Factors Limiting the Spread of the Protective Symbiont \u3cem\u3eHamiltonella defensa\u3c/em\u3e in \u3cem\u3eAphis craccivora\u3c/em\u3e Aphids

Many insects are associated with heritable symbionts that mediate ecological interactions, including host protection against natural enemies. The cowpea aphid, Aphis craccivora, is a polyphagous pest that harbors Hamiltonella defensa, which defends against parasitic wasps. Despite this protective benefit, this symbiont occurs only at intermediate frequencies in field populations. To identify factors constraining H. defensa invasion in Ap. craccivora, we estimated symbiont transmission rates, performed fitness assays, and measured infection dynamics in population cages to evaluate effects of infection. Similar to results with the pea aphid, Acyrthosiphon pisum, we found no consistent costs to infection using component fitness assays, but we did identify clear costs to infection in population cages when no enemies were present. Maternal transmission rates of H. defensa in Ap. craccivora were high (ca. 99%) but not perfect. Transmission failures and infection costs likely limit the spread of protective H. defensa in Ap. craccivora. We also characterized several parameters of H. defensa infection potentially relevant to the protective phenotype. We confirmed the presence of H. defensa in aphid hemolymph, where it potentially interacts with endoparasites, and performed real-time quantitative PCR (qPCR) to estimate symbiont and phage abundance during aphid development. We also examined strain variation of H. defensa and its bacteriophage at multiple loci, and despite our lines being collected in different regions of North America, they were infected with a nearly identical strains of H. defensa and APSE4 phage. The limited strain diversity observed for these defensive elements may result in relatively static protection profile for this defensive symbiosis

Almost There: Transmission Routes of Bacterial Symbionts between Trophic Levels

Many intracellular microbial symbionts of arthropods are strictly vertically transmitted and manipulate their host's reproduction in ways that enhance their own transmission. Rare horizontal transmission events are nonetheless necessary for symbiont spread to novel host lineages. Horizontal transmission has been mostly inferred from phylogenetic studies but the mechanisms of spread are still largely a mystery. Here, we investigated transmission of two distantly related bacterial symbionts – Rickettsia and Hamiltonella – from their host, the sweet potato whitefly, Bemisia tabaci, to three species of whitefly parasitoids: Eretmocerus emiratus, Eretmocerus eremicus and Encarsia pergandiella. We also examined the potential for vertical transmission of these whitefly symbionts between parasitoid generations. Using florescence in situ hybridization (FISH) and transmission electron microscopy we found that Rickettsia invades Eretmocerus larvae during development in a Rickettsia-infected host, persists in adults and in females, reaches the ovaries. However, Rickettsia does not appear to penetrate the oocytes, but instead is localized in the follicular epithelial cells only. Consequently, Rickettsia is not vertically transmitted in Eretmocerus wasps, a result supported by diagnostic polymerase chain reaction (PCR). In contrast, Rickettsia proved to be merely transient in the digestive tract of Encarsia and was excreted with the meconia before wasp pupation. Adults of all three parasitoid species frequently acquired Rickettsia via contact with infected whiteflies, most likely by feeding on the host hemolymph (host feeding), but the rate of infection declined sharply within a few days of wasps being removed from infected whiteflies. In contrast with Rickettsia, Hamiltonella did not establish in any of the parasitoids tested, and none of the parasitoids acquired Hamiltonella by host feeding. This study demonstrates potential routes and barriers to horizontal transmission of symbionts across trophic levels. The possible mechanisms that lead to the differences in transmission of species of symbionts among species of hosts are discussed

Invasion biology of spotted wing Drosophila (Drosophila suzukii): a global perspective and future priorities

The Asian vinegar fly Drosophila suzukii (spotted wing Drosophila [SWD]) has emerged as a major invasive insect pest of small and stone fruits in both the Americas and Europe since the late 2000s. While research efforts have rapidly progressed in Asia, North America, and Europe over the past 5 years, important new insights may be gained in comparing and contrasting findings across the regions affected by SWD. In this review, we explore common themes in the invasion biology of SWD by examining (1) its biology and current pest status in endemic and recently invaded regions; (2) current efforts and future research needs for the development of predictive models for its geographic expansion; and (3) prospects for both natural and classical (=importation) biological control of SWD in invaded habitats, with emphasis on the role of hymenopteran parasitoids. We conclude that particularly fruitful areas of research should include fundamental studies of its overwintering, host-use, and dispersal capabilities; as well as applied studies of alternative, cost-effective management techniques to complement insecticide use within the integrated pest management framework. Finally, we emphasize that outreach efforts are critical to effective SWD management by highlighting successful strategies and insights gained from various geographic regions.This is the publisher’s final pdf. The published article is copyrighted by Springer and can be found at: http://link.springer.com/journal/10340Keywords: Integrated pest management, Biological control, Drosophila, Frugivore, Invasion biolog

