Cydalima perspectalis is the box tree caterpillar and if you don’t already have it, very sorry, but it’s coming soon to a box ball near you.
It lays pale yellow eggs on the underside of the leaves, which hatch green and yellow caterpillars with black heads. These will munch away, leaving only crispy brown skeletons of leaves, and making you both angry and sad.
This caterpillar has no natural predator so it’s not like you can just leave the birds to eat them. In fact birds won’t eat more than a few due to the toxic alkaloids in the box (which is the sole diet of the caterpillar). That means that if we want to keep our box plants, we have to take action and control the pest.
Now of course, there are loads of sprays out there which will nuke the critters, including all other beneficial insects!! That’s not something I want to contribute to, and if you’re looking for chemical insecticide recommendations then this is not the place for you. Before you go though, I would beg you, for the sake of our planet, to take a look at the options below - they are all effective and most importantly, SAFE for other insects and wildlife, pets, your children and YOU!
Firstly, hand-removal. It’s time-consuming and gross, but effective if you do it regularly, remembering that this moth has more than one life cycle every year, and that the caterpillars over-winter on the plant.
A moth trap should be the first port of call for anyone dealing with this pest. It contains a lure which attracts the males and traps them, preventing them from reproducing. You may need more than one trap, depending on how large your garden is, and it’s essential that you follow the instructions, replacing the lure regularly etc, otherwise it simply won’t work (and for something that doesn’t look very nice, that would be a shame!) There is a discount code on the trap I’ve linked above - just put TODMAN10 into the box. If everyone with box had a trap and used it, we would effectively be able to control this pest.
Next is the nematode route, This is effective as long as you follow the instructions religiously. I haven’t used it but it is absolutely safe for use. I use nematodes to deal with vine weevil larvae and they are effective but as I said, do follow the instructions; nematodes are very temperature sensitive and you don’t want to waste your time or money.
Biological control - Topbuxus Xentari contains a microorganism that will kill caterpillars but not bees (indeed bee-keepers use it to control moths within hives). I use it and can vouch for it. The caterpillars stop eating your box within an hour and they die within a couple of days. Be aware that it is not officially sanctioned for use in the UK yet, but also be aware that this is more to do with huge chemical companies and their wish that you should continue to buy their stuff..
If you have a fair amount of box and are using Xentari I highly recommend a Trigger Sprayer - it makes the whole thing much easier and quicker and avoids blisters from repetitive spray injury! If that’s not your thing then of course a normal spray bottle will do; it needs to be at least a litre capacity as each sachet of Xentari needs a litre of water to mix with.
Finally, remember that this pest isn’t going away any time soon, so if you want box the you’ll have to commit to them…big time. Having found a solution that works I’m happy to put the work in, but if you’re not up for this lark, then stay tuned and I’ll put a blog up soon with my favourite box alternatives, all of which respond well to clipping, and none of which will (hopefully) break your heart.
Hope that was helpful. I am not a scientist - just an interested gardener, and sometimes I get things wrong. If you see anything here that you don’t agree with, please do let me know.