The evolution of host specialization in phytophagous insects is genera
lly thought to involve genetic trade-offs that prevent individuals fro
m maximizing fitness simultaneously on two or more hosts. Several hypo
theses, however, have suggested that trade-offs may not be evident in
experiments comparing larval performance on normal and novel hosts. Te
sts on survivorship, growth rate, and pupal mass among families of the
swallowtail butterfly Papilio oregonius on its normal host and on a n
ovel host provide support for these hypotheses, although they do not d
iscriminate among them. Families differed in their relative performanc
e on the hosts, but there was no evidence of a negative genetic correl
ation between hosts for any of the measures of performance. In additio
n, there were no correlations among the different measures, corroborat
ing an earlier result suggesting that these different components of pe
rformance in the P. machaon species group are under at least partially
separate genetic control. These results and similar results published
for other insects have now produced a body of studies indicating that
genetic trade-offs in individual components of larval performance may
not be a major factor preventing shifts onto novel host plants. Trade
-offs leading to the evolution of host specialization are more likely
to involve coordination among the various components of performance to
gether with ecological factors that allow higher fitness on one host t
han on others.