Fig wasps, evolutionary marvels

By Athayde Tonhasca

When people talk about keystone and indicator species, often what they mean is ‘my favourite species’, or ‘the important species I work with’. But one group of organisms truly deserves the label of keystone species: figs. The genus Ficus comprises over 900 species spread throughout the tropical and subtropical regions as shrubs, lianas (woody vines), or trees. Strangler trees – which don’t strangle anything – are one of the best known types of fig plants.

Many fig species produce fruit asynchronously throughout the year, so many animals have a steady supply of abundant and nutritious food. This is especially important during the dry season, when most plants do not fruit. Figs are often preferred even when other fruits are available because they are rich in calcium, a mineral usually in short supply. So figs are essential for a wide range of birds and mammals such as pigeons, toucans, parrots, macaws, bats, peccaries and monkeys. Over 1,200 vertebrate species feed on figs.

The strangler fig Ficus aurea © Forest Starr and Kim Starr, Wikipedia Creative Commons, and the diversity of fig characteristics © Lomáscolo et al. 2010. PNAS 107(33):14668-72

Figs support the diversity and functioning of ecosystems around the world, but they can only do that thanks to some tiny wasps.

Chalcid wasps are an enormous group of insects, estimated to contain over 500,000 species. Most of them are parasitoids of other insects, but a small group belonging to the family Agaonidae has one purpose in life: to get into a fig to reproduce. By engaging in fruit breaking and entering, these wasps, appropriately known as fig wasps, pollinate the fig plant. 

A female fig wasp © Robertawasp, Wikipedia Creative Commons

The mission is made immensely complicated by figs’ morphology. Botanically speaking, a fig is not a fruit but a type of inflorescence known as a syconium (from the Ancient Greek sykon, meaning ‘fig’, which originated ‘sycophant’, or ‘someone who shows a fig’; a term of curious etymology). A syconium is a fleshy, hollow receptacle containing simplified flowers or florets, and each floret will produce a fruit with seeds in it. A fig harbours dozens to thousands of florets and fruits, depending of the species. The crunchy bits of the fig we eat are not seeds but fruits.

Florets need pollination, not an easy proposition when they are bunched up and locked inside a container. So the fig wasp’s first hurdle is to get inside the fig. A female wasp does it through a hole at the bottom of the fig (the ostiole), which loosens when the fig is ready for pollination. 

Longitudinal section of a syconium. The inner wall of the hollow chamber is covered with florets, and the ostiole at the bottom is the door for female wasps © Gubin Olexander, Wikipedia Creative Commons

A receptive fig does not make life much easier for the female wasp. She has to chew her way through, pushing and squeezing, often having her wings and antennae snapped off in the process. She will find a floret, insert her long ovipositor into it and lay an egg. As she’s busy doing that, pollen grains attached to her body get rubbed off onto nearby florets, assuring their pollination. With the job done, the female wasp dies.

The ovules of florets that receive eggs will form galls in which the wasp larvae develop, while pollinated ovules turn into fruit. The adults chew their way out of the galls, males first. Sometimes they help females get out from their own florets and mate with them. Males will then chew a hole through the fig wall to let the females escape. Males stay behind: they couldn’t go anywhere, as they have no eyes and no wings. After an ephemeral life spent entirely inside a fig and marked by moments of glory such as fertilising females and setting them free, males die. 

A male (L) and a female fig wasp recently emerged from their galls. The male is using his mandibles to open a gall containing a female to let her escape and be the first to mate with her © van Noort et al. 2013. African Invertebrates 54(2): 381-400

A female collects pollen grains from intact florets or picks them up by accident before braving the world outside. She will follow the trail of chemicals released by a host plant to find another fig receptive to pollination and start the cycle again. But she must be quick: she has a few hours to three days to live, depending on the species. And to complicate things, not any fig will do. Each species of fig tree is pollinated by one or a few host-specific fig wasps, which is an outstanding case of coevolution. 

The great majority of female wasps don’t make it, but a few do: they catch rides on wind currents above the canopy to find host plants over 10 km away, farther than most pollinators. This is a remarkable achievement for such small, fragile, and short-lived insects. 

