Cultivar preferences of ovipositing wheat stem sawflies as influenced by the amount of volatile attractant

Identifying novel sources of resistance to wheat stem sawfly in five wild wheat species

BACKGROUND

The wheat stem sawfly (WSS, Cephus cinctus) is a major pest of wheat (Triticum aestivum) and can cause significant yield losses. WSS damage results from stem boring and/or cutting, leading to the lodging of wheat plants. Although solid-stem wheat genotypes can effectively reduce larval survival, they may have lower yields than hollow-stem genotypes and show inconsistent solidness expression. Because of limited resistance sources to WSS, evaluating diverse wheat germplasm for novel resistance genes is crucial. We evaluated 91 accessions across five wild wheat species (Triticum monococcum, T. urartu, T. turgidum, T. timopheevii, and Aegilops tauschii) and common wheat cultivars (T. aestivum) for antixenosis (host selection) and antibiosis (host suitability) to WSS. Host selection was measured as the number of eggs after adult oviposition, and host suitability was determined by examining the presence or absence of larval infestation within the stem. The plants were grown in the greenhouse and brought to the field for WSS infestation. In addition, a phylogenetic analysis was performed to determine the relationship between the WSS traits and phylogenetic clustering.

RESULTS

Overall, Ae. tauschii, T. turgidum and T. urartu had lower egg counts and larval infestation than T. monococcum, and T. timopheevii. T. monococcum, T. timopheevii, T. turgidum, and T. urartu had lower larval weights compared with T. aestivum.

CONCLUSION

This study shows that wild relatives of wheat could be a valuable source of alleles for enhancing resistance to WSS and identifies specific germplasm resources that may be useful for breeding. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Volatile Organic Compounds Mediate Host Selection of Wheat Midge, Sitodiplosis Mosellana (Géhin) (Diptera: Cecidomyiidae) between Preanthesis and Postanthesis Stages of Wheat

The orange wheat blossom midge, Sitodiplosis mosellana (Géhin) (Diptera: Cecidomyiidae), is a significant wheat pest in the Prairie Provinces of Canada and northern regions of the USA. Wheat phenology plays a critical role in wheat midge oviposition. We hypothesized that S. mosellana oviposition behaviour is influenced by volatile organic compounds (VOCs) emitted by wheat at two adjacent wheat growth stages: preanthesis and postanthesis. A higher number of S. mosellana eggs laid on preanthesis than postanthesis spikes in an oviposition choice experiment using the susceptible spring wheat cultivar 'Roblin'. In preanthesis, wheat emitted higher amounts of Z-3-hexenyl acetate (Z3-06:OAc) than at the postanthesis stage. Higher amounts of methyl ketones such as 2-tridecanone, 2-pentadecanone, and 2-undecanone were emitted by wheat in the postanthesis stage and these VOCs were sensitive to S. mosellana antennae used in the Gas Chromatography-Electroantennographic Detection. Females were attracted to synthetic Z3-06:OAc but were deterred by 2-tridecanone relative to the solvent control in the vertical Y-tube olfactometer. 2-Undecanone and 2-pentadecanone did not show any attractiveness or deterrence. In a no-choice oviposition experiment, fewer eggs were laid in preanthesis wheat exposed to a synthetic VOC blend of Z3-06:OAc, 2-undecanone, 2-tridecanone, and 2-pentadecanone at the concentrations released by postanthesis spikes. This study shows that the reduction of Z3-06:OAc, in the VOC mix, and possibly the increase in 2-tridecanone, are likely responsible for the reduction in oviposition on postanthesis wheat. These results elucidate for the first time the role of specific VOCs mediating S. mosellana oviposition in preanthesis and postanthesis wheat.

Development and identification of four new synthetic hexaploid wheat lines with solid stems

