Paying Attention to Aphids by João P. R. Joaquim
The first instalment in our project blog, where From Collection to Cultivation members share research insights, snippets, and ideas.
Most histories where insects have a significant role gravitate towards those critters that people either love or hate. But what about one that is often ignored—the aphid? As we will see, this came about indirectly as a result of my interest in the history of virus research.
A while ago, I was at a social sciences seminar on entomology and the convenor asked everyone to indicate a favourite insect, as in an insect that people had academic interest in: The hated tropical-disease-transmitting mosquito and the lovable honey-making bee were the most mentioned. This can be confirmed by having a look at the bibliography on this topic, even though there are some outliers and finding works on other insects is possible. The insects we tend to pay attention to are seen as public health threats (such as the flea) or agricultural pests (the Colorado potato beetle), are used in scientific research (fruit flies) or have particular cultural significance (butterflies).
Nevertheless, if one looks close enough, it becomes clear that despite insects being near omnipresent in human history, their impact is often underestimated. As noted by the anthropologist Hugh Raffles, even though “tiny insects”, such as “the aphids, the thrips, the microlepidoptera, the smallest of beetles, and the smallest of parasitic wasps” make up most of the known species and individuals among the class Insecta, little can be found about them outside of specialised entomological publications.[1] Insects are small and, in increasingly aseptic, urbanised and agriculturally-oblivious societies, often unfamiliar. Even some egregious agricultural pests—creatures that negatively impact crops at an economically significant scale—have managed to pass mostly unnoticed. Outside of farms, laboratories, and field stations, agricultural pests appear to be far removed from our daily lives. So why should we pay more attention to insects like aphids?
For one, they are a constant presence in our lives. Have you noticed that sometimes the pavement or any object left under certain trees gets sticky in spring? That is likely to be honeydew, excreted by aphids whilst they feed on sugary sap sucked from the vascular tissue of plants. Interestingly, this led to aphids developing a mutualistic relationship with certain species of ants which protect them from predators in exchange for the honeydew “milked” from them.
Aphids, also known as greenfly or plant lice, are no strangers to gardeners, who might be used to washing their rose bushes with soapy water to try limiting the depredations of these minute insects. Allotment keepers have most likely already encountered them. I know I have—last summer a plot of broad beans I was helping to attend to succumbed to the blackfly, another name by which aphids are known. Overall, Aphididae are a hardy but unassuming taxonomic family of sap-sucking insects—only a couple of millimetres long—that can be found all around the world and varies considerably in their colouration and even morphology.
Some species of aphids have followed human crops wherever they were taken and are now cosmopolitan. Theirfeeding habits encompass a wide variety of plants and they have proven themselves capable of prospering in agricultural landscapes. For example, aphids can overwinter in a variety of host plants, and, in colder climates, benefit from the warmth provided by human-made structures, e.g. glasshouses. Theyare also highly prolific, and their life cycle is complex with several generations per year parasitizing a succession of hosts (including food crops, ornamental garden plants, and weeds) and adopting changing reproductive strategies.
Most aphids you might come across are sedentary wingless females; they are asexual, breeding through viviparous parthenogenesis. That is to say, they give birth to small clones of themselves. Furthermore, aphids can produce telescopic generations, that is, yet unborn insects might already be pregnant. However, when resources become scarce and/or winter starts approaching, flying individuals, both male and female, are born. These can migrate over long distances and lay eggs capable of surviving low temperatures. To make matters worse for agriculturalists, these insects are believed to be one of the most notorious vectors of plant viruses, compounding their impact on crop health and agricultural productivity. As Kenneth M. Smith, an entomologist and early plant virus researcher, put it: “They are sluggish, slow-moving creatures, but are exceedingly prolific and constitute some of the worst insect pests of plants.”[2]
Aphids have benefited from the expansion of monoculture farming and the mass use of chemical insecticides: the former provides them with vast extensions of alternative plant hosts to parasitize and the latter disproportionately affects their natural enemies, ladybugs and parasitic wasps (which constitute key elements in biological control programs). Aphid populations grow very rapidly which allows them to both outnumber their predators and adapt by developing resistance to human-made pesticides. Significantly, research into new aphid control methods has never really stopped. In several ways, they are the plant world’s equivalent to the arbovirus-transmitting mosquito.
Unlike ants, humans appear to have failed to develop a commensal relationship with aphids. However, as I hope to demonstrate in my research, aphids came to occupy an important role in the study of viruses in the early twentieth century precisely because of the characteristics that make them agricultural pests. Hardy, prolific, unfussy eaters, and egregious vectors of plant viruses, aphids proved ideally adapted to the laboratories and glasshouses of virus researchers. At a time when viruses could only be studied through their means of transmission and the symptoms caused on their hosts, aphids became important tools in the arsenal of agricultural scientists like Smith.
Less charismatic than mosquitos or bees, aphids have nonetheless impacted our lives significantly, showcasing the challenges and perhaps the folly of attempting to bend nature to our needs. Moreover, they have unwittingly provided a bridge to the ultramicroscopic world of viruses, the importance of which is quite pertinent at the moment. Perhaps, we should follow the suggestion of the Royal Horticultural Society and “where possible tolerate populations of aphids”—as they have already proved in different ways to be more than mere annoyances.[3]
You can find out more about João's research on plant viruses at his research page here.