Inoculation order affects the outcome of interactions between Pandora blunckii and Zoophthora radicans in Plutella xylostella larvae

Pandora blunckii and Zoophthora radicans has been found infecting Plutella xylostella larvae in the field. Within-host interactions between the two species are very likely. Here we describe experiments where the interactions between both fungal species were studied. In dual inoculation experiments we tested the effect of the order of inoculation on outcome of infection in P. xylostella larvae. First, the relative virulence of five isolates of P. blunckii and four of Z. radicans from Mexico were estimated in comparison with isolate NW250 as a standard. Based on these results, two isolates from each species were selected for dual inoculation studies. Groups of larvae were first inoculated with P. blunckii isolates then with Z. radicans. The same experiment was repeated with Z. radicans inoculated first followed by P. blunckii. When Z. radicans was inoculated first, the overall P. xylostella larvae mortality was less than 50%, and the majority of cadavers from dual inoculations sporulated poorly. When P. blunkcii was inoculated first, the overall mortality was larger than 50%, however, the number of sporulating cadavers was larger (more than 50%) and only Z. radicans conidia were identified from these cadavers. The results suggest that prior inoculation with P. blunckii facilitates infection by Z. radicans inoculated later, thereby increasing overall mortality. The mechanism may involve effects on the host’s immune system allowing Z. radicans to infect and complete its cycle until sporulation, which did not happen when Z. radicans was inoculated first. Currently, more replicates of these experiments are in progress and the final results, including a formal statistical analysis, will be presented.