Compensatory feeding in Danaus plexippus (Lepidoptera: Nymphalidae) in response to variation in host plant quality

Because plant nitrogen content varies within and between plants, insects must often compensate for low nitrogen levels by consuming more or suffer decreased performance. To test the effects of varying nitrogen content and plant characteristics that are affected by nitrogen content on monarch butterfly (Danaus plexippus L.) growth and development, we applied high and low nitrogen fertilizer treatments to common milkweed grown in a glasshouse. High nitrogen fertilization resulted in increased leaf nitrogen and plant height, but also higher disease and pest levels. Monarchs fed plants in the low nitrogen fertilization treatment showed higher relative consumption rates. Monarch performance, measured with development time and relative growth rate, was affected differently in different larval stadia, but larvae of all stadia fed leaves from the low nitrogen fertilization treatment weighed as much as or more than those fed high nitrogen leaves. These results suggest that monarch larvae compensate for lower plant quality by consuming more. If increased consumption is costly because of increased exposure to natural enemies or increased expenditure on consuming and processing food, low nitrogen host plants may result in decreased fitness, despite the monarchs' ability to compensate.