Phenology and demography of Homalodisca coagulata (Hemiptera: Cicadellidae) in southern California citrus and implications for management

Predicting Phenology of Four Major Hemipteran Pests to Enhance Integrated Pest Management Programs in Potatoes in the Lower Columbia Basin

The potato crop (Solanum tuberosum L.) is affected by various hemipteran insect pests including Circulifer tenellus Baker, Lygus spp., Myzus persicae Sulzer, and Macrosiphum euphorbiae Thomas. These pests can cause direct foliage damage or vector plant pathogens, and consequently reduce potato yield. Gaining insights into which factors have the greatest impact on seasonal population growth of insect pests is key for improving integrated pest management strategies. Moreover, abiotic and biotic cues such as temperature and crop growth stage can strongly influence insect population growth. Hence, the seasonal population dynamics of C. tenellus, Lygus spp., M. persicae, and M. euphorbiae, and temperature, were monitored weekly throughout potato growing seasons in commercial fields located in the lower Columbia Basin (USA). Using a multi-year dataset, we developed phenology models of each pest based on the accumulated degree days (DD) and potato days (PD). Temperature-mediated population growth models suggest that C. tenellus and Lygus spp. are the first of the pests to colonize the potato crop fields, with 90% of cumulative catch by 2,823 and 1,776 DD, respectively. In contrast, M. persicae and M. euphorbiae populations increased more gradually over the course of the season, with 90% cumulative catch by 5,590 and 5,047 DD, respectively. PD-mediated population growth models suggest that 50% of the populations of C. tenellus, Lygus spp., and M. persicae can be collected at potato tuber growth stage, while 50% of the M. euphorbiae population at tuber initiation stage. The results presented here will help in improving hemipteran potato pests' management.

Models Applied to Grapevine Pests: A Review

Simple Summary

Mathematical models are developed to predict key aspects of insects harmful to many crops, including grapevine. Practical applications of these models include forecasting seasonal occurrence and spread over space in order to make decisions about pest management (e.g., timing of insecticide sprays). Many models have recently been developed to evaluate the spread of insect pests on grapevine under a climate change scenario as well as to forecast the possibility that alien species could settle into new environments. To make the published models available to vine-growers and their stakeholders, a holistic approach presenting these models within the frame of a decision support system should be followed.

Abstract

This paper reviews the existing predictive models concerning insects and mites harmful to grapevine. A brief conceptual description is given on the definition of a model and about different types of models: deterministic vs. stochastics, continuous vs. discrete, analytical vs. computer-based, and descriptive vs. data-driven. The main biological aspects of grapevine pests covered by different types of models are phenology, population growth and dynamics, species distribution, and invasion risk. A particular emphasis is put on forecasting epidemics of plant disease agents transmitted by insects with sucking-piercing mouthparts. The most investigated species or groups are the glassy-winged sharpshooter Homalodisca vitripennis (Germar) and other vectors of Xylella fastidiosa subsp. fastidiosa, a bacterium agent of Pierce’s disease; the European grape berry moth, Lobesia botrana (Denis and Schiffermuller); and the leafhopper Scaphoideus titanus Ball, the main vector of phytoplasmas agents of Flavescence dorée. Finally, the present and future of decision-support systems (DSS) in viticulture is discussed.

Effect of Different Fruit Developmental Stages and Biochemical Metabolites of Pomegranate (Ponicaceae) on Life History Parameters of Ectomyelois ceratoniae (Lepidoptera: Pyralidae)

Plant-herbivore interactions can be influenced by plant developmental stages. Effects of four different developmental stages of pomegranate fruit (Hazelnut size, Walnut size, Juicy, and Ripe) were investigated on life table parameters, nutritional yield, and energy contents (protein, glycogen, and lipid) of Ectomyelois ceratoniae (Zeller) under controlled conditions in two pomegranate cultivars, namely Shahvare-Danesefid and Esfahani-Daneghermez as susceptible and resistant cultivars, respectively. Biochemical characteristics of the four developmental stages were determined, and a correlation was made with life-history and nutritional responses of E. ceratoniae. Significant differences were observed in developmental time and adult fecundity of E. ceratoniae across various tested stages in both pomegranate cultivars. The highest value of intrinsic rate of increase (rm) was found in Juicy and lowest in Hazelnut-size stage of Shahvare-Danesefid. The highest rm value was on Walnut-size and Juicy stages, and the lower rate was obtained from the feeding of E. ceratoniae on Ripe and Hazelnut-size stages in Esfahani-Daneghermez. A higher relative growth rate (RGR) was observed in Juicy (in Shahvare-Danesefid) and Walnut-size stages (in Esfahani-Daneghermez) when compared with other developmental stages tested. Life-history parameters of E. ceratoniae were correlated with biochemical qualities of pomegranate stages. Results indicated Juicy (Shahvare-Danesefid) and Walnut-size stages (Esfahani-Daneghermez) were relatively susceptible pomegranate stages, and Hazelnut-size and Ripe stages were unsuitable for feeding of E. ceratoniae in Shahvare-Danesefid and Esfahani-Daneghermez, respectively. Findings of this research could aid in the development of integrated management programs of E. ceratoniae in pomegranate orchards.

Abundance, age structure, and voltinism of light brown apple moth populations in California

The light brown apple moth, Epiphyas postvittana (Walker), is native to Australia and first was detected in California in 2006. In this study, we regularly sampled populations on Leptospermum laevigatum (Gaertn.) F.Muell. at two sites in San Francisco and on Arctostaphylos densiflora M.S. Baker at two sites in Santa Cruz over a 2-yr period to monitor the abundance, age structure, and voltinism of this potential pest in relation to degree-days. Our results showed that larval abundance declined at two sites, cycled with peaks in midsummer at one site, and remained steady at one site. Generations overlapped at all four sites with the full range of larval instars being present for most of the year, although populations during the winter were predominantly mid to late instars. Accumulated degree-days predict an average of 3.27 and 4.58 generations per year in San Francisco and Santa Cruz, respectively, which matched our observed peaks of late-instar larvae in the field remarkably well. This new information on light brown apple moth phenology in coastal California will be invaluable for the development of effective monitoring and management strategies for this new invader in the studied region.

Distribution of glassy-winged sharpshooter and threecornered alfalfa hopper on plant hosts in the San Joaquin Valley, California

Homalodisca vitripennis (Germar) and Spissistilus festinus (Say) populations were surveyed bimonthly for 14 mo in Kern County, CA, at five agricultural sites made up of a variety of potential host plants. In addition, S. festinus populations were surveyed in four alfalfa, Medicago sativa L., fields in Kern and Tulare counties. Insects were collected by beats-sweeps and sticky traps. Data on host plant condition and phenology, and ground cover presence and composition were collected at the five agricultural sites, whereas data on mowing and insecticide use were collected at the four alfalfa sites. Populations of both insects persisted at the five agricultural locations despite insecticide applications applied as part of a H. vitripennis areawide management program and standard commercial operations. Plants colonized by H. vitripennis included eucalyptus (Eucalyptus L'Hér.), jojoba [Simmondsia chinensis (Link) C. K. Schneid.], and citrus (Citrus spp.). Populations of S. festinus were much greater in collections from alfalfa fields than from the five agricultural sites. Insects collected from the five mixed agricultural sites were negative for presence of X. fastidiosa. In laboratory tests, S. festinus did not acquire or transmit X. fastidiosa in tests with infected grape (Vitis spp.) as an acquisition source and grape, almond [Prunus dulcis (Mill.) D.A.Webb], and alfalfa as inoculation hosts. Recommendations for vector control, vegetation management, and targeted monitoring to reduce insect populations and inoculum potential are discussed.

Temporal patterns in Homalodisca spp. (Hemiptera: Cicadellidae) oviposition on southern California citrus and jojoba

A detailed study of the distribution of egg masses of Homalodisca vitripennis (Germar) and H. liturata Ball was done across a 2-yr period (2001-2003) on six host plants in southern California (Marsh grapefruit, Lisbon lemon, Washington navel, Dancy tangerine, rough lemon, and jojoba in Riverside; jojoba in Desert Center). The majority of egg masses in Riverside belonged to H. vitripennis (84-100%), whereas in Desert Center, all Homalodisca egg masses were H. liturata. Oviposition in Riverside occurred in two discrete periods, a late winter and spring period (mid-February to late May), followed by a short interval of very low oviposition during most of June, and then a summer period (late June to late September) followed by a relatively long period of very low oviposition in fall and early winter (October to mid-February). Levels of oviposition during the late winter-spring period were similar to those during the summer despite an observed larger population of adults during the latter period. Moreover, egg clutch size for H. vitripennis was generally greater in spring than during summer and was generally higher than that for H. liturata, especially on Riverside jojoba. Larger egg clutch size was seen on grapefruit than on lemon, navel, and tangerine during summer. There appeared to be temporal host shifts in oviposition; most evident was the shift from relatively high rates of oviposition on lemon and tangerine in late winter-early spring to relatively higher rates of oviposition on grapefruit and navel during summer.

