Biology, images, analysis, design...
|"It has long been an axiom of mine that the little things are infinitely the most important" |
Aphid wildlife in springtime Britain:
2 invasives, + 1 minor pest & 1 important pestHoplocallis picta Myzus hemerocallis Myzaphis_rosarum Rhopalosiphum padi
Following upon last month's theme of invasive aphids we consider two newcomers to Britain, the painted holm oak aphid (Hoplocallis picta) and the day lily aphid (Myzus hemerocallis). It seems at least possible that Hoplocallis picta has made it into Britain under its own steam (that is by flight) from southern Europe, rather than by travelling on imported plant material like most other 'invaders'. In contrast, Myzus hemerocallis seems to have arrived via the ornamental plant trade: in this case of day lilies. We then look at two very 'British' aphids (UKIP take note!) starting with the lesser rose aphid. Myzaphis rosarum is not a rare aphid, but may be overlooked because its feeding site is often hidden from view. Lastly we take a closer look at the bird cherry - oat aphid (Rhopalosiphum padi) on its primary host - the bird cherry (Prunus padus).
Painted holm oak aphid (Hoplocallis picta) - is climate change driving their extension northwards?
One bright day in early April we decided to check out how the common holm-oak aphid (Myzocallis schreiberi ) was faring, after a rather long cold winter, on an old holm oak (Quercus ilex) in East Sussex. Sexual forms are unknown for this species, so it overwinters not as eggs, but as parthenogenetic winged adults and nymphs. We soon found a few Myzocallis schreiberi (see picture below of a fourth instar alatiform nymph and a winged adult) on the apparently dry, hard oak leaves. They had the transverse abdominal bars characteristic of their species.
Our search also turned-up some nymphs that looked rather different (see second picture below).
We kept these latter nymphs alive on an oak cutting, until one moulted to a winged adult (see picture below). This was clearly not Myzocallis schreiberi! A short session using the keys in Blackman & Eastop's (2006) excellent 'Aphids on the World's Plants' showed we had found the painted holm oak aphid (Hoplocallis picta ).
Hoplocallis picta has only been reported on oak once before in Britain, when small numbers were observed in July 2006, feeding on the leaf undersides of three young Quercus ilex in Cardiff Bay (Baker, 2009 ). Their populations persisted at very low levels and, like the colony we found, coexisted with Myzocallis schreiberi. Hoplocallis picta is otherwise only known in the UK from a handful of suction trap records (sadly unpublished) from southern England (Richard Harrington, pers. comm. to Ed Baker).
There do not appear to be any detailed studies of the ecology of the painted holm oak aphid, although various authors have noted its occurrence. Pons et al. (2006) monitored numbers on holm oak in urban green spaces in France and found that numbers peaked in April and May. Paris et al. (2009) looked at honeydew collection and predation by the ants Lasius neglectus and Lasius grandis on Quercus ilex. Hoplocallis picta was rarely tended for honeydew, but was often predated, along with barkflies (Psocoptera) with which it cohabits (see picture above). Although its current presence is somewhat tentative, this southern european species is evidently extending its range northwards...
Day-lily aphid (Myzus hemerocallis) - what's hiding in that growing shoot?
The day lily aphid most likely used a different port of entry to Britain - hitching a lift on Hemerocallis imported from the USA. We came across Myzus hemerocallis on a potted plant on sale in a Sussex nursery glasshouse in early April 2015. No aphids were visible, but some of the leaves were covered in exuvia - a sure sign of a current (or past) aphid colony - moreover some ants were clearly interested in this particular plant, a new Hemerocallis hybrid: "Black magic".
Separating the leaves revealed a large colony of the day lily aphid (Myzus hemerocallis) shown in the pictures below. Myzus hemerocallis is a pest of East Asian origin, but is now also widely distributed on Hemerocallis in Australia, New Zealand, N&S America, Kenya and France. The UK Food and Environment Research Agency (FERA) has fourteen records of the species (ours is the 15th) - detected in England and Wales, mostly on Hemerocallis imported from the USA (pers. comm. Chris Malumphy). In July 2002 Rothamsted suction traps picked up winged adult Myzus hemerocallis at Starcross in Devon, and at Writtle in Essex (pers. comm. Mark Taylor via Ed Baker). Nevertheless we still cannot be sure this species has established itself in Britain.
