Correlation between "Candidatus Liberibacter solanacearum" infection levels and fecundity in its psyllid vector

The potato/tomato psyllid, Bactericera cockerelli (Šulc) transmits the bacterium, "Candidatus (Ca.) Liberibacter solanacearum" (Lso), also known as "Ca. Liberibacter psyllaurous", which causes zebra chip disease in potato and other solanaceous crops. The authors previously showed that fecundity and nymph survival is significantly reduced in Lso-infected psyllids compared to uninfected psyllids on tomato. However, it is not known whether the level of the pathogen is correlated with concomitant reduction in fitness of the psyllid vector. Using quantitative PCR assays, Lso levels were determined in adult female founders of isofemale lines for whom several life history traits were previously recorded. Analysis of psyllid isofemale lines revealed that Lso infection levels in founders or mothers was negatively correlated with 7-day fecundity, nymph survival percentage, and number of F1 progeny including eggs, nymphs and adults. There was a significant negative density-dependent relationship between Lso level and fecundity. That is, psyllids experienced decreasing levels in fecundity with increasing bacterial titer. There was no apparent negative density-dependent relationship between Lso copies and number of nymphs, nymph survival percentage and number of adults. The negative effect of Lso on psyllid fecundity is likely due to direct effects of the bacteria on the insect host and not via the host plant. Taken together, these findings suggest that the level of Lso in its psyllid vector correlates with reduction in psyllid fitness.