Stem solidness is an important agronomic trait for increasing the ability of wheat to resist lodging. In this study, four new synthetic hexaploid wheat with solid stems were developed from natural chromosome doubling of F₁ hybrids between a solid-stemmed durum wheat (Triticum turgidum ssp. durum, 2n = 4x = 28, AABB) and four Aegilops tauschii (2n = 2x = 14, DD) accessions. The solid expression of the second internode at the base of the stem was stable for two synthetic hexalpoid wheat Syn-SAU-117 and Syn-SAU-119 grown in both the greenhouse and field. The lodging resistance of four synthetic solid-stem wheats is stronger than that of CS, and Syn-SAU-116 has the strongest lodging resistance, followed by Syn-SAU-119. The paraffin sections of the second internode showed that four synthetic wheat lines had large outer diameters, well-developed mechanical tissues, large number of vascular bundles, and similar anatomical characteristics with solid-stemmed durum wheat. The chromosomal composition of four synthetic hexaploid wheat was identified by FISH (fluorescence in situ hybridization) using Oligo-pSc119.2-1 and Oligo-pTa535-1. At adult stage, all four synthetic hexaploid wheat showed high resistance to mixed physiological races of stripe rust pathogen (CYR31, CYR32, CYR33, CYR34). These synthetic hexaploid wheat lines provide new materials for the improvement of common wheat.

Plant Volatiles and Oviposition Behavior in the Selection of Barley Cultivars by Wheat Stem Sawfly (Hymenoptera: Cephidae)

Wheat stem sawfly, [Cephus cinctus (Hymenoptera: Cephidae)], females display complex behaviors for host selection and oviposition. Susceptible hollow stem wheat (Triticum aestivum L.) cultivars release a greater amount of attractive compound, (Z)-3-hexenyl acetate and receive a greater number of eggs compared to resistant solid stem wheat cultivars. However, barley (Hordeum vulgare L.) is becoming a more common host for C. cinctus in Montana. Therefore, how do host selection and oviposition behaviors on barley cultivars compare to what happens when encountering wheat cultivars? To answer this question, we carried out greenhouse experiments using two barley cultivars: 'Hockett' and 'Craft'. Between these cultivars at Zadoks stages 34 and 49, we compared host selection decisions using a Y-tube olfactometer, compared oviposition behaviors on stems, and counted the number of eggs inside individual stems. In Y-tube bioassays, we found a greater number of C. cinctus females were attracted to the airstream passing over 'Hockett' than 'Craft' barley cultivars. Although the frequencies of oviposition behaviors were similar between these cultivars, the number of eggs was greater in 'Hockett'. Volatile profiles indicated that the amount of linalool was greater in the airstream from 'Craft' than in 'Hockett' at Zadoks 34 while the amount of (Z)-3-hexenyl acetate was greater in airstream from 'Hockett' at both Zadoks 34 and 49. These results suggest that volatiles of barley plants influenced host selection behavior of ovipositing C. cinctus females, while other discriminating behaviors do not differ between cultivars.

Identifying aphid resistance in the ancestral wheat Triticum monococcum under field conditions

Wheat is an economically, socially, and nutritionally important crop, however, aphid infestation can often reduce wheat yield through feeding and virus transmission. Through field phenotyping, we investigated aphid resistance in ancestral wheat Triticum monococcum (L.). Aphid (Rhopalosiphum padi (L.), Sitobion avenae (F.) and Metopolophium dirhodum (Wlk.)) populations and natural enemy presence (parasitised mummified aphids, ladybird adults and larvae and lacewing eggs and larvae) on two naturally susceptible wheat varieties, Triticum aestivum (L.) var. Solstice and T. monococcum MDR037, and three potentially resistant genotypes T. monococcum MDR657, MDR045 and MDR049 were monitored across three years of field trials. Triticum monococcum MDR045 and MDR049 had smaller aphid populations, whereas MDR657 showed no resistance. Overall, natural enemy presence was positively correlated with aphid populations; however, MDR049 had similar natural enemy presence to MDR037 which is susceptible to aphid infestation. It is hypothesised that alongside reducing aphid population growth, MDR049 also confers indirect resistance by attracting natural enemies. The observed resistance to aphids in MDR045 and MDR049 has strong potential for introgression into commercial wheat varieties, which could have an important role in Integrated Pest Management strategies to reduce aphid populations and virus transmission.