Proximate Drivers of Migration and Dispersal in Wing-Monomorphic Insects

Gains in our knowledge of dispersal and migration in insects have been largely limited to either wing-dimorphic species or current genetic model systems. Species belonging to these categories, however, represent only a tiny fraction of insect biodiversity, potentially making generalization problematic. In this perspective, I present three topics in which current and future research may lead to greater knowledge of these processes in wing-monomorphic insects with limited existing molecular tools. First, threshold genetic models are reviewed as testable hypotheses for the heritability of migratory traits, using the sweet potato whitefly (Bemisia tabaci) as a case study of a behaviorally-polymorphic migratory species lacking morphological or physiological differentiation. In addition, both adaptive and non-adaptive explanations for the empirically variable relationship between egg production and flight in wing-monomorphic insects are discussed. Finally, with respect to the largest order of insects (Hymenoptera), the role of sex determination mechanisms for haplodiploidy as a driver for natal dispersal (for inbreeding avoidance) versus philopatry (such as in local mate competition) is discussed

Morphometric Measurements of Field and Laboratory-Reared Spotted-Wing Drosophila (2017-2018)

The data set contains wing length, wing width, and hind tibia length of laboratory-reared known summer and winter morphs of Drosophila suzukii. Summer morphs were reared on artificial diet at 16:8 (light:dark) and at 25°C. Winter morphs were reared on artificial diet at 12:12 (light:dark) and at 10°C. In addition, we recorded morphometric measurements on field-caught D. suzukii through-out the greater Twin Cities region in Minnesota.Winter and summer morph Drosophila suzukii can be difficult to distinguished based on a color scale. The purpose of this data were to find an alternative, quantitative method for identifying the two morphs using wing and/or hind tibia measurements.Minnesota Invasive Terrestrial Plants and Pests Center (through the the Environment and Natural Resources Trust Fund; mitppc.umn.edu

Field Evaluation of Eretmocerus eremicus Efficacy in the Control of Sweet Potato Whiteflies Infesting Melons

The effect of three different release rates (1x, 10x, and 20x the recommended rate of 10,000/acre) of Eretmocerus eremicus, a whitefly parasitoid, on sweet potato whitefly populations in cantaloupe were evaluated against populations in untreated control plots. Parasitoids were released from a point source in the center of each of nine treatment plots. All stages of whitefly development were monitored within a 10-m annulus surrounding each release point in all 12 plots, as were rates of parasitism. This occurred over a 52-d period from July 21 through September 11, 2001. The rates of sweet potato whitefly population increase during this time were equivalent and independent of the parasitoid release rate. Whitefly densities were not controlled in any of our treatment plots, nor in the controls. Moreover, rates of parasitism did not increase with time in any of the treatment plots and did not differ among the three release rates (22.0 ± 16.2%). Hence, Eretmocerus eremicus, by itself, is not efficient as a means to control whitefly populations in melon crops in the Southwest US. The ineffectiveness of E. eremicus to control whitefly populations in the field may be due to its propensity to dispersal at low host densities

Eggs of Eretmocerus eremicus, a Whitefly Parasitoid

Reproductive traits of wasp parasitoids are thought to be strong indicators of their success as biological control agents. Our study looks at the number of eggs produced by the whitefly parasitoid Eretmocerus eremicus. A series of experiments conducted on female wasps reared in the absence of whitefly hosts demonstrated that adult wasps emerge with a large (approximately 54) number of eggs that is retained during the first 2 days of adult life. Eggs are then absorbed steadily until at least 8 days following emergence. The results of this study suggest that the mode of egg production exhibited by E. eremicus is the type where they emerge with all, or nearly all, of their eggs, i.e. they do not produce additional eggs as they age. This information is significant when considering how they find their whitefly hosts and how effective they might be in controlling whitefly numbers

Horizontal transmission (from <i>R⁺</i> whiteflies to wasps) and vertical transmission (from <i>R⁺</i>wasps to progeny) of <i>Rickettsia</i> to males and females of <i>Er. emiratus</i> (top), <i>Er. eremicus</i> (middle) and <i>En. pergandiella</i> (bottom).

<p>‘P’ are <i>R⁻</i> wasps that were exposed to <i>R⁺</i> whiteflies for 24 hrs (horizontal transmission via host feeding and/or honeydew), ‘F₁’ are their resulting progeny that developed in <i>R⁺</i> hosts (also horizontal transmission), and ‘F₂’ are progeny of F₁ that were exposed to <i>R⁻</i> hosts (vertical transmission). The numbers above the columns are the sample size, <i>n</i>, from which the proportion of infected wasps was calculated. See also <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0004767#pone-0004767-g001" target="_blank">Fig. 1</a> for this experiment's set-up.</p