Weevil researcher Dr Chris Lyal elucidates on the darker side of weevil life-histories and they are not as friendly as you may have imagined…
Weevils are perhaps the most inoffensive of beetles – well, unless you’re a farmer, forester or horticulturalist, in which case you may take a rather dimmer view of them, since some species of this huge group are major plant pests. However, to focus on the animals themselves and ignore inconvenient economics, they seem to look out at the world through immense soulful eyes, and trundle rather erratically along like one of those clockwork plastic children’s toys with slightly more legs than are truly manageable.
As herbivores, they spend their lives up to their antennae in plants, nibbling at leaves and flowers, buds and roots. They may have a long projecting rostrum at the front of their heads, but they do not behave like horse-flies, bed-bugs or any of the rest of the blood-sucking brigade and try and force it through your skin and suck out your life-juices. Adult weevils are covered in scales and sometimes very brightly coloured, but they have a previous existence as a larva, chomping their vegetarian way inside fruit, stems, leaves or roots. Larvae are fat, white, legless comma-shaped beasts, almost blind and apparently interested only in food.
Again, not one of nature’s bad boys (unless, as I said, you are concerned with keeping plants alive, in which case I may be irritating you by now). However, not all is as it seems. Some weevils, it turns out, have a darker side to their nature. Some are killers. Some are cannibals.
Our first instance is rather sad, albeit with a shocking element. Most weevil females drill a hole in the host plant using their rostrum – the projection of the front of the head at the end of which is the mouth. They then turn around and carefully lay their egg in the bottom of the hole they have produced. This process, to the observer, can be tense – how does the female know where the hole is? Will she find it, probing blindly with her ovipositor?
Not always, it turns out. Several species of European Ceutorhynchine weevils, including the stem cabbage weevil Ceutorhynchus napi, occasionally lay their eggs too soon, and miss their carefully drilled hole. The larva would not survive, were the egg to even hatch. Pragmatically (though not sentimentally – but one can take anthropomorphism too far), rather than waste the resource the female will eat the egg, and therefore be able to use the nutrition to develop more eggs .
More deliberate is the elegant Ludovix fasciatus, which lays its eggs in the stems of the water hyacinth, Eichornia crassipes. This is no simple placement in the plant tissue – the female probes with her long slender rostrum until she finds eggs of the grasshopper Cornops, already laid inside the stem. On finding a clutch, she inserts her rostrum into one and, rather like drinking milk from a coconut through a straw, drains the contents. She then lays a single egg and the larva, when it hatches, eats the rest .
Even more extreme is Anthribus nebulosus, another European weevil, which has taken on some of the characteristics of a parasitoid. In this case the female searches out scale insects on coniferous trees, just after the scale produces eggs. The beetle chews a hole in the scale and lays an egg in the ovisac; when the larva hatches it stays where it is, feeding on the scale’s eggs and nymphs before pupating in the same place. When the adults emerge they feed on the remains of the scale, with the occasional pause to imbibe some honeydew as an accompaniment.
Scale insects are not the only Hemiptera to suffer at the mouths of weevils. Researchers in a lab in New Zealand a few years ago, studying resistance of grasses to the pest weevil Listronotus bonariensis, noticed that aphids accidentally included on the grass vanished during the experiments. Closer examination revealed the adult weevils, if they encountered an aphid as they walked across the plant, would ‘grasp and rupture the aphid with the mandibles, followed by mastication and ingestion’ . Nice
The instance that led me to this curculionoid underworld, however, is more extreme, and that was a paper that came out recently on a seed-feeding weevil, with an intriguing title: “Curculio Curculis lupus: biology, behavior and morphology of immatures of the cannibal weevil Anchylorhynchus eriospathae“.
Many weevils feed on plant seeds as larvae. This is a very good source of food, neatly packaged and concentrated. With one exception, the bizarre cycad-feeding brentid Antliarhinus (of which more, perhaps, another time) generally only one or, more rarely, a few, weevils can develop in a single seed. In fieldwork I sometimes find, on opening a seed, a weevil pupa fully occupying the interior, neatly packaged and waiting to emerge.
How weevils arrange this singular occupancy is not clear. In some cases, perhaps, females can detect if another female has already oviposited and avoid the fruit; in others, there may be so many fruit and so few weevils that competition is rare. Perhaps if there is more than one larva there is simply not enough food and one or both starve – so-called ‘scramble competition’. Maize weevils normally produce several adults from a single seed however many eggs are laid, and aggression between larvae has been seen by X-raying the seed. In the case of two weevils feeding on fruit of the palm Syagrus, however, the mechanism is known, and it’s not pretty. Most weevil larvae have broad triangular mandibles, suitable for chewing plant tissue.
