Response of resistant and susceptible Brachiaria spp. genotypes to simultaneous infestation with multiple species of spittlebugs (Hemiptera: Cercopidae)

The response of one susceptible and three resistant Brachiaria spp. (Hemiptera: Cercopidae) genotypes to individual or combined attacks by nymphs of Aeneolamia varia (F.), Aeneolamia reducta (Lallemand), Zulia carbonaria (Lallemand), and Zulia pubescens (F.) was studied. We assessed the effect of infesting plants of the susceptible check BRX 44-02 and of the A. varia-resistant genotypes CIAT 6294 and CIAT 36062 with A. varia, Z. carbonaria, or Z. pubescens either alone or in two-species combinations. In a second trial, we studied the performance of BRX 44-02, CIAT 6294, and the multiple resistant clone SX01NO/0102 exposed to individual or combined attack by A. reducta and Z. carbonaria. In a third trial, we compared the response of BRX 44-02, CIAT 6294, and CIAT 36062 to individual A. varia, Z. carbonaria, or Z. pubescens attack as opposed to a combined three-species attack. Plant damage scores and percentage of nymphal survival were recorded in all three trials. Data on percentage of survival indicated that competition between and among spittlebug species occurs. However, we found no evidence of interaction between species competition and different levels of resistance to spittlebug. Rather, host genotype reactions conformed to previously known categories of resistance regardless of the presence of more than one spittlebug species. Resistance rather than competition seems to have been the overriding factor determining nymph survival and resistance expression (damage scores) in these experiments. Our results corroborate the need to develop brachiariagrass genotypes with multiple resistance to spittlebugs.     

Sublethal effects of antibiosis resistance on the reproductive biology of two spittlebug (Hemiptera: Cercopidae) species affecting Brachiaria spp

Several greenhouse experiments were used to measure how high levels of antibiosis resistance to nymphs in two interspecific Brachiaria (brachiariagrass) hybrids affect life history parameters of the spittlebugs Aeneolamia varia (F.) and Zulia carbonaria (Lallemand), two of the most important spittlebug (Hemiptera: Cercopidae) species affecting Brachiaria production in Colombia. The A. varia-resistant hybrid CIAT 36062, the Z. carbonaria-resistant hybrid SX01NO/0102, and the susceptible accession CIAT 0654 were used to compare the effect of all possible combinations of food sources for nymphs and adults. Calculation of growth indexes showed a significant impact of antibiosis resistance on the biology of immature stages of both species. Median survival times of adults feeding on resistant genotypes did not differ from those recorded on the susceptible genotype, suggesting that factors responsible for high mortality of nymphs in the resistant hybrids did not affect adult survival. Rearing nymphs of A. varia on CIAT 36062 and of Z. carbonaria on SX01NO/0102 had deleterious sublethal effects on the reproductive biology of resulting adult females. It is concluded that high nymphal mortality and subsequent sublethal effects of nymphal antibiosis on adults should have a major impact on the demography of the two spittlebug species studied.     

Independence of resistance in Brachiaria spp. to nymphs or to adult spittlebugs (Hemiptera: Cercopidae): implications for breeding for resistance

Both nymphal and adult spittlebugs (Hemiptera: Cercopidae) cause serious economic damage to susceptible brachiariagrass [genus Brachiaria (Trin.) Griseb], pastures in tropical America. Both life stages are xylem feeders: nymphs feed primarily on roots and stems, whereas the adults feed mainly on foliage. Numerous interspecific brachiariagrass hybrids with high levels of antibiosis resistance to nymphs of several important spittlebug species have been obtained. Recent studies revealed major inconsistencies between reaction to nymphs and reaction to adults on the same host genotype. Because both insect life stages can cause severe economic damage on susceptible brachiariagrass pastures, a cultivar development strategy must take into account resistance to both life stages. To assess the degree of association between resistance to spittlebug nymphs and to adult feeding, we tested 164 hybrids and six check genotypes for resistance to both life stages of three spittlebug species: Aeneolamia varia (F.), Aeneolamia reducta (Lallemand), and Zulia carbonaria (Lallemand). Most hybrids tested were classified as resistant to nymphs. On the contrary, for all three species, the overall mean damage score of the 164 hybrids did not differ from the mean score of the susceptible checks. None of the hybrids was classified as resistant to adult feeding damage. Correlations between percentage nymph survival and adult damage scores were consistently low (r = 0.0104-0.0191). Correlations between nymphal and adult damage scores were also low (0.109-0.271), suggesting that resistances to the different life stages are largely independent. Chi-square analyses comparing frequency distributions of responses of the 164 breeding hybrids to nymphs or adults confirmed essential genetic independence of these two traits. We conclude that attention to improving genetic resistance specifically to adult feeding damage is warranted.     

Morphological and molecular identification of Prosapia simulans (Walker) (Hemiptera: Cercopidae), and screening and mechanisms of resistance to this spittlebug in Brachiaria hybrids

Prosapia simulans (Walker) is an important spittlebug species that attacks forage grasses of the genus Brachiaria (Trin.) Griseb. from Mexico to Colombia. This, and several other species of spittlebugs, cause important economic losses to the livestock production industry. Development of resistant cultivars is regarded as the best method of control. In the present study we used taxonomic keys, dissection of male genitalia and RAPD-PCR patterns to reconfirm the identity of P. simulans specimens collected in Colombia and Mexico. We were able to reconfirm that P. simulans occurs as a pest of Brachiaria from Mexico to Colombia. We also studied the levels and mechanisms of resistance present in 34 Brachiaria hybrids developed by CIAT. Infestations were made with six eggs per plant. We used 10 replications (plants) per genotype in a completely randomized design. Seven hybrids were found to be susceptible, 16 showed intermediate resistance and 11 were resistant. Antibiosis was the mechanism of resistance expressed in resistant hybrids as well as in the resistant checks CIAT 6294 and CIAT 36062. Tolerance was absent. The genotypes BRX 4402 and CIAT 0606 were classified as highly susceptible.     

Characterizing Herbivore Resistance Mechanisms: Spittlebugs on Brachiaria spp. as an Example

Plants can resist herbivore damage through three broad mechanisms: antixenosis, antibiosis and tolerance¹. Antixenosis is the degree to which the plant is avoided when the herbivore is able to select other plants². Antibiosis is the degree to which the plant affects the fitness of the herbivore feeding on it¹.Tolerance is the degree to which the plant can withstand or repair damage caused by the herbivore, without compromising the herbivore''s growth and reproduction¹. The durability of herbivore resistance in an agricultural setting depends to a great extent on the resistance mechanism favored during crop breeding efforts³.We demonstrate a no-choice experiment designed to estimate the relative contributions of antibiosis and tolerance to spittlebug resistance in Brachiaria spp. Several species of African grasses of the genus Brachiaria are valuable forage and pasture plants in the Neotropics, but they can be severely challenged by several native species of spittlebugs (Hemiptera: Cercopidae)⁴.To assess their resistance to spittlebugs, plants are vegetatively-propagated by stem cuttings and allowed to grow for approximately one month, allowing the growth of superficial roots on which spittlebugs can feed. At that point, each test plant is individually challenged with six spittlebug eggs near hatching. Infestations are allowed to progress for one month before evaluating plant damage and insect survival. Scoring plant damage provides an estimate of tolerance while scoring insect survival provides an estimate of antibiosis. This protocol has facilitated our plant breeding objective to enhance spittlebug resistance in commercial brachiariagrases⁵.     

Resistance of Sugarcane Cultivars to Mahanarva fimbriolata (Stål) (Hemiptera: Cercopidae)

The spittlebug Mahanarva fimbriolata (Stål) (Hemiptera: Cercopidae) is one of the most important pests of the sugarcane crop in Brazil. Despite of its importance, there is currently a lack of information regarding sugarcane cultivars’ resistance to the spittlebug. Therefore, our objective was to evaluate the response of sugarcane genotypes to this species. Two experiments were carried out under laboratory conditions using a random block design with treatments in a factorial arrangement of 2?×?13 (experiment 1) and 2?×?12 (experiment 2), with six replicates. The first factor included two levels of infestation (infested and noninfested plants with spittlebugs), while the second consisted of the cultivars. Nymph survival varied from 47.9 to 84.5%, indicating that there are different levels of antibiosis to M. fimbriolata among the tested cultivars. The highest degree of antibiosis was found in cultivars IACSP96-7586 and IACSP96-2008, in which nymph survival was close to 48%. IACSP96-7586 also presented some degree of tolerance, but IACSP96-7569 and IACSP97-6682 stood out as the most tolerant cultivars to the pest, showing the lowest reduction in weight of aboveground biomass. On average, spittlebug infestations caused a significant reduction in relative leaf chlorophyll content and aboveground biomass weight.     

