Michael Schneider

The fig and the wasp

Tall oaks from little acorns grow, but until recently no spreading Moreton Bay fig trees sprouted from the seedy fruit that local trees produce. Now, thanks to a tiny wasp—the tree’s indispensable pollinator—the situation has changed.

Written by       Photographed by Mike Brown, Arno Gasteiger and Michael Schneider

The moreton bayhe moreton bay fig tree casts a sizeable shadow. It is one of the largest and fastest growing exotic trees in New Zea­land, and some specimens just 50 years old boast canopies which would easily cover a suburban section. A Moreton Bay fig at Pahi on the Kaipara Harbour, which stands 28 metres tall and has a canopy spread of 50 metres (above), is regarded as one of the ten finest exotic trees in the country.

Until recently, these banyan-like botanical goliaths have been confined to such parks, seaside promenades and private gardens large enough to accommodate them. Their massive buttress roots, spreading out like tentacles across the ground (opposite), have served as picnic tables, playgrounds for clambering children and shady nooks for courting couples.

But something has changed in the world of the Moreton Bay fig which may see the tree spread far beyond its former manicured habitats—even becoming a dominant feature in the countryside.

The tree’s sole pollinating agent, a tiny wasp no bigger than an ant, has arrived in the country. As a result, this warm temperate Austral­ian native tree, which has been propagated in New Zealand solely from cuttings since it was introduced from Queensland and New South Wales last century, has begun to produce viable seed.

The complex relationship be­tween the giant fig tree (Ficus macrophylla) and the minute wasp (Pleistodontes froggatti) is one to marvel at. Each depends on the other for successful reproduction (an arrangement called mutualism) and the life cycle of each is attuned to the idiosyncrasies of the other.

Such dependence is by no means unique to the Moreton Bay fig, for every other species of Ficus so far studied has its own species of pollinating wasp. However, there are some 800 species of the genus Ficus worldwide—some with quite unu­sual reproductive behaviour, such as having underground flowers and fruit and it will probably take entomologists some time to identify all the pollinating agents.

How the Moreton Bay fig wasp arrived in this country is a matter of conjecture. It is possible that strong westerly winds could have carried the wasps across the Tasman. Butterflies and other insects regu­larly make the crossing, but for the fig wasp it would need to be an express trip, for adult females have a lifespan of only one to three days. Winds of sufficient strength occur, on average, on 21 days each year.

Another possibility is that the wasps arrived inside a fig brought in as a souvenir or imported by accident—in a child’s pocket, perhaps. However, the arrival of the Moreton Bay fig’s personal pollinator, by accident or otherwise, might have been predicted because a closely related wasp which pollinates the Port Jackson (or rusty-backed) 52 fig, Ficus rubiginosa, has been in the country for about 30 years.

The Port Jackson fig is an attrac­tive specimen tree found in North Island parks from Whangarei to Napier, but it lacks the impressive buttress roots of the Moreton Bay species, and does not achieve the same physical grandeur of trunk and canopy. That it has been producing viable seed over the last few decades is certain because individual plants have been turning up in places where they would never have been planted. One has grown into a sizeable tree beside the Symonds Street on-ramp to Auckland’s Southern Motorway, its roots scrambling over the basalt stone­work that flanks the road (above).

Such precarious perches are not uncommon for either Port Jackson or Moreton Bay figs, both of which naturally start their lives as epi­phytes in the rainforest canopy. The fig seeds are distributed in the droppings of fruit-eating birds. Should one germinate high up in a cupped branch axil where rain water can accumulate, the seedling will have the benefit of considerably more sunlight than is available to a seedling on the forest floor.

As the perching fig grows, some of its roots encircle the host tree for support while others reach down to the ground for nutrients. This growth form, best exemplified by the so-called “strangler figs” of the tropical rainforest, can occasionally be seen in New Zealand. In Auck­land’s Cornwall Park, several large palms have fig trees clinging to their trunks, their tap roots snaking towards the ground.

