"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

May 28, 2012

Macrodasyceras hirsutum

On this blog, we have featured many parasites that drastically alter the appearance and/or behaviour of their host, usually to make them more likely to be eaten by the next host in the parasite's life-cycle. But today, we are featuring a parasite that makes their hosts appear less appetising - a seed parasitoid that has other plans for its host - none of which involves being eaten.

From the perspective of the plants that produce them, fruits are a way to turn animals into willing seed couriers. By wrapping seeds up in a tasty package, plants can deposit their seeds temporarily inside the body of an animal that will carry them off to a new location. We have even featured a (parasitic of course) plant on this blog that uses beetles for such a purpose.

photo from Figure 1 of the paper
Unlike the rest of the plant, which is often indigestible and laden with defensive toxins, the fruit is supposed to be attractive and appetising to would-be animal dispersers. However seed parasitoids such as Macrodasyceras hirsute have other plans for the fruits - they do not care for the fruit's flesh - they are only after the nutritious seed. Unlike the parasite we featured in the last post, the gullet of a bird is a death sentence for the larvae of this parasitoid (though as always in nature, there are some exceptions), which is a bit of an inconvenience as the fruits it parasitises are meant to be eaten by birds.

Macrodasyceras hirsutum parasitises the fruit of the mochi tree Ilex integra and all it wants to do is to live out its larval stage munching on seeds and grow up to be a wasp. It would rather not have its life suddenly interrupted by a hungry bird feasting on the mochi tree's bright red ripe berries.

So to ensure that its home will not end up tumbling down the throat of a bird, M. hirsutum larvae counteract the berry's usual ripening process, and ensure that it stays green (and unappetising to birds, which disdain unripe berries). A team of Japanese scientists found that if they shielded the fruits from wasp attack, almost all the mochi berries ripened to red. But, if they are exposed to M. hirsutum, some of them stayed green, and all the berries that stayed green had M. hirsutum larvae living inside them. Furthermore, they found that the more larvae there are in the berry, the more intensely green the fruit becomes - M. hirsutum did not merely stop the berries turning from green to red, they actually turned the dial on the green tone all the way up.

This little wasp is not the only insect to do this. Holly berries infected with a species of midge also stay green. It is unknown how this wasp interferes with the berry's pigment production/development, though for the holly berry midge it has been suggested that a symbiotic fungi is responsible for maintaining the host fruit's green colour. The relationship between fruit-bearing plants and fruit-eating animals has evolved to be a mutually beneficial interaction whereby one party provides food (fruits) while the other returns with a service (seed dispersal). But, the actions of M. hirsutum and other such seed parasitoids tinkering away in the background can certainly undermine the effectiveness of this mutualistic partnership if they cause otherwise ripened fruits to go uneaten. The extent of the impact such seed parasitoids have on the ecology and evolution of such plant-animal interaction is currently unknown.

Takagi, E., Iguchi, K., Suzuki, M. and Togashi, K. (2012) A seed parasitoid wasp prevents berries from changing their colour, reducing their attractiveness to frugivorous birds. Ecological Entomology 37: 99-107.