THE UTILIZATION OF VARIOUS ASCLEPIAS SPECIES 
BY LARVAE OF THE MONARCH BUTTERFLY, 
DANA US PLEXIPPUS 
By James M. Erickson* 
Dept, of Entomology, Cornell University 
Ithaca, New York, 14850 
Introduction 
Plants of the genus Asclepias are well known to contain high 
concentrations of cardiac glycosides (cardenolides) . It is generally 
surmised that such cardiac stimulants function to protect plants 
containing them against insect and vertebrate herbivores (Euw et ah 
1967). In addition, some insects which are adapted to feed on 
Asclepias plants store cardiac glycosides apparently as a means of 
protection against vertebrate predators which find the compounds 
distasteful (Brower and Brower 1964, Brower 1969). It was by 
means of such an herbivore-predator interaction, bluejays feeding 
on monarch butterflies, Danaus plexippus L. (Brower 1969, Brower 
et ah 1968), that a spectrum of palatability was detected among 
monarch butterflies reared on various species of milkweed. It is 
surmised that this spectrum of toxicity to a vertebrate predator reflects 
a spectrum of concentrations of cardiac glycosides in the different 
species of the insects’ larval food plants (Brower and Brower 1964, 
Brower et al. 1967, Brower 1969). However, quantitative and 
qualitative data for cardenolides in Asclepias leaves are at best in- 
complete (Reynard and Morton 1942, Kupchan et al. 1964, Duffey 
1970, Singh and Rastogi 1970, Feir and Suen 1971, Duffey and 
Scudder 1972, Scudder and Duffey 1972, and Eggermann and 
Bongers 1972). 
The following experiments were undertaken to determine 
whether a differential response by D. plexippus larvae to their host 
plants could be detected by measuring their efficiency of food utiliza- 
tion and whether such a response would support the concept of a 
spectrum of toxicity. It seemed conceivable that detoxication or 
storage of cardiac glycosides might require expenditure of energy 
and could be detected by a lowered feeding efficiency. In addition, 
Brower et al. (1972) have speculated that there is a reduction in 
^Present address: Dept, of Biological Sciences, California State Univer- 
sity, Hayward, California 94542. 
M anuscript received by the editor June 18, 1973. 
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