A serious potential weakness of the Phylogenetic Constraints Hypothesis is that the pine sawflies (Hymenoptera: Diprionidae), commonly severe pests and a sister group to the common sawflies (Tenthredinidae), show the same constraint of a sawlike ovipositor. This caused Berryman (1997) to reject the hypothesis because he argued that phylogenetic constraints in common between diprionid and tenthredinid sawflies should result in the same emergent properties. This may prove to be a common conception which needs to be addressed in this chapter However, while the constraint may be the same in two or more families, the adaptive syndromes may be different, resulting in divergent population dynamics (cf. Price and Carr 2000). First, I will describe the differences between the families and then I will provide a hypothesis on why patterns in distribution, abundance, and population dynamics have diverged in these two sawfly families.

DIFFERENCES BETWEEN SAWFLY FAMILIES

One fundamental difference between tenthredinids and diprionids relevant to population dynamics is that a small percentage of tenthredinids are pest species (about 3 percent), whereas a large proportion of diprionids are serious pests in coniferous forests (about 40 percent in North America and 53 percent in Europe (cf. Larsson et al. (1993); for the largest genus in North America, Neodiprion, with 35 species, Arnett (1993) states that most are of economic importance). Many other differences in the families are evident (Table 8.1). Large tracts of conifer forest may be repeatedly defoliated, and trees eventually killed by diprionid sawflies.