The eastern population monarch butterflies has declined over the past decade. The Iowa Monarch Conservation Consortium (IMCC) was established to address the monarch decline in Iowa. Few studies have examined larval survival on milkweeds with overlapping ranges. This study was conducted to determine if larval monarch butterflies would feed and mature at equal rates on nine milkweed species native to Iowa. Milkweeds with a variety of habitat requirements were tested including A. exaltata, A. hirtella, A. incarnata, A. speciosa, A. sullivantii, A. syriaca, A. tuberosa, A. verticillata, and C. laeve.
First instar larvae were reared in a greenhouse on milkweeds grown without the use of pesticides at Iowa State University (ISU). Each neonate was added onto a 4 month old milkweed plant. Plants were arranged in a randomized complete block design. Larvae were fed ad libitum, and were monitored for survivorship. Plants were changed at days 5 and 10. Larvae were monitored daily for pupation starting on day 13. Twenty-four hours after pupation, pupal mass, length, and width were recorded. Twenty-four hours after eclosion, adult mass, forewing length, and hindwing length were measured. Adults were frozen and dried; lipids were extracted using petroleum ether at Sweet Briar College (SBC).
As expected, monarch larvae survived on all milkweeds; however, survivorship was significantly different among larvae that ate different milkweeds. Seventy-five percent of larvae survived to adulthood on A. tuberosa compared to 30.6% on A. hirtella and 36.1% on A. sullivantii. The duration of larval and pupal stages was not different among larvae. Pupal mass and adult lipid content were significantly different among larvae that fed on different milkweed species. There were no differences in pupal measurements or adult mass, but forewing length was different among monarchs that fed on different milkweed species as alrvae. Our results suggest that monarchs can survive on all of the milkweeds tested, but that some milkweeds may allow higher numbers of larvae to survive to adulthood than others.