Multiple decrement life tables of Cephus cinctus Norton (Hymenoptera: Cephidae) across a set of barley cultivars: The importance of plant defense versus cannibalism

Accurately estimating cause-specific mortality for immature insect herbivores is usually difficult. The insects are exposed to abiotic and biotic mortality factors, causing cadavers to simply disappear before cause of mortality can be recorded. Also, insect herbivores are often highly mobile on hosts, making it difficult to follow patterns for individuals through time. In contrast, the wheat stem sawfly, Cephus cinctus Norton, spends its entire egg, larval, and pupal period inside a host stem. Therefore, with periodic sampling stage-specific causes of mortality can be ascertained. Consequently, we examined C. cinctus mortality in eight barley, Hordeum vulgare L., cultivars in two locations in Montana from 2016 to 2018 by collecting stem samples from stem elongation to crop maturity at weekly intervals, and collecting overwintered barley stubs the following spring and summer from the same plots. If larvae were present, we examined larval status—dead or alive—and categorized dead individuals into one of 5 mortality categories: plant defense, cannibalism, parasitism, pathogens, and unknown factors. We used multiple decrement life tables to estimate cause-specific mortality and irreplaceable mortality (the proportion of mortality from a given cause that cannot be replaced by other causes of mortality). Plant defense (antibiosis) caused 85.7 ± 3.6%, cannibalism (governed by antixenosis) caused 70.1 ± 7.6%, parasitism caused 13.8 ± 5.9%, unknown factors caused 38.5 ± 7.6%, and pathogens caused 14.7 ± 8.5% mortality in the presence of all causes of mortality. Similarly, irreplaceable mortality due to plant defense was 22.3 ± 6.4%, cannibalism was 29.1± 4.2%, unknown factors was 6.2 ± 1.8%, pathogens was 0.9 ± 0.5%, and parasitism was 1. 5 ± 0. 6%. Antibiosis traits primarily killed newly emerged larvae, while other traits supported more favorable oviposition decisions by females, increasing mortality by obligate cannibalism. Our results suggest that breeding barley for resistance to C. cinctus targeting both categories of traits (antibiosis and antixenosis) is a highly valuable tactic for management of this important pest.

Effect of Precipitation and Temperature on Larval Survival of Cephus cinctus (Hymenoptera: Cephidae) in Barley Cultivars

Host plant traits strongly affect survivorship of insect herbivores, and host suitability is especially important for the wheat stem sawfly, Cephus cinctus Norton, which spends its entire egg, larval, and pupal periods in a single stem. Measuring larval survival inside stems from egg hatch through diapause-mediated dormancy is a potential measure of population size for the next year but is also useful in assessing effects of growing season precipitation and temperature. Larval growth is synchronized with host plant growth, and the larva cannot switch hosts. Thus, incorporating plant physiological time, as growing degree days (GDD), may yield a better prediction of larval survival. Therefore, we assessed wheat stem sawfly survival from early larval growth to the beginning of autumnal diapause in barley cultivars selected from across feed, forage, and two- or six-row malt groups. Field experiments were conducted in Gallatin and Chouteau counties, Montana, in 2016 and 2017. We used Kaplan-Meier estimation to assess larval survival among cultivars. We found that the survival of pre-diapause larvae was greatest in 'Hockett' (36.5%) and lowest in 'Celebration' (15.4%). Precipitation and temperature during the growing season affected temporal patterns for larval survival across study sites. Adjusting survivorship curves using site-specific GDD accumulation allowed cultivar-specific survivorship to be estimated more precisely for each site, despite differing environmental influences. Our findings suggest that measuring wheat stem sawfly survival across barley cultivars and standardizing by site-specific GDDs may provide better recommendations on barley cultivars that impede wheat stem sawfly population growth and reduce economic losses.

Antixenosis, Antibiosis, and Potential Yield Compensatory Response in Barley Cultivars Exposed to Wheat Stem Sawfly (Hymenoptera: Cephidae) Under Field Conditions

Wheat stem sawfly, Cephus cinctus Norton, is an economically serious pest of cereals grown in North America. Barley cultivars were previously planted as resistant crops in rotations to manage C. cinctus, but due to increasing levels of injury to this crop, this is no longer a valid management tactic in Montana. Therefore, we aimed to understand antixenosis (behavioral preference), antibiosis (mortality), and potential yield compensation (increased productivity in response to stem injuries) in barley exposed to C. cinctus. We examined these traits in eight barley cultivars. Antixenosis was assessed by counting number of eggs per stem and antibiosis was assessed by counting infested stems, dead larvae, and stems cut by mature larvae. Potential yield compensation was evaluated by comparing grain yield from three categories of stem infestation: 1) uninfested, 2) infested with dead larva, and 3) infested cut by mature larva at crop maturity. We found the greatest number of eggs per infested stem (1.80 ± 0.04), the highest proportion of infested stems (0.63 ± 0.01), and the highest proportion of cut stems (0.33 ± 0.01) in 'Hockett'. Seven out of eight cultivars had greater grain weight for infested stems than for uninfested stems. These cultivars may have compensatory responses to larval feeding injury. Overall, these barley cultivars contain varying levels of antixenosis, antibiosis, and differing levels of yield compensation. Our results provide foundational knowledge on barley traits that will provide a framework to further develop C. cinctus resistant or tolerant barley cultivars.

