BIOLOGY, ECOLOGY, AND MANAGEMENT OF PROSAPIA BICINCTA (HEMIPTERA: CERCOPIDAE) IN HAWAIʻI RANGELANDS

Abstract

Human mediated activities have resulted in the introduction of thousands of non-native organisms to the Hawaiian Islands, making this archipelago a hot spot for invasive species. Invasive species threaten the persistence of Hawaiʻi’s unique and diverse endemic species and native ecosystems and impact the state economy, local agriculture, water quality, nutrient cycling, culture, and human health. Substantial efforts have been devoted to the research, development, and implementation of invasive species management plans in Hawaiʻi to mitigate the detrimental impacts, conserve biodiversity and endemic species, and protect the limited agricultural resources. However, invasive species management continues to be a significant challenge across the islands. Spittlebugs (Hemiptera: Cercopoidea) are not native to Hawaiʻi, but three species have been unintentionally introduced. The twolined spittlebug, Prosapia bicincta, is the first of species in the family Cercopidae to invade Hawaiʻi. Since its initial detection in 2016, it has become a highly invasive and destructive forage grass pest, damaging thousands of hectares of pastures and threatening the persistence of the economically, ecologically, and culturally significant beef cattle industry in Hawaiʻi. In this study, I investigated the biology, ecology, and impacts of P. bicincta in Hawaiʻi rangelands by quantifying the geographic distribution and seasonal abundance, determining environmental factors influencing population dynamics, assessing spatiotemporal changes in the dynamics of rangeland plant communities, and investigating food plant associations. Five years of monthly field surveys in the Kona district of west Hawaiʻi island revealed season (wet vs. dry) and elevation were the most significant factors indicative of P. bicincta distribution and seasonal abundance patterns. Monitoring from 519 to 1,874 m above sea level (a.s.l.) indicated pastures located between 1,000 to 1,300 m a.s.l. had the highest abundance of P. bicincta and experienced the greatest decrease in grass cover. Prosapia bicincta modified rangeland plant communities by causing severe grass dieback of key forage grasses and facilitating the establishment of weedy forbs and shrubs. This pasture pest was most abundant between April-October when rainfall was highest in Kona. Majority of the food plant associations were grasses, but there were also detections on legumes, sedges, and forbs. These findings on the distribution and abundance of P. bicincta in Kona provide information on the seasonality and life cycle duration under local conditions which will improve the effectiveness of management by indicating when control tactics should be implemented to target vulnerable life stages (i.e., nymphs and eggs). Monitoring of seasonal pest abundance supports management decisions as it provides a measure of the severity of infestation and if management costs are justified, allows for early detection in new areas, helps predict areas at high-risk of invasion, and can be used to gauge the efficacy of management actions. Furthermore, I evaluated forage grass susceptibility to adult feeding to initiate research on the development of host plant resistance as management strategy for P. bicincta since it is one of the most suitable tools for spittlebug control in extensive pasture systems. Results indicated the incorporation of resistant grasses is a promising management strategy against this pest as multiple grass species and cultivars exhibited high levels of resistance in terms of foliar damage, above- and below-ground dry biomass, and estimated functional plant losses. Seven grass species and cultivars incurred little to no foliar damage during and after adult infestation in the greenhouse. Among these, six species/cultivars showed no significant difference in dry mass between infested and control plants, and five had considerably low functional plant losses. In contrast, Kikuyu grass, Hawaiʻi’s most widespread forage, was extremely susceptible and presented high levels of foliar damage, showed significant reductions in above- and below-ground dry biomass, and sustained the greatest amount of functional plant losses. These findings suggest Kikuyu grass pastures infested with economically important levels of P. bicincta will warrant replacement with more resistant forages such as those observed in this study. Confirmation of forage resistance to P. bicincta adults provides a foundation for developing and implementing host plant resistance as a management tactic. Additionally, a follow up study was initiated to investigate if the resistance observed in the greenhouse was also observed in the field and assess grass ability to establish across the highly variable growing conditions in Kona pastures. However, manual establishment of nine field plots (each containing 32 subplots of eight grass species/cultivars) distributed across three ranches proved to be extremely labor intensive and took significantly longer than anticipated, ultimately delaying data collection. Preliminary data based on six monthly surveys indicated the five most resistant varieties in the greenhouse only accounted for <10% of the total nymphs detected in the field plots. Lastly, I assessed the response of Kikuyu grass to increasing densities of P. bicincta by quantifying the foliar damage presented during and after infestation and comparing above- and below-ground dry biomass of infested and control plants. Results indicated 75 adults/m2 was a catastrophic threshold for Kikuyu grass and above- and below-ground dry biomass of Kikuyu infested with ≥108 adults/m2 differed significantly from the dry mass of control plants. These results have major implications for Hawaiʻi rangelands as it provides a reference value for estimating an action threshold for the most dominant forage grass. The action threshold indicates when management should be implemented to reduce P. bicincta populations before the level of 75 adults/m2 reached. This ensures the costs of management actions are justified based on the pest density and helps prevent the economic injury level from being reached. The findings presented in this dissertation demonstrate the impact of insect pests on forage crops and the dynamics of rangeland plant communities over time and space, highlight the major factors driving insect pest distribution and abundance, underscore the effects of different ecological conditions on the biology of an invasive species, and provide a foundation for management of a highly invasive pasture pest. These findings improve our understanding of insect pest biology, ecology, distribution, seasonal abundance, and impacts on plant communities beyond the native range, thus, highlighting the variation in pest impact, life cycle, and habitat associations across different regions. Ultimately, the knowledge gained from this work contributes to the development and implementation of pest management strategies in pastures.