Flies eating caterpillar eggs?

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  • #41335


    Daily eggs laid on our 40 or so plants seem to be disappearing before my eyes. From observation paper wasp numbers seem to be negligible, so can’t blame them. There is the occasional ant, steely blue ladybird and small cockroach, which may be responsible for some loss. What we have a lot of at the moment is flies, thanks to our 3 feathered friends (chickens). A swan plant nearest the chicken run had at least a dozen eggs laid on it a few days ago, but today I could only find one while brushing away the flies that were resting there. Could the flies be eating the eggs???

    Anyway, I’ve decided to put my caterpillar castle into action, collecting eggs as soon as they’re laid to raise until they’re reasonable sized caterpillars. I’m hoping to re-release them back onto plants when they’re past this vulnerable stage.

    Fly heaven, Hibiscus Coast

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  • #41383


    Apologies for the delay in getting back to this.

    The flies are blue/green bottles enjoying the chicken’s molasses based food second hand. They appear to be innocently resting on the swan plant. I’m glad they’re not robber flies (very fearsome looking predators), otherwise the caterpillars and butterflies wouldn’t stand a chance. While I was observing the flies’ antics the real villain came along – a large, very aggressive stripey paper wasp. It flew off into the neighbour’s, but will no doubt be back to remove all life from this plant. I occasionally get lucky and bring one down with some disinfectant spray, and then apply the jandal treatment:) So much for the “negligible” presence.

    Back to getting the caterpillar castle going. I’m hoping to re-release any caterpillars when they’re at least 15mm. Last summer after New Year we had no successful caterpillars making it through to the butterfly stage, and as a consequence there were none to tag in autumn. I don’t want a repeat of this.




    Maybe the flies are what’s known as Robber or Assassin flies (family Asilidae) which I have seen attacking admiral caterpillars on my nettle patch. They paralyse them (and other insects) and suck the insides out of them. I don’t know if they eat eggs too, but it’s possible. There are about 20 species in NZ, six of them threatened species. If you google the Common Robber Fly (Neoitamus melanopogon), you might find a photo of one to compare with your flies. If they are robber flies, then it wouldn’t be advisable to put the caterpillars back outside at all, as they’d be delicious fodder for the flies!



    Hi Susan – any chance you could upload a photograph of the fly or flies? Have never heard of this before, but I’m always learning.



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