Survival in Dalbulus leafhopper vectors improves after exposure to maize stunting pathogens

Using D. maidis and pathogen isolates collected at three field sites along an altitudinal gradient in Mexico, we compared survival in leafhoppers exposed to healthy maize to those exposed to maize infected with one of four isolates of maize stunting pathogens: two isolates of the corn stunt spiroplasma (CSS, Spiroplasma kunkelii) and two of the maize busby stunt phytoplasma (MBSP). Survival improved after exposure to either plant pathogen under both the cooler and warmer environmental conditions D. maidis is likely to encounter during the dry season. Survival varied among leafhoppers from the different field sites, suggesting that gene flow between these populations is limited. The leafhoppers responded differently to the four isolates (i.e., we noted significant population by exposure interactions), but we found no difference between MBSP and CSS exposure. Finally, we found evidence of local adaptation in one leafhopper population to sympatric, as compared to allopatric, plant pathogens. We have shown with this and our earlier study that aspects of the interaction phenotype in the association between D. maidis and the plant pathogens are mutualistic and that this association has considerable potential as a model for studies of local adaptation.     

Inheritance of resistance in maize silks to the corn earworm

The corn earworm,Helicoverpa zea (Boddie), is a perennial economic pest of field crops in the United States. Maize,Zea mays L., is the major host crop promoting the build-up of devastating corn earworm populations that limit full production of cotton, soybean, peanut, and grain sorghum. Resistance to the corn earworm in maize and in particular sweet maize, would provide an environmentally safe, economical method of control for this pest insect. Antibiotic effects of corn silks on this insect are: small larvae, extended developmental period, and reduced fecundity. Silks from individual maize plants of resistant and susceptible lines and progeny in six generations consisting of parents (P₁, P₂), F₁, F₂, and backcrosses BC_1.1 (F₁ × P₁) and BC_1.2 (F₁ × P₂) from each of four crosses were used to determine the genetic basis of the antibiotic resistance of silks to the corn earworm. In the cross of Zapalote Chico × PI340856, genes controlling resistance in the silks to the corn earworm larvae are dominant in PI340856 to those in Zapalote Chico. The cross of Zapalote Chico × GT114 involves parents differing in degree of resistance, and possibly differing for the genetic mechanism by which the resistance is inherited. The inheritance of resistance may involve non-additive (dominance and epistasis) genetic variance. A digenic 6-parameter model indicated (1) the resistance in this cross is controlled by more than one pair of genes and (2) some or all of the genes interact to cause non-allelic interaction. Thus, the resistance in this cross may be controlled by both dominant and recessive genes. The resistance of Zapalote Chico × CI64, an intermediate inbred, is influenced by additive gene effects. The digenic model adequately predicts all generation means of the cross of GT3 × PI340856 except for the F1. Thus, it appears that the additive-dominance model is not satisfactory for this cross involving susceptible and resistant parents. Generation mean analysis indicates that resistance to silk-feeding by corn earworm larvae is under genetic control, but gene action differs from one type of cross to another.     

Oviposition behavior of Coccidoxenoides peregrinus, a parasitoid of Planococcus ficus

We tested hypotheses concerning the specificity of interactions between insect vectors and mollicute plant pathogens in a 22-month study of leafhoppers collected at three agricultural field sites in Mexico. The common species collected, Dalbulus maidis, D. elimatus, D. gelbus, and D. guevari were equally likely to test positive for corn stunt spiroplasma (CSS) in ELISA, and to transmit maize bushy stunt phytoplasma (MBSP) to test maize seedlings. We documented intraspecific variation in the ability of D. maidis to transmit confirmed CSS infections. Dalbulus guevari and D. gelbus were less successful in transmitting CSS than D. maidis from the same population. Our results suggest this vector-plant pathogen interaction is not specific to a single Dalbulus-mollicute combination, and that both the range of potential vectors in agricultural fields, and intraspecific variation across populations of these vectors, should be the focus of future work.     

Genetic mapping shows intraspecific variation and transgressive segregation for caterpillar‐induced aphid resistance in maize

Plants in nature have inducible defences that sometimes lead to targeted resistance against particular herbivores, but susceptibility to others. The metabolic diversity and genetic resources available for maize (Zea mays) make this a suitable system for a mechanistic study of within‐species variation in such plant‐mediated interactions between herbivores. Beet armyworms (Spodoptera exigua) and corn leaf aphids (Rhopalosiphum maidis) are two naturally occurring maize herbivores with different feeding habits. Whereas chewing herbivore‐induced methylation of 2,4‐dihydroxy‐7‐methoxy‐1,4‐benzoxazin‐3‐one glucoside (DIMBOA‐Glc) to form 2‐hydroxy‐4,7‐dimethoxy‐1,4‐benzoxazin‐3‐one glucoside (HDMBOA‐Glc) promotes caterpillar resistance, lower DIMBOA‐Glc levels favour aphid reproduction. Thus, caterpillar‐induced DIMBOA‐Glc methyltransferase activity in maize is predicted to promote aphid growth. To test this hypothesis, the impact of S. exigua feeding on R. maidis progeny production was assessed using seventeen genetically diverse maize inbred lines. Whereas aphid progeny production was increased by prior caterpillar feeding on lines B73, Ki11, Ki3 and Tx303, it decreased on lines Ky21, CML103, Mo18W and W22. Genetic mapping of this trait in a population of B73 × Ky21 recombinant inbred lines identified significant quantitative trait loci on maize chromosomes 1, 7 and 10. There is a transgressive segregation for aphid resistance, with the Ky21 alleles on chromosomes 1 and 7 and the B73 allele on chromosome 10 increasing aphid progeny production. The chromosome 1 QTL coincides with a cluster of three maize genes encoding benzoxazinoid O‐methyltransferases that convert DIMBOA‐Glc to HDMBOA‐Glc. Gene expression studies and benzoxazinoid measurements indicate that S. exigua ‐induced responses in this pathway differentially affect R. maidis resistance in B73 and Ky21.     

Within‐field spatial distribution patterns of corn planthopper,Peregrinus maidis,and severity of hopperburn and Maize mosaic virus symptoms as influenced by sunn hemp intercropping

Habitat management (e.g., intercropping) may alter within‐field spatial distribution patterns of herbivores, from a typical pattern as observed in a monoculture, and may influence patterns of crop injury. Field trials were conducted to study the effect of intercropping maize, Zea mays L. (Poaceae), with sunn hemp, Crotalaria juncea L. (Fabaceae) strips on within‐field spatial distribution patterns of corn planthopper, Peregrinus maidis (Ashmead) (Hemiptera: Delphacidae), and combined severity of hopperburn and Maize mosaic virus (MMV) (Rhabdoviridae: Nucleorhabdovirus) symptoms. In each field trial, spatially explicit data on P. maidis counts and ratings of severity of symptoms were obtained by sampling maize plants at weekly intervals. These data were used to examine the spatial patterns of P. maidis and severity of symptoms in maize‐intercropped and monoculture plots with Spatial Analysis for Distance IndicEs (SADIE) methodology. Spatial aggregation patterns of P. maidis in each treatment plot were not consistent among the field trials and tended to be mediated by their population densities. Interpolation of local cluster indices showed that P. maidis were more often aggregated at the field edges, irrespective of treatment. At times of MMV incidence in field trials (fall 2010 and spring 2011), the patch clusters of P. maidis and symptomatic plants were located at the field edges, but were spatially unassociated in both treatment plots. The results provided an approximation of the unpredictability of P. maidis spatial patterns at different population densities and their association with severity of symptoms in two maize‐cropping systems. However, the gap clusters of symptomatic plants were primarily located at the field interiors and were larger in intercropped than in monoculture plots. Such spatial pattern of symptomatic plants resulted in the reduced incidence of MMV in the intercropped plot compared with the monoculture plot, suggesting intercropping sunn hemp can be a useful tool in the management of MMV in maize fields.     

Did maize domestication and early spread mediate the population genetics of corn leafhopper?

Investigating how crop domestication and early farming mediated crop attributes, distributions, and interactions with antagonists may shed light on today's agricultural pest problems. Crop domestication generally involved artificial selection for traits desirable to early farmers, for example, in creased productivity or yield, and enhanced qualities, though invariably it altered the interactions between crops and insects, and expanded the geographical ranges of crops. Thus, some studies suggest that with crop domestication and spread, insect populations on wild crop ancestors gave rise to pestiferous insect populations on crops. Here, we addressed whether the emergence of corn leafhopper (Dalbulus ma id is) as an agricultural pest may be associated with domestication and early spread of maize (Zea mays mays). We used AFLP markers and mitochondrial COI sequences to assess population genetic structuring and haplotype relationships among corn leafhopper samples from maize and its wild relative Zea diploperennis from multiple locations in Mexico and Argentina. We uncovered seven corn leafhopper haplotypes contained within two haplogroups, one haplogroup containing haplotypes associated with maize and the other containing haplotypes associated with Z. diploperennis in a mountainous habitat. Within the first haplogroup, one haplotype was predominant across Mexican locations, and another across Argentinean locations;both were considered pestiferous. We suggested that the divergence times of the maize-associated haplogroup and of the "pestiferous" haplotypes are correlated with the chronology of maize spread following its domestication. Overall, our results support a hypothesis positing that maize domestication favored corn leafhopper genotypes preadapted for exploiting maize so that they became pestiferous, and that with the geographical expansi on of maize farming, corn leafhopper colonized Z. diploperennis, a host exclusive to secluded habitats that serves as a refuge for archaic corn leafhopper genotypic diversity. Broadly, our results help explain the extents to which crop domestication and early spread may have mediated the emergence of today's agricultural pests.     

Tritrophic interactions among Bt maize,an insect pest and entomopathogens: effects on development and survival of western corn rootworm

Agricultural systems often provide a model for testing ecological hypotheses, while ecological theory can enable more effective pest management. One of the best examples of this is the interaction between host‐plant resistance and natural enemies. With the advent of crops that are genetically modified to produce insecticidal toxins from the bacterium Bacillus thuringiensis (Bt), a new form of host‐plant resistance has been introduced to agroecosystems. How Bt crops interact with natural enemies, especially insect pathogens in below‐ground systems, is not well understood, but provides a unique opportunity to study below‐ground tritrophic interactions. In this study, we used two species of entomopathogenic fungi and three species of entomopathogenic nematodes to determine how this community of soil‐borne natural enemies might interact with Bt maize (event 59122, expressing the insecticidal protein Cry34/35Ab1) to affect survival and development of western corn rootworm (Diabrotica virgifera virgifera), which is an obligate root feeder and a serious pest of maize. We ran two experiments, one in a greenhouse and one in a growth chamber. Both experiments consisted of a fully crossed design with two maize treatments (Bt maize and non‐Bt maize) and two entomopathogen treatments (present or absent). The community of entomopathogens significantly increased mortality of western corn rootworm, and Bt maize increased larval developmental time and mortality. Entomopathogens and Bt maize acted in an independent and additive manner, with both factors increasing the mortality of western corn rootworm. Results from this study suggest that entomopathogens may complement host‐plant resistance from Bt crops.     

Enhancement of resistance to aphids by introducing the snowdrop lectin genegna into maize plants

In order to enhance the resistance to pests, transgenic maize (Zea mays L.) plants from elite inbred lines containing the gene encoding snowdrop lectin (Galanthus nivalis L. agglutinin; GNA) under control of a phloem-specific promoter were generated through theAgrobacterium tumefaciens- mediated method. The toxicity of GNA-expressing plants to aphids has also been studied. The independently derived plants were subjected to molecular analyses. Polymerase chain reaction (PCR) and Southern blot analyses confirmed that thegna gene was integrated into maize genome and inherited to the following generations. The typical Mendelian patterns of inheritance occurred in most cases. The level of GNA expression at 0.13%-0.28% of total soluble protein was observed in different transgenic plants. The progeny of nine GNA-expressing independent transformants that were derived separately from the elite inbred lines DH4866, DH9942, and 8902, were selected for examination of resistance to aphids. These plants synthesized GNA at levels above 0.22% total soluble protein, and enhanced resistance to aphids was demonstrated by exposing the plants to corn leaf aphid (Rhopalosiphum maidis Fitch) under greenhouse conditions. The nymph production was significantly reduced by 46.9% on GNA-expressing plants. Field evaluation of the transgenic plants supported the results from the inoculation trial. After a series of artificial self-crosses, some homozygous transgenic maize lines expressing GNA were obtained. In the present study, we have obtained new insect-resistant maize material for further breeding work.     

Physical leaf defenses – altered by Zea life‐history evolution,domestication, and breeding – mediate oviposition preference of a specialist leafhopper

Plant anti‐herbivore defenses are known to be affected by life‐history evolution, as well as by domestication and breeding in the case of crop species. A suite of plants from the maize genus Zea (Poaceae) and the specialist herbivore Dalbulus maidis (DeLong & Wolcott) (Hemiptera: Cicadellidae) were used to test the hypothesis that anti‐herbivore defenses are affected by plant life‐history evolution and human intervention through domestication and breeding for high yield. The suite of plants included a maize (Zea mays ssp. mays L.) commercial hybrid, a maize landrace, two populations of the annual Balsas teosinte (Z. mays ssp. parviglumis Iltis & Doebley), and perennial teosinte (Z. diploperennis Iltis, Doebley & Guzman). Leaf toughness, pubescence, and oviposition preference were compared among the suite of host plants looking for effects of transitions in life history (i.e., from perennial to annual life cycle), domestication (i.e., from wild annual to domesticated annual), and breeding (i.e., from landrace to hybrid maize) on defense against D. maidis. Results on leaf toughness suggested that the life‐history and domestication transitions weakened the plant's resistance to penetration by the mouthparts and ovipositor of D. maidis, whereas results on pubescence suggested that this putative defense was strengthened with the breeding transition, contrary to expectations. Results on oviposition preference of D. maidis coincided with the expectation that life‐history and domestication transitions would lead to preference for Balsas teosinte over perennial teosinte, and of landrace maize over Balsas teosinte. Also, a negative correlation suggested that oviposition preference is significantly influenced by leaf toughness. Overall, the results suggested that Zea defenses against the specialist herbivore D. maidis were variably affected by plant life‐history evolution, domestication, and breeding, and that chemical defense may play a role in Zea defense against D. maidis because leaf toughness and pubescence only partially explained its host preferences.     

Do assortative mating and immigrant inviability help maintain population genetic structuring of an herbivore on a crop and a wild relative?

Population genetic structuring is common among herbivorous insects and frequently is associated with divergent host plants, such as crops and their wild relatives. Previous studies showed population genetic structuring in corn leafhopper Dulbulus maidis in Mexico, such that the species consists of two sympatric, host plant-associated populations: an abundant and widespread "pestiferous” population on maize (Zea mays mays), and a small and localized "wild" population on perennial teosinte (Zea diploperennis). a maize wild relative with a limited distribution. This study addressed whether assortative mating and immigrant inviability mediate genetic structuring of corn leafliopper by comparing the mating and reproductive successes of pestiferous and wild females that colonize their nonassociated host plants against the successes of females colonizing their associated host plants. Assortative mating was assessed by comparing mating frequencies and premating and mating times among females of each population on each host plant: immigrant inviability was assessed by comparing, across two generations, the fecundity, survival, development time, sex ratio, and population growth rate among leafhopper populations and host plants. Our results showed that on maize, and compared to resident, pestiferous females, wild females were more likely to mate, and greater proportions of their offspring survived to adult stage and were daughters;consequently, the per-generation population growth rate on maize was greater for immigrant, wild leafhoppers compared to resident, pestiferous leafhoppers. Our results suggested that wild leafhoppers emigrating to maize have a fitness advantage over resident, pestiferous leafhoppers, while immigrant pestiferous and resident wild leafhoppers on teosinte have similar fitnesses.     

Sorghum root length density and the potential for avoiding striga parasitism

Striga hermonthica is a serious root parasite of sorghum in the semiarid tropics. Successful parasitism is dependent on interactions of Striga seeds and host roots. Several sorghum cultivars have been found which resist parasitism. The basis of resistance is not well known. One possible method for reducing the chances of parasitism is by restricted host root development. This research was conducted to evaluate this hypothesis in sorghum known to possess resistance to parasitism by Striga.Root length density of 21-day-old pot-grown resistant cultivars, Framida, N-13, IS-9830, Tetron and P-967083, were compared to that of the susceptible check, Dabar, using the line intercept method of measuring root length. There was no significant difference between resistant cultivars and the susceptible cultivar Dabar. The RLD of resistant P-967083 however was significantly less than Framida, another resistant cultivar.The RLD of Dabar was compared to that of Framida and P-967083 in USA and Niger field trials. Root length density was determined on soil cores taken at flowering with a Giddings Soil Sampler. Each core was divided into 10-cm fractions for estimating RLD by the line intercept method. In the USA Dabar had significantly greater RLD than the two resistant cultivars in the upper 10-cm portion of the soil profile, but only significantly greater than P-967083 in the 10–20-cm portion. Significant differences in RLD between susceptible and resistant cultivars were not found at depths between 20–60 cm. In field trials in Niger, RLD of Dabar was significantly greater than either resistant cultivar in the (0 to 30 cm) portion of the soil core. These results suggest that part of the Striga resistance of P-967083 and perhaps Framida may be a result of avoiding interactions between parasitic seeds and host roots.     

Host plant adaptability and proteomic differences of diverse Rhopalosiphum maidis (Fitch) lineages

Corn leaf aphid Rhopalosiphum maidis (Fitch) can feed on various cereal crops and transmit viruses that may cause serious economic losses. To test the impact of both host plant species and age on R. maidis, as well as the proteomic difference of diverse populations, we first investigated the survival and reproduction of six R. maidis populations (i.e., LF, HF, GZ, DY, BJ, and MS) via a direct observation method in the laboratory on 10 and 50 cm high maize seedlings, and 10 cm high barley seedlings. Then a proteomic approach was implemented to identify the differentially expressed proteins from both aphids and endosymbionts of BJ and MS populations. Results indicated that the BJ population performed significantly better than the others on both barley and 50 cm high maize seedlings, while no population could survive on 10 cm high maize seedlings. The proteomic results demonstrated that the expression levels of myosin heavy chain (muscle isoform X12) (spot 781) and peroxidase (spot 1383) were upregulated, while ATP-dependent protease Hsp 100 (spot 2137) from Hamiltonella defensa and protein SYMBAF (spot 2703) from Serratia symbiotica were downregulated in the BJ population when compared to expression levels of the MS population. We hypothesize that the fatalness observed on 10 cm high maize seedlings may be caused by secondary metabolites that are synthesized by the seedlings and the MS population of R. maidis should be more stress-resistant than the BJ population. Our results also provide insights for understanding the interaction between host plants and aphids.     