You can learn much more about figs and fig wasps at Figweb from Iziko Museums of South Africa.

Perhaps nothing exemplifies better the wonders of fig pollination than the exploits of Ceratosolen arabicus in Namibia. This wasp pollinates the African fig tree (Ficus sycomorus) along the Ugab river in the North Namib desert. This is one of the most inhospitable and remote corners of the planet, famous for its Skeleton Coast, a place of shipwrecks and marooned sailors. African fig trees occur in isolated clumps along the riverbank, but that’s not a barrier for the wasp: it covers average distances of near 90 km and a maximum of 160 km over the desert at night in search of a receptive fig. As the wasp survives for 48 h or less, this quest must be quick and efficient.

Dry Ugab riverbed, Namibia © Theseus, Wikipedia Creative Commons

How do figs and fig wasps relate to us, denizens of fig-less countries? This pollination system has a profound influence on global biodiversity and ecosystem functioning, so it affects us as well, even if indirectly. The story of figs and wasps also illustrates the capabilities, drive and hardiness of minute, easily overlooked insects that are so important for us and nature.

Lights-out, please

By Athayde Tonhasca

As the human population increases and concentrates more and more in cities, the world becomes more illuminated. Artificial light at night (ALAN) is an ever growing phenomenon because of the lighting of streets, parking lots, roads, buildings, parks, monuments, airports, stadiums – basically any manmade structure. This artificial light is scattered into the atmosphere and reflected back, particularly by clouds, creating a night-time sky luminance known as ‘sky glow’. Excessive illumination and artificial sky glow spread way beyond urbanized areas, essentially contaminating the whole landscape with light: night-time darkness is disappearing.

Sky glow over Kent © Chris Isherwood, Wikipedia Creative Commons

The Milky Way galaxy has awed civilizations and inspired many philosophical thoughts about mankind’s insignificance, our place in the big scheme of things, the fleeting nature of life, and what it’s all about. But if young Europeans or Americans are asked to share their impressions about the Milky Way, chances are the responses will be limited to a shrug or a puzzled look: the Milky Way is hidden from about 60% of Europeans and 80% of North Americans because of light pollution. When Los Angeles went through a blackout in 1994 because of an earthquake, emergency services received several calls from nervous citizens about a giant, strange, silvery cloud in the dark sky. These Angelinos were seeing the Milky Way for the first time.

The Milky Way, unseen by many © Creative Commons Attribution 3.0 Unported license

Light pollution is an ecological disturbance with multiple consequences. ALAN disrupts natural day-to-night rhythms such as singing and migration of birds, the activity period of small mammals, mating of frogs, nesting of bats and the orientation of sea turtle hatchlings. There is increasing evidence that humans are also sensitive to ALAN: it affects our circadian rhythm (the sleep–wake cycle repeated approximately every 24 hours), resulting in irregular hormone production, depression, insomnia and other maladies.

Insects couldn’t be immune to the effects of ALAN since much of their behaviour is dependent on light. We don’t know how insects see the world, but they recognize forms, detect movements and discern colours based on lighting patterns. Insects can monitor the position of the sun by the polarization of light, so they can navigate with precision. Light detection helps them to keep track of the photoperiod (day length), which is fundamental to preparing for the winter months. 

Many beetles, flies, lacewings, aphids, dragonflies, caddisflies, wasps and crickets are drawn to light, but moths’ compulsive and apparently suicidal attraction to lightbulbs or flames is the most familiar case of positive phototaxis (moving towards a light source) among insects. Moths are our better known nocturnal pollinators, so naturally their possible vulnerability to killer lights is a matter of concern.

© Fir0002, Wikipedia Creative Commons

It turns out that moths’ fatal attraction doesn’t seem to be that fatal because they are only drawn to light at relatively short distances. A few moths come to a blazing end, but most of them are beyond light’s dangerous pull. This is not to say that moths are safe from ALAN. When it’s not sufficiently dark, the production of sex pheromones and egg-laying are inhibited for some species, so that their reproduction is affected. Also, the window of time for courtship and mating can be severely reduced. Light pollution interferes with moths’ perception of colours and shapes, signals necessary for flower location. It also makes them more vulnerable to parasites and predators, either because they are easier to find, or their defence mechanisms (e.g., bat avoidance manoeuvres) are less effective in over-illuminated environments.