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Unlike the succulent (loillestic fig, the fruits of the Moreton Bay fig have little gustatory attraction. In this country, scant notice has been taken of the grape-sized green-and‑purple spheres which litter the ground under the trees.

Two or three years ago, however, careful observers began to notice that beneath a few trees the fallen fruit were noticeably larger, softer and deep maroon in colour. Split . open, the fruit revealed a structure and aroma similar to that of the domestic fig, and the thin layer of flesh beneath the skin had a charac­teristic figgy flavour. For the first time in this country, the figs were maturing and ripening, because the flowers had been fertilised.

The term “flower” is a little deceptive when applied to figs. You will never smell or pick a fig flower, because they develop within sack-­like structures called syconia­”inside out” inflorescences, if you will. The tiny flowers—and there may be dozens or even hundreds in each sack—are very simple structures, each facing inwards towards the dark lumen of the sack.

Once fertilised, fig flowers start to form seeds, and, as the juicy flesh surrounding the seeds develops, the whole package swells to form the familiar seed-packed fruit.

The pollinators of fig tree flowers are tiny gall wasps belong­ing to several genera of the hy­menopteran family Agaonidae. Gravid female gall wasps enter a developing syconium through a minute pore (the ostiole) at the end opposite the stem (below).

Only when the female flowers inside are in prime condition for fertilisation will the opening be large enough for a wasp to squeeze through. Even then it is a tight fit, and wasps always lose their wings and often damage their antennae as they struggle through the small aperture and haul their bodies along a narrow canal to the central cavity lined with open female flowers.

Inside the syconium there is space for the wasp to move around fairly easily, and pollen that the wasp ferried in in special pockets sticks to the stigmas of the inwardly facing female flowers.

Soon after fertilisation has occurred, the fruiting body begins to swell and the entrance pore closes. It is quite common for more than one wasp to get into the syconium, and wasps clearly have a desperate urge to enter, for invariably fruit are found with several wasps caught in the entrance pore. There they remain, crushed by the closing aperture before they could force their way into the pleasure dome that lured them so strongly.

The lure is scent. Young figs, with their entrance pores fully open and mature female flowers M. prime condition for fertilisation, release a powerful cocktail of aromas that draw the pollinators to the source and tempt them to enter the flower chamber.

The Moreton Bay fig wasp is an extraordinary insect, with an elon­gated head and a long ovipositor. The head is about 40 per cent of the whole body length and tapers strongly towards the tip, where there is a terminal hook formed from the curled ends of a pair of exceptionally long, rasp-toothed mandibles.

This hook serves as a grappling anchor. As the head is thrust for­ward, the hook engages on tissue inside the fig and anchors the head while the wasp struggles to pull its body through the constricted ostiole. Along the walls of the fig’s entrance canal there are inwardly directed scales which serve as ratchet teeth against which the insect’s mandibular grappling hook and rasp teeth engage.

Once a wasp enters the pore there is no escape, for the scales and jaw teeth allow movement in only one direction: forward into the centre of the fig.

There is no room for the legs to provide thrust, and indentations on the underside of the body (see2 electron micrograph, left) suggest that during entry the legs are laid back straight in these accommodat­ing body grooves so they do not impede forward progress.

In the context of such an exqui­sitely organised biological mission, the loss of some body parts, such as wings and antennae, should perhaps he regarded as discarding superflu­ous equipment. Just as spacecraft jettison their booster rockets once their propulsive phase is complete, so the wasp can afford to lose the wings that propelled it to its planned landing site and drop off the sensitive antennae that guided the craft to its docking point, the fig’s ostiole.

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The syconium of the Moreton Bay fig has both male and female flowers, but when the wasp enters the fig the male flowers are still developing and only the female flowers are sexually mature. The bases of these flowers are several layers deep, but their styles all reach through to the lumen to receive pollen from the wasp’s body in the first phase of a give-and-take rela­tionship between wasp and tree.