Increased survival of Dalbulus maidis, a specialist on maize, on non-host plants infected with mollicute plant pathogens

The leafhopper Dalbulus maidis DeLong & Wolcott survived significantly longer on aster, Callistephus chinensis Nees, infected with any one of 3 strains of aster yellows (AY) mycoplasma-like organism (MLO) than on healthy asters. After 7 or more days on AY-diseased aster, females were conditioned to survive longer on healthy asters than were leafhoppers of the same age previously exposed only to maize. Females were also conditioned to survive longer on healthy aster by prior exposure to AY-MLO-infected celery (Apium graveolens L.). Males were not so conditioned. Leafhoppers injected with infectious extracts of AY-MLO dit not live longer on aster nor transmit the AY-MLO to aster. Conditioning on AY-diseased aster did not cause D. maidis to transmit AY-MLO and did not interfere with the transmission to maize of the mollicute (Spiroplasma kunkelii Whitcomb et al.) that causes corn stunt disease. Spiroplasma citri Saglio et al. infection of aster but not of turnip (Brassica rapa L.), Plantago major L. or periwinkle (Catharanthus roseus (L.)), improved the longevity of D. maidis on these plants and conditioned leafhoppers for enhanced subsequent survival on healthy asters.

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Résumé La cicadelle du maïs, D. maidis à survécu significativement plus longtemps sur aster (Callistephus chinensis Nees) contaminé par l'une des trois souches de l'agent mycoplasmique (MLO) de la jaunisse de l'aster (AY), que sur des asters sains. Après 7 jours ou plus sur des asters AY-MLO, les femelles (et non les mâles) étaient conditionnées de telle sorte qu'elles survivaient plus longtemps sur asters sains que des femelles du même âge précédemment sur maïs. Sur des céleris (Apium graveolens) atteints de AY, D. maidis a survécu plus longtemps que sur céleri sain, mais moins que sur aster AY. Des extraits, contaminés par AY injectés à D. maidis n'ont pas augmenté sa longévité sur aster, ni provoqué la transmission de AY-MLO. Le conditionnement sur aster AY n'a pas entrainé la transmission de AY-MLO par D. maidis, et n'a pas interferé avec son aptitude à transmettre l'agent du nanisme du maïs, Spiroplasma kunkelii. S. citri a aussi conditionné D. maidis sur aster, mais a été sans effet sur navet (Brassica rapa), sur plantain (Plantago major) et sur pervenche (Vinca).

     

Genetic variability in the egg-laying behaviour ofTrichogramma maidis

SomeTrichogramma maidis females [Hymenoptera: Trichogrammatidae] aggregate their eggs in adjacent hosts whereas others scatter them amongst distant host eggs. The individual variability of aggregated-attack behaviour in matedTrichogramma maidis females was studied by isolating them with a uniform patch of about 500Ephestia kuehniella [Lepidoptera: Pyralidae] UV killed eggs. Genetic analysis demonstrated that similar behaviour exists between sisters and that the trait is transmissible and relatively stable over 2 successive generations. The ethological basis and the ecological significance of this variability are discussed.      

Population genetic structure of a specialist leafhopper on Zea: likely anthropogenic and ecological determinants of gene flow

Corn leafhopper, Dalbulus maidis DeLong & Wolcott (Hemiptera: Cicadellidae), is a specialist herbivore on the genus Zea (Poaceae). The genera Dalbulus and Zea evolved in central Mexico. We sought to determine whether population genetic structuring is prevalent in corn leafhoppers inhabiting three of its host plants: (1) the highland species perennial teosinte (Zea diploperennis Iltis, Doebley & Guzman), (2) the mid‐ to lowland‐species Balsas teosinte (Zea mays ssp. parviglumis Iltis & Doebley), and (3) the ubiquitous domesticated maize (Zea mays ssp. mays L.). We used amplified fragment length polymorphisms to detect population structuring and genetic differentiation among corn leafhoppers on the three host plants in western‐central and ‐northern Mexico. Our results showed that corn leafhopper in Mexico is composed of at least two genetically discrete populations: an ‘Itinerant’ population associated with the annual hosts maize and Balsas teosinte, which appears to be widely distributed in Mexico, and a ‘Las Joyas’ population restricted to perennial teosinte and confined to a small mountain range (Sierra de Manantlán) in western‐central Mexico. Our results further suggested that population structuring is not due to isolation by distance or landscape features: Las Joyas and Itinerant corn leafhopper populations are genetically distinct despite their geographic proximity (ca. 4 km), whereas Itinerant corn leafhoppers separated by hundreds of kilometers (>800 km), mountain ranges, and a maritime corridor (Sea of Cortez) are not genetically distinct. Based on our results and on published ethnohistorical and archaeological data, we propose pre‐Columbian and modern scenarios, including likely ecological and anthropogenic influences, in which the observed genetic population structuring of corn leafhopper could have originated and could be maintained. Also, we hypothesize that after evolving on the lowland Balsas teosinte, corn leafhopper expanded its host range to include maize and then the highland perennial teosinte, following the domestication and spread of maize within the last 9 000 years.     

Identification of a New Race of Sporisorium reilianum and Characterization of the Reaction of Sorghum Lines to Four Races of the Head Smut Pathogen

Sporisorium reilianum is the causal agent of head smut on sorghum and maize. In order to effectively utilize host resistance to control this important disease in crops, it is necessary to monitor changes in disease dynamics and virulence of the pathogen. An outbreak of head smut was recently observed in a sorghum field, near Gaoping, Shanxi, China, and research was undertaken to characterize a putative new race of S. reilianum. A set of differential sorghum lines with resistance to several conventional races was used to characterize the newly collected isolate of S. reilianum. The reactions of differential cultivars/germplasm lines to the new isolate indicate that it is a new physiological race of S. reilianum. The new race is highly virulent on sorghum line A₂V4 and its hybrid, Jinza 12, that are known as resistant to all existing Chinese races of S. reilianum, including races 1, 2, and 3. The new isolate of S. reilianum is different from all of the described races of the pathogen; thus, it is designated as race 4 of S. reilianum. Furthermore, a collection of 34 sorghum genotypes including commercial cultivars and germplasm lines was evaluated for disease reaction to the newly described race and the three known races of the pathogen.     

Host plant dispersion, leaf hopper movement and disease transmission

Abstract.

• 1 The plant-to-plant movement of the corn leafhopper, Dalbulus maidis Delong & Wolcott, and the spread of the leafhopper-borne maize rayado fino virus were investigated in four patterns of maize (Zea mays) dispersion.
• 2 D. maidis was less abundant and the spread of the virus was slower in dense stands of maize than in sparse stands.
• 3 When plant density was held constant, leafhoppers were more abundant in maize stands with relatively equidistant plant spacing (uniform dispersion) than in stands with densely-sown rows (linear dispersion) or double-sown hills (clumped dispersion), but there was no difference in virus incidence among these plant dispersion patterns.
• 4 Leafhoppers were less likely to move to adjacent plants in uniform plant dispersion patterns than in either linear or clumped dispersion patterns. This result may explain the lack of higher virus incidence in uniform stands, despite higher leafhopper abundance.
• 5 Leafhopper movement was consistent with a simple rule: the shorter the distance to the next adjacent plant, the more likely a leafhopper is to move between plants.
• 6 These results demonstrate that host plant dispersion can affect the abundance and behaviour of highly mobile herbivorous insects even when plant density is constant.

     

Transmission of rayado fino virus of maize (Zea mays) by Dalbulus tnaidis

Rayado fino virus (RFV) of maize (Zea mays) was transmitted by the leaf-hopper Dalbulus maidis in a manner characteristic of viruses that multiply in their insect vectors. Individual insects fed on infected plants transmitted the virus after incubation periods of 8–22 days; males had shorter incubation periods than females but died sooner. Insects retained infectivity for 1–20 days. Transmission by most insects was intermittent. Inoculativity by D. maidis decreased with time, but the virus was recovered from insects that had lost their ability to transmit. Extracts of plants infected with RFV and viruliferous insects were injected into healthy insects, which became viruli-ferous. Infectivity of the extracts was not affected by tetracycline hydrochloride (Achromycin). D. maidis was able to transmit simultaneously RFV and the corn stunt agent. Other than maize, Teosinte (Euchlaena mexicana) was the only plant susceptible to the virus, among a number of species of Gramineae tested.     

Leafhoppers on leaves: An analysis of feeding behavior using conditional probabilities

Feeding behavior of four deltocephaline leafhoppers,Graminella nigrifrons, G. oquaka, Amblysellus grex, andDalbulus maidis on maize and johnsongrass was analyzed using an electronic monitoring device. Five distinct waveform patterns were identified: secretion of sheath saliva (salivation), nonvascular probing, nonsieve element ingestion, x-waveform, and phloem ingestion. Waveforms were associated with feeding activities by correlation with light microscopic examination of salivary sheath termination points in leaf tissue and analysis of honeydew excreted by monitored leafhoppers. In previous studies x-waveforms have been reported to occur only when the stylets of homopterans are in contact with the phloem; the function of x-waveforms is poorly understood. There were no differences in time spent salivating or ingesting from nonsieve elements amongG. nigrifrons, G. oquaka andA. grex on either plant.D. maidis differed from other species in phloem probing and feeding behavior; only a small proportion produced x-waveforms, although those that did spent significantly more time in this behavior than other species. Also,D. maidis spent more time than other leafhoppers ingesting from tissues other than sieve elements. Kinetic diagrams of transition probabilities show that probing activities of all species were not random regarding the sequence of behaviors culminating in phloem ingestion. Thirty-five percent ofG. nigrifrons x-waveforms were followed by nonsieve element ingestion. This was consistent with observations showing that salivary sheaths of leafhoppers producing x-waveforms sometimes do not terminate in the phloem, but rather in nearby cells. Phloem ingestion was always preceeded by x-waveforms. The quantitative differences in probing behavior are discussed in relation to ability of these leafhoppers to transmit the phloem-associated maize chlorotic dwarf waikavirus.     

The parasitoid Gonatopus bartletti reduces presence of plant-pathogenic Spiroplasma kunkelii within the leafhopper vector Dalbulus maidis

Homopteran vectors (e.g., leafhoppers) of plant pathogens are vessels for reproduction of cell wall‐free bacteria. These vectors also serve as hosts for larval parasitoid dipterans, hymenopterans, and strepsipterans. However, no study has explored the relationship among these wall‐free bacteria and parasitoid larvae within the insect host. We studied the corn stunt spiroplasma (CSS), Spiroplasma kunkelii Whitcomb (Mycoplasmatales: Spiroplasmataceae), a bacterium that originated from secondary symbionts that cause corn stunt disease in maize, Zea mays L., and its reproduction in the haemolymph of the corn leafhopper, Dalbulus maidis (Delong and Wolcott) (Homoptera: Cicadellidae). We also studied the dryinid parasitoid Gonatopus bartletti Olmi (Hymenoptera: Dryinidae), the larva of which feeds in the corn leafhopper haemolymph. Our results showed that when CSS and the wasp coexisted in D. maidis, the development of the parasitoid was not affected by S. kunkelii. Parasitoid development was successfully completed when leafhoppers acquired S. kunkelii before or after parasitism and when CSS had median (10 days) and long (20 days) incubation periods in the leafhopper before parasitization. The presence of S. kunkelii did not affect parasitoid development to the adult stage. However, polymerase chain reaction showed that the presence (survival) of S. kunkelii in the leafhopper was negatively affected by the parasitoid larva. Fewer leafhoppers had CSS before and after parasitization compared with leafhoppers that only acquired the CSS. This negative effect helps to explain the high parasitism rate by G. bartletti in D. maidis and the low presence of S. kunkelii in the corn leafhopper when CSS and the wasp parasitoid overlap throughout their geographic distribution. The parasitoid larva may negatively affect S. kunkelii by (1) producing antibacterial peptides that are toxic to CSS; (2) producing teratocytes that take nutrients from the host for larval development, but these nutrients are required by CSS; (3) affecting, indirectly, CSS through other symbiotic microorganisms; and (4) producing proteins with antibacterial activity that are present in the venom of the wasp parasitoid.