Overview
Aphids are successful, largely because of their high fecundity. Parthenogenesis, viviparity and polymorphism, together, have made possible the telescoping of generations and a consequently high reproductive rate, and a division of labour with some morphs concentrating on reproduction and others on dispersal. The development of complex life cycles and specialised host plant relationships have also contributed. Exceptions and variations are common but, basically, in the life cycle of an aphid, sexuality and oviparity are restricted to only one generation annually, the final one in autumn, when sexual, oviparous females (oviparae) and males are produced. The fertilised egg is the overwintering stage. The generation (of fundatrices) hatching from the eggs and all subsequent generations (viviparae) until the final one consist of parthenogenetic, viviparous females. Viviparae may be apterous or alate; the latter disperse to other hosts within the host range.
Heteroecy, or alternation of hosts, is practised by a small proportion of Aphididae, and involves migration of a species from its usually deciduous, woody (primary) host, following leaf ageing in early summer, to taxonomically unrelated herbaceous (secondary) host(s) and, following their deterioration in autumn, a return to the woody host for overwintering purposes.
Cavariella aegopodii
, the carrot-willow aphid (Aphidinae), is a typical heteroecious species. The fundatrix hatching in spring on the primary host,
Salix
(willow), is the foundress of generations of viviparae, which in the later generations are predominantly or exclusively alate; these abandon willow and fly to the secondary hosts, in this case, umbellifers such as carrot, parsley or fennel, on which several viviparous generations may be passed. Then, in autumn, migrant alate viviparae (gynoparae) and alate males are produced; these fly to the primary host, on which the gynoparae reproduce the oviparae. Males and oviparae mate, fertilised, overwintering eggs are laid, and the cycle is complete. In (monoecious and) heteroecious Pemphiginae, Hormaphidinae and Anoeciinae, males are apterous and are produced on the primary host, often from the same mother (a sexupara) as the oviparae.
Complete cycles (holocycles), involving eggs in winter diapause, are obligatory in cold climates, and for species monoecious on deciduous trees. Australian examples of the latter are the exotic
Calaphis
and
Euceraphis
on birch,
Drepanosiphum
(Drepanosiphinae) on sycamore and
Periphyllus
(Chaitophorinae) on maple. In more temperate regions, where the autumnal environment is not extreme enough to induce the production of sexuales, and where suitable host plant material may remain available throughout the year, continuous parthenogenetic reproduction is possible. Such anholocyclic reproduction is prevalent among the exotic species in Australia, e.g.
Aphis spiraecola
,
Aulacorthum solani
,
Brachycaudus helichrysi
,
Hyalopterus pruni
,
Macrosiphum euphorbiae
,
M. rosae
,
Metopolophemi dirhodum
(Aphidinae),
Cinara spp
. (Lachninae). Holocyclic and anholocyclic reproduction commonly co-exist in both monoecious and heteroecious species. Examples of the latter in Australia are the exotic
Myzus persicae
,
Cavariella aegopodii,
Capitophorus elaeagni, Hyperomyzus lactucae
(Aphidinae) and
Pemphigus bursarius
(Pemphiginae). Species such as
Rhopalosiphum padi
cannot be holocyclic, even though males are common, because the primary host is absent. In some species, sexuales are unknown (world-wide), e.g.
Aphis nerii,
Toxoptera
spp.,
Brachycaudus persicae, Myzus ornatus
and
M. ascalonicus.
Description
Wings tectiform or, less often, held flat in repose (e.g. Hormaphidinae). CuA
1
and CuA
2
usually separate, united basally in some Pemphiginae and Hormaphidinae. Paired siphunculi marginally on or near T5, varying from rings to long, cylindrical tubes; producing alarm pheromones. The modified apical segment, or cauda, variously shaped; serves to dispel excreted honeydew from body. Ovipositor absent. Sexuales alate (especially males) or apterous (females of most species), rarely larviform; larviform, arostrate and female uniparous in Pemphiginae. Monoecious or heteroecious.
Distribution
In Australia, the long, hot dry summer is a limiting factor for aphids and, in general, aphids are plentiful only in spring and autumn (Hughes et al. 1965). The indigenous species
Neophyllaphis brimblecombei
and
N. gingerensis
on
Podocarpus
and
Taiwanaphis furcifera
(Drepanosiphinae) on
Nothofagus
have overcome summer conditions by having a prolonged egg stage; sexuales are produced in spring and early summer concurrently with the viviparae and the resultant eggs do not hatch until the following spring (Carver and Hales 1974; Hales 1976). In Australia, sexuales and eggs of the Australasian
Schoutedenia lutea
(Greenideinae) on
Breynia
are produced concurrently with viviparae throughout summer and autumn (Hales and Carver 1976).