Today we have a story and photo contribution about weevils by Athayde Tonhasca Júnior. Athayde’s captions are indented, and you can enlarge the photos by clicking on them.
See no weevil, hear no weevil
As the story goes, J.B.S. Haldane (1892-1964), British/Indian geneticist, evolutionary biologist and mathematician, found himself in the company of a group of theologians. On being asked what one could learn about The Creator from studying his creation, the atheist Haldane is said to have answered ‘an inordinate fondness for beetles.’ Haldane may have said something like that, and indeed a Great Architect of the Universe would have had to be partial to the order Coleoptera. With nearly 400,000 known species, beetles lead the biodiversity table, making up about 25% of all known animal species. But if the Almighty Creator liked beetles, he was especially fond of weevils (superfamily Curculionoidea): there are over 97,000 described species, of which 76,761 are snout beetles (family Curculionidae) (Global Biodiversity Information Facility). But we know these figures are gross underestimates because in poorly studied areas, i.e., most of the world, the majority of weevil specimens collected are members of unknown species.
A circular tree of life for some described eukaryote groups (all organisms except bacteria and bacteria-like Archaea). Insects – in the left column – make up about 63% of the total [JAC: weevils are the black bar]. Vertebrates, together with other deuterostomes (animals for which the anus is formed before the mouth during embryonic development) are a mere ‘etcetera’ in the big scheme of life. Their biodiversity is comparable to weevils’ © Adam Dent, Wikimedia Commons:
Weevils are found practically everywhere, and almost all of them are plant eaters. They feed on plants from any terrestrial or freshwater habitats and on a range of tissues: roots, stems, phloem, fruits, flowers or seeds. Many species are among the most damaging pests of stored grain, field crops, orchards, ornamental plants and commercial forests. Weevils’ destructive potential can’t be overestimated. The boll weevil (Anthonomus grandis) wrecked the American cotton industry in the 1920s and 30s, then invaded South America in the 80s causing further mayhem. In the US, the Southern pine beetle (Dendroctonus frontalis) is able to wipe out thousands of hectares of pine in less than two years, while grain weevils (Sitophilus spp.) can completely destroy rice, maize, wheat, oats, and many other products stored in silos around the world. You may have had your own experience with weevils infesting a bag of flour or a box of pasta in your pantry.
Rice weevils (S. oryzae), a pest of stored grains and cereal products. Some weevils don’t have the long snout characteristic of the group, and not all long-snouted beetles are weevils © CSIRO, Wikimedia Commons.
Considering weevils’ charge sheet, we would be tempted to dump the lot in the ‘creepy crawlers’ category. But that would be hasty and unjustified. Only a tiny minority of weevils are harmful, while the great majority contribute to the functioning of ecosystems. One way they do this is by pollinating a range of plants.
Cantharophily (from the Greek word kántharos for beetle), or pollination by beetles, is not well understood or researched, despite being one of the first pollinating systems in the evolutionary history of flowering plants. With time, bees, flies and moths became the main pollinators, but many plants, especially of ancient lineages such as magnolias (Magnoliaceae), retained cantharophily. Some custard apple-related plants (Annonaceae), arums (Araceae), palms (Arecaceae) and orchids (Orchidaceae) are also pollinated by beetles.
Most beetles don’t handle pollination skilfully and gently: they plough through flowers, gobbling down nectar, pollen or petals, defecating as they go, often spilling more pollen than they eat – that’s why they are called ‘mess and soil’ pollinators. During these raids, beetles become contaminated with pollen grains, which are deposited on the next plant they visit. Weevils, however, have a more intimate and nuanced rapport with their hosts. They lay their eggs on the flowers, where their larvae will grow and mature by feeding on pollen, ovules, or other floral parts. By hosting weevils during a significant portion of their lives, plants are almost guaranteed being pollinated for the price of a fraction of their reproductive parts.
This type of mutualistic relationship is known as brood-site pollination or nursery pollination and it has been reported dozens of times for different groups of insects, mostly in the tropics; the interactions between figs and wasps and between yuccas and moths are two of the better known examples. In the case of weevils, hundreds of species have coevolved brood-site associations with a range of plants, but mostly with palms (family Arecaceae).
Pupa (A), egg (B) and larvae (C-F) of weevils growing in different inflorescence parts of palms. Credits: A, F: J. Haran, B-E: B. de Medeiros © Haran et al., 2023:
One instance of weevil-palm mutualism has particular relevance for its ecological and economic implications: the pollination of African oil palm (Elaeis guineensis) by the African oil palm weevil (Elaeidobius kamerunicus). Male weevils feed on the palm’s flowers and pollen, while females oviposit in the flower structures, in which the larvae feed and develop. You can watch the weevils in action.
When oil palm growers around the world, but mostly in Indonesia and Malaysia (the leading producers and exporters of palm oil), began to import the pollinating weevil in 1981, the industry changed radically. The beetle adapted well to its new habitats and boosted African oil palm pollination, which resulted in sharp increases in production, revenue and applications; palm oil made its way into margarines, chocolates, baked products, cooking oils, soap, detergents, cosmetics – you name it. The “million dollar weevil” had been found (Robins, 2021).
A female African oil palm weevil, and weevils clustering on palm flowers © Ken Walker, Museum Victoria, CABI (L) and Susenoqurnia, Wikimedia Commons:
But as sociologist Robert K. Merton warned us, purposeful actions are bound to have multiple outcomes, some of them unanticipated. This law of unintended consequences (flippantly identified as Murphy’s Law) suited the case of the million dollar weevil to a T. Large-scale oil palm production resulted in massive deforestation that is destroying the habitats of large numbers of plant and animal species, and increased levels of erosion and pollution.
Fortunately, the introduced African oil palm weevil is an isolated case of ecological mishap. All other known examples of beetle brood-site pollination are mutualisms that help maintain biodiversity. Seres & Ramirez (1995) estimated that more than 45% of palms and herbs in some cloud forests are beetle‐pollinated, and Haran et al. (2023) have recorded at least 600 cases or suspected cases of palm-weevil interactions: the true number is likely to be much larger. We have the vaguest understanding of the pollinating services played by these weevils, but it mustn’t be something to sniff at considering that the number of Curculionidae species alone is almost four times bigger than the number of bee species (~20,000).
Palms and many other types of plant pollinated by weevils are sources of food, building materials, cosmetics and medicines; a good portion of those products are consumed locally or sold abroad, generating much needed income to developing countries. Not so bad for those maligned big-conked characters.
According to biblical sources, Noah’s ark had ~42,500 m3 of available space, the equivalent of 570 standard railroad stock cars. We can deduce that Noah’s ship was not a run-of-the-mill zoo because most species competing for a berth comprised parasites and weevils. Art by Simon de Myle, 1570. Wikimedia Commons: