Girl power: sex and parthenogenesis in Vine Weevils

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

Vine weevils!

I do not like all bugs. I like my pot plants and that's why one of the villains in the garden is the Black Vine Weevil, Otiorhynchus sulcatus (adult above). The larvae of this weevil feed on the roots of many types of plants. In pots, especially indoors, where there are fewer parasites and predators and root growth is restricted they thrive and cause havoc. They overwinter in the soil as larvae and start feeding underground as temperatures increase. By the time they pupate most plants host to this species will be dead. If they are succulents, their root system is gone and the stems fall out of the pot one by one. I have a pot of strawberries from last year. At the end of the winter they had a few little offshoots which I planned to pot out. But they all died and shrivelled and there was only some ghost strawberry plants clinging to life left. I took the plants out today and came across the culprits: several larvae and dozens of pupae of Vine Weevil.

Three pupae and a fully developed larva of Black Vine Weevil

The adults emerge from the soil around June, they climb clumsily out and wander in search of food. They feed on leaves at night, leaving tell-tale C marks on the edge of leaves. They appear slow and harmless, and are unable to fly, but they can climb very well and feign being dead and drop to the ground if disturbed, so they tend to be left alone. Adults are all females which reproduce by parthenogenesis giving origin to genetically identical daughters. They can start reproducing by themselves as soon as they are ready, no need to waste time looking for a male and mating, and when they are, they go for it in earnest. At 21 oC, their optimal temperature, they can lay over 1000 eggs over their lifetime of four or five months. This mode of reproduction coupled with its preferred niche in the ornamental and agricultural trade has allowed this species to thrive and expand across the world from their native Europe, so, although I do not like it, I do marvel at its successful life history. What next? We should be getting some parasitic nematodes and watering the pots with them in the hope that they will infect the pupae and stop the invasion.