Biology, images, analysis, design...
|"It has long been an axiom of mine that the little things are infinitely the most important" |
Subspecies cytisorum (Broom aphid)
Subspecies sarothamni (Laburnum aphid)Identification & Distribution Biology & Ecology
Identification & Distribution:Aphis cytisorum is a very dark green aphid which appears greyish because of the strong wax secretion. The siphunculi and cauda are dark, but the antennae and tibiae are mostly pale. The body length of apterae is 1.4-2.5 mm.
There are two subspecies of Aphis cytisorum:
The only morphometric difference between these two subspecies is that the oviparae of Aphis cytisorum sarothamni have more swollen hind tibiae with many more scent plaques than Aphis cytisorum cytisorum.
Aphis cytisorum is one of three 'black-backed' Aphis species living on woody Fabaceae (the other two are Aphis craccae and Aphis craccivora ). The adult aptera has a dark sclerotized dorsal abdominal shield, clearly visible in the first micrograph below. The shield 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. Marginal tubercles may be present. The siphunculi are 1.2 to 2.2 times as long as the cauda. The combined length of the last two (fused) segments of the rostrum is 0.97 to 1.3 as long as the hind tarsi. The longest tibial hairs are a little longer than the least width of the hind tibiae.
Their alates also have areas of membranous cuticle along the side the segmented bands on their abdominal tergites, as can be seen in the second micrograph above. Aphis cytisorum 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).
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:
Both subspecies of Aphis cytisorum are facultatively ant attended. We have found ssp. sarothamni being attended by Formica lemani (see first picture below) and Lasius in Scotland, and by Formica rufa (see second picture below) and Lasius in southern England.
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 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 very few predators attacking Aphis cytisorum sarothamni, although the picture below shows a midge larva (Aphidoletes) predating an aphid in one colony.
The scarcity of predators is almost certainly 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.