Blackman & Eastop (2006) describe adult Myzus hemerocallis as pale yellowish green or yellowish white and attacking the basal (concealed) parts of young leaves of Hemerocallis and Agapanthus. We note that whilst immatures and freshly ecdysed adults are pale yellowish green, the mature wingless adults (see picture below) have an orange-brown hue anteriorly and posteriorly - which does not seem to have been noted in the literature.
When we first found the colony, there were very few alates present, but as the colony grew (in strict confinement to prevent escapees) and density increased, nearly all the adults were winged (see first picture below).
The aphids were 'loosely' attended by Lasius ants (see second picture above), although they mainly seemed to be collecting the honeydew from the leaves (termed gleaning) rather than collecting the honeydew from the aphids as it was exuded.
Lesser rose aphid (Myzaphis rosarum) - just how common is this 'minor pest'?
Many people, especially gardeners, are all too well acquainted with the common rose aphid (Macrosiphum rosae), but fewer recognise the lesser rose aphid (Myzaphis rosarum) - even though it is supposed to be quite common. Myzaphis rosarum lives on wild and cultivated roses, especially climbers. It has long slightly swollen siphunculi (although this is not always very evident in photos of the live aphid) and a long cauda. The lesser rose aphid feeds on shoot tops and, on either side of the rose leaf, along the midrib.
Like many aphid species, Myzaphis rosarum abundance can vary greatly from year to year, hence the standard texts (being based upon relatively few observations) are not always a reliable guide. We found Myzaphis rosarum for the first time in early April 2015, despite the fact it is regarded as common and a (minor) pest species. Moreover, we have not so far found it in subsequent weeks, and the early populations seem to have died out.
Jaskiewicz (2003) describes the species composition and number of aphids on rose bushes in urban conditions over three years. Myzaphis rosarum appeared on roses late May to early June, reaching maximum numbers in June or early July. They disappeared from one site in late summer, but persisted at the other into December. Lesser rose aphids comprised 6-26% of the aphids feeding on roses.
Bird cherry-oat aphid (Rhopalosiphum padi) - first find your bird cherry tree!
Although the bird cherry-oat aphid (Rhopalosiphum padi) is a fairly common cereal aphid in Britain, its winter host - bird cherry (Prunus padus) - is surprisingly rare, at least south of the Scottish border. This may explain why the aphid is not a serious cereal pest in England, whereas it certainly is in northern Europe (Leather, 1989 ).
We looked at Rhopalosiphum padi last spring when we found the stem mothers (fundatrices) in leaf galls of Prunus padus. Having found an infested tree, or what was left of it after the local Council's efforts, this year we decided to try to follow the aphids' development from egg-hatch to their migration to grasses and cereals. We spotted their eggs laid last autumn in the narrow gap between axillary buds and stem (see picture below).
Leather (1993) estimated there is only enough room in this site for 10-15 eggs to be adequately protected from predators, and has shown that post-oviposition there is a period of density-dependent mortality where eggs in suboptimal sites are predated. Predators include spiders and birds. After egg hatch, the newly emerged nymphs (see first picture below) move to the unfurling bird cherry leaves where they feed and develop (see second picture below).
If the bud they are beside has opened, this move is readily achieved, but sometimes the newly hatched nymphs have to find other, more advanced buds, to access suitable young growth. Such nymphs may well be subject to high mortality rates.
Once they have found suitable fresh growth, the nymphs develop rapidly into mature fundatrices. The picture below shows one of these developing fundatrices up-close-and-personal.
These fundatrices give rise to a second, and wax-covered, generation. On mature leaves, or in very crowded conditions, these wax-covered nymphs (see picture below) develop into alates which migrate to cereals and grasses.
But on younger leaves the wax-covered nymphs develop to large, highly fecund, wingless adults - which then give rise to winged emigrants in the subsequent generation. This flexibility in their life cycle enables aphid population to adapt to local conditions, and is one reason why aphids are such a successful group.