Late summer browning with Leaf Miners

Sarah -   Now, everything is getting very autumnal outside now, not just the cold weather that's come upon us but you may have noticed that there's some horse-chestnut trees went brown along time before the other trees starts to.  This is called late summer browning and is caused by the caterpillars of a leaf mining moth.  First noticed in Macedonia in 1984, the leaf miner has been making its way across Europe.  Emily Seward took me to find some caterpillars and see how they do so much damage.

 Emily -   So we're just walking along by the river here in Cambridge now and you can see its really dramatic, the trees that are healthy are still green.  The other tree, all the brown trees, are the horse-chestnuts and they're really looking very unhealthy.

 Sarah -   Yes, they're looking pretty bad.  I mean with - hundred meters away from these trees and you can already see that they just look dead.

 Emily -   It looks like winter has surrounded just those leaves and just those trees, and left the all the other ones intact.  So if we go a little bit closer though, you can see that it's not a normal autumnal change.  It's not the characteristic browning that you associate with late October or November, that sort of time.  It's really turning brown in little patches and these little patches are cause by leaf miner larvae.  And these are little larvae of moths that have burrowed in between the two layers of the skin of the leaf and have eaten away the leaf that are causing this brown lesions there.

 Sarah -   Wow!  They look even worse close up.  And there's these patchwork little spots and patches all over the leaves.  Some of them going about yellow and then the rest of it goes a bit brown.  And then some of them, they've been really badly affected; I suppose once the leaf miners have had their fill, they have actually just died because all of the inside of the leaf has just been eaten out.  I don't really want to go any closer in case I get hit on the head by a conker.

 Emily -   And now we're being attacked by conkers, they're falling down on us as we speak, but they're not looking too bad.  They're a little bit beaten up, but in general, they're quite in normal size.  So the leaf miners aren't killing the trees, they're just reducing the amount of leaf area, so the amount of area that the plants can have to convert sunlight into sugar.  And so, although it's not killing them, it is meaning they have less resources, they're more likely to get disease from other things and they have less energy to put into making the conkers.

 Sarah -   Shall we see if we can find leaves?

 Emily -   So Sarah is just pulling off a couple of leaves now.  First of all, just hold it up against the light and you can actually see the larvae inside.  You can see the outline of them just wriggling around in there.  See look here, that one.  It's moving around.

 Sarah -   Oh you can!

 Emily -   You can see it's like a little worm...

 Sarah -   Yes...

Emily -   ...in between the two layers of the leaf.

Sarah -   So it's like a little segmented caterpillar obviously because moths are related to butterflies, but it's about half a centimetre long maybe and sort of a beige colour with little black lines on it.

Emily -   So if you peel back the upper layer.

Sarah - I'm not sure I really want to peel it back and see it closer.

Emily -   So Sarah is just using her nails to take off just the upper layer and you can see within the leaf itself is this larvae and it's wriggling around and there are lots of them.  Each leaf has 10, 15 larvae in it, in these pockets protected inside.

Sarah -   So it's kind of like between the top and bottom layers of the leaf.  It's eating out the inside so it's got a nice little protective coating on the top, and a nice little protective coating on the bottom, so it's safe from predators, from birds that might want to come and eat them, and that sort of thing.

Emily -   And it's got an easy food supply.  It can just eat its way through the leaf and you can see it's sort of contained within the rib regions.  It will cocoon and turn into a different developmental stage and overwinter like that, ready next spring to come out and eat more leaves.

Sarah -   And do we think perhaps things like the harshness of the winter affects how much we see of the damage to the horse chestnuts?

Emily -   These are actually very, very resistant to the cold.  So they can survive really freezing temperatures so that's not going to affect them.  It's very difficult to get rid of them so they weren't seen a couple of years ago.  2002 I think was the first sighting, and now they're everywhere.  If you just look, you can see every tree that's brown at the moment is a horse-chestnut tree.

Sarah -   Is there any hope in the future that we might be able to find some kind of treatment or some way of killing off the moths so that they can't lay their eggs so they develop into the leaf miner and eat more horse-chestnuts?

Emily -   Well, it's really a good question and people are working on it at the moment but like you said it's little bit bleak that hidden from their nature predators like blue tits and that sort of thing.  And though you can spray the trees with chemicals, that's also going to kill all the other insects and it's such a wide spread problem that people are finding it very difficult to take responsibility and actually clear up the problem.  What you can do if you're worried about your horse-chestnut in your garden is you can collect up all of the leaves and burn them in a bonfire because that will kill the pupae that over wintering and hopefully reduce the number of moths the next year but it's really a much more wide scale problem.  It's taken 10 years for people to really start noticing how widespread a problem it is now and so maybe in another 10 years, we will find a solution and we'll be back to having lush trees all year round until winter takes its natural toll.

Sarah -   That was Emily Seward, introducing me to the horse-chestnut leaf miner caterpillar.  And they are fairly easy to find and pretty gross.