Brent V. Brodbeck

Feeding by the leafhopper, Homalodisca coagulata, in relation to xylem fluid chemistry and tension

Abstract Feeding by the adult leafhopper [Homalodisca coagulata (Say)], a polyphagous xylem feeder, was monitored in response to changes in xylem tension, and the concentration of amino acids, organic acids and sugars in xylem fluid. Models were constructed based on diurnal feeding patterns on a preferred host, Lagerstroemia indica L. Feeding rate (measured by excretion rate) was best correlated with the concentration of the amides. Correlation coefficients of other amino acids were low. Models were tested by monitoring feeding rates diurnally on Pyrus communis L. and Vitis hybrid cv. Suwannee, and during midday on L. indica subjected to water stress. Only total amino acids, amides and glutamine were correlated with feeding on P. communis and on Vitis. When L. indica were subjected to a high level of water stress, feeding during midday was associated with xylem tension, and not the concentration of chemical variables. Feeding rate decreased exponentially as a function of xylem tension; feeding ceased above a tension of about 2.1 MPa. Models were substantially improved when compounds were expressed on a ratio basis. Feeding rate was best correlated with the ratio of amides to total organic compounds (RAM) in the diurnal experiments. In the stress experiment, amides/xylem tension provided the best correlation. Net energy gain (Xneg) of an insect feeding on xylem fluid was described by the equation: Xneg = 0.70(Xe) - Excoef(Xt − P), where 0.70(Xe) = net energy content of xylem fluid, Excoef = the extraction coefficient, Xt = xylem tension and P = the Poiseuilles law-derived pressure to account for fluid movement through stylets.

Metabolism of amino acids, organic acids and sugars extracted from the xylem fluid of four host plants by adult Homalodisca coagulata

The efficiency of amino acid, organic acid and sugar metabolism was quantified for adult Homalodisca coagulata (Say) (Homoptera: Cicadellidae) by comparing chemical profiles of xylem fluid (food source) and insect exudate. Leafhoppers were confined in Parafilm® sachets to stems of 4 host plants: [Baccharis halimifolia (L.), Lagerstroemia indica (L.), Prunus salicina (Lindl.), Prunus persica (L.), Batsch]. Insect feeding rates (0.09‐0.27 ml h−1***), exudate osmolarity (7.8–12.8 mM) and exudate composition (mainly inorganic entities) were characteristic of a xylem feeder. Total organic solute concentration in the xylem fluid of B. halimifolia, L. indica, P. salicina and P. persica was ca. 9.4, 13,8, 5.5 and 1.8 mM, respectively. Nineteen protein amino acids, 7 organic acids and 3 or 4 sugars were identified in the xylem fluid. Total amino acids, organic acids and sugars were metabolized with ca. 99% efficiency. Glutamine, asparagine, arginine and citric, malic and succinic acids, the predominant organic compounds in the xylem fluid of all four plant species, were metabolized with greater than 99% efficiency. Cysteine (51%), methionine (74%) and oxalic acid (77%) were metabolized with the lowest efficiency. The primary nitrogenous waste was NH+4; uric acid or urea were not detected. Nitrogen retention was generally less than 60% of dietary nitrogen. High feeding rates, ammonotelism and an extremely high metabolic efficiency of organic compounds permit H. coagulata to subsist on the dilute and skewed chemical profile of xylem fluid.

Amino acids as determinants of host preference for the xylem feeding leafhopper, Homalodisca coagulata (Homoptera: Cicadellidae).

SummaryHomalodisca coagulata is a highly polyphagous xylem feeder with distinct seasonal patterns in its selection of host plants. These patterns were examined in relation to the amino acid content of the xylem for four common host species; Lagerstroemia indica, Baccharis halimifolia, Prunus persica, and Prunus salicina. Xylem fluid was collected from each host species at times when numbers of feeding leafhoppers were both low and high. In each case, concentrations of amino acids were greatest when numbers were high. Similarly, comparisons between host species at given times showed that concentrations of amino acids were positively correlated with host selection. In a second study, amino acids of xylem were manipulated by budding scions of a non-preferred host (P. persica) on rootstocks of preferred (P. salinica) and non-preferred (P. persica) hosts. Morphology and phenology of the budded trees were similar to that of the scion species yet the xylem composition of amino acids was primarily dependent on the rootstock. Concentrations of amino acids and the preference of leafhoppers were roughly two-fold greater for scions of the preferred than the non-preferred rootstock. In both studies, amides (glutamine plus asparagine) were the amino acids most highly correlated with host selection. These compounds are the predominant amino acids in xylem fluid, have high nitrogen to carbon ratios, and account for a high percentage of the caloric value in xylem fluid. Many of the less abundant amino acids were positively correlated with host preference, but the correlations were less consistent and correlation coefficients were generally lower.