Light pollution disturbs many aspects of moths’ physiology and behaviour, although we can’t tell whether whole populations are being harmed: not all species respond equally, and there are many variables to be considered about the light source, such as wavelength, intensity, polarization and flicker. But from the little we know, excessive illumination can be added to the list of pressures on our moth fauna and consequently on pollination services. 

At a time of growing concern about global warming, light pollution may sound like a secondary problem. But the more scientists look into it, the more they learn that this is a serious environmental threat. And while sorting out the climatic mess will be tricky and complex, the light pollution problem is relatively easy. The first, obvious and straightforward measure is to turn off unnecessary lights. When illumination is needed, it could be dimmed, shielded or limited to specific areas such as pavements or roads.

Light designs and their impact on nearby biodiversity © AlexCairns, Wikipedia Creative Commons

Light dimming is good for the environment and for the economy too. When in 2018 the city of Tucson, USA, converted nearly 20,000 of their street lights to dimmable LED lights, the city saved £1.4m from its annual energy bill.

Preserving and protecting the night time environment is an important but neglected aspect of conservation. A darker world would benefit moths and other species, and it would be good for us as well. We could sleep better or go stargazing again. 

European artificial sky brightness on an increasing scale (black, blue, yellow, red) © Falchi et al. 2016. Science Advances 2(6) e1600377

Phoenix rising from the sand

By Athayde Tonhasca

As the waters subside in Germany and the country recovers from July’s catastrophic floods, naturalists may soon be able to evaluate the damage to one species caught in the deluge: the grey-backed mining bee (Andrena vaga). This bee is at home on river flood plains, grasslands, meadows, coastal areas, anywhere with alluvial soils – soil derived from sand and earth deposited by running water – and plenty of willows (Salix spp.) nearby.

A grey-backed mining bee © Ocrdu, Wikipedia Creative Commons

Female grey-backed mining bees dig their nests on spots of firm, sandy soil with sparse vegetation. The species is solitary, although females tend to nest close to each other in aggregations that can be thousands strong. Willows are their only source of pollen, but nectar is taken from a variety of flowers.

A grey-backed mining bee nesting aggregation © Mohra et al. 2004. Solitary Bees: conservation, rearing and management for pollination

Calamitous floods aside, to build a home on flood plains seems like a disaster waiting to happen. Water levels rise and fall, waterways change courses, river banks are washed away: riparian habitats are fragile and ephemeral. But none of this is the end of the world for grey-backed mining bees. Although floods may destroy large numbers of nests or even wipe out whole populations, these bees are well-adapted to disperse and colonise new places. In fact, fragmented populations dispersed over large areas are genetically similar, which suggests free and frequent interconnections between them.

Seven grey-backed mining bee nesting aggregations (red dots) in Germany © Mohra et al. 2004. Solitary Bees: conservation, rearing and management for pollination

Moreover, finding a new neighbourhood has a health benefit. Local populations of grey-backed mining bees grow steadily over the years, with more and more females sharing a nice nesting spot. These agglomerations do not go unnoticed by predators and parasites such as the nomad bee Nomada lathburiana. This parasite invades mining bee nests and lays an egg in the host’s brood cell; the invader’s larva emerges, kills the host’s egg or larva, then eats its provisions. A grey-backed mining bee aggregation targeted by parasites may contract by 50% in four years. But these population crashes are not all caused by natural enemies; some females just up sticks to build new nests on parasite-free sites.

