Theory on plasticity driving speciation, as applied to insect–plant
interactions (the oscillation hypothesis), predicts more species in
clades with higher diversity of host use, all else being equal. Previous
support comes mainly from specialized herbivores such as butterflies,
and plasticity theory suggests that there may be an upper host range
limit where host diversity no longer promotes diversification. The
tussock moths (Erebidae: Lymantriinae) are known for extreme levels of
polyphagy. We demonstrate that this system is also very different from
butterflies in terms of phylogenetic signal for polyphagy and for use of
specific host orders. Yet we found support for the generality of the
oscillation hypothesis, in that clades with higher diversity of host use
were found to contain more species. These clades also consistently
contained the most polyphagous single species. Comparing host use in
Lymantriinae with related taxa shows that the taxon indeed stands out in
terms of the frequency of polyphagous species. Comparative evidence
suggests that this is most probably due to its nonfeeding adults, with
polyphagy being part of a resulting life history syndrome. Our results
indicate that even high levels of plasticity can drive diversification,
at least when the levels oscillate over time.