Drought stress affects interactions between potato plants, psyllid vectors, and a bacterial pathogen.

Transmission of insect-borne pathogens is mediated by interactions between insects and plants across variable environments. Water stress, for example, affects the physiology, defense, chemistry, and nutritional balance of plants in ways that alter their tolerance to herbivores and pathogens. However, few studies have explored interactions between water stress and insect-borne pathogens as well as the molecular mechanisms mediating these interactions. Here, we address these knowledge gaps by assessing effects of plant water stress on the transmission of a bacterial pathogen, Candidatus Liberibacter solanacearum (CLs), by the vector Bactericera cockerelli Sulc (potato psyllid). We hypothesized that plant water stress would promote pathogen transmission by inducing plant gene transcripts and phytohormones involved in defense. Our results showed water stress was associated with decreased CLs titer with two psyllid haplotypes. Our analysis of plant gene transcripts suggested water stress affected phytohormone pathways in ways that altered plant tolerance to the CLs pathogen. Our study shows that abiotic stressors like drought may mediate the spread of plant pathogens by altering plant signaling pathways in ways that affect pathogen transmission.

Alternative prey and farming system mediate predation of Colorado potato beetles by generalists.

BACKGROUND: Biological control by generalist predators can be mediated by the abundance and biodiversity of alternative prey. When alternative prey draw predator attacks away from the control target, they can weaken pest suppression. In other cases, a diverse prey base can promote predator abundance and biodiversity, reduce predator-predator interference, and benefit biocontrol. Here, we used molecular gut-content analysis to assess how community composition altered predation of Colorado potato beetle (Leptinotarsa decemlineata (Say)) by Nabis sp. and Geocoris sp. Predators were collected from organic or conventional potato (Solanum tuberosum L.) fields, encouraging differences in arthropod community composition. RESULTS: In organic fields, Nabis predation of potato beetles decreased with increasing arthropod richness and predator abundance. This is consistent with Nabis predators switching to other prey species when available and with growing predator-predator interference. In conventional fields these patterns were reversed, however, with potato beetle predation by Nabis increasing with greater arthropod richness and predator abundance. For Geocoris, Colorado potato beetle predation was more frequent in organic than in conventional fields. However, Geocoris predation of beetles was less frequent in fields with higher abundance of the detritus-feeding fly Scaptomyza pallida Zetterstedt, or of all arthropods, consistent with predators choosing other prey when available. CONCLUSION: Alternative prey generally dampened predation of potato beetles, suggesting these pests were less-preferred prey. Nabis and Geocoris differed in which alternative prey were most disruptive to feeding on potato beetles, and in the effects of farm management on predation, consistent with the two predator species occupying complementary feeding niches. © 2021 Society of Chemical Industry.

Efficacy of Naturally Occurring and Commercial Entomopathogenic Nematodes Against Sugar Beet Wireworm (Coleoptera: Elateridae).

Wireworms are the larval stage of click beetles (Coleoptera: Elateridae), and some of their species are serious pests of many crops. In the present study, we evaluated the efficacy of naturally occurring and commercial entomopathogenic nematode species against the sugar beet wireworm, Limonius californicus (Mannerheim), in the laboratory. First, efficacies of Steinernema feltiae (Filipjev) (Rhabditida: Steinernematidae) collected from an irrigated (S. feltiae-SSK) and a dryland (S. feltiae-SSC) field and the two commercial entomopathogenic nematode species, S. carpocapsae (Weiser) (Rhabditida: Steinernematidae) and Heterorhabditis bacteriophora Poinar (Rhabditida: Heterorhabditidae), were examined. Efficacies of the two field-collected S. feltiae isolates were also compared against a commercial S. feltiae strain. In the first bioassay, S. feltiae-SSK caused 63.3% wireworm mortality, followed by 30% caused by S. carpocapsae, 23.3% by S. feltiae-SSC, and 6.7% by H.bacteriophora. In the second assay, S. feltiae-SSK killed 56.7% of the wireworms, ≈2.1- and ≈5.7-fold higher than S. feltiae-SSC and the commercial isolate, respectively.

