Effects of Glassy-Winged Sharpshooter (Hemiptera: Cicadellidae) Feeding, Size, and Lipid Content on Egg Maturation

Evaluation of Alfalfa Fields and Pastures as Sources of Spissistilus festinus (Hemiptera: Membracidae): Quantification of Reproductive and Nutritional Parameters

The threecornered alfalfa hopper (Spissistilus festinus) is a pest of grapevine, with damage caused by transmission of grapevine red blotch virus. Because grapevine is not a preferred host of the threecornered alfalfa hopper, abundance in vineyards depends on proximity to source habitats and presence of preferred hosts in vineyard understories. The potential for alfalfa fields and pastures in the Central Valley of California to serve as sources of threecornered alfalfa hopper was evaluated by quantifying parameters associated with threecornered alfalfa hopper reproductive and nutritional status. Laboratory studies determined that the threecornered alfalfa hopper is synovigenic, emerging as an adult prior to initiation of oogenesis and that females have multiple rounds of egg production. Alfalfa fields, irrigated pastures, and vineyards were sampled monthly. Adults were observed year-round in alfalfa fields and pastures, with populations peaking in fall. Gravid females were observed from February through November. While rare, adult threecornered alfalfa hoppers were collected from 2 of 4 sampled vineyards. In spring, adults were observed in samples collected from vineyard ground cover. In fall, adults were observed in samples collected from vineyard ground cover and foliage samples. Samples collected from pastures and vineyards were male biased, whereas equal numbers of males and females were observed in alfalfa fields. Adults collected from alfalfa fields were larger, heavier, and had greater estimated energetic reserves than adults collected from pastures. Adults collected from vineyards were of above average size and had relatively high estimated energetic reserves. Results suggest that alfalfa fields are more likely to serve as sources of threecornered alfalfa hoppers than irrigated pastures and that differences in male and female behavior may affect rates of pathogen transmission.

Effect of vibrational mating disruption on flight activity and oviposition to control the grapevine pest, Scaphoideustitanus

The increasing demand for safe and sustainably produced food is leading to the development of strategies of pest control alternative to chemicals. One innovative method is Vibrational Mating Disruption (VMD) to disrupt insect communication in plants. VMD was proven effective in preventing mating of the grapevine pest Scaphoideus titanus, vector of flavescence dorée. However, the stress induced by VMD on the target species has the potential to influence other crucial aspects of the insect biology and ethology. Therefore, the goal of this study was to understand side effects of VMD on the flight activity and oviposition of S. titanus. The results of our experiments conducted in the greenhouse showed that in the presence of a receptive female, males fly more if exposed to vibrations than in the silent control but not differently from singles males in silence. Surprisingly, we found that also females subjected to VMD fly more than in the silence. Regarding oviposition, we found that mated females exposed to vibrations and single females (unmated) laid significantly fewer eggs than mated females in silence. In conclusion, this study shows the potential of VMD to interfere, besides with mating, with other important biological aspects of the pest species.

Nutritional and Physiological Regulation of Glassy-Winged Sharpshooter Oogenesis

The glassy-winged sharpshooter (Homalodisca vitripennis (Germar); Hemiptera: Cicadellidae: Cicadellinae) is an invasive insect that transmits the plant pathogenic bacterium Xylella fastidiosa Wells et al. (Xanthomonadales: Xanthomonadacae). While adult glassy-winged sharpshooter must feed to produce eggs, the role of nutritional status on initiating oogenesis is poorly understood. To determine the effects of glassy-winged sharpshooter nutrition on nymphal development, oogenesis, and fecundity, glassy-winged sharpshooter were reared on cowpea, sunflower, sorghum, and a mixture of the three plant species. Adults emerging from cowpea, sunflower, or plant mixture treatments had shorter development times, attained larger size, and had greater estimated lipid reserves than females reared on sorghum. In choice tests, nymphs avoided sorghum and preferentially fed on cowpea and sunflower. Adult females provisioned with a single plant species during the nymphal stage were provided with either the same host plant species or a mixture of host plant species (cowpea, sunflower, sorghum) for a 9-wk oviposition period, with 37% of females initiating oogenesis. Ovipositing females had greater juvenile hormone and octopamine levels than reproductively inactive females, although topical application of the juvenile hormone analog Methoprene did not promote oogenesis. Across nymphal diets, reproductively active females produced more eggs when held on plant mixtures than on single plant species. In choice tests, adult females were observed most frequently on cowpea, although most eggs were deposited on sorghum, the host least preferred by nymphs. Results suggest that fecundity is largely determined by the quality of the adult diet, although the stimulus that initiates oogenesis does not appear to be related to nutrition.

