It’s a sign of age to notice that many things appear smaller than they used to be: chocolate bars, coins… and conkers.
Hold on. Conkers? Surely the same trees, growing in the same places, can’t have suddenly started producing smaller fruit? Well it seems that they have, and I’m not the first to notice, although hard data on sizes over time has proven elusive. Even so, it’s pretty obvious to me that this year’s crop provides relatively few conkers that would be worth putting on a string. One wonders how the Conker World Championships (and yes, they do exist) are going to respond to this threat*.
Conkers come from the horse chestnut tree, Aesculus hippocastanum (I have a video about it). It’s not native to the British Isles, coming instead from the Balkans, although like many immigrants it has embedded itself in the culture of these islands such that it would be strange to imagine it not being here. There are something like half a million horse chestnut trees in the UK, but they aren’t often found in our woodlands. The overwhelming majority are planted in parks and urban streets, and therefore we encounter them frequently in our daily lives. They also naturally spread into disturbed habitats around towns such as railway sidings or abandoned land.
In the last few years horse chestnuts have started to suffer from two problems. One is the horse chestnut leaf miner, a species of moth that was first discovered in 1985 in northern Greece, and only described as a new species the following year. It then spread rapidly throughout Europe, reaching the UK in 2002, and since then has reached all parts of the country. This is the main reason why the foliage of horse chestnuts starts to turn brown in August (or sometimes even earlier), long before it would naturally begin to yellow at the onset of autumn. The caterpillars actually live inside the leaves, eating away at the green tissue while protected by the tough upper and lower layers.
The leaf miners aren’t a risk to the tree’s survival — afflicted trees still generate a fresh flush of leaves the following year. What the leaf damage does, however, is to restrict the resources available to trees just when they’re getting round to producing the year’s crop of conkers. For a tree, the year is like a marathon; they spend many months storing up energy, biding their time, and around the end of August launch into their sprint finish, culminating in the release of thousands of fruit. The arrival of the leaf miner is like hobbling them halfway through the race. They can still make it to the line, but they don’t have enough energy left for the grand finale.
If your trees have leaf miners, then there are some things you can do, such as removing the leaves that fall and composting them, burying them (at least 15 cm / 6 inches deep) or, if it’s the only option, burning them. This will kill the pupae of the moth which are overwintering, but it won’t prevent reinfection from other trees in the neighbourhood, which is highly likely to occur. Some birds such as blue tits have learnt to recognise and eat the leaf miners, but this is unlikely to control them effectively because the main damage is done later in the summer once the fledgelings have already left the nest, so there is less feeding pressure.
This isn’t the only issue affecting horse chestnuts though. A new disease known as horse chestnut bleeding canker was discovered around ten years ago, and is now also widespread. It may have infected around half of British horse chestnuts. This causes scars on the stem which ooze sap. Not all trees are killed by the disease; some survive, while others might be immune. Nevertheless, many trees are weakened by it, and a number have already died. This hidden problem might also be part of the reason why our trees are struggling to produce conkers of the same quality as before.
There is some debate over whether the two problems, the leaf miner and the bleeding canker, are related to one another. Some early experiments suggested that seedlings with leaf miners were more vulnerable to the effects of the disease, but more recent and large-scale surveys have found no association between the two. The balance of evidence at the moment therefore indicates that they are independent problems. There is at present no cure for the disease, although some very recent work published earlier this year implies a role for the bark microbiome in regulating the severity of the disease.
A citizen science project has been tracking these new arrivals throughout the UK, and in 2013 found that parasites of the leaf miner have been catching up. This is good news, as it might mean that natural biological control could eventually restrict the impact of the miners. They are still asking for data and there are lots of nice resources on the website which could be used in classrooom teaching to engage schoolkids in observing an interesting phenomenon**. Children might not be able to play conkers in quite the same way, but why not use this as an excuse to teach some exciting ecology!
* Interestingly, the game of conkers predates the arrival of the horse chestnut tree in the UK, and was formerly played with snail shells or other objects. This would imply a rather radical shift in the modern rules though.
** You should also look at the OPAL national tree health survey, which involves recording observations on a whole range of trees as well as horse chestnut.