The flax weevils of Mana Island : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Ecology at Massey University, Manawatu, New Zealand

Abstract

In 2004 and 2006 flightless endemic flax weevils (Anagotus fairburni) were deliberately translocated to Mana Island, New Zealand, as part of the island’s restoration plan. The flax weevil population grew exponentially and by 2018 the weevils were dispersing and had destroyed large areas of their host plant, flax (Phormium spp.), on which threatened bird and lizard species on Mana Island rely for both habitat and food. Little is documented about flax weevil ecology, so it wasn’t immediately obvious why the flax weevil population was growing beyond the ability of the flax to sustain them. Investigations by Colin Miskelly into possible natural enemies of the flax weevil led to the discovery of a strain of the entomopathogenic fungi Beauveria pseudobassiana, naturally occurring on the island that was highly virulent to flax weevil larvae. This native pathogen was prevalent at the flax weevil release site, but uncommon further down the island, which led to the hypothesis that the flax weevil was able to disperse ahead of its natural enemy on the island. We decided to investigate the feasibility of deliberately spreading B. pseudobassiana ahead of the flax weevils, to protect flax plants from collapse and death. In August 2018 a trial site was established in the centre of the island well ahead of known flax weevil sites and 80 mature flax plants were individually marked. A fauna monitoring programme was put in place to understand the flax plant animal community before and after any experimental changes at the site. An irrigation system was designed and installed to deliver supplementary water to some flax plants, to test whether access to water during the crucial dry summer months was a critical variable for survival of flax plants. In March 2020 marked flax plants were treated with a solution of the virulent B. pseudobassiana strain, or a control mixture, both delivered via water the base of the plant. These plants were then stressed by the deliberate introduction of 1600 flax weevils. Numerous novel observations on flax weevil ecology and behaviour were made throughout the duration of the flax plant trial. Flax weevils were found to go through 6 to 7 instars as larvae in the soil, were mobile at the trial site as adults, and could walk long distances from their flax release sites over time. An alternative feeding plant was observed for adult weevils, which were found to feed on the leaves of Muehlenbeckia complexa. The trial release of B. pseudobassiana failed to protect most flax plants at the experimental site, with 35 out of 43 marked plants now (2022) in a state of heavy collapse or death. We attribute this failure to an insufficient dosage rate, although it is possible that release timing or environmental conditions might also have led to an unsuccessful release. Possible future ecological scenarios for flax weevil on Mana Island are discussed.