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About a week ago I noticed that some of our Boxwood plants (Buxus Sempervirens) contain a green caterpillar and a few days later the caterpillars had spread rapidly to almost all other Boxwood plants.

After I bit of Googling I found out it's the Box Tree caterpillar, an invasive species that was accidentally spread from Asia to Europe (and my country The Netherlands) 5-10 years ago. Some say that spraying with certain pesticides may help (which I'm reluctant to do because I try to do organic gardening as much as possible), but others say that once the caterpillars are spreading nothing really helps anymore.

Affected Boxwood

I tried to remove all caterpillars I could find by hand (at least 150), but the next day I found about 20 caterpillars which I probably missed. I'm not sure if this approach is effective and I suspect it will only delay the inevitable. I also noticed that almost all Boxwood shrubs in at least a 1 km radius around our block are affected.

So my question is: is there anything (handpicking or pesticide) that really helps against the Box tree caterpillar when they have already spread around the neighborhood?

  • I have read about pheromone traps, but I can't remeber the name or the brand. I don't know if it helps. – Alina May 25 '17 at 19:05
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    gardening.stackexchange.com/questions/32783/… might be of help, but I'm not sure there's any organic solution other than just picking them off – Bamboo May 25 '17 at 19:38
  • @Bamboo Thanks, I read your other post. Any idea how effective the mentioned solutions are when the caterpillars are already spread out around the neighborhood? – THelper May 26 '17 at 14:28
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    It will likely be an ongoing fight - I'd use a pesticide now because they're in full flood, if you can find a suitable one, then aim to find a pheremone trap which will signal when they arrive next year, and I'd try the nematode solution too next year, if you can buy that where you are. Now you're aware of the problem, control rather than eradication is the best you can hope for. – Bamboo May 26 '17 at 14:50
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Well, there are some things you can do that are "Organic," and safe for the environment. #1 you are already doing.

Handpick caterpillars from plants as you find them. You may need to patrol your garden at night to find slugs and some caterpillars. Relocate the caterpillars to a different area, or kill them by dropping them into a bucket of soapy water. Or feed them to your birds.

Spray the foliage of plants with neem oil. Neem oil controls caterpillars and other pests, including whiteflies, leafminers, and thrips. It may also kill beneficial insects such as bees and earthworms, however just don't spray you plant when they ar are blooming, and you can avoid much of this. To make a longer-lasting mixture purchase Neem Oil and Agricultural Oil to mix with water. You can also, spray with peppermint, lavender, or other essential oils, but for your problem, this would get expensive.

Dust plant foliage with Bacillus thuringiensis, or Bt, powder. Bt is a natural bacteria that kills caterpillars, but it does not harm most other beneficial insects. One of my favorites.

Invite birds and predatory insects into your garden. Hang a bird feeder to attract birds to the area. Ducks and chickens also enjoy eating caterpillars. If your garden lacks predatory insects, such as praying mantises, you can usually purchase a box of them from a garden center. My favorite since I started feeding the birds is a troupe of sparrows that have taken up residence in my Boxwood Hedge, and there is no insect damage anymore. Also, introduce habitat for toads and frogs on your property, in the long run, you pest population will dwindle.

Plant herbs and flowers around your garden to repel caterpillars. Many insects dislike the scent of lavender, mugwort, sage and peppermint. I like to plant garlic cloves among my plants, which drives away pests.

Finally, predators and co-planting are the long term solutions. For now, I would spray and use the bacilli solution. Wasps are a great solution for caterpillars; one big wasp nest will eat a couple of pounds of caterpillars a week if you can stand them. Several plants weirdly attract wasps; my favorite is Sea Holly (Eryngium maritimum). Plant it around your border and see what happens. Remember to put up with some damage, but if this is an invasive species best to eradicate it.

  • Thank you for you answer. You mentioned several options that I was not aware of. Do you know anything about the effectiveness of handpicking or applying Bt powder? Because some people tell me that unless you catch the infection early on any measure is pointless, especially with infections in other near-by gardens. – THelper May 26 '17 at 14:07
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    Handpicking is effective as long as you return a couple of times a week, and check back once in a while. The Bt powder could work, but I've been unable to find out any info how effective it is against Cydalima perspectalis infestation in Europe is an epidemic. I would check with your local farming or horticulture organization for more information. Using Neem oil w/agricultural oil is your best short term solution. Perhaps, your local library and newspaper can provide more information. – CloneZero May 30 '17 at 1:15
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Spray the box plants with Zen Tari (available on line) Buy pheramone traps from dragonfli.co.uk

The lures in the trap attract the moth and this breaks the cycle. Zen Tari, sprayed onto the leaf means that the caterpillar dies within a day of eating the sprayed leaf. You have to be vigilant and check box plants regularly and spray again if necessary. Do this March-October I am afraid. Or swap your box for something they don't like!

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I have the most organic suggestion - do nothing. This will last just one year. Your boxwood will lose all leaves but will also recover the next season. More than half of your neigbors will take their boxes out, thinking they are dead. But they won' t be.

This is just one-year phenomenon, without long term effects at all!

  • Thanks for your answer, but I don't think that works. The moth has spread over the entire city as well as neigbour cities by now. Some people took your approach but the shrubs get eaten empty and almost never recover. The few that I managed to save myself were infected again the next year. – THelper Jun 17 at 5:51

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