Today we have another photo-plus-text contribution from Athayde Tonhasca Júnior; the subject is mangoes, my favorite fruit (and flies, my favorite group of insects). Athayde’s text is indented, and you can click on the photos to enlarge them.

The king and its flies

Germany has its Pumpkin Festival, Canada celebrates a Cranberry Festival, Spaniards go wild hurling over-ripe tomatoes at each other at the Tomato Festival, while Italians savour their winemaking heritage during the Marino Grape Festival. But among the many fruit- and produce-themed events around the world, few have the cultural magnitude of The International Mango Festival, held annually in Delhi.

Delhi’s mango festival: activities include mango eating competitions, mango quizzes and slogan-writing, mango carving, mango tasting and varieties contests, dances, plays and crafts © India’s Ministry of Tourism.

Mango (Mangifera indica) has been a cultural and religious symbol in India for millennia: grown for over 4,000 years, its earliest references date back to around 2,000 BC from ancient texts and scriptures. The fruit is associated with fertility, prosperity and devotion in Hindu and Buddhist mythologies and traditions. Mangoes symbolise the arrival of summer, appearing in folk songs, literature and art, and are used in religious ceremonies and offerings to the gods. When summer comes, Indians give mangoes to family, friends, customers and employees. The fruit’s flavours, juiciness and texture make it an effective tool for diplomatic relations: mangoes have been routinely offered to foreign dignitaries and were sent as gifts for the coronation of George VI.

The mango is more than an Indian icon: it is one of the most important fruits in tropical and subtropical areas around the world. Mangoes are a main source of vitamin A in Africa and Asia, and the tree’s bark and leaves have been used in folk remedies for centuries. The fruit is mainly eaten in natura, green or ripe, but is also liberally used in chutneys, pickles, curries, preserves, juices, ice-creams and a variety of dishes throughout Asia and Central and South America. Mangoes are grown commercially in more than 100 countries, and 65 of them produce over 1,000 million tonnes each a year. And there’s no problem selling all those fruits: mangoes are rapidly gaining in popularity in temperate countries, so demand is increasing. The cultural, nutritional and economic importance of the mango more than justify its title of ‘the king of fruits’.

The king of fruits. Mangoes sold in Britain don’t do justice to the fruit’s flavours © Obsidian Soul, Wikimedia Commons:

Mango trees produce panicles (branched inflorescences) bearing tiny flowers – and lots of them. A mature tree may have 200 to 3,000 panicles, each with 500 to 10,000 flowers. This abundance may suggest ample opportunities for pollination, but that’s not so. Depending on growing conditions and crop variety, 30 to 80% of flowers are staminate, that is, they lack functional pistils. These flowers are functionally male, therefore incapable of being fertilized. The remaining fertile flowers are vulnerable to a range of environmental stresses such as excessive rain and extremes of temperature that prevent fertilisation. To make things worse, each flower produces little nectar, relatively few pollen grains (200-300), and its stigma (the part that receives the pollen) is too small to be of great efficiency. As a result, up to 60% of the flowers receive no pollen, and a panicle may produce up to three fruits at most.

A mango panicle © Delince, Wikimedia Commons:

A single mango flower is not particularly rewarding, but massive numbers of them entice lots of non-specialised visitors. A range of flies, bees, wasps, butterflies, moths, beetles, ants, bugs and bats drop by for a small sip of nectar from each flower. By hopping from flower to flower, visitors greatly increase the chances of cross pollination – although the wind also plays a part.

Among all the flower visitors, one group makes up some of most efficient pollinators of mango varieties grown around the world: flies, especially blowflies, carrion flies, bluebottles (family Calliphoridae), flesh flies (family Sarcophagidae), hover flies (family Syrphidae), and the house fly (Musca domestica). Except for hover flies, they are not seen in a good light by the public. That’s understandable, since most of what we know about them relates to their roles as agricultural pests and vectors of human and animal diseases. But that’s a narrow take on their comings and goings. These flies, often categorised as “filth flies”, are enormously important as decomposers and recyclers, and are vital for food chains: numerous birds, bats and fish depend on them. Another role is becoming increasingly understood: their contribution to myiophily (or myophily), that is, pollination by flies (Orford et al., 2015).

The unappealingly named oriental latrine fly (Chrysomya megacephala) is an important mango pollinator © portioid, iNaturalist:

Blowflies, flesh flies and the like are relatively large and their bodies are covered with ‘hairs’ (setae), which are important pollen-carrying structures. These flies are abundant and persistent flower visitors throughout the blooming season, all desirable qualities for efficient pollination. Besides mango, blow flies, flesh flies and the house fly are known or suspected to pollinate avocado, blueberry, Brussels sprout, carrot, leek, macadamia, onion and strawberry (Cook et al., 2020). The common greenbottle (Lucilia sericata) and the bluebottle (Calliphora vomitoria) are reared commercially for the pollination of seed crops and vegetable crops, respectively (L. sericata is also reared for medical uses: because their maggots preferentially eat dead tissue, they have been used for the treatment of diabetic ulcers, bedsores and other chronic wounds).

A fly with pollen attached to its back © ninfaj, Maryland Agronomy News:

Mango farmers in Northern Australia hold blow flies in such esteem that some growers have installed ‘stink stations’ in their orchards, a practice also used by avocado farmers in Peru. Each station consists of a plastic container filled with fish or chicken carcasses, a concoction guaranteed to attract flies. It’s not clear whether these contraptions improve yields (Finch et al., 2023), but at any rate, farmers see foul-smelling orchards as a small price to pay for the possibility of bumper crops of juicy, fragrant and profitable mangoes.

‘Stink stations’ used by mango growers in the Northern Territory, Australia © Finch et al., 2023:

The mango is a case study of the ‘other’ pollinators, that is, those outside the better known and celebrated club of bees, hover flies and moths. We may be unenthusiastic about flies that are the happiest on carrion and dung, but that’s a reflection of our aesthetic prejudices. Farmers around the world who deal with the mango’s finicky floral biology are very grateful for those unloved insects that help them produce better and more of the king of fruits.

The Guimaras Mango Festival in the Philippines wouldn’t be so lavish without the contribution of some flies of ill repute © Ranieljosecastaneda, Wikimedia Commons: