Tradeoffs driving local adaptation in soapberry bugs: quantifying the costs of having the wrong beak length

Background/Question/Methods

Local adaptation often hinges on tradeoffs in the ability to use different local resources.  Understanding the specific mechanisms driving such tradeoffs is key to predicting when divergence will persist or break down.  Soapberry bugs (Jadera haematoloma) have shown repeated adaptation to novel host plants across the globe.  Soapberry bugs feed on the seeds of a subset of plants in the Sapindaceae which surround their seeds with inflated pods.  A widely observed pattern in this group is that beak length evolves to match local host pod size; however, costs of having the ‘wrong’ beak length have not been explicitly measured.  This study examined 7 populations of bugs across 3 hosts in Florida: the native host (Cardiospermum corindum) which has large closed seed pods, and two introduced hosts (Koelreuteria elegans and C. microcarpum), which have small pods that open during seed development.  We quantified feeding efficiency and reproductive output for females of naturally varying beak length in two feeding treatments: seeds in closed pods vs. seeds in open pods.  In order to disentangle correlation in beak length and adaptation to host quality, all bugs were fed on seeds from their local host affixed inside closed or open pods. 

Results/Conclusions

Preliminary results show population-specific responses in performance to treatment, with populations from the native host showing higher success feeding from closed pods and populations from the two introduced hosts showing higher success feeding from open pods.  This is consistent with the hypothesis that beak length evolved in response to pod condition in these populations.