The role of host associated differentiation (HAD) in facilitating the extraordinary diversification of phytophagous insects has attracted much attention. However, relatively little attention has been paid to how the diverse parasitoids of these insects respond to genetic diversification of their hosts, and whether HAD may cascade upward through trophic levels. Expanding on previous research analyzing host-plant related differentiation of goldenrod feeding herbivores, we have identified at least two cases in which parasitoids of these herbivores themselves exhibit differentiation among host lineages. In both cases host-associated forms are broadly sympatric and they may have diverged in sympatry. These findings indicate that effects of host plant related divergence in herbivores may ramify up to higher trophic levels and that cascading host associated differentiation may provide an important means of diversification in insect parasitoids.
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