Eucalypts have been grown in New Zealand since the 1860s for several end uses including pulp and paper production, and high-value hardwood. More recently plantations have aimed to provide ground-durable eucalypt timber as a sustainably grown alternative to Pinus radiata D.Don treated with chromated copper arsenate. Six Australian paropsine leaf beetles (Coleoptera: Chrysomelidae) have established in New Zealand so far. The Eucalyptus tortoise beetle, Paropsis charybdis Stål, was the first paropsine that established over 100 years ago, has spread throughout the country, and is currently the most destructive defoliator, causing severe damage to short-fibre plantations of Eucalyptus nitens (H.Deane & Maiden) Maiden and hardwood plantations of Eucalyptus quadrangulata H.Deane & Maiden. Several attempts of classical biological control with egg and larval parasitoids have been undertaken with some success. The Eucalyptus variegated beetle, Paropsisterna cloelia (Stål), was detected in 2016 and defoliation in hardwood plantations of E. bosistoana F.Muell. and E. tricarpa (L.A.S.Johnson) L.A.S.Johnson & K.D.Hill appears to be the most severe. This thesis aimed to quantify the phenology of Pst. cloelia and P. charybdis and the effect of general predators and parasitoids currently present in New Zealand on the two paropsine species.

In a comprehensive literature review, chapter one compares life history traits, distribution, host preferences, phenology, natural enemies, and potential pest impacts of Pst. cloelia and P. charybdis in New Zealand. Australian distributions of both species largely overlap, indicating similar climate tolerances. Hence, it can be assumed that Pst. cloelia will also spread throughout New Zealand over time. In New Zealand, host preferences within the Eucalyptus subgenus Symphyomyrtus seem to vary between the two paropsines, with P. charybdis preferentially feeding on eucalypt species from the section Maidenaria, whereas Pst. cloelia seems to prefer the section Adnataria. Paropsisterna cloelia has a higher reproductive output than P. charybdis and advantageous life strategies that may lead to a higher survival rate of immature stages. This gives it the potential for more frequent population outbreaks and subsequently more severe defoliation on preferred hosts. Based on a literature review and personal communications, knowledge gaps were identified and, accordingly, thesis goals determined as follows. Goal 1: Quantify the phenology, meaning the timing of different life stages and the number of generations completed per year, of Pst. cloelia and P. charybdis in Marlborough. Goal 2: Develop a species-specific molecular assay that detects the DNA of Pst. cloelia and P. charybdis within field-collected predatory arthropods. Goal 3: Identify the most abundant predatory arthropod species in the field and quantify the proportion that feed on Pst. cloelia and P. charybdis life stages. Goal 4: Quantify the impact of the two established paropsine egg parasitoids on Pst. cloelia and P. charybdis populations in the field.

In chapter two, the timing and abundance of different life stages (phenology) and the number of generations (voltinism) of Pst. cloelia and P. charybdis were quantified on E. bosistoana over two growing seasons at two field sites in Marlborough. Paropsisterna cloelia clearly dominated on the trees, whereas P. charybdis was practically absent. Results show that the availability of new foliage, facilitated primarily by the synchrony of warm temperatures and water availability, determined the number of generations, timing of life stages, and population size of both paropsine species. Timing of P. charybdis and Pst. cloelia life stages coincided at both field sites. As availability of new leaves in spring and mid-summer prompts oviposition, both species had two generations at the site where microclimate facilitated refoliation of trees after a first early summer defoliation event and one generation at the site where summer droughts prohibited refoliation of trees in mid-summer. Additionally, field observations indicate that both species have the potential for a third generation if the growth season is extended by warm spring and autumn temperatures and sufficient rainfall to facilitate production of new foliage earlier in the season as well as after the second summer defoliation event. These conditions may be met in the Northland and coastal Bay of Plenty regions. Consequently, according to current knowledge, Pst. cloelia does not produce more generations than P. charybdis and immature stages are not active for longer during the season.

In chapter three, two sensitive quantitative PCR (qPCR) assays with species-specific primers that target the cytochrome oxidase I (COI) gene region were developed. Sequences and/or specimens for five of the six closely related paropsine species present in New Zealand at the time of assay development (i.e., Trachymela sloanei (Blackburn), Dicranosterna semipunctata (Chapius), Trachymela catenata (Chapuis), Paropsisterna beata (Newman), Trachymela sp. Weise) were included as non-target species in the assay development and testing to avoid non-target amplification. Assays reliably detected the DNA of both paropsine species within the bodies of field-collected predatory arthropods, thus identifying paropsine predators. Moreover, a decontamination protocol for field-collected predator individuals was developed to avoid false positives due to potential surface contamination.

Natural enemies play an important role in top-down control of pest species. However, in New Zealand, P. charybdis and Pst. cloelia have escaped most of their natural enemies from their native range Australia. In chapter four, field collections quantified richness and abundance (number of individuals) of predator taxa in two E. bosistoana plantations. Additionally, a combination of qPCR analysis and field observations of predation events confirmed that six insect species, two mite species, and three spider families are paropsine predators and quantified their activity as the number of collected specimens that had consumed paropsines. Richness and abundance of predator taxa varied over time and between the two sites and was higher at the site with complete ground cover and neighbouring forestry trials. At the more predator-rich site, the most abundant insect predator, Oechalia schellenbergii (Guérin) (Hemiptera: Pentatomidae), was also the most active paropsine predator, whereas the most abundant spider family, Pisauridae, was disproved as a paropsine predator. The Tasmanian ladybeetle, Cleobora mellyi (Mulsant) (Coleoptera: Coccinellidae), was the second most active predator, but had low abundance in the field. The red mite Anystis baccarum (Linnaeus) (Acari: Anystidae) was abundant at both sites and, at times of high abundance, was frequently seen feeding on eggs and newly hatched larvae. Most predators fed on eggs, few on early-instar larvae, and only O. schellenbergii preyed as both nymphs and adults on all paropsine life stages from egg to adult. Based on abundance, field observations of predation events, and molecular analysis, O. schellenbergii, C. mellyi, and A. baccarum appear to be the most promising candidates to support future paropsine control in New Zealand.

In chapter five, parasitism of P. charybdis and Pst. cloelia egg batches collected from E. bosistoana and E. quadrangulata trees at two field sites was quantified to assess the capabilities of the two established egg parasitoids, Enoggera nassaui (Girault) (Hymenoptera: Pteromalidae) and Neopolycystus insectifurax Girault (Hymenoptera: Pteromalidae), to control both P. charybdis and Pst. cloelia populations in the field. Simultaneous to egg collections, egg batch abundance of both paropsines was quantified on the two eucalypt hosts, revealing competitive displacement and host separation between P. charybdis and Pst. cloelia in the field. Paropsisterna cloelia competitively displaced P. charybdis on its most preferred eucalypt hosts, such as E. bosistoana, and consequently, P. charybdis switched to other suitable eucalypt hosts less preferred by Pst. cloelia, such as E. quadrangulata H.Deane & Maiden, if present at the site. If no suitable alternative hosts for P. charybdis were available at the site, P. charybdis was extremely rare. Hatch rates show that the two egg parasitoids controlled the second P. charybdis generation, but did not control Pst. cloelia populations, especially not on host trees most preferred by Pst. cloelia where it was most abundant. Parasitoids increased Pst. cloelia egg mortality through oviposition scars only on less preferred host trees where P. charybdis was also present and where abundance of Pst. cloelia was generally low.

Finally, the main findings of all chapters are drawn together in a general conclusion to provide recommendations for integrated pest management of paropsine beetles and future research. Host separation and competitive displacement between P. charybdis and Pst. cloelia suggest that additive defoliation on the same host species is unlikely because P. charybdis appears to avoid direct competition by ovipositing on a different host than Pst. cloelia if available. Paropsisterna cloelia is not affected by the two established egg parasitoids and predators are generally not sufficiently abundant to have an impact on populations. Moreover, it has a higher reproductive output than P. charybdis and advantageous life strategies that may lead to a higher survival rate. The species may therefore have bigger population outbreaks and cause more damage than previously observed by P. charybdis on its most preferred species, such as E. bosistoana. To date, Pst. cloelia has not established in E. nitens or E. globulus plantations in New Zealand and the risk of it replacing P. charybdis there is questionable as the two paropsines prefer hosts belonging to different sections within the eucalypt subgenus Symphyomyrtus. To investigate Pst. cloelia’s future potential impacts, further research should concentrate on phenology in warm and wet regions of New Zealand, where it may have more than two generations. Identifying host preferences and quantifying development rates on different host species could indicate the eucalypt hosts that may be most impacted by Pst. cloelia.

To exert effective paropsine population control, populations of paropsine predators need to be enhanced by attracting and retaining more individuals into forest plantations, and this could be achieved by using semio-chemicals, artificial supplementary food, and/or habitat improvement to increase the abundance and diversity of alternative food sources and provide shelter and overwintering sites. Future research could explore how to practically implement these measures in a cost-effective way and whether they can successfully increase predator abundance and pest mortality. Additionally, there would be the option to introduce specialist parasitoids for classical biological control of Pst. cloelia.

The present study demonstrated that timing of life stages can vary considerably between sites even in the same geographic region and that monitoring the production of new foliage can provide guidance on how to time pest monitoring. Establishment of development models from laboratory development trials can facilitate predictions on the occurrences of paropsine life stages in the field for more effective timing of control measures. In the future, standardised monitoring procedures to assess pest abundance and defoliation need to be developed and economic thresholds for defoliation and pest populations determined. This is essential for an effective IPM programme and can increase financial benefits and reduce impact on the environment due to more efficient pesticide use.