Precariously perched on a young oak branch, a Black Vine weevil, Otiorhynchus sulcatus, the first of the year. I am really not looking forward to seeing them, but they have an intriguing life cycle, which is key to their success. The Black Vine weevil is a generalist feeder which in the early 19th century was only known in a small area in Central Europe and since then has expanded to much of the world aided by human activities and has become a pest of a range of agricultural and horticultural plants, despite being a flightless, sluggish insect.The remarkable thing about them is that all Black Vine Weevils are female. A weevil does not need to find a partner to mate before reproducing, she just emerges from the soil, stumbles to the next suitable patch and lay eggs. The next generation of daughters will be genetically identical to their mum, forming natural clones. Parthenogenetic lineages in general are good colonisers, and expand their population ranges more easily than their sexual relatives. Individuals might disperse the same distance during their lives, but in the parthenogenetic species a single individual can successfully colonise a new patch and establish a new population, while the sexual species needs at least two individuals of different sex - or a fertilised female - and then has to cope with the disadvantages of inbreeding if it succeeds. Most parthenogenetic insect species are flightless, which suggests that parthenogenesis is more advantageous if the insect is naturally a poor disperser.
But there is another card up the weevil sleeve. These beetles are triploids, instead of having the usual two sets of chromosomes (that is, being diploids, like us) they have three. Polyploids are often larger than diploids and may enjoy further ecological benefits such as more resistance to the cold or wider ecological tolerances. Ecologically, parthenogenetic weevils have moved much from their origins in the moist valleys of the Alps, and are now able to persist even indoors, munching the roots of plant pots.
They might not be the most attractive insects, but these weevil story of success shows that appearances can be deceiving and that, sometimes, girls rule.
More information
Lundmark M (2010). Otiorhynchus sulcatus, an autopolyploid general-purpose genotype species? Hereditas, 147 (6), 278-82 PMID: 21166797
