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Aphis asclepiadisDogwood-Milkweed AphidOn this page: Identification & Distribution Biology & Ecology Interspecific competition Ant attendance Other aphids on the same hostIdentification & DistributionAdult apterae of Aphis asclepiadis on their primary host, dogwood (Cornus), in spring, are dark yellow to green - with their siphunculi variably coloured from green to black. Apterae on the secondary hosts (mainly Asteraceae, Apiaceae and Apocynaceae) are pale green, yellowish green or deep olive green mottled with yellowish green - with black siphunculi (see first two pictures below). The abdominal dorsum usually has no dark markings anterior to the siphunculi apart from a few dark intersegmental sclerites (cf. Aphis fabae, which usually has some dark markings anterior to the siphunculi in addition to intersegmental sclerites). The third antennal segment is 1.05-1.95 (usually more than 1.3) times the length of the terminal process. The hind tibiae are pale for more than half their length (cf. Aphis nerii, which has entirely dark or dusky hind tibiae). Tergite VIII usually has 4-6 hairs (it ranges from 2-6) (cf. Aphis gossypii and Aphis spiraecola, which have 2 hairs (rarely 3) on tergite VIII). The siphunculi are 1.5-2.9 times as long as the cauda. The body length of adult apterae is 1.4-2.5 mm. Immature Aphis asclepiadis vary in colour from blue-green to yellow-green to yellow-orange, and fourth instar alatoid nymphs usually have a set of white wax spots.
Note: Aphis helianthi and Aphis carduella are both synonyms for Aphis asclepiadis.
All images above by permission, copyright Claude Pilon, all rights reserved.
The alate Aphis asclepiadis (see third picture above) has a green or yellow-green abdomen, dark intersegmental and marginal sclerites, postsiphuncular sclerites, and bands across tergites VII-VIII. Alatae have 15-40 secondary rhinaria on antennal segment III, & 0-2 on segment IV. The images below show clarified mounts of Aphis asclepiadis, aptera and alate.
Images of clarified mounts, above, copyright Brendan Wray under a Creative Commons Attribution-Noncommercial 3.0 License. Aphis asclepiadis is now known to host alternate with a sexual phase on dogwood (Cornus spp.) where in spring they twist and curl the leaves. In late spring / early summer they migrate to a wide range of summer hosts in the Asteraceae, Apiaceae and Apocynaceae. The dogwood-milkweed aphid is usually attended by ants. Aphis asclepiadis is found throughout USA and Canada, and has been introduced to South America (Brazil, Peru and Argentina). Biology & EcologyInterspecific competition / associationIn North America four species of aphids may coexist on common milkweed (Asclepias syriaca). These are: the peach-potato aphid (Myzus persicae), the oleander aphid (Aphis nerii), the dogwood-milkweed aphid (Aphis asclepiadis), and the common milkweed aphid (Myzocallis asclepiadis).
First image above: Aphis nerii, copyright Polinizador under a Creative Commons Attribution License. Those 3-4 species were thought by researchers (Smith et al. (2008); Zust & Agrawal (2015)) to provide a useful comparative group for research into the consequences of dietary specialization in relation to factors promoting coexistence, and in relation to population growth and sequestration of plant toxin. Our changed understanding of the level of dietary specialization of Aphis asclepiadis from specialist to generalist does cast some doubt on the interpretation of the studies, but comparisons between the monophagous Myzocallis asclepiadis) and the other polyphagous species remain valid. Smith et al. (2008) investigated the role of several factors in promoting coexistence of three of these aphid species. Species are thought to be able to coexist when intraspecific competition is stronger than interspecific competition whereas, when the converse is true, competitive exclusion occurs. Smith found that the activities of predators did not affect the relative levels of intra- and interspecific competition. Mutualists, however, did have an effect - ants reduced the population growth of Aphis nerii and Myzocallis asclepiadis, neither of which are attended by ants. In so doing, ants greatly reduced the negative effects of these competitors on ant-tended Aphis asclepiadis. Nevertheless, ants alone did not explain the persistence of competitively inferior Aphis asclepiadis as, even in the presence of ants, interspecific competition remained stronger than intraspecific competition. In the plant genotype experiment, both Aphis asclepiadis and Myzocallis asclepiadis were competitively inferior to Aphis nerii. Yet these effects differed among milkweed genotypes, and there were one or more plant genotypes for each aphid species where coexistence was predicted. Zust & Agrawal (2015) looked at population growth and sequestration of plant toxins for the four aphid species above on twenty genotypes of the common milkweed Asclepias syriaca. Dietary specialization in insect herbivores has long been hypothesized to predict tolerance of plant defences, with more specialized herbivores being highly tolerant of and sometimes sequestering plant secondary compounds. The chosen genotypes of milkweed consistently differed in functional traits, including concentrations of toxic cardenolides, while the diet breadths of the four aphids ranged from broadly generalized to monophagous. The researchers noted that (what were thought to be) the more generalized species (Myzus persicae & Aphis nerii) had the highest population growth rate overall, while growth rates decreased with increasing specialization. In contrast, honeydew exudation as a measure of phloem consumption increased with specialization; thus, resource-use efficiency was lower in specialist aphids. Myzus persicae & Aphis nerii grew best on genotypes with the highest plant growth rate, while the other species were not affected by plant growth. All four species contained apolar cardenolides in their bodies and excreted polar cardenolides, but only the most specialized aphid Myzocallis asclepiadis was negatively affected by increasing cardenolide concentrations of the host plant. Sequestration of cardenolides increased with diet specialization, with Myzocallis asclepiadis accumulating twice as much as any other species, perhaps explaining its susceptibility to plant cardenolides. Increased host specialization resulted in lower resource-use efficiency, increased phloem throughput and ultimately higher cardenolide sequestration. Ant attendanceIn mutualisms of ants with aphids, the ants typically feed on honeydew produced by aphids and, in turn guard and shelter aphid colonies from insect natural enemies. The ants collecting the honeydew (see picture below) do not necessarily consume the honeydew themselves. In many cases they will engage in trophallaxis - namely transfer of food through mouth-to-mouth feeding - with other members of their community, either back at the nest or on the periphery of the aphid colony.
Image above by permission, copyright Claude Pilon, all rights reserved. Nielsen et al. (2009) reported that Formica podzolica ants tending Aphis asclepiadis also protect aphid colonies from lethal fungal infections caused by an obligate aphid pathogen, Pandora neoaphidis. In field experiments, bodies of fungal-killed aphids were quickly removed from ant-tended aphid colonies. Ant workers were also able to detect infective conidia on the cuticle of living aphids and responded by either removing or grooming these aphids. Such behaviour may lead to reduced disease transmission in aphid colonies.
Image above copyright Ken Potter under a Creative Commons Attribution License. Other aphids on the same hostPrimary hostAphis asclepiadis has been found on 3 species of dogwood, Cornus (Cornus mas, Cornus sanguinea, Cornus sericea (= Cornus stolonifera)). Blackman & Eastop list 16 species of aphid as feeding on Cornus sericea worldwide, and provide formal identification keys (Show World list). Of those aphid species, Baker (2015) lists 7 as occurring in Britain (Show British list).
Secondary hosts
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