Rapid change in host specificity in a field population of the biological control organism .

In biological control, populations of both the biological control agent and the pest have the potential to evolve and even to coevolve. This feature marks the most powerful and unpredictable aspect of biological control strategies. In particular, evolutionary change in host specificity of the biological control agent could increase or decrease its efficacy. Here, we tested for change in host specificity in a field population of the biological control organism . is an obligate parasite of the plant parasitic nematodes spp., which are major agricultural pests. From 2013 through 2016, we collected yearly samples of from eight plots in a field infested with . Plots were planted either with peanut () or with a rotation of peanut and soybean (). To detect temporal change in host specificity, we tested samples annually for their ability to attach to (and thereby infect) four clonal lines of . After controlling for temporal variation in parasite abundance, we found that from each of the eight plots showed temporal variation in their attachment specificity to the clonal host lines. The trajectories of change in host specificity were largely unique to each plot. This result suggests that local forces, at the level of individual plots, drive change in specificity. We hypothesize that coevolution with local hosts may be one such force. Lastly, we observed an overall reduction in attachment rate with samples from rotation plots relative to samples from peanut plots. This result may reflect lower abundance of under crop rotation, potentially due to suppressed density of host nematodes. As a whole, the results show local change in specificity on a yearly basis, consistent with evolution of a biological control organism in its ability to infect and suppress its target pest.