Biology, images, analysis, design...
|"It has long been an axiom of mine that the little things are infinitely the most important" |
Crimson tansy aphidOn this page: Identification & Distribution Biology & Ecology Other aphids on the same host
Identification & Distribution:
Apterae of Uroleucon tanaceti are red, reddish brown or crimson, with yellowish antennae and black apices. Body hairs are long and placed on small, discrete scleroites. Antesiphuncular and marginal sclerites are absent. The legs are (usually) yellow with the apices of the tibiae black. The siphunculi are brown or black, often with the middle part paler brown giving a characteristic bicoloured appearance. The cauda is yellow. The body length of Uroleucon tanaceti is 2.2 to 3.4 mm.
The crimson tansy aphid is found on tansy (Tanacetum spp.), especially on the lower yellowing leaves. It also occurs on cultivated Chrysanthemum species. Winged males and wingless female oviparae occur in October. Uroleucon tanaceti is distributed throughout Europe to Siberia and Central Asia, and North America.
Biology & Ecology:
Our first encounter with Uroleucon tanaceti was when Nigel Gilligan (one of our regular contributors from the north of England) sent us some photos in 2014 (see bottom of page under identification requests). Its brick red colouration appears to be an example of aposematic colouration, warning intentional predators (such as birds) and unintentional predators (such as herbivores) that the insect is distasteful or otherwise nasty. A closely related aphid, Uroleucon nigrotubeculatum, gets it brick-red colouration from a quinone pigment named uroleuconaphin (Horikawa et al., 2006). We suspect that Uroleucon tanaceti uses the same, or a very similar, pigment.
Tansy, also known as cow bitter or bitter buttons, contains a volatile oil containing thujone, camphor and myrtenol, which give tansy its bitter taste to vertebrates. If consumed in large quantities the leaves and flowers are toxic to humans. The crimson tansy aphid is also unpalatable to invertebrate predators, namely Coccinella septempunctata and Chrysopa carnea. Mehrparvar et al. (2013) found that the larvae of neither of these predators could reach maturity when reared only on the crimson tansy aphid. When offered a mixture of these and more palatable tansy aphids such as Metopeurum fuscoviride and Macrosiphoniella tanacetaria, they could mature but fitness parameters were still lower than for aphids feeding only on a palatable host. Surprisingly it seems that the predators were unable to avoid feeding on the unsuitable (toxic?) aphid species.
Aposematically coloured insects normally display their colour as much as possible, and sure enough when Nigel first spotted the aphids many of them were spaced out in a near regular pattern of dispersion on the upper surface of the leaves (see picture below).
Guest image copyright Nigel Gilligan, all rights reserved.
But Mehrparvar et al. (2013) and other workers have reported that Uroleucon tanaceti lives hidden on the undersides of the lower leaves. Heie, 2009 points out that living in a concealed position is not one would expect from an insect displaying aposematic colouration, and describes it as an 'aphid mystery'.
Further examination of the plants by Nigel revealed that those up top were the adults and most of the colony (including all the young) were on the undersides of the leaves (see first picture below). It seems plausable that adults on the top of the leaves are 'sacrificial lambs' which predators might take (and then regret), thus ensuring that the great majority of the colony remained untouched. Possibly adults on top of the leaf were post-reproductive, although this would not be required to explain such behaviour given that all individuals in the colony were likely to be closely related.
Guest images copyright Nigel Gilligan, all rights reserved.
So why have some workers not found any on the upper surfaces? Well, on a repeat visit to the colony following some wet weather, Nigel found that found that all the aphids were on the undersides of the leaves. So do they move to the undersides of the leaves for shelter during rain? We suggested a simple experiment of spraying the aphids with water - and sure enough a light sprinkle of water on the aphids produced a rush for cover (see second picture above). Rain is acknowledged as an important factor responsible for aphid mortality (Wellings & Dixon, 1987), so it makes sense for vulnerable aphids to shelter on the leaf underside in wet weather. It seems their display is very dependent on environmental conditions - which at least may explain why other workers have not noticed it.
There is some evidence that Uroleucon tanaceti can manipulate plants' nutritional quality to make it more palatable (to them, that is). Infestation with this aphid increases the percentage of methionine plus tryptophan in the phloem tenfold, which causes premature leaf senescence hence improving its nutritional quality for the aphid (Nowak & Komor, 2010).
We have since found Uroleucon tanaceti ourselves, not in Sussex but in Hampshire.
In this case all the aphids were on the undersides of the leaves (see picture below)- the weather was unsettled with occasional rain.
Other aphids on same host: