Tuesday 3 May 2011

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.

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