Insect-to-insect horizontal transmission of a phytoreovirus in the absence of an infected plant host

Vertical transmission of Homalodisca vitripennis reovirus (HoVRV) from glassy-winged sharpshooter (GWSS, Homalodisca vitripennis (Germar)) females to progeny occurred in laboratory assays at frequencies too low (2.6%-15.4%) to account for HoVRV incidence (90-100%) in field populations resident in citrus. Because citrus is immune to HoVRV and no plant host is known, horizontal transmission of HoVRV from insect-to-insect was evaluated. Exposure of colony-reared, virus-free test nymphs to HoVRV-infected source adults held in the same cage for 10 days on virus-immune cowpea resulted in HoVRV transmission (13.3%-30.7%) to test nymphs. HoVRV was not transmitted when exposure was indirect and required passive movement of virions through the xylem of immune citrus seedlings. Collectively, these results demonstrate direct insect-to-insect horizontal transmission of HoVRV, providing a plausible explanation for high incidence of HoVRV in GWSS field populations in the absence of efficient vertical transmission or a plant host.

Effects of Nymphal Diet and Adult Feeding on Allocation of Resources to Glassy-Winged Sharpshooter Egg Production

The glassy-winged sharpshooter is an invasive insect capable of transmitting the bacterial pathogen Xylella fastidiosa. Pre-oviposition periods of laboratory-reared glassy-winged sharpshooters are variable. Here, two questions were addressed: does nymphal diet affect pre-oviposition period and how do allocation patterns of resources differ for females that produce eggs versus females that do not? Nymphs were reared on one of three host plant species: cowpea, sunflower, or sorghum. Half of the females were sacrificed at emergence. The remaining adult females were held on cowpea, a host plant species known to support egg maturation via adult feeding. Females were sacrificed on the day of first oviposition or after 9 wk if no eggs were deposited. Females reared as nymphs on sorghum had longer development times and were smaller (head capsule width and hind tibia length) than females reared as nymphs on cowpea and sunflower. However, nymphal diet did not affect percentage of dry weight that was lipid at emergence. Further, nymphal diet did not affect time to deposition of the first egg mass or total number of eggs matured at the time of first oviposition. Egg production reduced the allocation of resources to insect bodies, with body lipid content decreasing with increasing egg production. In general, females increased wet weight 1.4-fold during the first week after adult emergence, with wet weights plateauing over the remaining 9 wk that adults were monitored. Thus, it seems reasonable to hypothesize that resources required for egg production were acquired via adult feeding during the first week after adult emergence.

Effects of Adult Feeding and Overwintering Conditions on Energy Reserves and Flight Performance of Emerald Ash Borer (Coleoptera: Buprestidae)

Emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), an invasive beetle from Asia, spreads through human-mediated movement and active flight. The effects of adult feeding and overwintering conditions on A. planipennis energy reserves (e.g., lipid, glycogen, and sugars) and flight are poorly understood. We conjectured that the potential energetic demands associated with the production of cryoprotectants might affect dispersal capacity and partially explain slower spread of A. planipennis in Minnesota than in the other states. Two studies sought to measure the effects of adult feeding on lipid content and flight capacity. Adult A. planipennis were fed shamel ash, Fraxinus uhdei Wenzig, leaves for 0-20 d after emergence, and half were flown on a custom flight mill for 24 h, before being frozen for comparative lipid analysis with a control group. The second study compared the effects of adult feeding on energy reserves and flight capacity of A. planipennis that were originally from St. Paul, Minnesota but overwintered in infested logs placed in Grand Rapids, Minnesota (low winter temperature, -34°C) or St. Paul, Minnesota (-26.3°C). Live adults consumed foliage at a constant rate, but lipid content (percentage of fresh mass) did not change with increases in feeding or flight. Adult glycogen content declined with flight and increased only slightly with feeding. Overwintering location affected survival rates but not energy reserves or flight capacity. These results suggest that the flight capacity of A. planipennis is largely determined before emergence, with no differences in energy reserves after cryoprotectant investment.