Sampling plans, selective insecticides and sustainability: the case for IPM as 'informed pest management'

Integrated Pest Management (IPM) is considered the central paradigm of insect pest management and is often characterized as a comprehensive use of multiple control tactics to reduce pest status while minimizing economic and environmental costs. As the principal precursor of IPM, the integrated control concept formulated the economic theory behind pest management decisions and specified an applied methodology for carrying out pest control. Sampling, economic thresholds and selective insecticides were three of the critical elements of that methodology and are now considered indispensable to the goals of IPM. We examine each of these elements in the context of contemporaneous information as well as accumulated experience and knowledge required for their skillful implementation in an IPM program. We conclude that while IPM is principally about integrating control tactics into an effective and sustainable approach to pest control, this overarching goal can only be achieved through well-trained practitioners, knowledgeable of the tenets conceived in the integrated control concept that ultimately yield informed pest management.

Influences of temperature on Homalodisca vitripennis (Hemiptera: Cicadellidae) survival under various feeding conditions

Survival of the glassy-winged sharpshooter, Homalodisca vitripennis (Germar) (Hemiptera: Cicadellidae), was studied under various constant temperatures and feeding conditions. When provided a host plant (Citrus limon L. Burm. f.) to feed on during a 21-d trial, 100% mortality occurred at 0.1, 3.2, and 40.1 degrees C, whereas an average of 74-76% of adults survived in the 13.2-24.5 degrees C range. When individually confined with moist cotton, adult longevity was greatest (16.3 d) at 13.3 degrees C, but it was <3 d at -2.4 and 36.2 degrees C. In a companion study comparing the presence versus absence of a host plant, the presence of a host plant was not a significant factor influencing survival at temperatures < or =7.8 degrees C but was at temperatures > or =18.9 degrees C. The relationship between temperature and survival was described by a nonlinear function that estimated the optimum temperature in each feeding regimen: no host plant or moist cotton (5.5 degrees C), moist cotton (9.9 degrees C), and accessible host plant (25.1 degrees C). The model quantitatively predicted that H. vitripennis would survive longer periods at a wider temperature regimen when provided with a host plant than when provided with water alone (moist cotton) or when provided with neither plant host nor water. Our results suggest that continuous exposure to either low (<5 degrees C) or high (>30 degrees C) temperatures are detrimental for adult survival. Specifically, low temperatures caused early mortality because of inhibition of feeding activity and presumably this threshold lies between 7.8 and 13.2 degrees C. Furthermore, this study clearly shows that temperature may influence the survival of H. vitripennis adults regardless of feeding regimens, and its implications for population dynamics are discussed.

Egg load dynamics of Homalodisca vitripennis

Homalodisca vitripennis, the glassy-winged sharpshooter, poses a serious threat to grape production because of its ability to vector Xylella fastidiosa, the causal agent of Pierce's disease. The glassy-winged sharpshooter is native to the southeastern United States, and over the last 20 yr has expanded its range into Texas and California and more distantly into French Polynesia. A better understanding of the reproductive dynamics of H. vitripennis will aid in assessment of the invasiveness of this insect and may aid in refinement of control strategies. First, females of known age were dissected to determine egg maturation schedules. Females did not produce mature eggs until at least 1 wk after adult emergence. Oviposition reduced the number of mature eggs carried by females, suggesting a continuous cycle of egg deposition followed by egg maturation where females may experience transient egg limitation. Second, males and females were monitored over their entire lifetimes to determine longevity and fecundity. Males and females were long lived with an average lifespan of 4 mo. Females displayed one of three temporal patterns of oviposition: (1) no oviposition, (2) oviposition began < 40 d after emergence, or (3) oviposition began > 40 d after emergence. In general, oviposition was independent of female age. Finally, egg maturation rates of field-collected females were determined. Egg maturation rates varied with time of year and maximum egg maturation rates coincided with periods when oviposition was expected to be high. The highest egg maturation rate observed was five eggs per female per day.

