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(Broom aphid, Laburnum aphid)On this page: Identification & Distribution Biology & Ecology Life cycle Interspecific interactions Ant attendance Natural enemies Other aphids on the same host
Identification & Distribution
Aphis cytisorum is a very dark green aphid which may appear greyish because of the strong wax secretion (see pictures below). The adult aptera has a dark sclerotized dorsal abdominal shield, which is often rather fragmented and variable in size, and is especially reduced in small specimens. The dark sclerotic areas are strongly reticulate. There are areas of membranous cuticle along the side bounded by the mid-dorsal shield and the intersegmental muscle sclerites. The apical rostral segment (RIV+V) is 0.97 to 1.3 times the length of the second hind tarsal segment (HT II). Marginal tubercles may be present. The longest tibial hairs are a little longer than the least width of the hind tibiae. The siphunculi and cauda are dark, but the antennae and tibiae are mostly pale. The siphunculi are 1.2 to 2.2 times as long as the cauda. The body length of apterae is 1.4-2.5 mm.
Alatae (see third picture above) have the dorsal shield broken up into segmental bands, marginal sclerites and postsiphuncular sclerites, with areas of membranous cuticle along side the segmented bands on the abdominal tergites, as can be seen in the second micrograph below.
Aphis cytisorum is one of three 'black-backed' Aphis species living on woody Fabaceae (the other two are Aphis craccae and Aphis craccivora). It is very similar morphologically to Aphis ulicis (which only feeds on gorse) from which it differs only by the length to basal-width ratio of the apical rostral segment. This ratio is less than 3 for Aphis cytisorum and more than 3 for Aphis ulicis.
There are two subspecies of Aphis cytisorum:
The only morphological difference between these two subspecies relates to the oviparae (see life cycle below).
Aphis cytisorum does not host alternate. The laburnum aphid (Aphis cytisorum cytisorum) lives on the leaves, stems and seed pods of laburnum (Laburnum anagyroides) or Spanish broom (Spartium junceum). The broom aphid (Aphis cytisorum sarothamni) lives on broom (Cytisus scoparius). Both subspecies are usually ant attended. Sexual forms occur in autumn. Aphis cytisorum is found throughout most of Europe eastward to Russia and Turkey. It is also found in North Africa, China, North America and parts of South America.
Biology & Ecology:
Colonies of the broom aphid (Aphis cytisorum sarothamni, see picture below) can be found in most months of the year. Their overwintering eggs hatch in spring, and aphid populations build up on the stems, leaves and flowers of the host plant. Often the branching point of a younger stem with the plant axis (= main stem) is the most heavily infested (Wink et al., 1982).
In late June the plant channels nutritional resources including soluble nitrogen to the seed pods of the broom which then become a favoured feeding site for Aphis cytisorum (see picture below).
Alatae (see picture below) are produced in summer enabling the aphids to disperse to other broom bushes.
The sexual forms of Aphis cytisorum develop in autumn. The males (not pictured) are winged. The oviparae of subspecies cytisorum have rather weakly swollen hind tibiae bearing relatively few scent plaques, but those of subspecies sarothamni (see picture below) have strongly swollen hind tibiae and numerous scent plaques.
The eggs of Aphis cytisorum (see picture below) are deposited singly on the stems of broom.
We have so far only found the other subspecies, the laburnum aphid (Aphis cytisorum cytisorum) on one occasion, in August at Rye Harbour in East Sussex. Its apparent scarcity compared to subspecies sarothamni most likely results from a shortage of laburnum trees in the areas we visit rather than any difference in abundance.
Interspecific competition / association
Rather few aphids feed on broom, probably because of the high levels of quinolizidine alkaloids (see below). Of those that do feed on broom, most are specialists which have evolved means of dealing with these toxins. One such specialist is the broom hedgehog aphid ( Ctenocallis setosa), a much rarer aphid than Aphis cytisorum and only previously recorded from Essex, but now known to be present in several locations in East Sussex.
In July 2020 we found several broom bushes in and around Flatropers Wood in East Sussex with quite high populations of both Aphis cytisorum and Ctenocallis setosa (the latter is usually rare and near-solitary). Both species may occur on the same stem of broom (see picture above), but colonies of the two species usually occupy different parts of the bush.
Both subspecies of Aphis cytisorum are facultatively ant attended. We have found ssp. sarothamni being attended by Formica lemani (see first picture below) and a Lasius species in Scotland.
In southern England, the attending ants were the southern wood ant (Formica rufa, see picture below) and a Lasius species.
The phloem sap of Cytisus scoparius contains high levels of quinolizidine alkaloids which is most likely toxic to ants. Presumably most of this is removed and detoxified or stored by the aphids - otherwise the (polyphagous) ants would not be able to harvest the honeydew. However, Szentesi & Wink (1990) showed that Lasius niger which collected honeydew from an Aphis cytisorum colony contained about 45 ug/g fresh weight quinolizidine alkaloids, mostly cytisine. The effect of such levels of alkaloids on the ants is unknown.
We have found rather few predators attacking Aphis cytisorum sarothamni on broom with cecidomyiid larvae being probably the most frequent. The picture below shows a cecidomyiid larvae predating a nymph of Aphis cytisorum in the north of Scotland.
The scarcity of predators may be partly because of the presence of high levels of quinolizidine alkaloids, which these aphids accumulate from the broom. The monophagous Aphis cytisorum sarothamni is able to detoxify or otherwise survive the effects of the alkaloids. Nevertheless Wink et al., 1982 showed that the alkaloid content of aphid infested plants was about 50% lower than that of uninfested plants, indicating that even Aphis cytisorum avoids plants with the highest levels.
Some syrphid species lay their eggs on broom including Syrphus luniger, Syrphus vitripennis, Syrphus nitidicollis and Platycheirus scutatus (Dixon, 1959). These are known to feed on Acyrthosiphon pisum which is also common on broom, but we have not been able to determine whether they also take the heavily waxed Aphis cytisorum.
Coccinellids may also 'check out' Aphis cytisorum colonies, although they soon attract the attention of the attending ants which attempt to drive the beetles away.
Other aphids on same host: