Completing development to a reproductively mature adult is energetically expensive, and environmental stress can impose substantial fitness tradeoffs during developmental processes. We tested the prediction that high larval rearing density and food stress should compromise monarch, Danaus plexippus L. (Lepidoptera: Nymphalidae), larval growth and the production of melanin or other pigments in the adult wings. We reared focal larvae on caged, potted plants in control, food stress and high density treatments. Food stressed larvae experienced food shortages but no crowding (n = 1 larva per plant), whereas high density larvae were crowded (n = 5 larvae per plant) but had constant access to food. We used digital image analysis to measure the size and coloration (black and orange) of the resulting adult forewings. Larval rearing treatment affected adult size, but not wing coloration or development time. Adults emerging from the high density treatment were significantly larger than adults from the control or food stress treatments. This suggests that larval crowding may stimulate monarch larvae to increase their feeding rate, producing larger body size when there is not a subsequent food shortage caused by the crowding. Food stress did not affect adult size, wing coloration or development time, which suggests that monarch larvae can overcome moderate levels of food stress without experiencing compromised development. Sibling group affected forewing area, development time, and several aspects of wing coloration, which is consistent with previous research that has demonstrated a heritable component to size and melanism in monarch butterflies.
digital image analysis