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Today sees the return of Athayde Tonhasca Júnior, who has a photo-and-text story about aphids. His text is indented, and you can enlarge the photos by clicking on them.

Pollinators’ emergency rations

If you are in Britain and have been gardening, visited a park, or gone for a hike in the countryside during the recent hot and sunny days, you may have been mobbed by tiny, greenish creatures. They don’t bite or swarm around; they seem to be minding their own business, but you can’t help bumping into them because they are all over the place. These are aphids (aka greenflies, blackflies or plant lice), soft-bodied insects in the family Aphididae (order Hemiptera). They are usually green, some are black, pink or mottled. If you have sharp eyes and examine one of them, you will notice ‘exhaust pipes’ sticking out from the backside of their pear-shaped bodies. These are cornicles, which exude a defensive fluid and alarm pheromones.

An aphid about to release fluid from its cornicles © Sanjay Acharya, Wikimedia Commons:

There are some 5,000 aphid species worldwide, but while most animal and plant groups reach greater diversity in the tropics, aphids are primarily temperate insects. They are all sap feeders: they insert their mouthparts into phloem (plant vessels) from leaves, buds, stems or roots and suck out the sap. Around 400 species are agricultural, horticultural and forestry pests because of the direct harm caused by loss of sap and especially the transmission of a range of pathogenic viruses.

Black bean aphids (Aphis fabae) can be a pest of more than 200 species of cultivated and wild plants © Alvesgaspar, Wikimedia Commons:

Aphids have complex and outlandish life cycles. They reproduce asexually most of the year, with wingless females giving birth to female nymphs that are essentially mum’s clones. Viviparity (giving birth to live young instead of laying eggs) is just the start: the young’uns are born pregnant, and their embryos may already have the developing embryos of granddaughters. This Russian dolls scheme is known as telescoping generations.

When temperatures drop and daylength decreases as the season progresses, or during any other change in environmental conditions such as overcrowding, or reduced food quantity or quality, winged females (alates) are produced. They fly to another host plant and give birth to more wingless females. As the end of the summer approaches, males finally are born. Mating takes place, and the females lay eggs that will overwinter on the host plant. These patterns vary considerably depending on the species and climatic conditions and food quality, so alate aphids can be around anytime.

An aphid giving birth to a fully formed daughter © MedievalRich, Wikimedia Commons.

Aphids are aphid-making machines: each female is reproductive for up to 30 days, giving birth to 60 to 100 live nymphs. These figures add up quickly, so that aphids could theoretically build up enormous populations in no time. Botanist Charles François Antoine Morren (1807-1858) – who coined the term ‘phenology’ – estimated that one Aphis sp. had the potential to create a quintillion individuals after ten generations. Thomas Henry Huxley (1825-1895), biologist, anthropologist and ‘Darwin’s Bulldog’, had a go at estimating the outcome of this growth: I will assume that an Aphis weighs 1/1000 of a grain (1 grain ~ 64.8 mg), which is certainly vastly under the mark. A quintillion of Aphides will, on this estimate, weigh a quatrillion of grains. He is a very stout man who weighs two million grains; consequently the tenth brood alone, if all its members survive the perils to which they are exposed, contains more substance than 500,000,000 stout men—to say the least, more than the whole population of China! (Huxley, 1858).

As we are not wading in layers of aphids, these projected growth spurts evidently don’t materialise. Bad weather and natural enemies wipe out most aphids way before they become noticeably abundant. However, favourable conditions such as an unusually mild spring, like the one we just experienced, will cause a spike in their numbers. We can only imagine what may happen as warm springs become the norm on a warmer planet.

So far we haven’t seen a connection between aphids and pollinators – but there is one.

Phloem sap is rich in sugars but relatively poor in what aphids require the most – amino acids. To get the nitrogen they need from amino acids, aphids must ingest far more water and sugars than they can absorb. They solve this problem by pumping out the excess intake from their bodies in the form of a sugary solution we know as honeydew.

An aphid secreting a droplet of honeydew © Amada44, Wikimedia Commons:

Honeydew is a waste product for aphids and other phytophagous hemipterans such as scale insects (coccids), whiteflies and jumping plant lice (psyllids). But some ants not only feed on the sticky stuff but also ‘milk’ aphids by tapping them with their antennae to induce the secretion of more honeydew. Ants also protect aphids against natural enemies.

Such a nutritious resource is not ignored by other insects such as hoverflies and parasitic wasps. Bees such as the European honey bee (Apis mellifera), bumble bees (Bombus spp.) and a few solitary bees are also known to be aphids’ customers, but their relationships have been barely investigated. Honeydew is not as good as floral nectar because of lower nutritional value and possible presence of harmful secondary metabolites. But these factors were not obstacles for buff-tailed bumble bees (Bombus terrestris) seen feeding on the honeydew secreted by giant willow aphids (Tuberolachnus salignus), who in turn fed on white willow (Salix alba) in Cornwall, England. Apparently, unusually hot weather increased evaporation, concentrating honeydew’s sugars and thus making it more attractive to the bees (Cameron et al., 2019). In a chaparral habitat of Southern California, 42 species of solitary bee were recorded feeding on honeydew produced by scale insects colonising chamise (Adenostoma fasciculatum) during early spring, when there are few flowers available to pollinators (Meiners et al., 2017). Besides functioning as emergency food, honeydew could be an life-saving alternative in a warmer world with increased frequency and intensity of droughts, which reduce nectar’s volume and sugar content (Phillips et al., 2018).

A: Giant willow aphids on a branch of white willow. B: A buff-tailed bumble bee collecting honeydew from leaf litter beneath branches bearing aphids © Cameron et al., 2019.

Aphids are the top subject of queries to the Royal Horticultural Society, and one of the main reasons for gardeners reaching for the insecticide canister. But the degree of alarmism caused by these insects is unjustified, because only severe infestations are harmful. Besides, aphids are food to bats, tits, warblers and finches. They are also preyed on by spiders, ladybirds, hoverflies, lacewings and many others. Aphids do have a major role in food chains, and are also likely to have a minor but not negligible part in pollination ecology, even though details of these workings are yet to be revealed.

This tale was inspired by and dedicated to the late Professor Simon Leather, an expert on aphids and author of the excellent entomological blog Don’t Forget the Roundabouts.

A tree bumble bee (Bombus hypnorum) feeding on honeydew © Orangeaurochs, Wikimedia Commons: