Horizontal Transmission of "Candidatus Liberibacter solanacearum" by Bactericera cockerelli (Hemiptera: Triozidae) on Convolvulus and Ipomoea (Solanales: Convolvulaceae)

“Candidatus Liberibacter solanacearum” (Proteobacteria) is an important pathogen of solanaceous crops (Solanales: Solanaceae) in North America and New Zealand, and is the putative causal agent of zebra chip disease of potato. This phloem-limited pathogen is transmitted to potato and other solanaceous plants by the potato psyllid, Bactericera cockerelli (Hemiptera: Triozidae). While some plants in the Convolvulaceae (Solanales) are also known hosts for B. cockerelli, previous efforts to detect Liberibacter in Convolvulaceae have been unsuccessful. Moreover, studies to determine whether Liberibacter can be acquired from these plants by B. cockerelli are lacking. The goal of this study was to determine whether horizontal transmission of Liberibacter occurs among potato psyllids on two species of Convolvulaceae, sweet potato (Ipomoea batatas) and field bindweed (Convolvulus arvensis), which grows abundantly in potato growing regions of the United States. Results indicated that uninfected psyllids acquired Liberibacter from both I. batatas and C. arvensis if infected psyllids were present on plants concurrently with the uninfected psyllids. Uninfected psyllids did not acquire Liberibacter from plants if the infected psyllids were removed from the plants before the uninfected psyllids were allowed access. In contrast with previous reports, PCR did detect the presence of Liberibacter DNA in some plants. However, visible amplicons were faint and did not correspond with acquisition of the pathogen by uninfected psyllids. None of the plants exhibited disease symptoms. Results indicate that horizontal transmission of Liberibacter among potato psyllids can occur on Convolvulaceae, and that the association between Liberibacter and Convolvulaceae merits additional attention.     

Different host plant species modifies the susceptibility of Bactericera cockerelli to the entomopathogenic fungus Beauveria bassiana

Bactericera cockerelli (Sulc.) is a serious pest of solanaceous crops and a vector of the plant pathogen Candidatus Liberibacter psyllaurous. Entomopathogenic fungi are the most important biological control alternatives for this pest. Host plant species, however, can modify the outcomes of insect–pathogen interactions. We conducted laboratory experiments to quantify the virulence of two isolates of the entomopathogenic fungus Beauveria bassiana (Bals. [Vuill.]), BB40 and BB42, against third instar B. cockerelli nymphs maintained on chilli pepper plants. Owing to the lack of difference in virulence against B. cockerelli nymphs on chilli pepper between the two B. bassiana isolates, only BB42 was used to: compare virulence against nymphs maintained on either chilli pepper, potato or tomato; and in vivo conidia production from nymphs maintained on different host plants. Virulence of the two B. bassiana isolates against B. cockerelli nymphs was similar. Bactericera cockerelli nymphs maintained on tomato were more susceptible to B. bassiana than nymphs maintained on potato or chilli peppers. Infected nymphs maintained on chilli peppers produced the greatest number of conidia followed by infected nymphs maintained on tomato and potato. Host plant affected the susceptibility of B. cockerelli to B. bassiana isolate BB42 and subsequent conidia production. The implications of our results for microbial control of B. cockerelli by B. bassiana are discussed.     

Sampling and interpretation of psyllid nymph counts in potatoes

Development of effective management practices for insect pests relies heavily on sampling methods to accurately detect and quantify emerging populations. Herein we describe a novel method to extract and count nymphs of potato psyllid, Bactericera cockerelli (?ulc) (Hemiptera: Triozidae), from leaves in commercial fields of potato, Solanum tuberosum L. (Solanaceae). The proposed sampling method (referred to as the leaf washing method, LWM) consists of: (1) immersing samples of infested leaves in cold water to remove dust and sand; (2) immersing leaves in >85 °C water for 5 s; (3) extracting psyllid nymphs from heated water by passing it through a sampling unit composed of a funnel with a fine mesh‐organza fabric inside and attached to a vacuum pump; and (4) removing organza fabric and count psyllid nymphs under a stereoscope. With five sampling units operating simultaneously, one person is able to process and count immatures from about 400 leaf samples in two normal work days. As examples of applications, the LWM was used to characterize: (1) the vertical distribution of potato psyllid nymphs in the potato canopy; and (2) the spatio‐temporal distribution of potato psyllid nymphs in three potato varieties. Using LWM, we showed that psyllid nymphs were most predominant in the middle portion of the canopy and that spatio‐temporal distributions of nymphs varied among potato varieties.     

Urbanization,land tenure security and vector-borne Chagas disease

Modern cities represent one of the fastest growing ecosystems on the planet. Urbanization occurs in stages; each stage characterized by a distinct habitat that may be more or less susceptible to the establishment of disease vector populations and the transmission of vector-borne pathogens. We performed longitudinal entomological and epidemiological surveys in households along a 1900 × 125 m transect of Arequipa, Peru, a major city of nearly one million inhabitants, in which the transmission of Trypanosoma cruzi, the aetiological agent of Chagas disease, by the insect vector Triatoma infestans, is an ongoing problem. The transect spans a cline of urban development from established communities to land invasions. We find that the vector is tracking the development of the city, and the parasite, in turn, is tracking the dispersal of the vector. New urbanizations are free of vector infestation for decades. T. cruzi transmission is very recent and concentrated in more established communities. The increase in land tenure security during the course of urbanization, if not accompanied by reasonable and enforceable zoning codes, initiates an influx of construction materials, people and animals that creates fertile conditions for epidemics of some vector-borne diseases.     

Spatial Distribution of Nymphs Populations Bactericera cockerelli Sulc in Tomato Crops (<i>Physalis ixocarpa</i> Brot)

Tomato crops (Physalis ixocarpa Brot.) are produced in almost all Mexico, part of the United States and Central America. Recently the tomato production has suffered economic losses of 70% to 80% due the presence of yellowing and floral abortion, whose causal agent has been attributed to the presence of phytoplasma; an insect vector of these phytoplasma is Bactericera cockerrelli Sulc. Alternative control of this psyllid has lacked effectiveness because their spatial distribution is unknown within tomato plots. This study aimed to determine the spatial distribution of populations of nymphs of B. cockerelli in four tomato plots, the determination of the spatial distribution was performed by means of geostatistics. The experimental semivariogram was determined to adjust to a theoretical model (spherical, exponential or Gaussian) through the program Variowin 2.2, the adjustment was validated with the method of cross-validation and aggregation maps of the pest were obtained through Kriging with Surfer 9.0 program. The short-term time-space stability of the pest was determined through the tests Crámer-von Mises. The results showed that populations of nymphs of B. cockerelli have a distribution of aggregate type, which was corroborated by density maps. Infestations are not present in 100% of the surface of the experimental plots, which helps to direct control measures on specific areas of infestation.     

RNA Interference towards the Potato Psyllid,Bactericera cockerelli,Is Induced in Plants Infected with Recombinant Tobacco mosaic virus (TMV)

The potato/tomato psyllid, Bactericera cockerelli (B. cockerelli), is an important plant pest and the vector of the phloem-limited bacterium Candidatus Liberibacter psyllaurous (solanacearum), which is associated with the zebra chip disease of potatoes. Previously, we reported induction of RNA interference effects in B. cockerelli via in vitro-prepared dsRNA/siRNAs after intrathoracic injection, and after feeding of artificial diets containing these effector RNAs. In order to deliver RNAi effectors via plant hosts and to rapidly identify effective target sequences in plant-feeding B. cockerelli, here we developed a plant virus vector-based in planta system for evaluating candidate sequences. We show that recombinant Tobacco mosaic virus (TMV) containing B. cockerelli sequences can efficiently infect and generate small interfering RNAs in tomato (Solanum lycopersicum), tomatillo (Physalis philadelphica) and tobacco (Nicotiana tabacum) plants, and more importantly delivery of interfering sequences via TMV induces RNAi effects, as measured by actin and V-ATPase mRNA reductions, in B. cockerelli feeding on these plants. RNAi effects were primarily detected in the B. cockerelli guts. In contrast to our results with TMV, recombinant Potato virus X (PVX) and Tobacco rattle virus (TRV) did not give robust infections in all plants and did not induce detectable RNAi effects in B. cockerelli. The greatest RNA interference effects were observed when B. cockerelli nymphs were allowed to feed on leaf discs collected from inoculated or lower expanded leaves from corresponding TMV-infected plants. Tomatillo plants infected with recombinant TMV containing B. cockerelli actin or V-ATPase sequences also showed phenotypic effects resulting in decreased B. cockerelli progeny production as compared to plants infected by recombinant TMV containing GFP. These results showed that RNAi effects can be achieved in plants against the phloem feeder, B. cockerelli, and the TMV-plant system will provide a faster and more convenient method for screening of suitable RNAi target sequences in planta.     

Landscape Diversity Influences Dispersal and Establishment of Pest with Complex Nutritional Ecology

We studied the effects of landscape structure on species with resource nutritional partition between the immature and adult stages by investigating how food quality and spatial structure of a landscape may affect the invasion and colonization of the insect pest, Diabrotica speciosa. To this end, we formulated two bidimensional stochastic cellular automata, one for the insect immature stage and the other for the adult stage. The automata are coupled by adult oviposition and emergence. Further, each automata site has a specific culture type, which can affect differently the fitness attributes of immatures and adults, such as mortality, development and oviposition rates. We derived the mean-field approximation for these automata model, from which we obtained conditions for insect invasion. We ran numerical simulations using entomological parameters obtained from laboratory experiments (using bean, soybean, potato, and corn crops), and we compared the results of the automata with the ones given by the mean-field approximation. Finally, using artificially generated landscapes, we discussed how the structured heterogeneous landscape can affect dispersal and establishment of insect populations.     

Spatial distribution and sampling plans for Thrips palmi (Thysanoptera: Thripidae) infesting fall potato in Korea

The spatial distribution of adult and immature Thrips palmi Karny on fall potato, Solanum tuberosum L., on Cheju Island, Korea, was studied over a 2-yr period by visually inspecting potato leaves. The majority of thrips collected from the leaves were observed in the top one-third of the plant. The within-field spatial patterns of adults and immature thrips were aggregated. The slopes and intercepts of Taylor's power law did not differ among adults and immature thrips. A fixed-precision-level sampling plan was developed using the parameters from Taylor's power law and was tested with resampling simulations using eight independent data sets. Over a wide range of densities, the simulation demonstrated that actual sampling precision (d = SEM/mean) values at d = 0.25 averaged < 0.24 in all cases. A binomial sampling plan for estimating mean density was developed using an empirical model evaluated at tally thresholds (the minimum number of insects present before a leaf is considered infested) of one, three, five, and eight thrips per leaf. Increasing sampling size had little effect on the precision of the estimated mean regardless of tally threshold (T). However, increasing T had a dramatic effect on precision. The best tally threshold for estimating thrips density based on the applicable density ranges and the precision of the model was T = 5. A binomial sampling plan with a tally threshold of five and a fixed sample size of 30 leaves should be an effective replacement for enumerative counts when thrips average < 10 per leaf.     

Analysis of the spatio-temporal distribution of Eurygaster integriceps (Hemiptera: Scutelleridae) by using spatial analysis by distance indices and geostatistics

Eurygaster integriceps Puton (Hemiptera: Scutelleridae) is the most serious insect pest of wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) in Iran. In this study, spatio-temporal distribution of this pest was determined in wheat by using spatial analysis by distance indices (SADIE) and geostatistics. Global positioning and geographic information systems were used for spatial sampling and mapping the distribution of this insect. The study was conducted for three growing seasons in Gharamalek, an agricultural region to the west of Tabriz, Iran. Weekly sampling began when E. integriceps adults migrated to wheat fields from overwintering sites and ended when the new generation adults appeared at the end of season. The adults were sampled using 1- by 1-m quadrat and distance-walk methods. A sweep net was used for sampling the nymphs, and five 180° sweeps were considered as the sampling unit. The results of spatial analyses by using geostatistics and SADIE indicated that E. integriceps adults were clumped after migration to fields and had significant spatial dependency. The second- and third-instar nymphs showed aggregated spatial structure in the middle of growing season. At the end of the season, population distribution changed toward random or regular patterns; and fourth and fifth instars had weaker spatial structure compared with younger nymphs. In Iran, management measures for E. integriceps in wheat fields are mainly applied against overwintering adults, as well as second and third instars. Because of the aggregated distribution of these life stages, site-specific spraying of chemicals is feasible in managing E. integriceps.     

恩施烟区无翅桃蚜在烤烟田空间动态的地统计学分析

恩施烟区是湖北省最大烟叶生产基地,桃蚜Myzuspercicae(Sulzer)是恩施烟叶最重要的害虫之一,桃蚜在田间的发生以及传播的病毒病害逐年加重,给烟业生产带来巨大损失。进一步了解桃蚜发生动态和空间分布规律,将提高对桃蚜的预测效果并为其综合防治提供理论依据。烟区和烟田之间的迁移以有翅蚜为主,田块内部的种群动态和发生规律,无翅桃蚜发挥着更加重要的作用。受寄主生理生化特性影响,不同烟叶生育期,桃蚜空间结构的差异需要进一步验证。经典的统计学方法以纯随机变量为基础,而昆虫种群的田间分布存在空间相关性,地学统计学承认空间相关性的存在,为区域化变量的空间分布分析提供新的理论和方法。在烟叶不同生育期进行无翅桃蚜的田间密度调查,运用地统计学的方法分析了其空间特征和发生动态,模拟了无翅桃蚜在烟叶不同生育期的田间分布图,并对无翅桃蚜在不同烟叶生育期田间分布格局的相关性进行了分析。结果表明:无翅桃蚜在烟叶苗期密度最小为(5.59±4.07)头/株,烟叶旺长期虫口密度最大为(14.5±9.6)头/株;种群密度变异系数均较大(0.6147-0.7281),表明其空间分布的不均匀性,并随密度的增大而减小,表明种群密度的增大一定程度上提高了种群结构的稳定性。烟叶苗期的种群分布曲线峰度最大,表现出更高的聚集性。无翅桃蚜在烟叶苗期的135°方向和团棵期的45。方向表现为随机分布。烟叶苗期的0°方向和45°方向可用线性有基台模型拟合,其他均可用球形+指数套合模型拟合,判断球形+指数套合模型是无翅桃蚜田间分布的主要模型,属于聚集型分布的范畴。块金值、基台值和变程均随田间虫口密度的增大而增大,苗期的随机程度(0.1905—0.7186)明显大于其他时期(0.0116—0.1620)。无翅桃蚜空间分布模拟图可以清晰地看出无翅桃蚜苗期迁移,旺长期后逐渐稳定的特性。无翅桃蚜的田间分布在烟叶苗期与团棵期无明显相关性,而团棵期与旺长期以及旺长期与成熟期显著相关,再次证明烟叶苗期到团棵期,无翅桃蚜的田间分布发生较大迁移,而团棵期以后基本定殖。烟叶苗期的无翅桃蚜高度聚集在少数烟株上,及早预防可以减少烟叶苗期虫口基数,有利于桃蚜种群数量的控制。首次将平面坐标系划分为4个方向,更加准确、全面地描述昆虫种群的空间分布特征。     

A New Huanglongbing Species, “Candidatus Liberibacter psyllaurous,” Found To Infect Tomato and Potato, Is Vectored by the Psyllid Bactericera cockerelli (Sulc)

A new huanglongbing (HLB) “Candidatus Liberibacter” species is genetically characterized, and the bacterium is designated “Candidatus Liberibacter psyllaurous.” This bacterium infects the psyllid Bactericera cockerelli and its solanaceous host plants potato and tomato, potentially resulting in “psyllid yellowing.” Host plant-dependent HLB transmission and variation in psyllid infection frequencies are found.     

Cover Caption

Bactericera cockerelli (Hemiptera: Psyllididae) has caused millions of dollars loss in solanaceous crop production in many countries, particularly by causing Zebra Chip (ZC) disease, so named for the characteristic symptoms that develop in fried chips from infected potato tubers. B. cockerelli feeding causes significant changes in free amino acid (FAA) composition in potato plant tissues. The FAA changes in plant metabolism indicate that B. cockerelli feeding may cause the plant stress and the stressed plants are more vulnerable to ZC disease than the unstressed ones. See pages 663–670. Photo taken by Tong‐Xian Liu.     

Site-Specific Management of Nematodes Pitfalls and Practicalities

The greatest constraint to potato production in the United Kingdom (UK) is damage by the potato cyst nematodes (PCN) Globodera pallida and G. rostochiensis. Management of PCN depends heavily on nematicides, which are costly. Of all the inputs in UK agriculture, nematicides offer the largest potential cost savings from spatially variable application, and these savings would be accompanied by environmental benefits. We mapped PCN infestations in potato fields and monitored the changes in population density and distribution that occurred when susceptible potato crops were grown. The inverse relationship between population density before planting and multiplication rate of PCN makes it difficult to devise reliable spatial nematicide application procedures, especially when the pre-planting population density is just less than the detection threshold. Also, the spatial dependence found suggests that the coarse sampling grids used commercially are likely to produce misleading distribution maps.     

Population Structure of the Chagas Disease Vector Triatoma infestans in an Urban Environment

Chagas disease is a vector-borne disease endemic in Latin America. Triatoma infestans, a common vector of this disease, has recently expanded its range into rapidly developing cities of Latin America. We aim to identify the environmental features that affect the colonization and dispersal of T. infestans in an urban environment. We amplified 13 commonly used microsatellites from 180 T. infestans samples collected from a sampled transect in the city of Arequipa, Peru, in 2007 and 2011. We assessed the clustering of subpopulations and the effect of distance, sampling year, and city block location on genetic distance among pairs of insects. Despite evidence of genetic similarity, the majority of city blocks are characterized by one dominant insect genotype, suggesting the existence of barriers to dispersal. Our analyses show that streets represent an important barrier to the colonization and dispersion of T. infestans in Arequipa. The genetic data describe a T. infestans infestation history characterized by persistent local dispersal and occasional long-distance migration events that partially parallels the history of urban development.     

Aggregation patterns ofBemisia tabaci in response to insecticide applications

We measured the effect of insecticide applications on the distribution patterns of the sweet potato whitefly,Bemisia tabaci (Gennadius), in fields of cantaloupe,Cucumis melo L., in Yuma, Arizona. Whitefly infestations were measured by counts of adults, eggs, first to mid/fourth instars, and late-fourth instars (‘red-eyed’) nymphs. Adults were sampled from the entire leaf, and immature stages were counted in a 1-cm² area of a leaf. The indicesb (Power Law), β (Patchiness regression) and I _d (Morisita) indicated that all life stages were aggregated, but results for the three indices were not similar for determining the relative aggregation levels between treatments. In general, indices β andI _d indicated higher aggregation in insecticide-treated fields, whereasb had mixed results. The Morisita index was sensitive to a few unusually high means among a series of low densities in the treatment plots, what could be attributed to refuges due to failure in the insecticide applications. Despite the usefulness of the Power Law and the Patchiness regression for describing the relationship between spatial or temporal variability and mean densities, we suggest thatI _d is more appropriate for expression of spatial distribution because it is based on a precise definition of aggregation.