02 September 2010

Predatory and parasitic wasps

To butterfly enthusiasts, wasps are the vespa non grata of the insect world.  The first problems arise from predatory wasps - the kind that pick up caterpillars whole and carry them off.  I had a frustrating summer trying to raise Cabbage Whites on Brassica plants, even up on a balcony away from ants and slugs.  Eggs would appear, then small caterpillars, then bigger ones, then... gone.  They weren't wandering - they were being abducted, presumably by wasps carrying them off to burrows (although I never saw the malefactors, so it's possible that birds or other predators may have been responsible).

To cope with predators of that sort, the cheapest response is to wrap the host plant in tulle - the same fabric used for wedding decorations.  Shown above is tulle covering a flat of milkweed and wrapped over a pot of cabbage.

A much more difficult problem arises when dealing with parasitic wasps.  I posted a gruesome but utterly fascinating video back in January, showing the larvae of parasitic wasps bursting out of the body of a caterpillar, whose brain has been hijacked as well, so that it spins a protective cocoon over the emerged larvae.  And a little over a year ago I posted two photos of the remarkable Ichneumon wasp (click on the lower of those two pix to see her magnificent ovipositors).

I had originally thought that the tulle wrapping of the host plants would keep out the parasitic wasps (it would certainly exclude an adult Ichneumon wasp).   I was therefore somewhat dismayed to see Ed Yong's report earlier this year about the sensory skills of Trichogramma wasps.  The part about anti-aphrodisiacs was interesting, but what opened my eyes was this photo -
- showing the parasitic wasp to be smaller than the eyeball of the butterfly.  I don't think my tulle (or my more recently-acquired paint-straining fabric) has any chance against that little bugger.

The other photo at Ed's article shows that the wasps are smaller than the butterfly's eggs, and that's part of the problem, because even when I bring butterfly eggs in to an enclosure to raise them, it's possible that they were parasitized even before they hatch.  More about that later when I show the havoc wrought by parasitic flies on immature Red Admirals.


  1. Your posts are just fascinating. I learn so much from them. Thank you!

  2. It's funny to read about parasitic wasps in this negative context, because we do everything we can to encourage them to live and be happy here. They eat all kinds of worms we don't want taking over our vegetable garden.

    We must have so many butterflies that the parasitic wasps can't get ahead of them. The wasps have never been a problem that way.

  3. Texan99, I certainly didn't mean to imply that there is anything evil or "wrong" about parasitic wasps. Their life-cycle is as valid as that of any other organism, and perhaps equally beautiful from a different perspective.

  4. Oh, shoot, I didn't think you were being mean to my wasp buddies. I just thought it was funny that your experience with them is negative, while we find them so important and valuable that we're inclined to coddle them. Just depends on what you're trying to raise, huh? We're so covered up in butterflies that we don't give a moment's anxiety to things that might be trying to feed on their caterpillars. We have several hundred square feet of passion-flower (maypop) vines, which the caterpillars seem to love, and we welcome their help in keeping the vines under control.

  5. I put tulle over my fennel this summer to keep the marauding wasps off the Black Swallowtail caterpillars. It seemed to help a bit. I am always disappointed when they get almost large enough to pupate and then just disappear, which happened both before and after the tulle application. I removed it because I witnessed a hummingbird become entangled in it one afternoon as I was tending the zinnias. I'd left holes in the tulle just in case something like this happened, but the little fellow zipped under and up and couldn't find a way out. It's little feet became caught in the tulle and I had to remove it by hand. Thank goodness I was there to free it. I've never held a humminbird in my hand before. An amazing experience. It made peeping sounds until I could release it. The tulle came down immediately thereafter and I went back to the tulle butterfly nursery cage. However, it is still a good idea for smaller plants and potted arrangements such as yours.

  6. Very interesting, bunnits. I'm a little surprised that hummers would be interested in your fennel; ours seem to ignore the fennel and the dill and favor instead the larger, more tubular flowers. Perhaps it was after some of the insects that like to visit the fennel blossoms.

    I agree holding one must be an amazing experience. :.)

  7. We're on the Coastal Bend of Texas, which is a huge hummingbird migration path. In fact, we're just coming into peak hummer season, with twenty feeders up and many hundreds of birds coming to our porch. They sometimes get into trouble flying against a window. Our neighbors have found them stunned and have held them (cat-proof) for a few minutes until they come to their senses and fly away.

    At the Hummingbird Festival every year in mid-September, experts come and do bird-bandings. The bands are tiny threads that weigh practically nothing. The birds fly into a soft-sided cage rather like a lobster trap. The banders catch them one at a time, weigh them, blow a little air on their stomachs with a straw, and assess several fat-collecting areas before banding them, or before recording information on any band they may already be carrying. All this time the birds are lying on their backs in the bander's hand, a position in which they seem naturally to freeze. When the bander's done, he turns them over and waits a moment until they suddenly fly off. Children in particular will watch this process in a trance until the adults make them leave, but even the adults are pretty amazed.

  8. There was a very interesting Nat Geo show a few years ago http://www.youtube.com/watch?v=-s2NvJajetI in which a group of entomologists was, and I assume still is, trying to cultivate the fire ants natural enemy which is a parasitic fly. They had to go to Brazil/Argentina, bring this tiny fly back and try to raise enough of them to release on fire ant nests in Mineral, Tx. The recent drought nearly killed all the ants and therefore the fly but they were able to find one surviving nest in a woman's garden and luckily a few flies.

    It is interesting that we humans have the ability to kill nearly everything on Earth but it takes a fly no bigger than dot to hopefully stop the fire ant invasion.


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