Ten facts about aphids
I'm pleased to present another post in the 'ten facts' series here on Expiscor. This one comes from the all-around fabulous entomologist and aphidologist Dr. Simon Leather. You can find Simon's Blog here.
1. Aphids are true bugs, Hemiptera. When I was an undergraduate they were in the sub-order Homoptera, nowadays they are classified in the sub-order Sternorrhyncha.
2. As true bugs they are characterised by having their mandibles and maxillae sheathed within a modified labium, the rostrum. Their mouthparts are somewhat akin to a hypodermic needle, and are known as stylets. These are capable of piercing plant tissue and are used to remove the plant sap upon which aphids feed. Many people refer to these as piercing and sucking mouthparts, but aphids, as they feed mainly on the phloem which is under pressure, actually spend most of their time regulating the flow of liquid into them, otherwise they would explode.
3. Aphids, except just prior to overwintering, give birth to live young (viviparity), and without the need of a male (asexual reproduction /parthenogenesis). Thus for most of the time when you look at an aphid, you are looking at one member of a clone i.e her sister-self-daughter. Not only that, but you are looking at not only the aphid in front of your eyes, but at her daughters and her daughter’s daughters, all of which are neatly lined up in tidy rows within the ovarioles of their respective mothers. You could imagine them like a set of Russian dolls.
4. Aphids are thus highly fecund and Thomas Henry Huxley (1858) famously said “In a season the potential descendants of one female aphid contain more substance than 500 million stout men“. More recently Richard Harrington (1984) of Rothamsted Research Station, England, said “In a year aphids could form a layer 149 km deep over the surface of the earth. Thank God for limited resources and natural enemies".
5. There is a common misconception aided and abetted by their common name of greenfly and blackfly, that aphids are either green or black. Although many aphids are indeed green, many are other colours. In fact it is possible, with a little imagination, to construct an aphid rainbow.
6. Aphids can have very complex life cycles. There are basically two types of aphid life-cycles, non-host alternating (autoecious, monoecious) and host alternating (heteroecious). Autoecious aphids spend their entire life cycle in association with one plant species (or group of related plant species), whereas heteroecious aphids divide their time between two very different species of host plant, usually a tree species (the primary host) on which they overwinter, and an herbaceous plant species (the secondary host) on which they spend their summer.
7. Approximately 10% of aphid species are heteroecious. The ancestral aphid life cycle is thought to have been winged, egg laying and autoecious on a woody host plant almost certainly conifers and the oldest families of woody angiosperms e.g. Salicaceae (Mordwilko, 1928; Moran, 1992).
8. Aphid life cycles can also be described as holocyclic, in which cyclical parthenogenesis occurs, with aphids reproducing sexually in the autumn of temperate regions and parthenogenetically during spring and summer. Some aphids are anholocyclic where the clone is entirely asexual reproducing by parthenogenesis throughout the year. This is often seen in locations where winter conditions are mild, in the tropics for example or as a bit of an oddity around hot-springs in Iceland.
9. Although aphids typically feed on plants, they have been known to act cannibalistically. Banks et al., 1968, in which the authors described not only incidents of aphids acting as cannibals and feeding on each other, but also of aphids acting as predators and feeding on the eggs of ladybirds and lacewings, a reverse of the normal situation. They also described an even more interesting report of what might have been biological control of hop feeding mites by aphids. One of the authors also reported seeing many incidents of cannibalism by the aphid Megoura viciae, during some of his experiments. They also noted that there was a report of an aphid species that was capable of causing swellings and rashes on people in what is now Taiwan. I tracked the paper down (Takahashi, 1930) and had it translated by a PhD student and the paper was indeed entitled An aphid that bites people. Apparently the gall forming aphid, Ceratoglyphnia (Astero) styracicola is well known to attack people who stand or sit underneath the snowbell tree Styrax suberifolium,. The result of being ‘bitten’ by the aphid is a red swelling that disappears after an hour or so, but leaves a very itchy rash that can persist for two to three days. In fact this phenomenon is so common that people avoid passing underneath infested trees. More recently my research group has described cannibalistic behaviour in the pea aphid, Acyrthosiphon pisum (Cooper, et al., 2014) This appears to be in response to poor food quality.
10. Some aphids exhibit a form of eusociality and have what could be termed soldier morphs. Pemphigine aphids are renowned for their gall forming activities, many of which are formed along the leaf petiole. The position of the gall along the petiole is quite important, the closer the gall is to the base of a leaf the better the quality of the food that the gall inhabitants experience. There is thus a premium to be gained by either being first to form a gall, or to usurp the aphid stem mother that got there first (Whitham , 1979). Newly emerged aphids (Pemphigus betae) fight each other, for the best position on the petiole . Incredibly, these fights can last for up to two days.
In another related species, Epipemphigus niisimae, where galls have already been formed, the aphids fight to take over already formed galls. These fights can actually result in the death of the loser (Aoki & Makino, 1982).
Aoki, S. &Makino, S. (1982). Gall usurpation and lethal fighting among fundatrices of the aphid Epipemphigus niisimae. Kontyu 50, 365-376.
Whitham, T. G. (1979). Territorial behaviour of Pemphigus gall aphids. Nature 279, 324-325.