Secretion of phytase from the roots of several plant species under phosphorus-deficient conditions

Phosphorus (P) deficiency increased the secretion of phytases from roots of various plant species. The secretory phytases were collected with a dialysis membrane tube for 24 hours from roots of sixteen plant species grown with low or adequate supply of P in nutrient solutions. The activity of not only secretory phytase, but also acid phosphatase, increased with the low P treatment in all of the plant species examined. Secretion of phytase by the roots under P-deficient conditions was highest in Brachiaria decumbens CIAT 606, Stylosanthes guianensis CIAT 184 and tomato, moderate in Brachiaria brizantha CIAT6780, Stylosanthes guianensis CIAT 2950, alfalfa, white clover and orchard grass, and lowest in Andropgon gayanus CIAT 621, Stylosanthes capitata CIAT 10280, upland rice, timothy, redtop, alsike clover, red clover and white lupin plants. An immunoreactive protein band that reacted with a polyclonal antibody raised against wheat bran phytase, corresponding to molecular weight 35–40 kD, could be detected in seven of the species tested. These results indicate that the secretory phytase may provide an efficient mechanism for certain plants to utilize inositol hexaphosphate in soil.     

Tolerance of antibiotic and susceptible cereal seedlings to the aphids Metopolophium dirhodum and Rhopalosiphum padi

Some cereal seedlings exhibit antibiotic and antixenotic resistance to the aphids Metopolophium dirhodum (Walker) and Rhopalosiphum padi (L.), because the seedlings contain hydroxamic acids or gramine. The association between tolerance to aphids and aphid antibiosis was investigated for three cereals, Dollarbird wheat Vulcan wheat and Yagan barley. The dry biomass gained by the aphids and the simultaneous reduction in the biomass of the plants (biomass conversion ratio) quantified tolerance. Biomass production and the density dependence of biomass production by the aphids quantified antibiosis more effectively than fecundity. Vulcan wheat, which has more hydroxamic acid than Dollarbird wheat showed the highest level of antibiosis, and the barley was not antibiotic for either aphid. The biomass conversion ratio was a constant; the biomass of an infested plant was reduced by 3 mg for each mg of aphid biomass gained, regardless of aphid species, plant cultivar, or aphid density. The three plants showed no differential tolerance to the aphids, and therefore tolerance is not associated with antibiosis in this case.     

Categories of Resistance to Greenbug and Yellow Sugarcane Aphid (Hemiptera: Aphididae) in Three Tetraploid Switchgrass Populations

Switchgrass, Panicum virgatum L., has been targeted as a bioenergy feedstock. However, little is currently known of the mechanisms of insect resistance in this species. Here, two no-choice studies were performed to determine the categories (antibiosis and tolerance) and relative levels of resistance of three switchgrass populations (Kanlow–lowland ecotype, Summer–upland ecotype, and third generation derivatives between Kanlow?×?Summer plants, K×S) previously identified with differential levels of resistance to the greenbug, Schizaphis graminum (Rondani), and yellow sugarcane aphid, Sipha flava (Forbes). No-choice studies indicated that Kanlow possessed multi-species resistance, with high levels of antibiosis to both aphid species, based on aphid survival at 7 and 14 days after aphid introduction and cumulative aphid days, while K×S possessed low-to-moderate levels of antibiosis to S. flava. Further, functional plant loss indices based on plant height and biomass indicated that tolerance is an important category of resistance for Summer plants to S. graminum. These studies also indicated that Summer lacks both tolerance and antibiosis to S. flava, relative to the other switchgrasses tested, whereas K×S lack tolerance and antibiosis to S. graminum. These studies are the first attempt to analyze the categories of resistance in switchgrass and provide critical information for characterizing the biological mechanisms of resistance and improving our knowledge of the plant–insect interactions within this system.     

Characterization of greenbug (Homoptera: Aphididae) resistance in synthetic hexaploid wheats

Twelve greenbug (Schizaphis graminum (Rondani)) biotype E-resistant synthetic hexaploid wheats synthesized by crossing Triticum dicoccum Schrank. and Aegilops tauschii (Coss.) Schmal. were evaluated for the three known insect resistance categories, including antibiosis, anti-xenosis, and tolerance. Different methods were evaluated for calculating antibiosis and tolerance. Calculating intrinsic rate of population increase and measuring leaf chlorophyll content with a SPAD chlorophyll meter proved to be time- and labor-efficient for antibiosis and tolerance determination, respectively. The resistance in all synthetic hexaploids proved to be the result of a combination of antibiosis, antixenosis, and tolerance, which makes them valuable sources of greenbug resistance. To assist plant breeders in selecting the best germplasm for greenbug resistance, a plant resistance index was created that revealed differences among the synthetic hexaploid wheats.     

Good mothers,bad mothers,and the nature of resistance to herbivory in <Emphasis Type="Italic">Solidago altissima</Emphasis>

Evidence of poor correspondence between an insect herbivore’s oviposition preferences and the performance of its offspring has generally been attributed either to maladaptive behavior of the insect mother or inadequate measurement by the researcher. In contrast, we hypothesize that many cases of “bad mothers” in herbivores may be a byproduct of the hierarchical way natural selection works on resistance in host plants. Epistatic selection on the components of resistance (i.e., antixenosis and antibiosis) may generate negative genetic correlations between the resistance components, which could counteract the efforts of herbivores to oviposit on the best hosts for the performance of their offspring. In common garden and greenhouse experiments, we measured aspects of antixenosis and antibiosis resistance in 26 genets of tall goldenrod, Solidago altissima, against two common herbivores: the gall-inducing fly Eurosta solidaginis and the spittlebug Philaenus spumarius. Goldenrod antixenosis and antibiosis were positively correlated against E. solidaginis and negatively correlated against P. spumarius. Analogously, population-wide preference–performance correlations were positive for the gall flies and negative for the spittlebugs. Several natural history differences between the two insects could make gall flies better mothers, including better synchrony of the phenologies of the flies and the host plant, the much narrower host range of the gall flies than the spittlebugs, and the more sedentary lifestyle of the gall fly larvae than the spittlebug nymphs. If these results are typical in nature, then negative genetic correlations in antixenosis and antibiosis in plants may often result in zero or negative population-wide correlations between preference and performance in herbivores, and thus may be an important reason why herbivorous insects often appear to be bad mothers.     

Host plant resistance in Brachiaria grasses to the spittlebug Zulia colombiana

Twelve forage grass accessions including 11 accessions of Brachiaria Griseb, were evaluated in a glasshouse for host plant resistance to nymphs and tolerance to feeding damage caused by adults of Zulia colombiana (Lallemand) (Homoptera: Cercopidae). Resistance to nymphs was evaluated with a technique that provided uniform environmental conditions and abundant feeding sites. B. brizantha Stapf (cv. Marandú) was the most resistant of the accessions tested based on nymphal mortality, duration of nymphal stadia, and weight of adult females. Andropogon gayanus Kunth, resistant to spittlebug attack in the field, was susceptible under the conditions of this study. While growth habit and rooting characteristics may contribute to field resistance, other resistance factors are present within the genus Brachiaria, particularly in the case of B. brizantha cv. Marandú. The number of insect-days causing severe damage in the most tolerant species (B. dictyoneura Stapf and B. humidicola Schweick) was approximately six times greater than that necessary to cause the same level of damage to the most susceptible species (B. ruziziensis Germain & Evrard and B. decumbens Stapf). No difference was found in regrowth capacity between infested and noninfested plants within accessions. There was a significant positive correlation between number of insect-days causing severe damage (tolerance) and regrowth of infested plants.

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Résumé Brachiaria est une graminée fourragère prometteuse pour les sols tropicaux acides, saturés d'aluminium. Z. colombiana est un Cercopidae très répandu, limitant l'utilisation de Brachiaria en Amérique Latine. La résistance (antibiose et tolérance) à Z. colombiana, de Brachiaria d'origines diverses a été examinée. B. brizantha cv. Marandù s'est révélé le plus résistant d'après la forte mortalité larvaire, la prolongation du développement larvaire, et le poids réduit des femelles adultes de Z. colombiana. Andropogon gayanus, résistant dans la nature, s'est révélé sensible. Ces résultats suggèrent que cette résistance de A. gayanus dans la nature pourrait être due à la structure du végétal et à son mode de croissance. Dans le cas de B. brizantha cv. Marandù, des facteurs supplémentaires de résistance, mis en évidence à partir de différents modes de croissance, ont été éliminés, de façon à identifier les mécanismes de l'antibiose présents chez Brachiaria. Une grande gamme de résistance aux attaques alimentaires a été observée chez Brachiaria. Les plus résistants ont besoin de 6 fois plus de jours d'attaque par Z. colombiana pour provoquer les dégâts observés sur individus sensibles.

     

Identification of distinct quantitative trait loci associated with defence against the closely related aphids Acyrthosiphon pisum and A. kondoi in Medicago truncatula

Aphids are a major family of plant insect pests. Medicago truncatula and Acyrthosiphon pisum (pea aphid, PA) are model species with a suite of resources available to help dissect the mechanism underlying plant-aphid interactions. A previous study focused on monogenic and relatively strong resistance in M. truncatula to PA and other aphid species. In this study a moderate resistance to PA was characterized in detail in the M. truncatula line A17 and compared with the highly susceptible line A20 and the more resistant line Jester. The results show that PA resistance in A17 involves both antibiosis and tolerance, and that resistance is phloem based. Quantitative trait locus (QTL) analysis using a recombinant inbred line (RIL) population (n=114) from a cross between A17 and A20 revealed that one locus, which co-segregated with AIN (Acyrthosiphon-induced necrosis) on chromosome 3, is responsible for the reduction of aphid biomass (indicator of antibiosis) for both PA and bluegreen aphid (BGA, A. kondoi), albeit to a lesser degree for PA than BGA. Interestingly, two independent loci on chromosomes 5 and 3 were identified for the plant biomass reduction (indicator of plant tolerance) by PA and BGA, respectively, demonstrating that the plant's tolerance response to these two closely related aphid species is distinct. Together with previously identified major resistant (R) genes, the QTLs identified in this study are powerful tools to understand fully the spectrum of plant defence against sap-sucking insects and provide opportunities for breeders to generate effective and sustainable strategies for aphid control.     

The taxonomy and ecology of the vascular plant flora of Singapore: a statistical analysis

A total of 2277 species of vascular plant in 868 genera and 158 families are recorded as native to the Republic of Singapore. This flora consists of 174 pteridophytes (7.6% of the total), eight gymnosperms (0.4%), 590 monocots (25.9%) and 1505 dicots (66.1%).Median values of species per family, genera per family and species per genus are 5, 2 and 1, respectively. The most abundant 'families' and genera in terms of species are Orchidaceae (196 spp.), Pteridophyta (174 spp.), Rubiaceae (140 spp.), Euphorbiaceae (116 spp.), Gramineae (100 spp.) and Eugenia (46 spp.), Ficus (43 spp.), Dendrobium (33 spp.), Bulbophyllum (29 spp.) and Cyperus (29 spp.). Trees constitute 37.2% of the flora, shrubs 9.8%, herbs 25.9%, climbers 14.0% and epiphytes 13%.Euphorbiaceae and Rubiaceae dominate the woody flora; pteridophytes, Gramineae and Cyperaceae the herbs; Rubiaceae, Palmae and Annonaceae the climbers and Orchidaceae and pteridophytes the epiphyteS. Species confined to coastal habitats comprise 11.1% of the total flora; 73.5% are lowland forest species and the remaining 15.5% are non-tree species of open and frequently-disturbed sites. The vascular plant of flora of Bukit Timah Nature Reserve, the largest remaining patch of primary rain forest in Singapore, is also analysed. Its flora is similar to that of the forest component of the complete Singapore flora. The Singapore flora, in comparison with other tropical floras, is highly diverse, but appears depauperate in epiphytes and climbers.     

Evaluation of turfgrass species and cultivars for potential resistance to twolined spittlebug (Hemiptera: Cercopidae)

Potential resistance to the twolined spittlebug, Prosapia bicincta (Say), was evaluated among 56 turfgrass genotypes. Greenhouse, laboratory, and field bioassays identified differences in spittlebug survival and development, host preference and damage levels, and turfgrass tolerance to and ability to recover from pest induced injury. All centipede grasses demonstrated high levels of susceptibility, followed by bermudagrasses, seashore paspalums, and zoysiagrasses. Average nymphal survival to the adult stage ranged from 1.5 to 78.1%. Development required 38.1-62.0 d under greenhouse conditions, depending on plant taxa. Among seashore paspalums, nymphal survival to the adult stage was lowest and duration of development was longest on HI-1, 'Sea Isle 2000', 561-79, and 'Mauna Kea'. Reduced spittlebug survival and increased developmental times were also observed on the bermudagrasses BERPC 91-15 and 'Tifway'. Although zoysiagrasses supported spittlebug development and survival to the adult stage, developmental times were extended on the zoysiagrass cultivars 'Emerald' and 'El Toro'. Spittlebug preference varied with generation evaluated. First-generation spittlebugs inflicted the greatest damage on TC201 (centipede grass), 'Primavera' (bermudagrass), and 'Emerald' (zoysiagrass) in choice tests. In the fall, second-generation spittlebugs damaged TC201 (centipedegrass) and 'Sea Isle 1' (paspalum) most severely, whereas 561-79 (paspalum) and 'Emerald'(zoysiagrass) were less severely affected. Among taxa included in field trials, HI-1, 'Mauna Kea', 'Sea Isle 2000',and AP-14 paspalums, 'Tifway' bermudagrass, and 'Emerald' zoysiagrass were most tolerant (demonstrated the best regrowth potential following twolined spittlebug feeding).     

An endophyte of the tropical forage grass Brachiaria brizantha: isolating, identifying, and characterizing the fungus, and determining its antimycotic properties

Brachiaria, predominantly an African genus, contains species, such as B. brizantha, an apomictic C4 grass, that are commercially important forage grasses in tropical America, where they now cover about 55 million hectares. From B. brizantha accession CIAT 6780, we isolated an endophytic fungus that may be economically significant. The fungus was identified as Acremonium implicatum (J. Gilman & E.V. Abott). 18S rDNA and ITS rDNA sequences were used to characterize isolates of the endophyte, and showed that they belonged to the Acremonium genus, being close to A. strictum and A. kiliense. Using the random amplified polymorphic DNA (RAPD) technique, involving arbitrary primers of 10 bases, we showed that the isolates were highly similar to each other. Antiserum produced from a monoconidial culture of A. implicatum isolated from B. brizantha 6780, differentiated the isolates consistently in line with the DNA data. When we compared endophyte-free with endophyte-infected B. brizantha CIAT 6780 plants, both artificially inoculated with the pathogenic Drechslera fungus, we found that the endophyte-infected plants had fewer and smaller lesions than did the endophyte-free plants. Sporulation of Drechslera sp. on artificially inoculated leaf sheath tissues was also much less on tissue infected with the endophyte.     

Foliar Chlorosis in Symbiotic Host and Nonhost Plants Induced by Rhizobium tropici Type B Strains

Rhizobium tropici CIAT899 induced chlorosis in the leaves of its symbiotic hosts, common bean (Phaseolus vulgaris L.), siratro (Macroptilium atropurpureum Urb.), and Leucaena leucocephala (Lam.) de Wit. Chlorosis induction by strains CIAT899 and CT9005, an exopolysaccharide-deficient mutant of CIAT899, required carbon substrate. When the bacteria were added at planting in a solution of mannitol (50 g/liter), as few as 10³ cells of CIAT899 were sufficient to induce chlorosis in bean plants. All carbon sources tested, including organic acids and mono- and disaccharides, supported chlorosis induction. The addition of a carbon source did not affect the growth rate or the population density of CT9005 in the bean plant rhizosphere. Cell-free filtrates of cultures of CT9005 did not induce detectable chlorosis. All type B strains of R. tropici tested also induced chlorosis in common bean. Type A strains of R. tropici and all other species of bacteria tested did not induce chlorosis. Several lines of evidence indicated that nodulation was not required for chlorosis induction. Strain RSP900, a pSym-cured derivative of CIAT899, induced chlorosis in wild-type P. vulgaris. In addition, NOD125, a nodulation-defective line of common bean, developed chlorosis when inoculated with CIAT899, but did not develop nodules. CIAT899 consistently induced severe chlorosis in the leaves of the nonhost legumes alfalfa (Medicago sativa L.) and Berseem clover (Trifolium alexandrinum L.), and induced chlorosis in 29 to 58% of the plants tested of sunflower, cucumber, and tomato seedlings, but it did not induce chlorosis in the leaves of corn or wheat. Chlorosis induction in nonhost plants also required carbon substrate. The data are consistent with the hypothesis that R. tropici type B strains produce a chlorosis-inducing factor that affects a wide range of plant species.     

