I’m sitting in a Polish farmhouse, watching the rain fall outside. A tree creeper is working his way busily up the furrowed trunk of a big horse chestnut just outside the window, probing his long curved beak into every crevice. He moves quickly, as if he is trying to dodge the raindrops. I’m thinking how much better would be his chances of living through the coming winter if his species shared the intelligence and curiosity of the great tits in the branches above him. They have recently learned how to peel open the blisters on horse chestnut leaves, and eat the juicy grub inside. This newly arrived pest of horse chestnuts (the horse chestnut leaf mining moth) has swept across Poland in the last 5 years , and is just now making an appearance in South East England. You will be hearing a good deal about it in the popular press no doubt.
I have been fascinated to watch the highly intelligent and inventive great tits learning how to exploit this new food source. First one adult would visit the tree and we would watch him or her ( they look the same) peeling back the skin of each infected leaf. The leaf-mining grub inside each blister is about ¼” long, and considering that each leaf held several grubs and the large tree bears many tens of thousands of leaves, the total weight of protein-rich grubs on just one tree must be considerable. Later last summer the same bird, or perhaps another of the new leaf-aware clan, could be seen teaching a large brood of newly-fledged babies the same trick. I am reminded of the media coverage given to British great tits when I was a boy , which were then learning the very similar peeling trick to get at the cream under the (then) new foil caps of milk bottles on doorsteps. In exactly the same way , this highly adaptable species has learned quickly to exploit a new food source in 1960’s UK and 2000’s Poland. A successful pair can raise several large broods in a single summer , so these new sicills are quickly spread through the population, and this intelligence and inquisitiveness has ensured that parus major ( the great tit) is one of the commonest birds on all Europe and Asia. (and also North America where it is known as the Black Capped Chickadee).
But to return to the horse chestnut leaf mining grubs- also apparently an increasingly successful species. An affected leaf shows a characteristic (see picture) brownish blister – perhaps 1 ½ inch long by about ¼ “ wide. One leaf may have several such. It is quite easy to peel back the skin and find the grub inside. The affected part of each leaf will become useless for photosynthesis, so presumably a heavily infested tree will be at some disadvantage, and grow more slowly, and affected leaves are certainly unsightly, but I’d be surprised if any tree is actually killed by this creature. Leaves are temporary organs , and most are heavily damaged by a host of creatures, fungi and diseases by the time they are shed in autumn. Nevertheless, if you want to clip the wings of the new pest, the best advice is to sweep up and burn fallen horse-chestnut leaves. The grubs apparently pupate over winter in the fallen leaves. My wife did that last year around our house in Poland and this year there are definitely less blistered leaves on the tree. But whether it was my wife or the great tits which destroyed most grubs, I guess we’ll never know. So please keep a look out for this new pest in our own numerous horse chestnuts (the RHS would like reports – or tell me and I will pass them on). But I must admit, the first reports of English great tits exploiting this excellent new food source will excite me even more.