Sunday, 21 September 2014

Keep your eyes open for "Popcorn on sticks"

Knopper galls - look a bit like brown Popcorn on sticks!
If you go down to the woods today and stand under a common Pedunculate Oak (which differ from our other native Oak – the Sessile Oak - by having acorns on stalks or “peduncles”), take a look at the ground to see if you can find any fallen acorns. You may well spot a few, but you will almost certainly also find plenty of Knopper galls, as it seems to have been a particularly good year for them.

The Knopper Gall is caused by a tiny gall wasp, Andricus quercuscalicis, which only arrived in this country in the 1960s, but has already colonised the whole of England and Wales and has moved into southern Scotland too. This little insect has a rather interesting life cycle.

In the spring asexual females lay their eggs into the male flowers of the introduced Turkey oak (Quercus cerris). The hatching larvae induce tiny flask-shaped galls to develop on the catkins in April and May giving rise to a sexual generation.  Males and females of the sexual generation emerge in late May to early June of the same year. 

The sexual females then lay their eggs into the female flowers of our native Pedunculate oak (Quercus robur), where an asexual generation develops, producing galls on the acorns in the summer and autumn.

These large galls are very conspicuous and consist of a mass of ridged, pyramidal-shaped growths, looking rather like brown Popcorn! Often there are two or three growths on an individual acorn which completely cover it.  When growing they are often coloured red or russet-green and are quite sticky. After hardening, the galls turn brown and drop to the ground, usually around this time of year.

Inside the gall is a chamber which contains just one larva.  This asexual generation overwinters in the hard Knopper gall, hidden away amongst the fallen leaves.  Around the start of February, the adult asexual female gall wasp emerges through a vent at the top of the gall and flies off to find a Turkey oak, so that the cycle can start all over again.

This little gall wasp therefore needs both the non-native Turkey oak, which we have planted all over the place, and our native Pedunculate oak in the same locality if it is to breed successfully.


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