Diapause by seed predators and parasitoids in Chionochloa mast seeding communities
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Chionochloa, a genus of snow tussock grasses native to New Zealand, exhibits pronounced mast seeding. Chionochloa suffers very high levels of pre-dispersal flower and seed predation by three main insects: Eucalyptodiplosis chionochloae, a cecidomyiid midge, which is formally described here; Megacraspedus calamogonus, a gelechiid moth and Diplotoxa similis, a chloropid fly. Seven species of parasitoids that attack these seed predators were discovered. Four species parasitize M. calamogonus (one tachinid fly and three hymenopteran wasps), one parasitizes D. similis (Hymenoptera: Eulophidae) and two parasitize E. chionochloae, (a pteromalid wasp Gastrancistrus sp. and a platygastrid wasp Zelostemma chionochloae, which is given a formal description here). The abundance, predation levels by each of the insect species, and interactions between all the organisms in the community were studied across three elevations at Mount Hutt over three summer seasons. M. calamogonus was most abundant at 450 m altitude during all three seasons. D. similis was most common at 1070 m altitude, while its predation levels peaked in low flowering seasons and decreased in high seasons. E. chionochloae was abundant in all three altitudes and increased its predation levels with increasing flowering intensity. E. chionochloae was confirmed to use prolonged diapause of at least three years. Prolonged diapause was also confirmed in its two parasitoids. Chionochloa plants were manipulated with various treatments to test the effect on diapause in E. chionochloae and its two parasitoids. Treatments included plant warming, root pruning, gibberellic acid sprayed on the plants and combinations of these treatments. All three insects changed their emergence in response to some treatments and therefore it was suggested that combined with risk-spreading diapause, they may use some predicting to emerge from prolonged diapause. E. chionochloae control their diapause following some of the cues that Chionochloa use for flowering, while Z. chionochloae and Gastrancistrus in some cases follow their host’s cues and in others use similar cues as Chionochloa plants. Emergence or diapause predictions differed across elevations and plant species in all three insect seed/flower predators. E. chionochloae had female-biased sex ratios in different populations even after prolonged diapause. There was week evidence that both parasitoid species are female-biased in the first emergence year and male-biased after more than one year in diapause. Therefore it was suggested that diapause is not more costly for females of E. chionochloae and its parasitoid than for males. Females of all three species were not found to be better predictors (i.e, more likely to respond to treatments by not entering extended diapause) than males. The complex interactions of all the organisms in this web are thought to be sensitive to climate, and it was suggested that the global climate change may alter this sensitive system.