Developmental instability in a stem-mining sawfly: can fluctuating asymmetry detect plant host stress in a model system?

Fluctuating asymmetry (FA) may be a sensitive indicator of the stress experienced by organisms during their development. Its use in this manner is an intuitively appealing, frequently proposed, and potentially powerful tool but remains controversial partially because its underlying premise rarely has been critically tested. Such tests should include direct comparisons among individuals for which levels of FA, stress and fitness have been unambiguously quantified. We assessed the use of FA as a bioindicator of the stress experienced during egg-to-adult development by the stem-mining sawfly, Cephus cinctus Norton. Sawflies were reared in a common garden from seven different wheat cultivars, which were selected to represent a gradient of stem solidness, a key factor imposing stress on sawflies during development. In this model system, stress was quantified by the weight of emergent adults. Fitness was quantified by counting the number of eggs in dissected females, which emerge with their full lifetime complement. FA was measured for wing length, three wing cells, and three wing veins using image analyses. The greatest amount of stress was induced by solid-stemmed cultivars from which the adults were significantly smaller than those developing in hollow-stemmed hosts. In turn, adult weight was positively correlated with fitness. The net effect was a 25-fold variation in sawfly fitness, which gave a reasonable expectation that FA levels would differ across cultivars. However, FA levels of all the traits were similar among cultivars and there was no negative relationship between FA and fitness. These results: (1) document the failure of FA as an indicator of stress in this model system, (2) identify adult weight as a satisfactory indicator of plant-induced stress and sawfly fitness, and (3) add to the growing body of literature questioning the value of FA as a biomonitor tool of developmental stress.