & Hogue (1990) compared four ground cover treatments in a young apple orchard to evaluate their effects on Aphis pomi and its predators. The treatments were (1) herbicided fall rye, (2) white clover/ grass mixture; (3) herbicide strips with grassed alleys and (4) woven black plastic strips with grassed alleys. Seasonal tree terminal growth and leaf nitrogen in the clover-grass treatment were significantly lower than in the other three treatments probably because of ground cover competition with the trees for nitrogen. Trees in the clover-grass treatment had total seasonal aphid and predator densities more than fourfold lower than trees in the other treatments.
Wyss (1995) looked at the effects of weed strips on aphids and aphidophagous predators in an apple orchard in Switzerland. Selected weeds were used to attract predators such as anthocorid bugs (shown in the picture above). In the year before the experiment, Dysaphis plantaginea and Aphis pomi and aphidophagous predators were homogenously distributed in the orchard. Weed strips were sown between tree rows and along the border parallel to the first and the last row of trees in one part - the other part served as control. In both parts of the orchard, randomly chosen trees were monitored at weekly intervals in 1992 and 1993. During flowering of weeds more aphidophagous predators were observed on the apple trees within the strip-sown area than in the control area. The most abundant aphidophagous predators were spiders, predaceous Heteroptera, Coccinellidae, and Chrysopidae. Both species of aphids were significantly less abundant in the area with weed strips than in the control area during the period.
The current conventional wisdom for commercial orchards is that Aphis pomi normally causes little injury except in nurseries and on young trees (see Bayer Crop Science - Aphis pomi ) Population densities can, however, increase rapidly if natural enemies appear late or are ineffective, e.g. because of weather conditions or an unusual intensity of ant attendance. In this case, damage can be economically significant. Careful pruning and minimized nitrogen fertilization will reduce water sprouts and thereby remove favourable feeding sites for the aphids. Any pesticides should be chosen and applied with care to avoid disrupting predators.