Daily dynamics of contrasting spring algal blooms in Santa Monica Bay (central Southern California Bight).

Protistan algae (phytoplankton) dominate coastal upwelling ecosystems where they form massive blooms that support the world's most important fisheries and constitute an important sink for atmospheric CO2 . Bloom initiation is well understood, but the biotic and abiotic forces that shape short-term dynamics in community composition are still poorly characterized. Here, high-frequency (daily) changes in relative abundance dynamics of the metabolically active protistan community were followed via expressed 18S V4 rRNA genes (RNA) throughout two algal blooms during the spring of 2018 and 2019 in Santa Monica Bay (central Southern California Bight). A diatom bloom formed after wind-driven, nutrient upwelling events in both years, but different taxa dominated each year. Whereas diatoms bloomed following elevated nutrients and declined after depletion each year, a massive dinoflagellate bloom manifested under relatively low inorganic nitrogen conditions following diatom bloom senescence in 2019 but not 2018. Network analysis revealed associations between diatoms and cercozoan putative parasitic taxa and syndinean parasites during 2019 that may have influenced the demise of the diatoms, and the transition to a dinoflagellate-dominated bloom.

Novel multitrophic interactions among an exotic, generalist herbivore, its host plants and resident enemies in California.

What happens when an exotic herbivore invades and encounters novel host plants and enemies? Here, we investigate the impacts of host plant quality and plant architecture on an exotic generalist herbivore, Epiphyas postvittana (Lepidoptera: Tortricidae) and its interactions with resident parasitoids in California. Using artificial diet and five plant species, we found significant effects of diet on the fitness of E. postvittana under laboratory conditions. In the field, based on a common garden experiment with host plants of nine species, we found that larval parasitism varied among plant species by a factor of 2.1 with a higher risk of parasitism on shorter than taller plants. Parasitism of egg masses varied by a factor of 4.7 among plant species with a higher risk of parasitism on taller than shorter plants. In the laboratory, the foraging time of a resident egg parasitoid on excised leaves varied among plant species, but did not correspond to observed egg parasitism rates on these same plants in the field. On leaves of Plantago lanceolata, the probability of egg parasitism decreased with trichome density. Overall, there was a significant effect of host plant on the intrinsic rate of increase of E. postvittana and on the extent of parasitism by resident parasitoids, but no correlation existed between these two effects. The recent decline of E. postvittana in California may be due to the low quality of some host plants and to the many resident enemies that readily attack it, perhaps due to its phylogenetic relatedness to resident tortricids.

Pathogenicity, prevalence and intensity of a microsporidian infection by Nosema fumiferanae postvittana in the light brown apple moth, Epiphyas postvittana, in California.

While biological invasions are increasing, in some cases exotic species exhibit an initial phase of population growth and spread, followed by a subsequent phase of natural decline. The light brown apple moth, Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae), provides a unique opportunity to examine potential mechanisms for the natural suppression of an exotic insect species that has become established in coastal California. We recently discovered a microsporidian pathogen, Nosema fumiferanae postvittana, from E. postvittana in its novel range. In the laboratory, we examined the pathogenicity and latent period of this microsporidium, and in the field we determined its prevalence and intensity in five locations using quantitative real-time PCR (qPCR). In the laboratory, when comparing healthy larvae to larvae infected with up to 10(5) spores, we found a reduction in juvenile survivorship (from 100% to 26%), a prolongation of juvenile development time (of up to 9-10 days), a reduction in viable lifetime fecundity (from 788 to 1) and a reduction in the intrinsic rate of increase (from 0.18 to 0.008). The median lethal dose (LD50) was estimated to be 1.8 × 10(4) spores, and the mean latent period for infections with 10(3) spores was 12.67 days. Our field sampling revealed that E. postvittana populations have further declined from previously reported densities in San Francisco and Santa Cruz. We detected N. fumiferanae postvittana in all five locations with an overall prevalence of 5%, which did not appear to be influenced consistently by either host density or season. Mean microsporidian intensity in field-infected individuals was 226 spores. Although the laboratory results demonstrated the potential for host suppression, the field sampling indicated that the prevalence and intensity of microsporidian infection were too low to account for the continued decline in population densities of E. postvittana in coastal California.

Pathogenicity, morphology, and characterization of a Nosema fumiferanae isolate (Microsporidia: Nosematidae) from the light brown apple moth, Epiphyas postvittana (Lepidoptera: Tortricidae) in California.

We recently discovered infections by a microsporidium closely related to Nosema fumiferanae in field populations of the light brown apple moth, Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae), in the San Francisco region of California. E. postvittana originates from Australia and was first detected in California in 2006; therefore, our aim was to identify and determine the origin of the Nosema isolate. We characterized the pathogenicity, transmission pathways, and ultrastructure of this new Nosema isolate. In addition, we sequenced fragments of commonly used genetic markers (ITS, SSU, and RPB1), and examined the phylogenetic relationships between the Nosema isolate and other microsporidian species commonly found in lepidopteran hosts. The pathogenicity of the Nosema isolate was investigated by infecting second instar larvae of E. postvittana. Larval and pupal survivorship were reduced by 7% and 13% respectively, and pupation occurred 1-2d later in infected individuals than in healthy individuals. Emerging infected females died 5d earlier than healthy females, and daily fecundity was 22% lower. Hatch rate also was 22% lower for eggs oviposited by infected females. Vertical transmission was confirmed; spores were present in 68% of egg masses and 100% of the surviving larvae from infected females. Ultrastructure images, together with sequences from selected genetic markers, confirmed the Nosema isolate to be a member of the Nosema fumiferanae species complex (Nosema fumiferanae postvittana subsp. n.). The association of this pathogen with E. postvittana contributes further to the biotic resistance that E. postvittana has experienced since its introduction to California.

Physiology governing diatom vs. dinoflagellate bloom and decline in coastal Santa Monica Bay.

Algal blooms on the Southern California coast are typically dominated by diatom and dinoflagellate taxa, and are governed by their physiological responses to environmental cues; however, we lack a predictive understanding of the environmental controls underlying the establishment and persistence of these distinct bloom events. In this study, we examined gene expression among the numerically dominant diatom and dinoflagellate taxa during spring upwelling bloom events to compare the physiological underpinnings of diatom vs. dinoflagellate bloom dynamics. Diatoms, which bloomed following upwelling events, expressed genes related to dissolved inorganic nitrogen utilization, and genes related to the catabolism of chitin that may have prolonged their bloom duration following nitrogen depletion. Conversely, dinoflagellates bloomed under depleted inorganic nitrogen conditions, exhibited less variation in transcriptional activity, and expressed few genes associated with dissolved inorganic nutrients during their bloom. Dinoflagellate profiles exhibited evidence of proteolysis and heterotrophy that may have enabled them to bloom to high abundances under depleted inorganic nutrients. Taken together, diatom and dinoflagellate transcriptional profiles illustrated guild-specific physiologies that are tuned to respond to and thrive under distinct environmental "windows of opportunity."

Into the weeds: Matching importation history to genetic consequences and pathways in two widely used biological control agents.

The intentional introduction of exotic species through classical biological control programs provides unique opportunities to examine the consequences of population movement and ecological processes for the genetic diversity and population structure of introduced species. The weevils Neochetina bruchi and N. eichhorniae (Coleoptera: Curculionidae) have been introduced globally to control the invasive floating aquatic weed, Eichhornia crassipes, with variable outcomes. Here, we use the importation history and data from polymorphic microsatellite markers to examine the effects of introduction processes on population genetic diversity and structure. We report the first confirmation of hybridization between these species, which could have important consequences for the biological control program. For both species, there were more rare alleles in weevils from the native range than in weevils from the introduced range. N. eichhorniae also had higher allelic richness in the native range than in the introduced range. Neither the number of individuals initially introduced nor the number of introduction steps appeared to consistently affect genetic diversity. We found evidence of genetic drift, inbreeding, and admixture in several populations as well as significant population structure. Analyses estimated two populations and 11 sub-clusters for N. bruchi and four populations and 23 sub-clusters for N. eichhorniae, indicating divergence of populations during and after introduction. Genetic differentiation and allocation of introduced populations to source populations generally supported the documented importation history and clarified pathways in cases where multiple introductions occurred. In populations with multiple introductions, genetic admixture may have buffered against the negative effects of serial bottlenecks on genetic diversity. The genetic data combined with the introduction history from this biological control study system provide insight on the accuracy of predicting introduction pathways from genetic data and the consequences of these pathways for the genetic variation and structure of introduced species.