In exchange for pollinating the flowers, the gravid female wasp uses some of them as nurseries for its young, thrusting its ovipositor like a hypodermic needle down their styles to deposit a single egg into each flower’s ovary. Like other hymen­optera, fig wasps are selective about which eggs are fertilised with stored sperm as they leave the body. The fertilised ones develop into female wasps, and the remainder into males.

The wasp’s ovipositor is only long enough to reach down to the ovaries of flowers with short styles—those in the centre of the syconium—so flowers in the outer layers of the fruit develop seeds in the normal way. Most of the flowers in the inner layer are sacrificed to the wasps. Each forms gall tissue around the developing grub, which the grub then feeds on to nourish its growth.

The combined duration of the larval and pupal stages in New Zealand is unknown, and probably varies with seasonal temperatures, but development is synchronised with the opening of the fig’s male flowers—the next stage of the wasp­and-tree interaction.

Adult male wasps are the first to emerge from their pupal cases. They are small, brown, blunt-headed, wingless insects, often carrying their abdomen doubled up under the thorax and head. They will never see daylight, for their role is just to fertilise female wasps and assist them to escape from the fruit. On emerg­ing from their galls the males burrow through the fig flesh and break into galls containing females, mating with them just as they are ready to emerge as winged adults.

By this stage the fig has swollen and started to soften into a ripening fruit, and this softening makes it easier for the males to make escape tunnels for the females to reach the outer skin. The males’ burrowings are essential because the females must emerge from the fig with undamaged wings in order to fly away and search for developing figs elsewhere, and so keep the life cycle turning.

As the males tunnel, the females mill around in the centre of the fig, where they become dusted with pollen from the freshly opened male flowers. Unlike most wasps, which have hairy bodies for transporting pollen, fig wasps are almost hairless. Even if they were hirsute, pollen would be wiped from their bodies either as they burrowed through the sticky ripe flesh while escaping from their natal fig or as they squeezed through the narrow entrance passage of a developing syconium to reach its flowers. Instead, some species have a large pollen pouch and others also have a pair of pollen pockets recessed into the underside of the body just behind the head.

When the wasp is wriggling through a tight space, the forelegs are folded back into its sculpted body grooves, and in this position they neatly cover the pockets and pouch to prevent pollen being lost.

A fringe of bristles which serve as grooming combs along the edges of the second joints of the forelegs (visible in the scanning electron micrographs) and fine hairy brushes along the distal joints indicate how the wasps gather loose pollen and pack it into their storage pouches prior to leaving their natal fig for their few days of adult life.

Small holes visible in the purple skin of ripe fruit show that the females have departed.

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Figs belong to the mulberry family, Moraceae. Though found worldwide, most figs have a tropical or subtropical distribution, and many would struggle to survive in New Zealand, where temperatures fall below their usual range of tolerance. The natural southern limit of the hardiest species extends to about the 38th parallel, the same latitudinal line below which such trees as kauri, mangroves and taraire are seldom found.

A few of the hardier species have been introduced to this country as ornamental specimen trees in parks, gardens and public reserves, but without their appropriate pollinating wasps they cannot set viable seed and can be propagated here only from cuttings. Less common than the Moreton Bay and Port Jackson figs, but with a similar growth form, are Ficus microcai pa, Fiats superba and Fiats obliqua.

In large spreading species like the Moreton Bay fig—referred to as “banyan” types—the main horizon­tal branches produce aerial roots which, on reaching the ground, thicken to become supporting props. In the banyan itself, Fiats betighaknsis, these roots form veritable columns, and the tree grows like a huge pergola. Indian merchants used to carry out their business in the shade of these trees, and the word banyan actually derives from the Sanskrit for merchant.

The world’s largest banyan, growing in Sri Lanka, has 350 large trunks and more than 3000 small ones.

The related bo or bodhi tree, Fiats religiosa, is India’s sacred tree. It was while meditating in the shade of a bo that the Buddha achieved enlightenment.

