Dear Readers, my friend A gave me some ‘manky apples’ from the tree in her garden, but she hoped that I wasn’t squeamish, as the fruit had been ‘got at’ by at least one insect. And, dear readers, I hope that you aren’t squeamish either, because I have spent the last half an hour cutting the edible bits off of the fruit for a crumble, and taking photos of the inhabitants of the other bits. If you don’t like photos of caterpillars, look away now :-).
The major pest of apples in the UK, as you probably know, is the codling moth( (Cydia pomonella). Another stipulation for my receipt of the apples was that I was not to wax lyrical about the creature, but here is a picture of the adult, and I think it’s not unattractive in a mothy kind of way.
However, the caterpillars are obligate frugivores – in other words they cannot digest leaves, like other larvae, and are wholly adapted to eating fruit. Apples are their favourite, but they can also eat pears, walnuts and even apricots. The adult moth lays her eggs on either the twigs or the fruit of a tree, depending on the time of year (there are two generations, one in the spring and one later in the summer). It’s thought that the caterpillars navigate towards the light once hatched, and at some speed too, because nearly everything else in the garden, from blue tits to ants, eats codling moth larvae. As most trees produce fruit on the end of their branches, this behaviour means that the caterpillars are quick to find a suitable apple. It takes the larva about 45 minutes to burrow into the fruit, and then a further 15 minutes to spin a cap for the tunnel out of silk. Once in the apple, it heads into the centre where the seeds are. Amazingly, it then bites the seeds, which stops the natural development of the fruit and causes it to start to ripen – ripe fruit provides the caterpillar with more nutrients. Often, one apple will be enough, but some especially hungry critters will then start the process all over again with another apple. No wonder farmers love them so much.
Once the caterpillar is snug inside its fruity home, it makes a chamber, which rapidly goes black and fills with frass (droppings).
And then, when the caterpillar is ready to pupate, it either launches itself into the air on a silky thread, or crawls down the trunk, looking for a suitable chink in the bark or other secure spot. Some moths will hatch just ten days after pupation, while others will ‘hibernate’ over the winter. One old method of deterring codling moth was to put a sticky band around the tree – I always thought this was to stop the caterpillars climbing up (silly me) but in fact it catches the caterpillars climbing down to pupate. Once there is a suitable ‘haul’ the band can be removed and destroyed. However, my money is on the codling moths in my friend’s garden, as this very morning we saw some caterpillars abseiling down from the branches like tiny green SAS men about to remedy a hostage situation.
Some fruits, as is the nature of things, have fought back. Some varieties have thickened skins, and hairy skins, such as those of peaches and apricots, are also shown to deter codling moth. Others have responded by producing ‘stony cells’ in the layer of flesh that surrounds the seeds, making it much more difficult for the caterpillar to munch through. In short, the battle between codling moth and stone fruit has probably been going on for as long as both have existed.
Various methods to reduce the predations of codling moth on human food have been tried, including the usual ‘spraying with insecticides’ method, which has resulted in insecticide-resistant codling moths, as we might have expected. Less harmful techniques for the garden apple tree include clearing away fallen fruit so that any caterpillars still in situ don’t have a chance to pupate, pruning to get more sun into the heart of the tree (the caterpillars require humidity to survive), and even scraping the bark to make less sites for pupation. Plus, encouraging birds such as tits into the garden and being tolerant of ants (who are excellent predators of caterpillars before they set up home in the apple) helps. Woodpeckers are skilled at finding codling moth pupae under the bark of trees. However, once the caterpillar is ensconced it is very well protected from practically everything, because to destroy the insect you’d have to contaminate the fruit.
However, the adults are strongly attracted to the smell of ripening apples, and this can be used as a way of catching large numbers and disposing of them – the scent can be distilled into moth traps, and both males and females are likely to turn up. Like most pheromone traps, they are lined with a sticky substance like flypaper, which feels to me like a most unpleasant way to die. Still, at least it is very specific, and hopefully other species are not also enticed. It’s surely an improvement on insecticide.
So, I now have some perfect chunks of apple, and some that are full of caterpillars and holes. I imagine that for most of human history this was how it was. There would be good years and bad years, and on the bad years the apples wouldn’t keep as well, and more would have to be discarded. Still, it’s interesting that the bits of apple that haven’t already been nibbled are absolutely delicious, in a way that only garden apples seem to be these days. I’m happy to share my fruit with the little guys, but then my life and my livelihood don’t depend on it. I might feel differently if they did.
Photo One by By Olei – Self-published work by Olei, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=810865
Photo Two by By Slaunger – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=20440883