Exuding Abundance

© Ákos Lumnitzer

Tim Low examines an overlooked but vitally important food source for Australian birds—the sticky substances that exude from our plants known as manna, lerp and honeydew. 

Before the modern age of plastics arrived, much value was placed on substances oozing from trees. Ships hauled cargoes with names that sound fanciful today: tragacanth, sandarach, caoutchouc, gamboge, gum arabic, dragon’s blood. These substances—exudates—ooze from woody plants after injury by people or animals. They found their way into medicines, cosmetics, glue, paint, varnish, rubber, and other industrial products, and even into chewing gum, gramophones and golf balls.

Australia was colonised when interest in exudates ran high. The proof resides in the name of the dominant trees: ‘gum trees’. Eucalypt secretions served well against dysentery, becoming one of our first exports, sold in London as ‘Botany Bay Kino’. Two substances that excited interest in those times are extremely important to birds. In 1855, Lieutenant-Colonel G.C. Mundy described one:

It sounds strange to English ears—a party of ladies and gentlemen strolling out in a summer’s afternoon to gather manna in the wilderness: yet more than once I was so employed in Australia. This substance is found in small pieces on the ground under the trees at certain seasons, or in hardened drops on the surfaces of the leaves; it is snowy white when fresh, but turns brown when kept like the chemist’s drug so called, is sweeter than the sweetest sugar, and softer than Gunter’s softest ice-cream.

© Dean Ingwersen

Others who sampled it were reminded of sweetened flour, sugar-plum and wedding cake icing. About 60 per cent sugar, it could be bountiful on as well as under trees when there was no rain to dissolve it. Men told of “upwards of 20 lb” procured from one tree, of “many bushels” gathered “in a short space of time,” of Aborigines collecting a pound in a quarter of an hour.

The Old Testament manna can’t be identified with certainty today, but may have been the sweet congealed fluid secreted by tamarisk trees under attack from bugs. Australian manna is soft and white, appearing after insect attack on the foliage or trunks of particular eucalypts, especially Manna Gum (Eucalyptus viminalis) and Brittle Gum (E. mannifera). The insects responsible in Australia have not been identified with certainty, and what was said in 1849 remains true today: “the question of its origin becomes of very great difficulty.” A chemist in the 1960s—Ralph Basden—could not coax it from trees despite 600 attempts that included “punching holes in leaves, scarifying twigs, cutting leaves in half, drilling a hole in the trunk or cutting a blaze in the tree.”        

The other exudate to interest birds as well as people is lerp. In 1845, a stockman stranded in mallee groves north-west of Melbourne fed happily upon it for a day or two. “Lerp is very sweet,” wrote Robert Cay afterwards, “in size and appearance like a flake of snow, it feels like matted wool, and tastes like the ice on a wedding-cake.”

In a charming piece written for the Microscopical Society of Victoria in 1880, ‘How the Lerp Crystal Palace is Built’, W. Wooster explained what lerp was. Tiny insects “in the larval state protect themselves from the sun and their enemies by building over themselves little tents, or rather crystal palaces, composed of a gummy and sugary secretion, which is exuded in a semi-liquid state from the tube at the hinder end of the body.” Lerp is modified bug excrement produced by aphid-like bugs called psyllids. It contains starch, something few animals synthesise. The word is indigenous, from the mallee lands of Victoria.

Australian lerp and manna became minor marvels of the age, depicted in detail in academic journals across Europe. There were visions of lerp industries providing sugar for the table, malt for brewing, and starch for stiffening shirts. Manna gum plantations were proposed. This is long forgotten today, but lerp and manna remain important as foods for millions of birds.

Awareness of this was late in coming. Naturalists in Europe and America never saw birds eat such things, and perhaps for this reason they escaped much notice in Australia. Lerps are usually less than six millimetres wide and difficult to see up in trees. Honeyeaters were often seen eating ‘scale’ but its importance was not divined.

