Weevils (Coleoptera: Curculionoidae)

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Along with the Chrysomelidae (leaf beetles) and Tenebrionidae (darkling beetles), the superfamily Curculionidae (weevils, bark beetles and ambrosia beetles; from now on collectively referred to as “weevils”) are some of the most damaging pests in agriculture. As larvae, some may feed on roots, others develop in roots (stealing nutrients); some cause leaf or stem galls; many burrow into leaves and stems. Adults may lay eggs in seed pods or undeveloped flower buds, while many adults attack trees, irrespective of the health of the tree. Many eat fruit. Basically, nobody with a plantation wants weevils – a fact recognised metaphorically as far back as ~200 BC, in the Pluteus play Curculio, where Curculio (the main character) is a social parasite interested only in food. On the other hand, some species can be exploited as biocontrol agents against weeds and other invasive pest plants.

In the second chapter of “The Voyage of the Beagle”, Darwin noted the abundance of weevils in the tropics (back then, they were known as Rhynchophora, sensu Latreille (1825)), and in letters and personal notes, he was impressed by their ubiquitous presence and diversity. In fact, weevils are found globally wherever there is vegetation, even in deserts and the Arctic. It’s only in northern regions, such as Canada or even as far south as Central Europe, that the weevils are outnumbered by other beetles (staphylinids, who are adapted for cold).

Weevils are known primarily for looking like a joke, the insectan counterpart to the platypus (in fact, the name curculio may have been derived from its meaning of little penis – maybe Linnaeus established a trend of only masculine weevil genus names, a trend reinforced by Carl John Schoenherr (1772-1848), who changed all non-masculine genus names of the time to masculine ones; this is now mostly forgotten because nobody learns proper Latin anymore, and Schoenherr’s corrections are invalid anyway, but it’s still fun to to note). Their clownish appearance is actually a hint that they are highly-derived. Phylogenetically, their sister group is the leaf beetles, together forming the Phytophaga (~130000 spp., or 40% of beetle species); in all they belong to the cucujiformian supergrouping (characterised by a specific type of Malpighian tubule arrangement, cone ommatidia with an open rhabdom, no functional spiracles on the 8th abdominal segment and the same spiracle closing mechanism in adults and larvae – most of these are adaptations to dry conditions). They are the most species-rich superfamily of beetles (New Guinea alone has 2824 species! In total, ~60000 species), a fact recognised even as far back as Linnaeus 250+ years ago, who (if we update his classification to modern understanding) included 15.8% of all the beetles in his book as weevils. This diversity is courtesy of their lifestyle: as small, not-so-mobile exclusive plant feeders, they have plenty of opportunities for niche specialisation, in other words each species can specialise on one single plant. These interactions, once established, can last for as long as no extinction comes in the way: for example, nemonychid and belid weevils develop in the cones of araucariaceans and have been doing so since the Late Jurassic. Similarly, weevils have been integral cycad pollinators since the the mid-Mesozoic (a fairly recent discovery, earlier it was thought that cycads are wind-pollinated); some of the associated weevils even possess special cavities in their mandibles used to carry pollen grains. In Australia, the cyclomine weevils have undergone a radiation in coevolution with Eucalyptus and Acacia trees. Plant feeding is ancient in beetles, arising merely 50 Ma after the first Permian beetles (~230 Ma is the earliest phytophagan beetle) and much of the recent diversity is due to the angiosperm coevolution since the Tertiary. The weevils that radiated in relation to angiosperms are the stricter curculionid group; the older, gymnosperm-associated ones are the curculionoids. See the picture above (Oberprieler et al., 2007) for a general outline of weevil evolution.

