Comparative phenology of Paropsisterna cloelia and Paropsis charybdis in Marlborough
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Executive Summary: Paropsine leaf beetles (referred to as paropsines) are the most damaging group of eucalypt foliar pests in Australian plantations and elsewhere where eucalypts are grown. Six eucalypt-feeding paropsine leaf beetles from Australia have established in New Zealand (NZ) to date. Paropsis charybdis has historically been the most damaging defoliator of eucalypt species throughout NZ and is currently regarded as the number one eucalypt pest. Paropsisterna cloelia was first detected in March 2016 in New Zealand, in Hawke’s Bay. The current known range of Pst. cloelia in NZ spans the central North Island to the upper South Island, including the districts of Gisborne, Taupo, Hawke’s Bay, Manawatu-Wanganui, Wellington, Marlborough, Nelson, and North Canterbury. The Australian distributions of P. charybdis and Pst. cloelia largely overlap and extend from subtropical Queensland to cool temperate Tasmania. This suggests a similar climate tolerance of the two species. Paropsis charybdis generally produces two generations per year in NZ, however, no systematic study of Pst. cloelia phenology (i.e., seasonal presence and abundance of life stages) has been completed. In Australia, Pst. cloelia produces two to five generations depending on climatic region.
This project aims to understand Pst. cloelia phenology in NZ. This contributes to our understanding of the potential risks posed by Pst. cloelia compared to P. charybdis. Moreover, knowledge of a pest species’ phenology informs the development and implementation of more effective and efficient control methods, such as integrated pest management and biological control.
A total of 15 assessments were made at regular two-week intervals between September 2021 and March 2022 on Eucalyptus bosistoana at the NZDFI Dillon site in Marlborough. Immature life stages (egg batches and four larval stages) and adults beetles of Pst. cloelia and P. charybdis were counted. Defoliation and the production of new leaves was measure using the Crown Damage Index (CDI) and New Leaf Score (NLS), respectively. The southern distribution of Pst. cloelia along the eastern coast of the South Island was quantified in an ad hoc, opportunistic assessment.
Paropsisterna cloelia clearly dominated P. charybdis on E. bosistoana and accounted for 96% of all immature stages and 88% of all adult beetles counted throughout the sampling season. Both species produced two generations with simultaneous peaks of immature life stages in November and January for the first and second generation, respectively, and the start of a third generation with egg batches again being found in March. The total time that immature stages were discovered in the field differed between species: first Pst. cloelia egg batches were observed on 28 September and first P. charybdis batches on 26 October. At the end of the season, however, immature life stages of both species were still found on the last sampling date on 29 March. The second larval generation was smaller for both species with second-generation last-instar larvae being 35 and 4% of the first generation for Pst. cloelia and P. charybdis, respectively. Additionally, survival rate of last-instar larvae in the first generation was 46% and 88% for Pst. cloelia and P. charybdis, respectively, but only 20% for both species in the second generation. New Leaf Score declined severely from an average of 2 to 0.1 from beginning to end of the first larval generation (November/December), but showed the highest seasonal peak of 2.5 in the middle of the second larval generation (January/February). Hence, food limitation is an unlikely driver of the higher larval mortality. We tentatively attributed low larval survival in the second generation to an increase in predation. Since its first discovery in the South Island in Nelson/Marlborough in 2019, Pst. cloelia has spread by at least 120 km in a direct line southward to North Canterbury.
We conclude that it is likely that Pst. cloelia will establish throughout NZ where preferred eucalypts occur and will dominate or potentially even outcompete P. charybdis. Paropsisterna cloelia has the potential to become a damaging pest to its preferred eucalypt hosts, but it remains unknown whether its abundance and impact on the trees will exceed that previously observed by P. charybdis. The threat that Pst. cloelia may pose to eucalypt plantings in the warm northern regions will depend on a complex interaction between climate, the ability of trees to produce new leaves in response to herbivory, and the strength of top-down control by natural enemies on populations.