Does fluctuating asymmetry of wing traits capture relative environmental stress in a lepidopteran?

Fluctuating asymmetry (FA) may be a useful predictor of population
canalization, especially for organisms at risk from environmental change.
Identification of traits that meet statistical criteria as FA measures
remains a challenge. In the present study, a laboratory experiment
subjected immature butterflies (Vanessa cardui) to a range of diet and
temperature conditions of varying stress levels. Variation in dietary
macronutrient ratio (protein: carbohydrate) and rearing temperature
(optimal: 25°C; elevated: 32°C) were introduced as stressors. Individuals
subjected to stressful conditions were predicted to show elevated FA of
three wing size traits. While FA of all three traits proved measurable, it
did not vary across diet and temperature treatments. Instead, treatment
levels impacted viability: the combined incidence of death prior to
eclosion and expression of significant wing malformations increased in
treatment levels predicted to increase FA. Variation in adult dry mass
also reflected predicted stress levels. Results suggest that predicted FA
variation was not found because individuals predicted to display increased
FA either died or displayed gross developmental aberrations. This
experiment illustrates important constraints on the investigation of FA,
including selection of appropriate traits and identification of
appropriate levels of stressors to avoid elevated mortality. The latter
concern brings into question the utility of FA as an indicator of stress
in vulnerable, natural populations, where stress levels are rarely
controlled, and mortality and fitness effects are often not quantifiable.