Diet-dependent effects of gut bacteria on their insect host: the symbiosis of Erwinia spec. and western flower thrips

Studies on bacteria in the gut of insect species are numerous, but their focus is hardly ever on the impact on
host performance. We showed earlier that Erwinia bacteria occur in the gut of western flower thrips, most
probably acquired during feeding. Here, we investigate whether thrips gain a net benefit or pay a net cost
because of these gut bacteria. On a diet of cucumber leaves, the time to maturity is shorter and the oviposition
rate is higher in thrips with bacteria than in thrips without (aposymbionts). When fed on cucumber
leaves and pollen, aposymbionts develop faster and lay more eggs. So Erwinia bacteria benefit or parasitize
their thrips hosts depending on the diet, which is in accordance with theoretical predictions for fitness of
organisms engaged in symbiotic interactions. Possibly, the transmission of gut bacteria has not become
strictly vertical because of this diet-dependent fitness variability.