1973 ] 
Erickson — Danaus plexippus 
241 
Recently, Brower et al. (1972) have shown a general decrease in 
cardiac glycoside content of migrating monarch butterflies as the 
specimens are collected farther south. They feel selection may be 
operating against high cardiac glycoside content (larvae reared on 
A. curassavica or A. humistrata ) since, although high concentrations 
of cardiac glycosides in the butterfly confer greater protection from 
predators, these authors surmise that these high concentrations de- 
crease the viability of the insect. In this experiment, there was no 
significant difference in either the number of eggs deposited or the 
number of these that were fertile among the adult females reared 
on the four Asclepias species (Table 4). It does not appear at least 
over a couple of generations, that larvae reared on A. curassavica are 
any less viable than larvae reared on much less toxic Asclepias species. 
The importance of nitrogen for larval growth and development 
cannot be overemphasized. House (1961, 1962) and Dadd (1973) 
have discussed the qualitative requirements of proteins and amino 
acids for larval development. There appears to be an optimal nitro- 
gen level, which varies from species to species, that produces maximal 
larval growth (Dadd 1961, House 1959, Vanderzant 1958). The 
fecundity of Dacus dorsalis Hendel increased with an increase in the 
protein content of the diet (Hagen 1958), whereas low protein 
levels greatly prolonged the developmental period of Drosophila 
melanogaster (Sang 1956). In this experiment, larvae reared on 
A. tuberosa had the second fastest developmental time of the 4 
groups of larvae, yet this plant species contained a little more than 
one half the nitrogen content in the leaves than was contained in 
the other leaves of the other host plants. Larvae of the monarch 
butterfly appear, therefore, to have an adaptive strategy which al- 
lows them to best utilize a resource that is in limited supply. Al- 
though ‘wasteful’ in the caloric sense, these larvae are able to secure 
the necessary supply of nitrogen needed for the later adult stage by 
increasing their total intake of food (Table 3). A similar situation 
has been demonstrated in this laboratory involving the utilization of 
crucifer plants by Pieris rapae (Slansky and Feeny, in preparation). 
This ability of an insect to compensate for decreased nutrient con- 
tent of its food is discussed by House (1965) and McGinnis and 
Lasting (1966). It thus appears that the low nitrogen content of 
A. tuberosa has a somewhat limiting effect on the monarch larvae 
but this low nutrient content only limits or regulates the efficiency 
with which food is utilized by the larvae and does not limit the 
larval growth or consumption rates to a significant degree. 