The parasitic bee Nomada lathburiana © James K. Lindsey, Wikipedia Creative Commons

The grey-backed mining bee has been recorded intermittently in Britain since the 1930s, although its identification has not been confirmed until 2014. Currently this bee is confined to a few colonies is southern Britain: its populations may expand or be eliminated if nest aggregations are to be hard hit by rising waters. But even in this doomsday scenario, the grey-backed mining bee is not likely to be gone for long: wandering females in continental Europe should have no problem in crossing the English Channel and making themselves at home in Britain. This ‘here today, gone tomorrow’ lifestyle helps explain the difficulty in tracking the grey-backed mining bee and assessing its conservation status: it was labelled ‘endangered’ in 1987, ‘believed extinct’ in 1991, and ‘data deficient’ in 2020. As a species fine-tuned to transitory and unstable habitats, and highly adept at dispersing and colonising new territories, this unassuming bee takes natural disasters in stride; they are just facts of life.    

Spooked hikers and choughs chuffed to bits

By Athayde Tonhasca

During these unprecedented/strange/challenging times (pick your favourite cliche), many Britons have swapped their holidays abroad for the domestic great outdoors. This shift may help explain a spike in the number of bee swarm sightings along trails and in open spaces. In most cases, people are witnessing the comings and goings of the heather colletes (Colletes succinctus). This bee is usually found on heathlands, drier parts of moorland and coastal dunes – places with abundant heather (Calluna spp.) and heath (Erica spp.), its main sources of pollen.

A heather colletes © gailhampshire, Wikipedia Creative Commons

The heather colletes nests underground in bare or thinly-vegetated south-facing spots; sandy banks quickly warmed by the sun are particularly favoured. Each female digs a burrow, stocks it with pollen and lays an egg on the pollen mass. The larva feeds on the pollen and emerges as an adult the following year. 

Like many solitary bees, heather colletes nest close to each other, usually cheek by jowl. These aggregations can be massive: in one case, 60 to 80,000 tightly packed nests along a 100-m stretch of a river bank. Not surprisingly, these concentrations of swarming bees have prompted many a rambler to turn back or give a wide berth to the restless mob of honey bee lookalikes. Such precautions are unnecessary because these bees are harmless. They are not at all aggressive, and their stings are too weak to penetrate human skin. People who stop to admire them may catch a sight of clusters of bees rolling around. These are mating balls, comprising several males jostling furiously to mate with a female, who is hidden in the middle of the melee. As soon as a male succeeds – usually the larger one – the mating ball breaks apart. Females are monandrous, that is, they have one mate at a time, so they are not receptive to other males. 

Nest aggregations, which are common among several species of mining bees, are a bit puzzling because of the risks they represent. The abundance of provisions (pollen and nectar) stored by female bees, and so many juicy larvae and pupae in the same place are godsends to predators and parasites. 

So why do bees aggregate? It could be that adequate nesting sites are scarce: the ground has to be within certain physical specifications for secure tunnelling – the right type of soil, texture, drainage, slope and temperature – so several local bees may be attracted to the few good spots. Nesting needs help explain why many solitary bees display natal philopatry, which is the tendency to return to the site of their birth. It makes sense for a newly emerged bee to stick around: why take chances somewhere else when its place of birth ticks all the boxes? So the colony keeps growing, sometimes for decades. Aggregation may also result from having food nearby: broods have better chances of success if their mothers had easy access to pollen and nectar.

Whatever the reason, aggregations are quite handy for farmers who take advantage of the crop pollinating skills of some species such as the alkali bee (Nomia melanderi).

Alkali bee nests at the edge of an alfalfa field in America © Jim Cane, Agricultural Research Service, US Department of Agriculture

On the island of Colonsay, another species appreciates heather colletes nest aggregations: the red-billed chough (Pyrrhocorax pyrrhocorax). This bird feeds mostly on arthropods, and ants, beetles, moths and spiders are its usual prey. The choughs on Colonsay have learned to excavate heather colletes nests to get a nutritious, plentiful meal. 

A red-billed chough, a heather colletes predator © gailhampshire, Wikipedia Creative Commons

Hungry choughs are not likely to threaten heather colletes populations, considering the small number of birds on the island and their preference for farmland food such as crane flies and dung beetles; bee meals are probably opportunistic.

The heather colletes has experienced a small decline throughout Britain in the last 10 years or so for unknown reasons, but the population is still widespread and abundant in many places. They will probably carry on amazing and sometimes unintentionally startling nature ramblers for years to come.