You can recognise weevils instantly (see picture above (Grimaldi & Engel, 2005)) by their prolonged snout (rostrum) on which the 11-segmented clubbed antennae are attached. The mouthparts are at the end of the rostrum. The rostrum varies in shape (look up pictures and compare the rostra of Anthribidae, Apionidae, Ithyceridae, Attelabidae or Rhynchitidae); sometimes it is even absent (e.g. in Platypodidae). Its use is primarily for feeding and egg-laying (some pierce leaves with it and lay eggs inside). Weevils (and other beetles that live on plants, e.g. chrysomelids) have enlarged tarsomeres – the last leg segment. Crowson (1981) suggested this may be an adaptation leading to better attachment on plants – and further postulated that they secrete sticky substances from them. He was right. Their larva is characteristic, termed the curculionoid larva: thick, C-shaped and legless (although note that several families have secondarily gained larval legs, including the Anthribidae, Nemonychidae, Ithyceridae, Antliarhinidae and Brentidae).

In his “Descent of Man”, Darwin notes on sexual dimorphism in the form of small horns “on the lower surface of the body”, referring later to a passage by Wallace (1869) and describing that even weevils, seemingly “ill-fitted for fighting … nevertheless engage in conflicts for the possession of females”, a testament to the ubiquity of sexual selection. Sexual dimorphism in the weevils is best seen in the Brentidae, where the rostra of females are straight with minute mandibles, while males have flattened and broadened rostra with enlarged mandibles.

Fighting in male weevils is bound to be hilarious, as they use their prolonged rostrum to do it, as well as their legs, which can get modified for specific functions, for example elongated forelegs to push the opponent off the vegetation or swollen leg segments to use as clubs. One subfamily of weevils, the Baridinae (~9000 spp.), has horns on their prosternum (i.e. at the frontmost part of the body, on the underside, and they stick out forward – think of the rostrum as an elephant’s tusk and the horns sticking out like elephant teeth). When fighting, these horns lock together and sometimes one of the weevils will get lifted off and flipped over and lose, although the actual fighting involved is supposed to be a variant of sumo wrestling: it’s a match of pushing strength. These horns are not ancestral but evolved convergently multiple times along with stridulatory organs (structures that make sounds, used to deter opponents, also part of the sexual selection).

The immense diversity of weevils is reflected by their higher-level taxonomy, which doesn’t really exist in any standardised form. I will highlight some families so you can google for pictures.
The Anthribidae (~3860 spp.) are the fungus weevils and can be found in weedy fields and on dead/diseased branches, where they feed on growing fungi.
I already mentioned the Brentidae (~4000 spp.) and their sexual dimorphism; when not fighting between themselves, they attack wood-feeding insects – if you are a pacifist and want to punish them for their needless aggression, you can find them under bark or in decaying wood.
The Attelabidae (~2500 spp.) are the leaf-rolling weevils, so called because they construct cigar-like nests out of rolled leaves and lay their eggs inside; each roll is for one egg; once it hatches, the larva feeds on the leaf then digs into the ground for pupation.
The Rhynchitidae (~1000 spp.) are easily identifiable by their mandibles, which are spread apart and have teeth on both the inner and outer edges; they are also quite vicious: some rhynchitids find attelabid nests, kill the previous occupants (and their babies!) and thus invade and use the leaf rolls as their own nests.
The Nemonychidae (~76 spp.) are possibly the most plesiomorphic of the living beetles – i.e. the ones that most closely represent the ancestral weevil.
The Belidae (~375 spp.) is another relict family, as is the Caridae (~6 spp.). They are both found predominantly in the southern hemisphere, a possible hint at a Gondwanan distribution.
The Curculionidae (~51000 spp.) is by far the most successful weevil family, comprising 80% of the weevils, occurring globally in every possible habitat and feeding on every possible plant type and tissue. The source of their diversity is the angiosperm radiation, whereby they first fed on monocots (grasses) before moving on the other angiosperms. Their taxonomy is, of course, a mess, but several subfamilies include the Dryophthorinae (~1200 spp.), Platypodinae (~1500 spp.), Brachycerinae (~1200 spp.), Entiminae (~12000 spp.), Molytinae (~10000 spp., most wood-boring weevils) and Scolytinae (~6000 spp.).