Seasonal Population Dynamics of Potato Psyllid (Hemiptera: Triozidae) in the Columbia River Basin.

Understanding factors that affect the population dynamics of insect pest species is key for developing integrated pest management strategies in agroecosystems. Most insect pest populations are strongly regulated by abiotic factors such as temperature and precipitation, and assessing relationships between abiotic conditions and pest dynamics can aid decision-making. However, many pests are also managed with insecticides, which can confound relationships between abiotic factors and pest dynamics. Here we used data from a regional monitoring network in the Pacific Northwest United States to explore effects of abiotic factors on populations of an intensively managed potato pest, the potato psyllid (Bactericera cockerelli Sulc), which can vector Candidatus Liberibacter psyllaurus, a bacterial pathogen of potatoes. We assessed effects of temperature on psyllid populations, and show psyllid population growth followed predictable patterns within each year, but there was considerable variation across years in psyllid abundance. Examination of seasonal weather patterns suggested that in 2017, when psyllid populations were less abundant by several orders of magnitude than other years, a particularly long and cold period of winter weather may have harmed overwintering populations and limited population growth. The rate of degree-day accumulation over time, as well as total degree-day accumulation also affected trap catch abundance, likely by mediating the number of psyllid generations per season. Our findings indicate that growers can reliably infer the potential magnitude of risk from potato psyllids using monitoring data, date of first detection, seasonal weather patterns, and population size early in the growing season.

Seasonal Population Dynamics of Three Potato Pests in Washington State.

Pest phenology models allow producers to anticipate pest outbreaks and deploy integrated pest management (IPM) strategies. Phenology models are particularly useful for cropping systems with multiple economically damaging pests throughout a season. Potato (Solanum tuberosum L.) crops of Washington State, USA, are attacked by many insect pests including the potato tuberworm (Phthorimaea operculella Zeller), the beet leafhopper (Circulifer tenellus Baker), and the green peach aphid (Myzus persicae Sulzer). Each of these pests directly damages potato foliage or tubers; C. tenellus and M. persicae also transmit pathogens that can drastically reduce potato yields. We monitored the seasonal population dynamics of these pests by conducting weekly sampling on a network of commercial farms from 2007 to 2014. Using these data, we developed phenology models to characterize the seasonal population dynamics of each pest based on accumulated degree-days (DD). All three pests exhibited consistent population dynamics across seasons that were mediated by temperature. Of the three pests, C. tenellus was generally the first detected in potato crops, with 90% of adults captured by 936 DD. In contrast, populations of P. operculella and M. persicae built up more slowly over the course of the season, with 90% cumulative catch by 1,590 and 2,634 DD, respectively. Understanding these seasonal patterns could help potato producers plan their IPM strategies while allowing them to move away from calendar-based applications of insecticides. More broadly, our results show how long-term monitoring studies that explore dynamics of multiple pest species can aid in developing IPM strategies in crop systems.

Organic agriculture promotes evenness and natural pest control.

Human activity can degrade ecosystem function by reducing species number (richness) and by skewing the relative abundance of species (evenness). Conservation efforts often focus on restoring or maintaining species number, reflecting the well-known impacts of richness on many ecological processes. In contrast, the ecological effects of disrupted evenness have received far less attention, and developing strategies for restoring evenness remains a conceptual challenge. In farmlands, agricultural pest-management practices often lead to altered food web structure and communities dominated by a few common species, which together contribute to pest outbreaks. Here we show that organic farming methods mitigate this ecological damage by promoting evenness among natural enemies. In field enclosures, very even communities of predator and pathogen biological control agents, typical of organic farms, exerted the strongest pest control and yielded the largest plants. In contrast, pest densities were high and plant biomass was low when enemy evenness was disrupted, as is typical under conventional management. Our results were independent of the numerically dominant predator or pathogen species, and so resulted from evenness itself. Moreover, evenness effects among natural enemy groups were independent and complementary. Our results strengthen the argument that rejuvenation of ecosystem function requires restoration of species evenness, rather than just richness. Organic farming potentially offers a means of returning functional evenness to ecosystems.