© Chris Tzaros

It was left to an astute PhD student in the 1970s, David Paton, to notice that the honeyeaters he was studying were often busy among leaves or bark. This was nothing unusual, experts having decided long ago that many honeyeaters fed largely on insects. But David saw that sugar and starch were attracting these birds, not meals on legs. In one manna gum stand he saw six species take manna. Some were eating nothing else. He also saw honeyeaters taking honeydew, a sweet fluid secreted by tiny bugs hiding in bark cracks, on branches that were sometimes sticky to touch. In Victoria and South Australia he found exudates everywhere: “In all areas I found honeydew, manna or lerp within ten minutes of searching for them and often saw birds collect them.” A student was seeing what all before had missed

In what proved a watershed article, David announced in 1980 that manna, honeydew and lerp were important honeyeater foods, “but the significance of this has been ignored.” Today, it is widely recognised but still remains underappreciated

Lerp and honeydew exist because there is far more sugar in tree sap than amino acids, forcing sap-suckers to imbibe more sugar than they need so as to meet their protein needs. Excess sap flows from their anuses as sweet liquid, or is sculpted into lerp ‘palaces’. The lerps of value to birds (Glycaspis species) usually look like limpets, and occur singly or in clusters

Other biologists have helped join up the dots. In the 1980s another PhD student, John Woinarski, concluded that pardalotes specialise on lerps and manna. Pardalote beaks are stout for levering sweets from leaves. A flock I approached sounded like rain from all the clicks of tiny beaks. The white spots on three of the four species suggest that evolution has produced birds that wear pictures of their foods. John concluded that pardalotes, by controlling psyllid outbreaks, “play an important role in maintaining the health of the ecosystem.”

Another lerp specialist that produces a not-so-healthy outcome is the Bell Miner, which is notorious for depleting ecosystem health. With its tongue it levers lerp off leaves, usually without harming the bug beneath, which is then free to secrete another starchy cap. But because Bell Miners are so effective at repelling other birds, which might eat the psyllids, these bugs reach such high densities that trees sicken and often die. Bell Miners have been portrayed as birds that farm their food, but their farming is unsustainable.

The honeyeaters in genus Melithreptus also include lerp and manna specialists, with short beaks and unusual braced jaws designed, it seems, for levering lumps from leaves. One member of this group, the White-naped Honeyeater, is faring poorly around Adelaide from koalas defoliating local Manna Gums.

John Woinarski conducted one study in which newly-caught birds were caged for twenty minutes with lerp-infested foliage. Of the 29 species offered lerp, an amazing 22 accepted. All the small foliage birds took it, every one. Thornbills, Weebills, whistlers, pardalotes and various honeyeaters ate large amounts, while Silvereyes, fantails and gerygones took a little. Any small bird seen among gum leaves may have lerp in mind. It contains more starch than sugar, hence its broad appeal. The insects that produce it, psyllids, are the most common insects on eucalypts in southern Australia, one study found. Birds usually eat the bugs with the lerp, gaining protein as well as starch.

© Dean Ingwersen

An amazing range of Australian birds feed on lerp, not that any list has ever been compiled. As well as the small birds mentioned, many parrots eat it, and also finches, bowerbirds, trillers, currawongs, choughs, Apostlebirds, woodswallows, shrike-tits, and Crested Pigeons. Reed-warblers and Purple Swamphens will seek it up in trees. “They look pretty awkward but they can do it,” said David Paton about the swamphens he has seen.

Lerp can be especially plentiful on mallee eucalypts in Victoria and South Australia. On mallee trees in Brookfield Conservation Park I have seen clusters large enough for me to live on. The one biologist I located who has witnessed a large outbreak, Joe Benshemesh, told me of fallen lerp looking like drifts of light snow. For some months a Malleefowl he was studying obtained half its sustenance from this windfall. “Everything grows fat on lerp when they outbreak,” he told me. “It would be hard to overestimate the potential importance of such events in the ecology of mallee.” He suspects that Malleefowl raise young only in years when something like lerp provides a boost.

The River Red Gum (E. camaldulensis), Australia’s most wide-ranging tree, is another important source. David Paton has seen 30–50 lerps per leaf, enough to fill a cereal bowl in half an hour. In Australia, lerp varies in abundance by season, location and tree species, but some birds enjoy a year round supply.  

