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St John's wort aphid, Hypericum aphidOn this page: Identification & Distribution Biology & Ecology Habitat Colour Natural enemies Biological control of weeds Other aphids on the same host
Identification & Distribution
Aphis chloris is rather bright green, pale yellow-green or dark green and is a member of Aphis frangula group. The antennal terminal process is rather short, being 1.6-2.1 times the length of the base of antennal segment VI. The fused terminal rostral segment is also short being only 0.9-1.0 times the length of the second hind tarsal segment. The abdominal dorsum is usually entirely membranous with few if any sclerotized areas. Tibial hairs are short, at most 0.7-1.2 times as long as the least width of the hind tibia, and all femoral hairs are much shorter than the least tibial width. Aphis chloris has dark siphunculi but a slightly paler cauda. The siphunculi are 0.83-1.63 times the length of the cauda. The body length apterae is 1.0-1.8 mm.
The St John's wort aphid does not host alternate. It lives basally on Hypericum spp. especially St John's Wort (Hypericum perforatum) where it may be attended by ants. Oviparae and dull green apterous males are produced in the autumn. In Britain it has been little recorded (till now only from Surrey, Hertford and Buckingham), but Stroyan (1984) commented that it was probably widespread in southern Britain. Aphis chloris is found throughout Europe and into Asia.
Biology & Ecology
We have searched for Aphis chloris many times, but only found it in 2019 for the first time on the stem bases of Hypericum perforatum (St John's wort) growing in the disused hoverport at Pegwell Bay in Kent. There was some evidence of past ant-tenting and a few colonies were ant-attended.
Stroyan (1984) provided a detailed description of the morphology of Aphis chloris and stated that it lived in ant-attended colonies at stem bases of Hypericum perforatum just below the soil surface or occasionally on aerial parts.
The colour of Aphis chloris apterae has been described as bright green, pale yellow-green or dark green. We have noted that much of the variation in colour can be accounted for by the age of the aphids. In the colonies at Pegwell Bay the immatures (see picture below) were invariably a uniform pale yellow-green.
Adult apterae, on the other hand, usually had darker green shading on the thoracic tergites extending to a greater or lesser extent over the rest of the dorsum.
It seems likely that the paler green alate in the first image below is less mature than the darker green alate in the second image below.
Although the ant-tending seemed rather sporadic, there was little evidence of any predators attacking the colonies. They had however been attacked by parasitoids, most likely by Lysiphlebus fabarum (found parasitising Aphis chloris by Feraru et al., 2005) or Lysiphlebus testaceipes (found parasitising Aphis chloris by Costa & Stary et al., 1988). Feraru et al. (2005) also found the hyperparasitoid Syrphophagus aphidivorus emerging from some of the Aphis chloris mummies.
Biological Control of Weeds
St John's wort (see picture below) is a yellow-flowered perennial herb native to Europe and toxic to livestock. It was introduced into Australia in the mid to late 19th century, and has become a serious weed of pastures, poisoning stock and displacing more productive pasture species. It has also spread into natural vegetation beneath open eucalypt woodlands, where it threatens floral diversity (Briese & Cullen, 2012).
Biological control of St Johns Wort by introduction of insect herbivores from Europe began as far back as 1928 when a laboratory was set up in Buckinghamshire, England to find herbivores that could be released in Australia. Aphis chloris was identified as such at this early stage, but it was found that the English population overwintered in the egg stage and cold-requirements for subsequent emergence of nymphs were not met in Australia. Over fifty years later a French strain of Aphis chloris that reproduced parthenogenetically year round was found and released at multiple sites in Australia. (Briese, 1997)
The aphid quickly established throughout south-eastern Australia. Local populations built up to levels at which migratory alates were produced within the first season. Dispersal of these alates over large distances led to widespread colonization of St John's wort infested areas within 2-3 years. Detailed observations at various sites showed that aphid populations undergo a seasonal cycle of population growth throughout the summer, followed by a migratory phase with alate production and dispersal in autumn and a population decline over winter. Unfortunately over this cycle populations never built up to sufficiently high levels to damage the plants. Natural enemies, competition from existing biological control agents, climatic factors and emigration appeared sufficient to limit such outbreaks. While caged plants did show a decline in vigour and seed production under heavy aphid attack, such damage was intermittent in open field situations. Briese & Jupp (1996) concluded that, while Aphis chloris had become widespread, it would not make an important contribution to the overall control of St John's wort.
Other aphids on same host:
Blackman & Eastop list 10 species of aphid as feeding on St John's wort (Hypericum perforatum) worldwide, and provide formal identification keys. Only Aphis chloris normally feeds below the soil surface (Show World list). Of those aphid species, Baker (2015) lists 8 as occurring in Britain: (Show British list).