Differential Stem Proteomics and Metabolomics Profiles for Four Wheat Cultivars in Response to the Insect Pest Wheat Stem Sawfly

Common wheat (Triticum aestivum L.) is a global staple crop, and insect pests can impact grain yield. The wheat stem sawfly (Cephus cinctus, WSS) is a major wheat pest, and while partial resistance has been deployed by breeding for a solid-stem trait, this trait is affected by environment. Here, a proteomics and metabolomics study was performed on four wheat cultivars to characterize a molecular response to WSS infestation. The cultivars Hatcher (hollow-stem partially tolerant), Conan (semisolid-stem-resistant), and Denali and Reeder (hollow-stem-susceptible) were infested with WSS, and changes in stem proteins and metabolites were characterized using liquid chromatography-mass spectrometry. The proteome was characterized as 1830 proteins that included five major biological processes, including metabolic processes and response to stimuli, and the metabolome (1823 metabolites) spanned eight chemical superclasses, including alkaloids, benzenoids, and lipids. All four varieties had a molecular response to WSS following infestation. Hatcher had the most distinct changes, whereby 62 proteins and 29 metabolites varied in metabolic pathways involving enzymatic detoxification, proteinase inhibition, and antiherbivory compound production via benzoxazinoids, neolignans, and phenolics. Taken together, these data demonstrate variation in the wheat stem molecular response to WSS infestation and support breeding for molecular resistance in hollow-stem cultivars.

Effects of Chemical Insecticide Imidacloprid on the Release of C6 Green Leaf Volatiles in Tea Plants (Camellia sinensis)

Chemical insecticides are widely used for pest control worldwide. However, the impact of insecticides on indirect plant defense is seldom reported. Here, using tea plants and the pesticide imidacloprid, effects of chemical insecticides on C₆-green leaf volatiles (GLVs) anabolism and release were investigated first time. Compared with the non-treated control plants, the treatment of imidacloprid resulted in the lower release amount of key GLVs: (Z)-3-hexenal, n-hexenal, (Z)-3-hexene-1-ol and (Z)-3-Hexenyl acetate. The qPCR analysis revealed a slight higher transcript level of the CsLOX3 gene but a significantly lower transcript level of CsHPL gene. Our results suggest that imidacloprid treatment can have a negative effect on the emission of GLVs due to suppressing the critical GLVs synthesis-related gene, consequently affecting plant indirect defense.

Plant defense elicitors: plant fitness versus wheat stem sawfly

The wheat stem sawfly (WSS), Cephus cinctus Norton, is an important wheat pest in the Northern Great Plains of the USA. No single control measure effectively suppresses WSS damage. This study provides information on the effects on the WSS adult settling preference behavior on wheat plants under laboratory conditions from treatment with both synthetic plant defense elicitors (Actigard® and cis-jasmone) and a botanical insecticide (Azadirachtin®). In addition, field experiments were performed to determine whether these chemicals impact the WSS fitness (larval mortality and larval body weight), winter wheat plant fitness (infestation, stem lodging, yield, and quality), adult population of WSS and Bracon spp., and larval parasitism levels. Our lab results showed that there were no significant differences in adult settling behavior on plants exposed separately to each chemical and control. In contrast, when adults were exposed simultaneously to treated and untreated plants, there was a significant reduction in the percentage of adults settling on Actigard® and Azadirachtin® treated plants compared to plants sprayed with water in the same cage. However, in field situations, regardless of application timing and field location, none of the chemicals significantly reduced adult population or stems damage. The exception was two times applications of Actigard® had significantly lower WSS infested stem damage levels at 30 days after initial treatment applications at Knees and 50 days at Choteau locations compared to control, but without effect at the Conrad location. The field study indicated that two times applications of Actigard® significantly increased diapausing larval mortality percentages and lowered stem lodging levels compared to untreated controls at Knees and Choteau locations, while no effects at Conrad location. Larval body weight was significantly lower in plots treated with Actigard® at Knees and Conrad, but no effects at Choteau. No significant differences were found in wheat yield and quality in plots treated with chemicals and controls at any location. Bracon spp. adult population and parasitism levels were not negatively affected by the use of chemicals. In conclusion, this study offers insights on what treatments should be emphasized in more detail despite variable findings.