Lerp is most plentiful in Australia, but does occur elsewhere. Mopane Trees in southern Africa become infested with a lerp that looks and tastes remarkably like ours. Explorer David Livingstone saw villagers consuming it. I have trawled through the scientific literature but found only one article, from Botswana, that documented birds (starlings, doves, bulbuls, francolins) eating it. I have seen it in Kruger National Park, but baboons rather than birds were taking it as food. The Mopane Trees that produce it are deciduous, which rules out Africa having specialised lerp birds.   

Manna appears to be a food of birds only in Australia. A search of the global database, Web of Science, brings up scores of journal articles about it, but only those emanating from Australia mention birds. Honeydew is eaten elsewhere; ornithologists in the 1990s expressed surprise at finding warblers and hummingbirds eating it in Latin America, describing this as an unusual phenomenon “most often reported from Australia.” Honeydew is important in New Zealand as well, where Kakas and honeyeaters obtain it from beeches (Nothofagus). On the Honeydew Walk in Nelson Lake National Park I have watched New Zealand Bellbirds chasing Silvereyes from this food.

In Australia, mammals as well as birds take exudates. Large amounts are eaten by all the smaller gliders and also by the threatened Leadbeater’s Possum. Flying foxes like lerp. Gliders also eat wattle gum, which is usually too sticky for birds. Outside Australia, Madagascar is the one place where some mammals (lemurs) specialise on some exudates.

Why is Australia so different? In a major overview article, ‘Ecology of Australia’, Gordon Orians and Antoni Milewski portray sap and other sweet liquids as something cheap and superfluous that eucalypts can afford to waste, because nutrient-poor soils constrain their use of the sugars they produce. Where Europe and America have many aphids, Australia has larger bugs (psyllids and coccids) sucking tree sap instead, producing honeydew at higher rates. They are better than aphids at processing nutrient-poor fluid because their guts have a bypass filter to quickly remove water and sugar while keeping amino acids and other rare nutrients. Unlike aphids, they conceal themselves beneath bark or lerps, thereby escaping the drying winds that gust through flimsy eucalypt foliage. Australia’s evergreen foliage allows psyllids to feed and breed year round, and their numbers sometimes explode, creating feasting and breeding opportunities for birds. Aphid honeydew, wherever it is found, mainly feeds ants.

Honeydew is most plentiful in cold places, probably because sugar plays a role as anti-freeze in sap. On Ben Lomond in Tasmania I have seen Mountain White Gums aglitter from all the congealing honeydew catching the afternoon sun; it was sweet and thick enough to lick, behind almost every bark strip. In Snow Gum forest on Mount Wellington I watched Crescent Honeyeaters on feeding runs, dabbing their beaks at points on small branches where I could peel back bark and find minute bugs beside glints of liquid. The bugs are thought to benefit from birds removing fluid that might otherwise drown them or attract moulds. Ants remove it as well.

The raggedness of the Australian woods was something early visitors, including a travel-weary Charles Darwin, saw fit to complain about. Why did the trees shed not their leaves but their bark? We now know that trees in very poor soil invest scarce nutrients in leaves that are tough and long-lived rather than soft and deciduous. Smooth-barked eucalypts (those called ‘gums’) have been found to stay cooler during fires than eucalypts such as stringybarks that retain old bark as a shaggy (but flammable) coat, suggesting a reason why bark is often shed in such a fire prone land. Birds benefit twice over, from evergreen leaves that supply lerp year round and from honeydew bugs (and edible insects) sheltering behind slowly shedding bark.

Manna, like honeydew, is mainly found in southern Australia, and lerp is also more plentiful away from the tropics. This may reflect the eucalypt composition in each region, exudates occurring on some species rather than others. In northern Australia the harsh monsoon climate may inhibit sucking bugs.

The implications of exudates are profound. Australia has the world’s biggest and most plentiful nectar birds. Wattlebirds and lorikeets are giants compared to the sunbirds and hummingbirds found elsewhere, and exudates help sustain them. Les Christidis has observed that when studied closely most honeyeaters are found to consume some “alternative carbohydrate foods.” Reliable foods are needed when birds breed, and exudates often fill gaps in the flowering calendar. Australia has been called the land of nectar, but it is equally the land of exudates, a place where birds obtain unusual sugary and starchy treats from the dominant trees.

Tim Low is an environmental consultant and author of six books about nature and conservation. He is currently working on a book about bird ecology and evolution.