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Identification & Distribution:The Hyalopterus pruni aptera is a small to medium sized aphid, 1.5 - 2.6 mm long, with an elongate shape. It is usually pale green with a fine darker green mottling, covered with wax meal (see first picture below). The antennae are quite short, between 0.5 - 0.75 times the body length. The siphunculi are very short, and are thicker and darker towards the apex; they are also flangeless and rounded at apex. The cauda is 1.5 - 3.0 times longer than the siphunculi. The Hyalopterus pruni winged form (see second picture below) is green with white wax patches on the dorsum of each abdominal segment.
The short siphunculi are almost concealed by the wax, but are visible in the preserved specimens below.
The mealy plum aphid host alternates between its winter host - Prunus species, mainly plum but also peach, apricot and almond, and its summer host - mainly reeds (Phragmites), but also giant cane (Arundo donax) and some other wetland grasses. Some aphids remain on plum all the year round. Hyalopterus pruni is cosmopolitan, but may have geographical races or subspecies.
Biology & Ecology:
Life cycleThe eggs of Hyalopterus pruni overwinter on Prunus species. They hatch in April, usually by the white bud stage on plum. They seldom cause much curling of the leaves, but can live inside curls of newly expanding leaves. The picture below shows a colony on plum in mid-May.
As the season progresses, Hyalopterus pruni colonies can literally 'pave' the underside of the leaves (see picture below).
The population of Hyalopterus on plum continues to increase through June and July, and many aphids may remain on plum all year rather than migrating to the secondary host. This results in an accumulation of exuvia on the leaves as shown in the picture below.
Nevertheless some alatae start to develop in early June, with the peak of migration of alatae to the secondary host occurring between early July and early August. The main summer host is the common reed Phragmites australis, although it has been recorded on other grasses. Moericke (1969) demonstrated that Hyalopterus pruni emigrants are attracted to colour in the range orange-yellow-green, but only when these colours are unsaturated tints. This is a specific adaptation to the unsaturated colour of the leaves of Phragmites.
When an alate starts a colony on a grass blade, she appears to deposit wax around herself presumably providing some protection against predators to the young nymphs. The result of this is that initially each colony sits within a wax circle on the leaf (see picture below).
As on Prunus, high numbers can build up on Phragmites in summer.
The return migration to Prunus spp. comprising winged male and winged sexuparae begins in September, and is usually quite small. These supplement the part of the population that has remained on plum all summer. We have found sexuales being produced on the secondary host well into October as shown in the picture below.
The dark yellow alate with some black abdominal markings is the male. The mated oviparae then lay eggs on the trunks and branches of the primary host.
Colour crypsis & polymorphismThroughout its life cycle some or all of the population of Hyalopterus pruni is cryptically coloured. The crypsis on the primary host (see below) is especially impressive.
On the summer host the species exhibits colour polymorphism, with two colour forms - green and dark red.
The biological significance of this colour polymorphism is (as is so often the case) unknown, but the polymorphism may be maintained by balanced selection from two predatory species as is the case for Acyrthosiphon pisum (Losey et al., 1997 ).
Ant attendanceWe have found no evidence of active ant attendance of Hyalopterus pruni (in the sense of ants antennating the aphid to induce the excretion of more honeydew), but ants do sometimes visit colonies to collect (glean) honeydew from the leaves.
Ants are not the only insects to glean honeydew excreted by mealy plum aphid colonies. The common wasp (Vespula vulgaris) can often be found gleaning honeydew from Hyalopterus pruni colonies on plum trees.
Predators and parasitoids attack the mealy plum aphid on both its winter and summer host. Basky (1982) surveyed the predators and parasitoids of Hyalopterus pruni and Hyalopterus amygdali living on plum, peach and reed in Hungary. Syrphidae were the most abundant predators, the most common of which were Episyphus balteatus (see below) and Metasyrphus corollae.
Basky found that Coccinella septempunctata was the commonest coccinellid, with fewer Adalia bipunctata, Coccinella 5-punctata and Coccinella 11-punctata also present. In recent years we have found the invasive harlequin beetle (Harmonia axyridis) on infested plum leaf in June.
The most unusual predator we have found attacking Hyalopterus pruni was a silverfly larvae (Chamaemyiidae). The picture below shows a larval chamaemyiid sitting quietly in a colony of the mealy plum aphid.
Much like Aphidoletes larvae, chamaemyiid larvae use what Fréchette et al. 2008 described as a "furtive predation strategy". They sit quietly within the aphid colony, occasionally consuming an aphid, but cause little defensive reaction among the aphids - or disruption of the colony.
Other parasitoids attacking the mealy plum aphid are Ephedrus plagiator and Aphidius transcaspicus (Starý & Havelka, 2008 ). The ectoparasitic mites Allothrombium triticium and Erythraeus ankaraicus can also be found on Hyalopterus pruni. Bayram & Çobanoglu (2005) found parasitism rates were between 1% and 5%, and suggested that higher parasite loads increased aphid host mortality. Note the mites are true parasites rather than parasitoids as they do not invariably kill the host.
Damage and control
In Europe Hyalopterus pruni is only a pest on its winter hosts, in particular plums. When it occurs in large numbers on young leaves (see picture below) it causes significant feeding damage, but leaf curling only seems to occur when very young leaves are attacked.
It combines this damage with excreting honeydew on to lower leaves, which become dark with sooty moulds growing in the resulting sticky film. These fungi reduce the plant's ability to photosynthesise. This species is 'weak' vector of Plum pox potyvirus.
The situation in America, where Hyalopterus pruni is a non-native invasive species, is rather different. Not only is it a pest on plum, but there is also concern about its effects on its summer host Phragmites australis (the common reed). An aggressive, non-native haplotype (distinct genetic lineage within a species) of Phragmites australis from Europe is invading brackish and freshwater systems in the eastern United States, potentially displacing native haplotypes. Both Lambert & Casagrande (2007) and Park and Blossey (2008) have found significantly higher aphid populations on native haplotypes than on the non-native haplotype. Moreover Lambert & Casagrande (2007) found that aphid feeding caused chlorosis and death of native stems, whilst the non-native plants remained relatively undamaged. There is therefore concern that non-native aphid infestation may indirectly affect the ability of these native plants to compete with non-native plant populations, ultimately contributing to the decline of native haplotypes. The two haplotypes of Phragmites australis have each recently been accorded subspecific status.