Horsehair worms pull the strings

Posted on March 27, 2012


Horsehair worms are parasites of terrestrial insects such as crickets and grasshoppers, entering their hosts as microscopic larvae and growing into adults that can take up most of the host’s body cavity. In some cases the presence of the parasite leads to erratic behaviour by the host, which causes it to jump into a stream, whereupon the adult worm emerges and takes up a free-living existence. The ability of horsehair worms to manipulate their host can have marked impacts on stream ecosystems. A study of a Japanese headwater stream found that crickets and grasshoppers were 20 times more likely to enter the water if they were infested by horsehair worms. Infested hosts were very common in the stream, especially in summer and autumn, and made up 60% of the annual food supply for trout. Crickets and grasshoppers had much higher energy value than alternative types of prey (aquatic insects and benthic invertebrates). This is the first quantitative evidence that manipulative parasites can modify energy flow in ecosystems. It raises some interesting possibilities – for example, the trout in the study stream are members of an endangered species (Salvelinus leucomaenis japonicus) that may actually rely on horsehair worms for its survival. Also, the high dependence of trout on crickets and grasshoppers reduces fish predation pressure on benthic invertebrates. This could increase the number of adult insects emerging from the stream, and thus the abundance of prey available for terrestrial species such as birds and lizards.

References:

Sato, T., Watanabe, K., Kanaiwa, M., Niizuma, Y., Harada, Y. & Lafferty, K.D. 2011. Nematomorph parasites drive energy flow through a riparian ecosystem. Ecology, 92(1), 2011, pp. 201–207. http://www.lifesci.ucsb.edu/eemb/labs/kuris/pubs/Satoetal2011Ecology.pdf

Cricket “suicide” video: http://www.ird.fr/la-mediatheque/videos-en-ligne-canal-ird/un-parasite-manipulateur/un-parasite-manipulateur

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