Effects of Energy Reserves and Diet on Glassy-Winged Sharpshooter Egg Maturation

The glassy-winged sharpshooter is an invasive insect capable of transmitting the plant pathogen Xylella fastidiosa. As rates of pathogen spread are a function of vector abundance, identification of factors contributing to glassy-winged sharpshooter egg production will aid in predicting population growth. Here, effects of stored energy reserves and adult diet on glassy-winged sharpshooter egg maturation were evaluated. To estimate energy reserves available to adult females at the beginning of feeding assays, residuals from a regression of wet weight on size were used. Analysis of a subset of females sacrificed at the beginning of feeding assays, demonstrated that females with a positive residual wet weight had higher lipid content and carried more eggs than females with a negative residual wet weight. To evaluate effects of diet and energy reserves on egg maturation, energy reserves available to females entering feeding assays on cowpea and grapevine were estimated. For females held on cowpea, residual wet weight and quantity of excreta produced over a 6-d feeding period affected egg production. In contrast, for females held on grapevine, only residual wet weight affected egg production. Comparison of cowpea and grapevine xylem sap determined that eight amino acids were more concentrated in xylem sap from cowpea than from grapevine. Collectively, the results suggest that glassy-winged sharpshooter population growth within crop monocultures will not depend solely on the nutritional quality of the specific crop for producing mature eggs but also on the quantity of energy reserves accumulated by females prior to entering that crop habitat.

Effects of nutritional deprivation on development and behavior in the subsocial bee Ceratina calcarata (Hymenoptera: Xylocopinae)

By manipulating resources or dispersal opportunities, mothers can force offspring to remain at the nest to help raise siblings, creating a division of labor. In the subsocial bee Ceratina calcarata, mothers manipulate the quantity and quality of pollen provided to the first female offspring, producing a dwarf eldest daughter that is physically smaller and behaviorally subordinate. This daughter forages for her siblings and forgoes her own reproduction. To understand how the mother's manipulation of pollen affects the physiology and behavior of her offspring, we manipulated the amount of pollen provided to offspring and measured the effects of pollen quantity on offspring development, adult body size and behavior. We found that by experimentally manipulating pollen quantities we could recreate the dwarf eldest daughter phenotype, demonstrating how nutrient deficiency alone can lead to the development of a worker-like daughter. Specifically, by reducing the pollen and nutrition to offspring, we significantly reduced adult body size and lipid stores, creating significantly less aggressive, subordinate individuals. Worker behavior in an otherwise solitary bee begins to explain how maternal manipulation of resources could lead to the development of social organization and reproductive hierarchies, a major step in the transition to highly social behaviors.

Effects of Xylem-Sap Composition on Glassy-Winged Sharpshooter (Hemiptera: Cicadellidae) Egg Maturation on High- and Low-Quality Host Plants

Glassy-winged sharpshooters must feed as adults to produce mature eggs. Cowpea and sunflower are both readily accepted by the glassy-winged sharpshooter for feeding, but egg production on sunflower was reported to be lower than egg production on cowpea. To better understand the role of adult diet in egg production, effects of xylem-sap chemistry on glassy-winged sharpshooter egg maturation was compared for females confined to cowpea and sunflower. Females confined to cowpea consumed more xylem-sap than females held on sunflower. In response, females held on cowpea produced more eggs, had heavier bodies, and greater lipid content than females held on sunflower. Analysis of cowpea and sunflower xylem-sap found that 17 of 19 amino acids were more concentrated in cowpea xylem-sap than in sunflower xylem-sap. Thus, decreased consumption of sunflower xylem-sap was likely owing to perceived lower quality, with decreased egg production owing to a combination of decreased feeding and lower return per unit volume of xylem-sap consumed. Examination of pairwise correlation coefficients among amino acids indicated that concentrations of several amino acids within a plant species were correlated. Principal component analyses identified latent variables describing amino acid composition of xylem-sap. For females held on cowpea, egg maturation was affected by test date, volume of excreta produced, and principal components describing amino acid composition of xylem-sap. Principal component analyses aided in identifying amino acids that were positively or negatively associated with egg production, although determining causality with respect to key nutritional requirements for glassy-winged sharpshooter egg production will require additional testing.

Sequence polymorphism in an insect RNA virus field population: A snapshot from a single point in space and time reveals stochastic differences among and within individual hosts

Population structure of Homalodisca coagulata Virus-1 (HoCV-1) among and within field-collected insects sampled from a single point in space and time was examined. Polymorphism in complete consensus sequences among single-insect isolates was dominated by synonymous substitutions. The mutant spectrum of the C2 helicase region within each single-insect isolate was unique and dominated by nonsynonymous singletons. Bootstrapping was used to correct the within-isolate nonsynonymous:synonymous arithmetic ratio (N:S) for RT-PCR error, yielding an N:S value ~one log-unit greater than that of consensus sequences. Probability of all possible single-base substitutions for the C2 region predicted N:S values within 95% confidence limits of the corrected within-isolate N:S when the only constraint imposed was viral polymerase error bias for transitions over transversions. These results indicate that bottlenecks coupled with strong negative/purifying selection drive consensus sequences toward neutral sequence space, and that most polymorphism within single-insect isolates is composed of newly-minted mutations sampled prior to selection.