This is the case of the older larvae of weevils in the genera Revena, a baridine, and Anchylorhynchus, a curculionine. In both cases, however, the first instar larva is different. The mandibles are long, slender and pointed – predator’s mandibles. With such mandibles chewing plant material would be difficult, but piercing and killing other insects – that is where one sees this morphology in other beetles.
The first intimation of what was happening was in a paper by Cecilia Alves-Kosta and Chrisoph Knogge in 2005 , where they discovered the first instar larvae attacked and killed one another, should more than one egg be laid in a fruit. The larva remained in this killer instar until the endocarp of the fruit hardened and no more eggs could be laid, after which it moulted into the more ‘normal’ second instar. The story was elaborated more recently by Bruno Souza de Medeiros and his colleagues who last year published the paper mentioned above on the ‘weevil wolf’, Anchylorhynchus eriospathae.
Like Revena, the 2nd, 3rd and 4th instar larvae have blunt triangular mandibles and, like Revena, those of the first instar are long, slender and pointed. In this case the eggs are laid on the flowers before the fruit are formed, and the larvae, flattened and with long setae to detect their competitors and prey, slide between the sepals and petals of the flower and fruit, fighting and killing others they find.
Unlike Revena they then eat them, a so-far unique observation of cannibalism in weevils. Some cases have been seen of more than one later instar in a fruit, but in this case the ignore one another, other than feeding at opposite ends of the fruit in scramble competition to mature earlier than their competitor – other weevil larvae entering the fruit may still be killed, however.
The two beetles showing this amazing development of the first instar are not closely related, and similar adaptations have not been seen elsewhere in seed-feeding weevils (other than in congeneric species on the same hosts). On the other hand, not many people have looked. In fact, we apparently need to look even more widely.
Since I wrote the text above another paper has revealed intraspecific aggression in weevils with a totally different lifestyle, where the larvae live externally on the plant – members of the subfamily Hyperinae. Jiří Skuhrovec and his colleagues found that fighting to the death can occur in cultures of two different hyperfine species, Hypera postica and Brachypera vidua . In this case there does not seem to be cannibalism or modification of the mouthparts (although they have introduced some wonderful terms: ‘offensive larva’, ‘defensive larva’ and ‘combat ball’).
What would lead to the evolution of the behaviour and morphology in these weevils, especially those attacking the Syagrus seeds? The leaf-feeding Hyperinae only demonstrate the behaviour when there is insufficient food. For the seed-feeders one perhaps critical factor is the very high seed-predator load of the plant; it is not unusual for 100% of the seeds to be attacked. This would lead to intense competition, driving the weevils to develop means of eliminating other larvae competing for the same resource – and maybe obtain some extra nutrient at the same time. One thing is clear – there can be only one.
 Kozlowski, M.W., 2003, Consumption of own eggs by curculionid females (Coleoptera: Curculionidae: Curculioninae, Ceutorhynchinae) – Weevil News: http://www.curci.de, No.10, 4pp., CURCULIO-Institut: Mönchengladbach (ISSN 1615-3472). http://www.curci.de/weevilnews/no/10/
 Zwolfer, H. & Bennett, F.D., 1969, Ludovix fasciatus Gyll. (Col., Curculioninae), an entomophagous weevil. Entomologists Monthly Magazine, 105: 122-123
 Barker, G.M., 2006, Predation on aphids by the herbivorous weevil Listronotus bonariensis (Kuschel) (Coleoptera: Curculionidae: Brachyceridae). The Coleopterists Bulletin, 60(2), 164-165.
 de Medeiros et al. (2014), Curculio Curculis lupus: biology, behavior and morphology of immatures of the cannibal weevil Anchylorhynchus eriospathae G. G. Bondar, 1943. PeerJ 2:e502; DOI 10.7717/peerj.502
 Alves-Costa CP, Knogge C. 2005. Larval competition in weevils Revena rubiginosa (Coleoptera: Curculionidae) preying on seeds of the palm Syagrus romanzoffiana (Arecaceae). Naturwissenschaften 92:265-268 DOI 10.1007/s00114-005-0620-6.
 Jiří Skuhrovec, Pavel Štys & Alice Exnerová (2015) Intraspecific larval aggression in two species of Hyperini (Coleoptera: Curculionidae), Journal of Natural History, 49:19-20, 1131-1146, DOI: 10.1080/00222933.2014.974704