Abstract
Aphids have intimate associations with facultative symbionts that can help their hosts against natural enemies or heat shocks. However, these partnerships with symbionts could also induce costs for the hosts. This study focused on such associations to unearth induced costs and coevolution. Our data indicated that two strains of Regiella insecticola had negative effects on aphid fecundity, developmental time of the 3(rd) instar nymphs, and the age at first reproduction. Subsequently, the role of the R. insecticola was examined in the aphid's resistance to a parasitoid, Aphidius gifuensis (Ashmaed), a major agent used in biological control of the grain aphid Sitobion avenae. Results revealed that a significant reduction in emergence rate of the wasp, but not the percentage of parasitism. These results provided more evidences on a hypothesis that aphid individuals hosting facultative symbionts have a fecundity cost associated with parasitoid resistances. Additional tests were conducted to determine fitness costs of harboring the symbiont on the remaining unparasitized aphid individuals, showing no difference in aphid fecundity and survival between aphids with and without the symbiont. However, when compared to the absence of the wasp, the reproduction rate of aphids showed a strong increase under the stress of the parasitism in three aphid lines, particularly in the aphids harboring R. insecticola. This may suggest a potential evolutionary direction. Overall, the study enhanced our knowledge of the role of symbionts in host and parasite coevolution and insect pest control.