Elytra color as a signal of chemical defense in the Asian ladybird beetle Harmonia axyridis

Carotenoid- and melanin-based colors are valuable indicators of quality in many vertebrates, but their signaling role in invertebrates remains relatively unexplored. The Asian ladybird beetle Harmonia axyridis is an ideal organism for studies of this nature because males and females exhibit highly variable red and black colors on their elytra and are chemically defended with an alkaloid (harmonine). We used digital photography to quantify elytra coloration and absorbance spectrophotometry and gas chromatography mass spectrometry analyses to quantify pigment and alkaloid concentrations, respectively, in wild-caught male and female H. axyridis. We predicted that extensive or intense coloration would be an aposematic signal of high-alkaloid stores. We found that carotenoid pigments largely controlled variation in red elytra coloration. There was no relationship between alkaloid content and either elytra redness or carotenoid pigment concentration in either sex. However, we found a positive correlation between the extent of elytra coloration and alkaloid content. Animals with proportionally more red (or less black) on the elytra were more alkaloid-rich; this relationship was particularly strong in females. We also found that females with lighter black spots had greater amounts of harmonine than those with darker spots. These results suggest that elytra color patterns have the potential to reveal information about chemical defensiveness to mates or predators. Prior studies in this species show that nonmelanic forms are typically less active and yet more sexually attractive than melanic forms, and both results may be explained by the fact that nonmelanics are better chemically defended.