Seedling and adult plant resistance to Sitobion avenae (Hemiptera: Aphididae) in Triticum monococcum (Poaceae), an ancestor of wheat

Cereal aphids are important pests of wheat, Triticum aestivum L. and Triticum durum Desf. Crop resistance is a desirable method for managing cereal aphids in central North America, where the dominant crop, spring-sown wheat, has a low value per unit area. A diploid ancestor of wheat, Triticum monococcum L., is reported to be partially resistant to Sitobion avenae (Fabricius), the most damaging cereal aphid in the region. To identify potential sources of resistance, 42 accessions of T. monococcum and three cultivated wheats were infested with aphids, seedlings for six days and adult plants for 21 days. Overall resistance was estimated by the biomass loss of foliage and spikes in relation to uninfested control plants. Antibiosis was estimated by the gain in biomass of aphids during infestation, and tolerance was estimated as a biomass conversion ratio, overall resistance divided by antibiosis. A few T. monococcum accessions exhibited partial resistance. No relationship was found between seedling and adult plant resistance: the former exhibited primarily antibiosis and the latter primarily tolerance. Two accessions with antibiosis reduced aphid biomass by 60% compared with commercial wheats. Tolerance was correlated with growth potential, and was useful only in accessions with high growth potential. Four accessions exhibited tolerance levels at least 30% greater than commercial wheats. Highly susceptible accessions also were identified, which would be useful for investigating the inheritance of antibiosis and tolerance.     

Genotypic variability and mode of action of Colorado potato beetle (Coleoptera: Chrysomelidae) resistance in seven Solanum species

Accessions from seven wild Solanum species were evaluated in the field for resistance to the Colorado potato beetle, Leptinotarsa decemlineata (Say). The multivariate insect population density data were analyzed using factor analysis. The factors extracted corresponded to relevant phases of the insect's life cycle and provided information on the mode of resistance (antixenosis and antibiosis) of the plant species. S. berthaultii, S. capsicibaccatum, S. jamesii, S. pinnatisectum, and S. trifidum demonstrated both antixenosis and antibiosis but expressed different levels of resistance. The mode of resistance of S. polyadenium seemed to be antibiosis and that of S. tarijense antixenosis. Genetic variability and heritability of insect resistance traits within accessions was trivial or inconsistent for all Solanum species studied.     

Identification of antibiosis and tolerance in rice varieties carrying brown planthopper resistance genes

Brown planthopper (BPH) [Nilaparvata lugens (Stål) (Hemiptera: Delphacidae)] is a major pest in rice [Oryza sativa L. (Poaceae)] production. Identification of resistance genes and development of BPH‐resistant varieties is an economical and effective way to control this pest. In this study, BPH honeydew excretion, survival rate, and emergence rate were used as indicators to detect the antibiotic level, whereas the relative growth rates of plant height (R_H) and fresh weight (R_W), and the number of days until yellowing were used to identify the level of tolerance to BPH in rice varieties. Rice varieties Swarnalata and B5, which showed high levels of antibiosis and tolerance to BPH, thus were highly resistant in the seedling bulk test; Mudgo and T12, which showed moderate resistance to the insects, had a high level of tolerance and moderate antibiosis to BPH. Varieties Rathu Heenati, ARC 10550, and Chin Saba were identified to be susceptible to BPH, showing a moderate level of tolerance and no antibiosis. In comparison to the evaluation methods of BPH resistance, the honeydew excretion and survival rate could be used to detect the antibiotic level, and the R_H, R_W, or leaf yellowing days could be employed as indicators to evaluate the rice varieties’ tolerance. Overall, a combined application of these indicators can effectively identify the levels of antibiosis and tolerance to BPH in rice varieties, and BPH‐resistance levels of the varieties were mainly determined by the antibiosis level. The results should help in understanding BPH‐resistance categories of rice varieties and for resistance breeding.