Effects of citrus and avocado irrigation and nitrogen-form soil amendment on host selection by adult Homalodisca vitripennis (Hemiptera: Cicadellidae)

Host plant water status is thought to influence dispersal of the xylophagous leafhopper Homalodisca vitripennis Germar, especially where plants are grown under high evaporative demand. Preference by adult H. vitripennis for plants grown under different water deficit and nitrogen form fertilization regimens was studied under laboratory conditions. Leafhopper abundance and ovipositional preference were studied on potted 'Washington navel' orange and 'Haas' avocado in cage choice tests, and feeding rate was estimated using excreta produced by insects confined on plants. A similar study compared responses to citrus treated with 1:1 and 26:1 ratios of fertigated nitrate-N to ammonium-N. The insects were more abundant, oviposited, and fed significantly more on surplus-irrigated plants than on plants under moderate continuous deficit irrigation except avocado feeding, which was nearly significant. Plants exposed to drought became less preferred after 3 and 7 d in avocado and citrus, respectively. Citrus xylem fluid tension at this point was estimated at 0.93 MPa. A corresponding pattern of decline in feeding rate was observed on citrus, but on avocado, feeding rate was low overall and not statistically different between treatments. No statistical differences in abundance, oviposition, or feeding were detected on citrus fertigated with 26:1 or 1:1 ratios of nitrate-N to ammonium-N. Feeding occurred diurnally on both plant species. Discussion is provided on the potential deployment of regulated deficit irrigation to manage H. vitripennis movement as part of a multitactic effort to minimize the risk of disease outbreaks from Xylella fastidiosa Wells et al. in southern California agriculture.

Comparative genomic characterization of citrus-associated Xylella fastidiosa strains

Background

The xylem-inhabiting bacterium Xylella fastidiosa (Xf) is the causal agent of Pierce's disease (PD) in vineyards and citrus variegated chlorosis (CVC) in orange trees. Both of these economically-devastating diseases are caused by distinct strains of this complex group of microorganisms, which has motivated researchers to conduct extensive genomic sequencing projects with Xf strains. This sequence information, along with other molecular tools, have been used to estimate the evolutionary history of the group and provide clues to understand the capacity of Xf to infect different hosts, causing a variety of symptoms. Nonetheless, although significant amounts of information have been generated from Xf strains, a large proportion of these efforts has concentrated on the study of North American strains, limiting our understanding about the genomic composition of South American strains – which is particularly important for CVC-associated strains.

Results

This paper describes the first genome-wide comparison among South American Xf strains, involving 6 distinct citrus-associated bacteria. Comparative analyses performed through a microarray-based approach allowed identification and characterization of large mobile genetic elements that seem to be exclusive to South American strains. Moreover, a large-scale sequencing effort, based on Suppressive Subtraction Hybridization (SSH), identified 290 new ORFs, distributed in 135 Groups of Orthologous Elements, throughout the genomes of these bacteria.

Conclusion

Results from microarray-based comparisons provide further evidence concerning activity of horizontally transferred elements, reinforcing their importance as major mediators in the evolution of Xf. Moreover, the microarray-based genomic profiles showed similarity between Xf strains 9a5c and Fb7, which is unexpected, given the geographical and chronological differences associated with the isolation of these microorganisms. The newly identified ORFs, obtained by SSH, represent an approximately 10% increase in our current knowledge of the South American Xf gene pool and include new putative virulence factors, as well as novel potential markers for strain identification. Surprisingly, this list of novel elements include sequences previously believed to be unique to North American strains, pointing to the necessity of revising the list of specific markers that may be used for identification of distinct Xf strains.

Amino acid fluctuations in young and old orange trees and their influence on glassy-winged sharpshooter (Homalodisca vitripennis) population densities

The glassy-winged sharpshooter (GWSS) is an invasive insect pest in California that proliferated in citrus while incorporating many other plant species in its host range. Field studies were conducted from September 2002 to September 2003 in adjacent young and old orange groves to determine the influence of amino acid concentrations in the xylem fluid on relative densities of the xylophagous GWSS. Beginning in early September 2002, higher densities of adult GWSS were observed on young compared to old trees. This difference became increasingly pronounced through late October in conjunction with increasing concentrations of essential amino acids in xylem fluid of the young trees. Greater densities of adult GWSS remained on young trees through early February 2003. Thereafter, the population crashed unexpectedly, leaving only negligible numbers of GWSS on either young or old trees. Mean concentrations of the essential amino acids for insect growth and development were higher in young compared to old trees. Besides these essential amino acids, asparagine, serine, glutamine, and tyrosine were also significantly higher in young trees during the September-February period when GWSS was present. The pattern of elevated amino acid concentrations in young trees continued through the entire year irrespective of the presence or absence of GWSS. Principal component analysis followed by factor analysis revealed three factors for young or old orange trees that accounted for 81.6 and 78.9% of the total variation in the young and old tree analyses, respectively. Various groups of amino acids in different factors displayed peak or elevated levels in young trees corresponding to the increased densities of GWSS. The potential roles of these amino acids in GWSS host selection are discussed.