Which native milkweeds are acceptable host plants for larval monarch butterflies (Danaus plexippus) within the Midwestern U.S?

The eastern population of migratory monarch butterflies has declined over the past decade.  The Iowa Monarch Conservation Consortium (IMCC) was established in March 2015 to address the monarch decline in Iowa.  This study was conducted to determine if larval monarch butterflies would feed and mature at equal rates on leaf material and whole plants from nine milkweeds native to Iowa.  Species tested included 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 the laboratory.  Each neonate was added singly to a petri dish containing one species of milkweed or to a 6 month old milkweed plant in the greenhouse.  Petri dishes and plants were arranged in a completely randomized block design. Petri dishes were kept in an incubator (28°C; 40% relative humidity); plants were kept in a greenhouse. Larvae were fed ad libitum and were monitored for survivorship.  After five days, head capsule width, larval instar, and larval mass were measured.  Neither head capsule width nor instar was significantly different among milkweed species.  Larval weight was significantly different among species.  Larvae fed leaves of A. hirtella weighed less than those fed leaves of A. incarnata, A. syriaca, A. tuberosa, or A. verticillata.  Larvae that fed on C. laeve plants weighed the least of all nine species. Our results suggest that all nine milkweeds are acceptable larval host plants for the monarch butterfly but there may be some differences in growth rates among milkweed species.