Physiological and behavioral adaptations of three species of leafhoppers in response to the dilute nutrient content of xylem fluid

Abstract Physiological and behavioral processes of three species of leafhoppers were examined in relation to the extremely dilute content of their food source (xylem fluid). Segregated sexes of Homalodisca coagulata (Say), Homalodisca insolita (Wlk.) and Cuerna costalis (F.) were caged on two hosts, Lagerstroemia indica L. and Catharanthus roseus (L.). Excreta was analyzed and compared to the composition of xylem fluid of the hosts during day and night feeding. Both xylem fluid and insect excreta were analyzed for amino acids, organic acids and sugars and excreta was also analyzed for common excretory products. Efficiencies of metabolic conversion of organic compounds (defined by quantity ingested versus quantity excreted), and total nitrogen and carbon retention were calculated. Diurnal feeding patterns as well as total daily ingestion rates were recorded. Four primary similarities were noted in diet utilization regardless of leafhopper species, host species or sex: (1) excretion of ammonia as the primary waste product; (2) high efficiency of conversion for all organic compounds ingested from xylem fluid; (3) high daily feeding rates (100–300 times dry body weight per day and (4) adjustment of feeding rates to diurnal fluctuations in xylem fluid chemistry. Although these processes were operative for all leafhoppers examined, the extent of their usage and subsequent energy gain and nitrogen accumulation varied greatly with host species and insect sex. These processes maximize efficiency of diet utilization and are discussed in relation to nutritional and caloric requirements of xylem feeding insects.

Flower nitrogen status and populations of Frankliniella occidentalis feeding on Lycopersicon esculentum

In a 2‐year study, we manipulated flower chemistry of field grown Lycopersicon esculentum in North Florida and examined subsequent effects on populations of Frankliniella occidentalis. We examined variation in amino‐acid profiles and total nitrogen concentrations of flowers, flower number and plant size seasonally and in response to variable rates of nitrogen fertilization (202 kg ha−1 and 309 kg ha−1). Populations of F. occidentalis sampled every 6 to 10 days were then related to seasonal and treatment effects on plant variables. For both years, F. occidentalis was abundant on L. esculentum tomatoes for only short intervals (2–3 weeks), and populations were significantly higher on hosts receiving higher rates of nitrogen fertilization. Seasonal trends in F. occidentalis on L. esculentum were correlated to number of flowers per host plant, as well as concentrations of total nitrogen in flowers. Moreover, treatment (fertilization) effects were consistent with changes in the nitrogen status of flowers. Plants subjected to higher fertilization rates produced flowers that had higher nitrogen content as well as variations in amino‐acid profiles during the period of peak thrips populations. Abundance of F. occidentalis (particularly adult females) were most highly correlated to flower concentrations of phenylalanine during population peaks. Our results are consistent with beneficial effects of high dietary nitrogen to thrips populations, and also support the hypothesis that aromatic amino acids (e.g., phenylalanine) play a central role in the nutritional ecology of F. occidentalis.

Population Dynamics of Frankliniella spp. and Tomato Spotted Wilt Incidence as Influenced by Cultural Management Tactics in Tomato

Abstract We investigated the effects of ultraviolet (UV)-reflective mulch and two rates of nitrogen fertilization on populations of Frankliniella spp. thrips and on the incidence of tomato spotted wilt in field-grown tomato in northern Florida. The higher of the two soil nitrogen fertilizer treatments significantly increased populations of Frankliniella occidentalis (Pergande), whereas mulch reflectance significantly decreased populations of F. occidentalis. Populations of Frankliniella tritici (Fitch) were decreased only by UV-reflective mulch. Decreased thrips populations in UV-reflective mulch plots were probably due to disruptions in host-finding behavior. Increased thrips populations in tomatoes treated with the higher nitrogen fertilization rate were probably due in part to increased nutrients available in flowers. Incidence of tomato spotted wilt was significantly decreased in tomatoes grown on UV-reflective mulch, whereas disease incidence was significantly greater in increased nitrogen-fertilized plots. This research reveals that cultural practices resulted in up to 45% reduction in the numbers of vector and nonvector species of flower thrips and up to 50% reduction in tomato spotted wilt.

Differential utilization of nutrients during development by the xylophagous leafhopper, Homalodisca coagulata

Utilization of nutrients at different stages of development was examined for the xylophage, Homalodisca coagulata (Say). Survivorship and consumption rates of second‐instar, fourth‐instar and adult leafhoppers were measured daily on the hosts Lagerstroemia indica L. and Euonymus japonica Thumb. Rates of consumption, assimilation efficiencies and daily assimilation of nitrogen, carbon, and individual organic compounds were calculated based on chemical analyses of xylem fluid and insect excreta. Gross growth efficiencies of diet utilization were also estimated by comparing biomass of young adults to estimates of nutrient utilization of the two host species.

