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.

Methods
Data collection was performed on dead adult butterflies that were killed by freezing. Upon thawing, all four wings of each butterfly were cut as close to the abdominal attachment as possible. The remainder of the butterfly was saved, desiccated, and later weighed (this weight is represented as the variable “drywt” in the data set).

Detached wings were laid flat and were encased in clear packaging tape so that the full wing area could be displayed for imaging.

Photographs of each wing were produced with a mounted camera. Images were imported to ImageJ, where wing traits were measured. The wing traits “forewing area” and “hindspot area” were generated using the “polygon” tool. The wing trait “hindwing vein” was generated using the “line” tool. A subset of measurement values from each treatment level were randomly selected for measurement replication to both ensure the fidelity of measurement and to calculate the magnitude of measurement error.