White butterfly (Pieris rapae) and the white rust Albugo candida on Cook’s scurvy grass (Lepidium oleraceum)

Once widespread, Cook’s scurvy grass (or nau, Lepidium oleraceum) is now confined to a few offshore populations. Classed as nationally endangered by the New Zealand Department of Conservation, populations of Cook’s scurvy grass are threatened by a number of factors, including introduced herbivorous insect species such as the white butterfly (Pieris rapae) and white rust infection caused by the oomycete Albugo candida. In this paper, we investigate the occurrence of white butterfly on Cook’s scurvy grass and possible interactions with the white rust infection on the northernmost of the Matariki Islands in the Firth of Thames, New Zealand. We found that larger host plants were more likely to be infested with white butterfly. The occurrence of white butterfly eggs and larvae also decreased as levels of white rust increased. Twenty-eight percent of the white butterfly larvae collected and reared in the laboratory were parasitised by the braconid wasp species Cotesia rubecula. We also reared a hyperparasitoid belonging to the super-family Chalcidoidea from one of the parasitoid cocoons. Further studies on the trophic interactions between Cook’s scurvy grass, Albugo candida and white butterfly and its parasitoids could improve the understanding of the threats posed by plant pathogens and insect herbivores to populations of Cook’s scurvy grass, which in turn may lead to new management strategies for conservation.