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Myzocallis asclepiadisCommon milkweed aphidOn this page: Identification & Distribution Biology & Ecology Life cycle Colour & Aposematism Other aphids on the same host Damage & ControlIdentification & DistributionAll adult viviparae of Myzocallis asclepiadis are alate (the apterae referred to by Blackman, shown in BugGuide, are more likely to be oviparae - see below). The alate Myzocallis asclepiadis vivipara is very distinctive (see pictures below). The body is yellow-green or brown green with blotches of red, orange or yellow, overlain with a complex set of black markings including narrow dorsal bands. The wings are conspicuously mottled between the veins with brownish or blackish patches, with the markings not restricted to the bordering of the veins or the costal area. The antennae are pale but ringed with black. The siphunculi and areas around their bases are pale. The cauda is knobbed and the anal plate is bilobed.
First image above by permission, copyright Claude Pilon, all rights reserved. In spring the immatures (see three pictures below) are pale green, but as summer progresses an increasing proportion of immatures are marked with yellow, orange or red blotches on the dorsum, with some individuals appearing more uniformly red or orange. They have numerous long capitate hairs borne on paired spinal and marginal tubercles.
First image above copyright Kenneth Frank under a Creative Commons License. Myzocallis asclepiadis is found on the undersides of leaves of milkweed (Asclepias species) especially common milkweed (Asclepias syriaca). Sexual forms develop in October, and the population overwinters in the egg stage. Hottes & Frison (1931) note it is very common in all parts of Illinois. The common milkweed aphid is found in eastern USA and Canada. Biology & EcologyLife cycleThe overwintering eggs of Myzocallis asclepiadis hatch in spring to give the fundatrices which initiate the aphid colonies. The mature fundatrices reproduce parthenogenetically, giving birth to live nymphs which on reaching maturity also reproduce parthenogenetically, enabling a rapid increase in population size. The summer adults (= virginoparae) are always winged. It is possible that in mid-summer aestivating nymphs are produced "whose bodies are covered and fringed with minute plates giving them the appearance of a tortoise" (Zust, in pers. comm. to Beatriz Moisset on BugGuide). After aestivation, and triggered by lower temperatures, these nymphs mature into wingless oviparae (see first picture below). Both images above copyright Beatriz Moisset under a Creative Commons License. As with most Myzocallis species, the oviparae have very long capitate hairs arising from paired spinal and marginal tubercles which are borne on large dark sclerites. But unusually for Myzocallis species, the oviparae are brightly-coloured orange or yellow (regarding colour and aposematism see below). The second picture above shows the male. The wings are again mottled with brownish or blackish patches between the veins, although not to the same extent as on the alate vivipara. The male has dark transverse bands and marginal sclerites on each abdominal segment. After mating the ovipara deposits eggs on the senescing milkweed plants which hatch the following spring. Colour and AposematismThe colouring of the ovipara in autumn shown above is strongly suggestive of aposematic colouration. Järvi et al. (1981) define aposematism as the combination of distastefulness and easy recognition by potential predators. Myzocallis asclepiadis is known to sequester high levels of toxins, specifically apolar cardenolides, accumulating twice as much as either of the other two species (Aphis nerii and Aphis asclepiadis) which specialise on milkweed (Zust & Agrawal, 2009). It is generally accepted that the ease of recognition is aided by bright or conspicuous coloration of prey, and the black and orange/yellow pattern of the ovipara is similar to that used by other aphids to warn predators of their unpalatability (see for example Panaphis juglandis). If ovipara coloration is aposematic, one might expect their behaviour to differ from that of viviparae, for example by feeding and/or egg-laying on the upper side of the leaves where they can better 'advertise' their unpalatability. Such differences in behaviour have not yet been reported. We should also consider the colour of the nymphs and the alate viviparae through the year. This aphid has all winged adults, and the nymphs are very mobile, exhibiting what has been called a "run" defense, rather than the "fight" response of Aphis nerii (by aposematic warning of toxicity) or the "hide" response (via cryptic colouration) of Aphis asclepiadis. Early in the year the nymphs are pale green providing additional protection by crypsis, but later in the year nymphs of Myzocallis asclepiadis display brightly coloured yellow-orange-red blotches on the dorsum. McMartin & Malcolm (2008) found that the proportion of orange aphids in a natural population increased from none in early July to around 37% in the third week of August, which prompted them to suggest that the species may be evolving an aposematic strategy with changing day length being the primary factor determining colour form. Image above by permission, copyright Seth Ausubel, all rights reserved. Another aspect of the evolution of aposematism is gregariousness. Gregariousness is thought to enhance the effect of aposematic coloration, and McMartin & Malcolm (2008) did demonstrate that decreasing nearest neighbour distances by the end of the summer coincides with higher levels of orange aphids. It may be the case that the bright colours both of oviparae and of nymphs in late summer are aposematic. But one should also remember that late in the year leaves tend to turn multiple shades of yellow, orange and brown (see milkweed leaves turning). Hence it is possible that the changing colour of the nymphs (which have no conspicuous dark markings) is driven more by crypsis than aposematism - or, for avian predators, possibly both (they may look cryptic at a distance, but aposematic close-up). Other aphids on the same hostMyzocallis asclepiadis has been recorded on 3 species of Asclepias (Asclepias purpurascens, Asclepias syriaca, Asclepias tuberosa).
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