Acorn weevils – Curculio glandium
Alright, not adult Curculionidae but the grubs. Weevils are a massive group – more species in that family than anything else (see that link) so clearly they are very good at what they do. When the acorns fall around mid November and you pick up the freshly fallen ones, you may spot that quite a few have a neatly bored hole in them. Obviously acorns are scrummy food for lots of insects, but many of these holes will be where an acorn weevil has deposited eggs. I picked up a number of acorns – ‘as you do’ (!) – to grow some oaks, but over the course of a week or two I discovered that the grubs had emerged from many of them, and were lying in a slightly sad way under the acorns in the bowl. I put them outside. I have not researched this properly but the grubs burrow into the earth to spend the winter and pupate before emerging next year. Or in a period of a couple of years.
I did read that “the acorn weevil Curculio glandium kills a significantly greater proportion of small acorns than of larger acorns from the same tree”, quoted by Professor Michael J.Crawley in ‘Insect herbivores and plant population dynamics,’ Ann. Rev. Entomol. 1989. 34:531-64 from Forrester, G. 1 989. The population ecology of acorn weevils and their influence on natural regeneration of oak. PhD thesis. Univ. London. Crawley also says in another article, “The two principal acorn-feeding insects (the alien cynipid gall wasp Andricus quercuscalicis and the native weevil Curculio glandium) killed between 30% and 90% of the acorn crop in given years and between 0% and 100% of the acorn crop on individual tree”, and I would say 20-30% of the acorns I gathered were affected – see ‘Alternate Bearing, Predator Satiation and Seedling Recruitment in Quercus Robur L.’ M. J. Crawley and C. R. Long, Journal of Ecology, Vol. 83, No. 4 (Aug., 1995), pp. 683-696.
Here are some adults –
The proboscis – rostrum – is longer in the female than the male.
Here some work on Curculio phylogeny – and unsurprisingly the nearest relative is the hazelnut weevil, Curculio nucum –
Michael J.Crawley has written a lot on seed predation http://www3.imperial.ac.uk/people/m.crawley/publications
I bet many a grub dies in the acorn when the acorns are eaten by squirrels and birds etc.
Wouldn’t it be interesting to see if there is any genetic distance between weevils in Kent and those in Pas-de-Calais, though 10,000 years or so since they were cut off is not so long genetically I suppose. It seems they are poor dispersers according to one French Phd (Oberli, 2001) so perhaps they are mostly spread as the oaks on which they predate are spread? I should on reflection, have kept them in some soil then seen them emerge, (C.elephas spend more than one year in the soil as a sort of ‘hedge bet’ against the problems of oaks having a ‘mast year’ when they produce a bumper crop, followed by one or two relatively fallow years – see Venner et.al. What I have yet to discover is what the grubs eat in the soil – or do they just pupate in a period of stasis for a long time? Fascinating. A project for next autumn!