Biology of insects that feed in the inflorescences of Chionochloa (Poaceae) in New Zealand and their relevance to mast seeding

Inflorescences of New Zealand Chionochloa species are attacked by at least three insects, two flies and a moth. There has been disagreement about the identity of various life stages of these insects. We followed the seasonal pattern of occurrence of the two fly species that fed in Chionochloa pallens inflorescences in a population on Mt Hutt, Canterbury. Eggs and larvae of Diplotoxa similis (Diptera: Chloropidae) appeared in the inflorescences as soon as they emerged. The larvae are principally flower feeders, and most D. similis individuals had pupated by the end of the flowering period. Diplotoxa similis adults emerged from the puparia at the end of the season, and probably overwintered as adults. The second fly was an undescribed cecidomyiid (Diptera: Cecidomyiidae). Eggs of the cecidomyiid are laid into the C. pallens florets at the time of flowering, and hatch into mobile, translucent, early-instar larvae. Late-instar larvae were less mobile and opaque orange. and probably dropped from the inflorescences late in the season. The third species, Megacraspedus calamogonus (Lepidoptera: Gelechiidae) has large mobile caterpillars which appear early in the season, but there is doubt about its egg morphology and oviposition sites. On two dates there was a negative correlation between densities of the two fly species among plants. Since D. similis appears first, it may be able to usurp resources and reduce densities of the cecidomyiid. A review of known occurrences of the three insects suggests that. compared to D. similis and M, calamogonus, the cecidomyiid (1) has a greater geographic range and (2) occurs on more Chionochloa species; however, these trends might be due to poor sampling of D. similis and M. calamogonus early in the season. The cecidomyiid appears to be less easily satiated than D, similis by masting in Chionochloa.