1. Despite the profusion of Lepidoptera and other potential hosts in the tropics, the Ichneumonidae are not more species-rich in the lowland tropics than in temperate communities. Alternate but partially complementary hypotheses emphasizing resource fragmentation, predation, or chemical defence have been proposed to explain this paradox.
2. Because different diversity gradients occur in the various subgroups of the Ichneumonidae, as well as among their hosts, general explanations are unlikely to account for this ‘anomalous diversity.’ Hypotheses of ecological causation are best tested at the taxonomic level where the pattern of interest applies.
3. Phylogenetic trends in the distribution, abundance, and palatability of butterflies (Papilionoidea), a group that increases dramatically in species richness towards the tropics, are presented as a case study for evaluating hypotheses of parasitoid species richness. Evidence is presented that supports the ‘nasty host hypothesis’ as an explanation for the lack of diversity of associated parasitoids.
4. An examination of hypotheses and the approaches used to test them suggests that the nasty-host hypothesis is best supported by the limited evidence available, while evidence in favour of other hypotheses is either lacking or ambiguous. In particular, use of the koinobiont/idiobiont dichotomy and parasitoid assemblage-size comparisons may result in contradictory conclusions.