When any Ficus—indeed, any mulberry—is broken or cut, white sticky sap oozes out, which dries to form a rubbery protective skin over the wound. One east Asian species, aptly named Ficus elastica, became the original Indian rubber tree, from which the sap or latex was tapped for industrial processing. It was subse­quently discovered that the South American tree Hevea brasiliensis, belonging to the spurge family, was a superior source of latex, and that tree is now the sole commercial source.

It was once common to find small pot-bound specimens of the glossy oval-leafed India rubber tree in many homes and offices. Fashion has seen this species displaced by tub specimens of the larger-leafed fiddle-leaf fig, Ficus lyrata, and by Ficus benjamina, which has small wavy-edged leaves. The latter has become popular in the foyers of office buildings and the atria of shopping malls.

Figs are popular indoor plants because of their slow growth and the tenacity with which they cling to life in the face of unfavourable condi­tions (poor light, infrequent water­ing and the drying effects of air conditioning).

In a small number of public and private gardens around Auckland can be found arresting specimens of a deciduous fig, Ficus auriculata, which has leaves the size of dinner plates and doughnut-sized figs sprouting directly from the trunk and main branches (below). The tree is a spectacular ornamental speci­men, for, when new foliage breaks from the bare branches, it is tinted raspberry pink for the first month.

Less easily recognised as figs is the creeper Ficus pumila, which has small, crinkly leaves and is often found covering walls of concrete, brick and stonework—for example, the toilet block in Auckland’s Cornwall Park (opposite page, upper photographs).

To anchor its stems securely to these hard surfaces the plant uses sticky secretions of sap exuded from the flattened tips of short specialised roots which spring from the stems. Occasionally the plant produces pale purplish-brown figs which look like elongated versions of the edible fig.

The domestic fig, Ficus carica, is thought to be indigenous to Persia, Asia Minor and Syria. Through trade and travel it spread Mediterranean countries, and was brought to ancient Britain by the Romans, or possibly earlier visitors. Subsequently, it has been cultivated in most of the world’s subtropical and warm temperate countries.

Curiously, while it is the most familiar fig species, it is also the least typical in having palmate leaves—like a maple—rather than the smooth, simple ovate leaves that are typical of so many species of Ficus (opposite, below).

Early cultivators knew that fertilisation of the flowers inside the edible fig caused the fruit to become fleshy and ripen, and was related to tiny insects entering through the terminal orifice. However, it is clear that they did not understand the mechanism fully, because they used artificial “fertilisation” procedures—such as twirling a feather in the ostium—which would have been of little use because of the long delay between the opening of the female and male flowers.

Perhaps our ancestors attempted such methods in order to overcome a horticultural quandary: to become plump and succulent, figs need to be pollinated, but natural pollination by fig wasps also results in gall forma­tion around the ovaries in which the insects lay their eggs. The galls are hard and gritty, and the fact that they contain insects spoils the palatability of the fruit (to say nothing of its desirability).

The problem was solved by the chance appearance of a new cultivar in which fruit developed without the flowers being pollinated—a phe­nomenon called parthenocarpy. Such figs still contain seeds, but they are not viable. It is not recorded when this sport was discovered, but all modern domestic figs—including the varieties available in New Zealand—are parthenocarpic cultivars presumably developed from this stock.

So when you bite into a sweet, ripe fig this summer, you need have no fear that among the seeds you crunch some will be moving. But if your preference is dried figs . .. well, let the vegetarian beware. For even parthenocarpic fig cultivars still attract fig wasps. In the regions of the Middle East where figs are dried for export, wild figs are still fertilised by their own species of fig wasp, and although in the parthenocarpic varieties the pistils of the flowers are longer than the wasp’s ovipositor (so she cannot lay her eggs in the required spot at the base of the pistil) she can still get into the developing fig—but is destined to remain entombed therein.

For the Moreton Bay fig wasp, though, a kinder fate is in store. Released from her sarcophagus by the burrowing male, she will live to fly another day, pollinate another flower and play her small but vital part in the promise of great trees to come.