You are what you eat: the effect of Euphydryas phaeton’s novel host plant use on its morphology

Background/Question/Methods

The butterfly Euphydryas phaeton (Baltimore Checkerspot) has historically specialized on the host plant Chelone glabra, but has recently adopted an exotic host plant, Plantago lanceolata. Niche expansions to exotic hosts are not uncommon, but the the eco-physiological implications of such expansions are unclear. Here we investigate the effect of a larval host expansion on the morphology of adult E. phaeton.

In May 2012, 24 outdoor enclosures were deployed at two sites in Massachusetts. Both sites supported natural populations of C. glabra, P. lanceolata, and E. phaeton. Enclosures were deployed over naturally occurring host plants, half enclosing P. lanceolata and half C. glabra and inoculated with E. phaetonlarvae.  Enclosures were monitored to ensure host was not exhausted and during the flight period emerging adult were measured, marked and released.  Measurements included weight to the nearest milligram, body length to the nearest 0.1 mm, and wings digitally photographed against graph paper for calculation of wing area. 

Results/Conclusions

As expected, we found clear sexual-size dimorphism with females larger than males in both wing area and weight. Unexpectedly, we found male and female morphologies were differentially affected by host plant. Males reared on C. glabra averaged 37% larger wing areas than those reared on P. lanceolata. However, male weight was not affected, and there was no relationship between male weight and wing area (R² = 0.03046). In females, host plant affected neither wing area nor weight. Instead, our results indicate that female wing area increases as a function of weight (R² = 0.23601).

It is unclear how P. lanceolata causes reduced wing size in males or why females are unaffected, but our results suggest that males prioritize their developmental resources into wing area after body mass reaches a certain threshold. Larger wings in males reared on C. glabra suggest that males might gain a competitive advantage in defending territories and attracting mates with larger wings to drive more powerful flight. Preliminary mark-recapture data suggest males with larger wings are superior at defending territories. If this translates into more matings, the pattern we show suggests that the interaction between sexual selection and host plant nutrition might maintain host plant specialization.