Are your box plants (Buxus sempervirens) covered in a messy, sticky webbing ? Do they all of a sudden look dreadful, almost disappearing before your eyes? This is the work of the box moth caterpillar. This May and June (2025) in south Dublin they have become more prevalent then ever, with customers from all over south Dublin reporting them in their gardens. If you have a beautiful box hedge, or some old topiary, they can be devastating. The insects are a recent arrival, completely non-native, so they don't have an obvious predator.
Box moth caterpillars eat through foliage at a rapid rate, often stripping plants within a couple of days. What can you do against the box moth caterpillar if you find them munching through your box plants?
A first action can be to spray the living dayllights out of your plants with tapwater from your hose. This will dislodge the caterpillars and flush some of them off the plants. It won't kill them, but it will wash them and their larvae (eggs) onto the ground and some of them wont make it back again. Do that several times and every couple of days and you will reduce the damage.
If you just have a few plants, you can poke around in the plants and pick them out. Once your eyes becomes accustomed to finding them it becomes easier. Throw them into a jar of very hot water and they wil die instantly. It's an unpleasant job, but effective.
If you would prefer a less labour intensive way of doing it, purchase a 'bug killer spray' (note, we ony recommend options that are naturally-derived, organic in their composition, but no less effictive because of that). Browse the range...
We sell two options that are made from plant-based chemicals. The first is 'Bug Free bug & larvae killer' from Neudorff. It uses Pyrethrum as its active ingredient. This chemical is toxic to insects and is certified organic. The concentrate will go further than the ready to use spray. Spray the plants, inside and out. Once the spray touches the caterpillar or its larvae, they will die. We recommend spraying in the early eveing when you will not inadvertently spray beneficial insects such as bees.
The second option is to use Neem oil. Neem oil is extracted from an Indian tree (the chemical compound is called azadirachtin) and has a toxic effect on insects. Mixed with water and a few drops of soap, it can be sprayed onto plants. It's a highly effective treatment for greenfly, blackfly etc and we use it in our shops and at home on our houseplants and on our roses.
Against box moth caterpillars, Neem oil takes a couple of days and a couple of applications to be effective. However, it is inexpensive and a small bottle will go a long way. Customers have given us good reports regarding its effectiveness aginst box moth caterpillars. It's important to remember that it will need to be applied several times over several days. Spraying may only reach certain parts of the plants, and these small creatures have short life cycles. The secret is in repeated application. Again, do this in the early evening to avoid spraying beneficial insects.
As Neem oil is a thick oil, here are a few tips on how to mix it with water to make a spray. Warm the bottle of oil in a mug of warm water. It will not pour if it's below 20ºC. Mix a small amount with water and a a similiar amount of liquid soap such as washing up liquid (approx 40 drops per litre). Shake the mix well before use. This will break down the oil into smaaller particles that can be sprayed.
There are also biological controls against the box moth caterpillar such as XenTari from TopBuxus, but these are not presenlty licensed to be sold in Ireland.
A box moth caterpillar just after being spayed with a Neem oil solution.