Effects of total dietary nitrogen and nitrogen form on the development of xylophagous leafhoppers

The influence of primary nutrients on the development of the leafhopper Homalodisca coagulata (Say) was examined using Glycine max (L.) Merrill subjected to treatments using different forms of nitrogen fertilization (urea and nitrate) and inoculation with Rhizobium bacteria. Concentrations of amino acids, organic acids, carbohydrates, and ureides in xylem fluid varied with treatment; differences were most pronounced in young plants (6 weeks after planting), and subsided as plants aged. Plants receiving urea fertilization had lower concentrations of organic nitrogen and carbon, but more balanced profiles of amino acids. Leafhoppers on urea-fertilized plants had much higher success in development (62 to 87% survival to adult stage) than H. coagulata developing on nitrate-fertilized plants (31-42%). The sex ratio of successfully developed adults varied greatly with treatments, indicating that female survival was much higher on urea-fertilized plants. Males and females utlilized diets differently as indicated by the carbon and nitrogen content of adult carcasses. Consumption rates and nitrogen assimilation efficiencies were higher for young (third instar) leafhoppers on urea-fertilized plants. Instar duration and total time of development were not affected by treatment. Highest rates of success in development on diets providing the lowest (yet most balanced) concentrations of total nitrogen, amino acids, and organic compounds underscored the importance of nutrient balance for leafhopper development. Arch. Copyright 1999 Wiley-Liss, Inc.

Utilization of primary nutrients by the polyphagous xylophage, Homalodisca coagulata, reared on single host species

Feeding experiments were conducted to assess development of the polyphagous leafhopper, Homalodisca coagulata, on single host species. Insects were reared from eggs on two cultivars of Euonymus japonica, two cultivars of Lagerstroemia indica, and two species of Prunus. Only insects on nonvariegated E. japonica and L. indica cv. Osage survived to maturity, although a substantial proportion of those on P. salicinia developed to the fourth instar. Chemical profiles of food source (xylem fluid), insect excreta and body composition, and consumption rates were used to estimate assimilation efficiencies, daily assimilation rates, and efficiency of conversion of food into biomass for all primary nutrients identified in the xylem fluid. Rates of successful maturation were greatest on the highest nitrogen and carbon source (nonvariegated E. japonica) and were associated with low consumption rates, high assimilation efficiencies, and prolonged instar durations. Developmental period on L. indica was shorter and consumption rates were higher, yet assimilation efficiencies were reduced and young nymphs had lower survivorship. Low consumption rates, coupled with the low nutritional value of P. salicinia, precluded sufficient accumulation of nutrients to complete development. Insects fed on hosts with xylem fluid containing high ratios of amides to total organic compounds (indicative of low carbon to nitrogen ratios) had higher consumption rates but less efficient assimilation of primary nutrients. Carcass analyses of newly enclosed adults suggest that regulation of consumption rates and assimilation efficiencies to provide adequate accumulation of essential amino acids may be pivotal for successful development.

Comparative Nutrition and Developmental Biology of Xylem-Feeding Leafhoppers Reared on Four Genotypes of Glycine max

Abstract A simplified protocol for rearing the glassy-winged sharpshooter, Homalodisca coagulata (Say) (Homoptera: Cicadellidae), was tested on four genotypes of soybean, Glycine max (L.) Merrill. Growth and development of leafhoppers were examined on three glabrous isolines (D88-5320, D88-5328, and D90-9216) and one pubescent genotype (Hagood). All three glabrous isolines were adequate hosts producing an average of >100 adults from eight original mating pairs of leafhoppers. Pubescent Hagood produced significantly fewer adults with longer developmental times. Isoline D88-5328 produced 30% more adults than the other glabrous isolines. Genotype also had significant effects on total developmental times, developmental rates within instars, growth rates, and adult masses at eclosion with insect performance being enhanced on isoline D88-5328. All genotypes of G. max had similar xylem chemistry at the initiation of the rearing experiment (6-week-old plants), but as G. max matured (14–18 wk), isoline D88-5328 provided significantly more organic nitrogen. Chemical analyses of xylem fluid and developmental data support the hypothesis that young H. coagulata required balanced profiles of organic nitrogen for development but benefited from increased organic nitrogen as they approached maturation. Poor performance on pubescent Hagood was not related to xylem chemistry and was assumed to result from the inhibitory effect of trichomes. Xylem fluid analysis suggests that some plants may have eventually experienced nitrogen deprivation from high leafhopper loads; this, in turn, resulted in slow development and small body size at maturation for some insects. Although this protocol provides a simplified technique for rearing H. coagulata, it is likely that manipulation of xylem chemistry via fertilization would further maximize growth and developmental rates.