UC Riverside

With respect to California and the relatively recent irruption and spread of P. perseae in 2017, knowledge gaps were identified regarding the effects of temperature on the development and reproductive biology of P. perseae, population dynamics of these more damaging populations and resulting leaf damage from feeding, natural enemy associations in commercial Hass avocado orchards, use preferences across different avocado cultivars, and the distribution of haplotypes present in California in both native and invaded regions. Work presented in this thesis investigated these critical areas with the aim of better equipping the California avocado industry with updated information on the biology, phenology, damage, and natural enemy associated so that more effective management strategies for this pest can be developed within an IPM framework. In Chapter One, temperature-driven degree-day models were developed to provide insights into how temperature affects the development and survival rates of different P. perseae life stages. This work analyzed the effects of six fluctuating temperature profiles (with daily averages of 15°C, 20°C, 25°C, 30°C, 32°C, and 35°C) on the life history traits (i.e., development times and longevity) of P. perseae. The aim of this work was to enhance understanding of the role of temperature on the development and reproductive biology, and phenology of P. perseae. These data are crucial for developing degree-day models for P. perseae and for understanding how high temperature events, such as heat waves and Santa Ana winds may affect pest populations in the field. Degree-day models developed in Chapter 1 provide a tool for predicting temperature-based population growth trajectories which can aid in the optimal timing of management strategies, such as insecticide applications or natural enemy releases, to control damaging populations of P. perseae in commercial Hass avocado orchards in California. Chapter Two investigated P. perseae phenology by examining monthly variations in population density and leaf infestation rates of P. perseae across four commercial Hass avocado orchards in northern San Diego County (two orchards were in cooler coastal zones and two were in warmer interior regions) over a period of two years on five different avocado cultivars (i.e., Hass, Lamb Hass, Fuerte, Bacon, and Gem). This information will assist in predicting the timing of population peaks and the likelihood of leaf damage, which will aid in the optimal scheduling of control measures. As part of monthly sampling efforts, densities of generalist predators were monitored to provide insights into the species identities of natural enemies, and the potential level of biological control provided by these biological control agents. Furthermore, leaf area damage from P. perseae and the perseae mite, Oligonychus perseae (Acari: Tetranychidae), feeding, along with tip burn resulting from too much chloride salt in irrigation water, were estimated using image analysis software to gain insight into the overall necrotic damage on leaves and to quantify the increase in leaf damage by the addition of P. perseae to the leaf feeding pest guild. This study took advantage of monthly leaf collections to examine an unusual egg-laying behavior, "egg-dumping," in P. perseae, where females oviposit into pre-existing clumps of eggs laid on leaves by conspecifics. “egg-dumping” is a trait observed in sub-social tingids (Tallamy, 1985; Tallamy and Horton, 1990). In Chapter Three, molecular analyses were carried out to genetically characterize P. perseae populations from original infestation zones that established in southern San Diego County (i.e., National City and Chula Vista) sometime around 2004, newly infested groves (circa 2017) in northern San Diego, Los Angeles County, and Hawai'i (2019), with additional samples from the presumptive native regions in Florida and México, including Veracruz, Quintana Roo, Colima, and Baja California. These efforts aimed to discern whether new introductions of P. perseae had occurred in California sometime around 2017 or if the original population from 2004 exhibited a lag phase (i.e., a delayed expansion) and whether California's more “aggressive” P. perseae population had spread to Hawai’i sometime around 2019. Additionally, molecular analyses were conducted on the endosymbiont Wolbachia, which influences reproductive compatibility, and this work complemented controlled mating trials between different California P. perseae populations. These cross-mating trials were undertaken to determine if reproductive barriers existed between P. perseae populations from the original 2004 invasion areas in southern San Diego County (these still existed and are still restricted to urban avocados and were identified by mitochondrial haplotype) and the new more “aggressive” populations of P. perseae infesting commercial avocado orchards in northern San Diego Country (these populations have a distinct mitochondrial haplotype that separates them from the 2004 populations). Mating incompatibility, if it exists, may explain, in part, observed variations in pest severity in different avocado growing areas of California.