Castration parasites attack and destroy the sexual organs of their hosts, thereby preventing host reproduction and causing host resources to be reallocated to growth and ⁄ or survival, which directly benefits the parasite.

Previous theory suggests that spatial structuring should select for intermediate levels of virulence parasites, but empirical tests are rare and have never been conducted with castration (sterilizing) parasites.

To test this theory in a natural landscape, Kunming Institute of Zoology (KIZ) researchers, Douglas W. Yu in collaboration with scientists from Hungary and UK constructed a spatially explicit model of the symbiosis between the ant-plant Cordia nodosa and its two, protecting ant symbionts, Allomerus and Azteca. Allomerus is also a castration parasite, preventing fruiting to increase colony fecundity. Limiting the dispersal of Allomerus and host plant selects for intermediate castration virulence. Increasing the frequency of the mutualist, Azteca, selects for higher castration virulence in Allomerus, because seeds from Azteca-inhabited plants are a public good that Allomerus exploits.

These results are consistent with field observations and, provide the first empirical evidence supporting the hypothesis that spatial structure can reduce castration virulence and the first such evidence in a natural landscape for either mortality or castration virulence.