Insect strategies for avoiding parasitic infection

Parasites have some remarkable strategies with which they can manipulate the behaviour of their hosts in order to increase their chances of reproducing. For example, fungi of the genus Cordyceps produce chemicals which alter the host’s navigational senses. These chemicals induce the host to climb to the top of a plant, and secure itself to a stem with its mandibles. The fruiting body of the fungus then grows from the host’s head, and releases spores. The height at which this happens increases the area over which the spores are distributed. Similarly, the gordian worm produces chemicals which disrupt the geotactic sense of its terrestrial host, causing the insect to jump into water so that the mature worm, which is aquatic, can emerge.

But in this fascinating evolutionary struggle, insects have evolved their own strategies to avoid parasitic infection. Jane Brockman, a zoologist at the University of Florida, and her colleagues, have discovered one such host strategy. The findings are reported in two papers, one in the journal Ethology, the other in Animal Behaviour.

The insect host investigated by Brockman’s team was the Southeastern U.S. field cricket (Gryllus rubens), which is native to North Florida. Male crickets attract mates by producing a song. However, the song of the male cricket also attracts unwelcome visitors – a species of parasitic fly called Ormia ochracea.

Female flies deposit hundreds of tiny larvae on or near the host. The larvae enter the insect’s body by burrowing into an exposed membrane located between the pro- and mesothorax. Inside the host, the larvae moult and feed on non-essential organs for a week. The larvae then emerge from the host, pupate and metamorphose into adult flies. Normally, the cricket dies soon after the emergence of the larvae, maybe because the parasite empties the toxic contents of its gut before leaving the host.

copy-of-ormiacricket_sm.jpg

(Image from Ronald Hoy’s lab at Cornell University, film clip from Andrew Mason’s lab at the University of Toronto)

The flies use highly specialized hearing organs to locate their hosts. The organ consists of two tympana (ear drums) fashioned out of thin sections of the chitinous exoskeleton. These miniscule ears are about 1 mm-squared in size and are highly sensitive to the frequencies at which crickets sing. The tympana are very close to each other and, because they are mechanically connected by a semi-rigid bridge, can resolve nanoscale time differences in an incoming sound and have hyperacute directional selectivity. With these tiny ears, the flies can accurately locate singing male crickets from distances of up to several metres, and drop larvae on them while flying overhead.

In field experiments, Brockman’s team captured male crickets in containers and recorded their songs. They have found that the crickets sing less during the autumn (fall) months, when there is an abundance of parasitic flies, than in spring. 75% of males captured in the spring sang, compared to just 43% of those captured in the autumn. In another set of experiments, female crickets were captured, and their behaviour observed when they heard recorded male songs being played back. It was found that females were more reluctant to approach males in the autumn, perhaps to avoid becoming hosts to the flies.

Although fewer males sang in autumn, those that did sing did so for longer periods of time. This seems paradoxical, as it increases the probability of attracting the flies. Brockman speculates that this may be because there are far more female crickets in autumn than in spring.

For the male crickets, this strategy poses somewhat of a dilemma: singing increases its likelihood of finding a mate, but also of attracting the parasitic flies. Some males appear to have another behavioural adaptation that may have arisen as a result – some do not sing at all, but instead found mates by walking around.

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5 thoughts on “Insect strategies for avoiding parasitic infection

  1. Together with the November 20th post on the same, great post. One thing that neurophiles might wonder about is what the mechanism is for the evolution of these intra- and inter-specific adaptations. Is it simply genetic selection (with an extended phenotypic gloss, a la Dawkins), or standard individual selection, or the until-recently-maligned group selection? Or “all of the above”? I think this turns out to be harder to answer, and surprisingly harder the more empirical evidence you get. For those interested in one context in which that’s been explored, see my
    Genes and the Agents of Life
    , especially chapter 9, which takes this up for the well-mined case of the myxoma virus in rabbits.

  2. Parasitic fungus sounds about right. I contacted this ‘parasite’ whilst driving semi truck throughout the US. My husband did not. I have had it for 2 years now. Doctors shoo me away with antihistimines (so I don’t itch). I’ve practically given up.

    Self medicating–I peel these things off my skin with sanitizer gel. I soak with salt water. Lice treatments work only temporarily. Better success with Off bug spray. This keeps these worms/fibers at bay. They seem to attach themselves to the kind of cloth that’s covering your body, leaving shiny synthetic like twisted coiled threads on your body. At the same time–they seem to turn black–perhaps when they die. As if this isn’t insane enough, when they accumulate they appear as white specks that are on top of your pants and t shirts–and brush off like lint or dandruff.

    I have seen them move when doused with an alcohol cotton square. They seem to reach toward heat. Most often they are bothersome more at night.

    Stuff floats off my body in the bathtub–that were never there before.

    I’m 52 year old married female who is a medical transcriptionist professionally. I semi-retired with my husband to drive flatdeck from Calgary Alberta Canada to Vancouver, Nova Scotia, New York City, Wisconsin, Texas, California, Louisiana, Florida–you name it I’ve just about been there.

    This disease is part of the reason for quitting over a year ago. At least I can apply stuff and soak almost daily in the tub and almost lead a normal life.

    I am predisposed to depression and have read this could be a commonality.

    I hope I outlive it before it eats my brain. Oddly enough, antibiotics seem to slow this down–my pimple like rash almost disappears and I am not as itchy for several days.
    I had real g ood like at first with digesting a supplement ‘Paradex”. Before I used this I had cotton ball sized stuff dropping from my hair onto my shoulders and the crawling skin sensation was unbearable.

    I hope some bright-minded scientist can isolate and detain these creatures/fungus, etc.

    Deb Newbery
    Tees, Alberta Canada

  3. Hi Deb,

    Please call me.416-546-8944 .i know someone who might be able to help you.I think you have a stealth virus.It is also called viteria (virus/bacteria)

    linkinghub.elsevier.com/retrieve/pii/S0014480005000158 .if that doesn’t work copy this to Google search.

    Citation: Alternative cellular energy pigments from bacteria of stealth virus infected individuals
    Experimental and Molecular Pathology, , Volume 78, Issue 3, June 2005, Pages 215-217
    W. John Martin

    Catherine Hutton Toronto
    416-546-8944

  4. Pingback: 2008-12-08 - Cordyceps ed altri parassiti « Paleonews

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