Scolytinae and Platypodinae (bark and ambrosia beetles) are notable for having evolved agriculture. Bark beetles build elaborate tunnel systems (“galleries”) in tree bark and carry fungal spores inside – the fungal spores are carried in specific structures called mycangia, which are kind of like vessels in the cuticle with glandular secretions maintaining a fungal colony (like a cell culture in a lab). Once planted in the galleries, the fungi develop and serve as food; alternatively they make the tree sick, and the weevils then feed on the tree. Ambrosia beetles go one step further and lay their eggs in the fungal garden, and the larvae then feed on the spores. Related to the evolution of agriculture is certain degrees of sociality – it’s a team effort, not individual – and ambrosia beetles are even eusocial (like ants or bees). Unlike in the attine ants, which also have a form of fungus cultivation, this evolved convergently multiple times in these weevils (in the attine ants, it has a single ancestral origin).

All Recent weevil families are known from the fossil record, as is one now-extinct family (the Ulyanidae). The fact that body fossils are found in all typical insect fossil localities (most of them being from lake environments, amber excepted) is an example of taphonomic bias: they sink easily and their chitin exoskeleton is quite tough, so they are easier to preserve. That said, the bulk of insect fossils are not body fossils, but isolated wings, and weevil wings are reduced, so they are also unevenly represented in the fossil record. Fossil weevils are known from the Triassic, but some weevil-like beetles are already there from the Jurassic. For those interested in history, Lev Arnoldi (1903-1980), Vladimir Zherikhin (1945-2001) and Badim Gratschev (1963-2006) were the most important fossil weevilologists. Here’s some weevil fossil porn.

Fossil weevil from Enspel, Late Oligocene of Germany. Source: Wedmann et al., 2010.
Abrocar brachyorhinos, Late Jurassic/Early Cretaceous (age disputed) of China. Source: Liu & Ren, 2006.

Fossil weevil, Lower Cretaceous of Brazil. Source: Bechly, 2001.

Stenommatus pulvereus, Dominican amber. Source: Davis & Engel, 2006.

Leptocar polychaetus, Late Jurassic/Early Cretaceous (age disputed) of China. Source: Liu & Ren, 2007.

Literature Cited:

Bechly, G. 2001. Ur-Geziefer: Die faszinierende Evolution der Insekten. Stuttgarter Beiträge zur Naturkunde C 49, 6-96.

Crowson, R. A. 1981. The biology of the Coleoptera.

Darwin, C. 1845. Journal of researches into the natural history and geology of the countries visited during the voyage of H.M.S. Beagle round the world, under the Command of Capt. Fitz Roy, R. N.

Darwin, C. 1871. The Descent of Man, and selection in relation to sex.

Davis, S. R. & Engel, M. S. 2006. Dryophthorine weevils in Dominican amber (Coleoptera: Curculionidae). Transactions of the Kansas Academy of Sciences 109, 191-198.

Grimaldi, D. & Engel, M. S. 2005. Evolution of the Insects.

Latreille, P. A. 1825. Familles Naturelles du Règne animal.

Liu, M. & Ren, D. 2006. First fossil Eccoptarthridae (Coleoptera: Curculionoidea) from the Mesozoic of China. Zootaxa 1176, 59-68.

Liu, M. & Ren, D. 2007. New fossil eccoptarthids (Coleoptera: Curculionoidea) from the Yixian Formation of western Liaoning, China. Science in China D 50, 641-648.

Oberprieler, R. G., Marvaldi, A. E. & Anderson, R. S. 2007. Weevils, weevils, weevils everywhere. Zootaxa 1668, 491-520.

Wallace, A. R. 1869. The Malay Archipelago.

Wedmann, S., Poschmann, M. & Hörnschemeyer, T. 2010. Fossil insects from the Late Oligocene Enspel Lagerstätte and their palaeobiogeographic and palaeoclimatic significance. Palaeobiology and Palaeoenvironments 90, 49-58.

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