Farming system and wheat cultivar affect infestation of, and parasitism on, Cephus cinctus in the Northern Great Plains

BACKGROUND

Cephus cinctus infestation causes $350 million in annual losses in the Northern Great Plains. We compared infestation and parasitism of C. cinctus in spring (including Kamut; Triticum turgidum ssp. turanicum) and winter wheat cultivars grown in organic and conventional fields in Montana, USA. In the greenhouse, we compared C. cinctus preference and survival in Kamut, Gunnison, and Reeder spring wheat cultivars.

RESULTS

Stems cut by C. cinctus varied by farming system and the seasonality of the wheat crop. No stems of Kamut in organic fields were cut by C. cinctus, but 1.5% [±0.35% standard error (SE)] of stems in conventional spring wheat, 5% (±0.70% SE) of stems in organic winter wheat, and 20% (±0.93% SE) of stems in conventional winter wheat fields were cut by C. cinctus. More larvae of C. cinctus were parasitized in organic (27 ± 0.03% SE) compared with conventional (5 ± 0.01% SE) winter wheat fields. Cephus cinctus oviposition, survival, and the number of stems cut were lowest in Kamut compared with Gunnison and Reeder.

CONCLUSION

Cephus cinctus infestation was more common in winter wheat than in spring wheat. Organic fields with fewer cut stems also supported more parasitoids. Kamut is a genetic resource for developing C. cinctus-resistant cultivars. © 2018 Society of Chemical Industry.

Genome Sequence of the Wheat Stem Sawfly, Cephus cinctus, Representing an Early-Branching Lineage of the Hymenoptera, Illuminates Evolution of Hymenopteran Chemoreceptors

The wheat stem sawfly, Cephus cinctus, is a major pest of wheat and key ecological player in the grasslands of western North America. It also represents the distinctive Cephoidea superfamily of sawflies (Symphyta) that appeared early during the hymenopteran radiation, but after three early-branching eusymphytan superfamilies that form the base of the order Hymenoptera. We present a high-quality draft genome assembly of 162 Mb in 1,976 scaffolds with a scaffold N50 of 622 kb. Automated gene annotation identified 11,210 protein-coding gene models and 1,307 noncoding RNA models. Thirteen percent of the assembly consists of ∼58,000 transposable elements partitioned equally between Class-I and Class-II elements. Orthology analysis reveals that 86% of Cephus proteins have identifiable orthologs in other insects. Phylogenomic analysis of conserved subsets of these proteins supports the placement of the Cephoidea between the Eusymphyta and the parasitic woodwasp superfamily Orussoidea. Manual annotation and phylogenetic analysis of families of odorant, gustatory, and ionotropic receptors, plus odorant-binding proteins, shows that Cephus has representatives for most conserved and expanded gene lineages in the Apocrita (wasps, ants, and bees). Cephus has also maintained several insect gene lineages that have been lost from the Apocrita, most prominently the carbon dioxide receptor subfamily. Furthermore, Cephus encodes a few small lineage-specific chemoreceptor gene family expansions that might be involved in adaptations to new grasses including wheat. These comparative analyses identify gene family members likely to have been present in the hymenopteran ancestor and provide a new perspective on the evolution of the chemosensory gene repertoire.

Characterization of Resistance to Cephus cinctus (Hymenoptera: Cephidae) in Barley Germplasm

Most barley cultivars have some degree of resistance to the wheat stem sawfly (WSS), Cephus cinctus Norton (Hymenoptera: Cephidae). Damage caused by WSS is currently observed in fields of barley grown in the Northern Great Plains, but the impact of WSS damage among cultivars due to genetic differences within the barley germplasm is not known. Specifically, little is known about the mechanisms underlying WSS resistance in barley. We characterized WSS resistance in a subset of the spring barley CAP (Coordinated Agricultural Project) germplasm panel containing 193 current and historically important breeding lines from six North American breeding programs. Panel lines were grown in WSS infested fields for two consecutive years. Lines were characterized for stem solidness, stem cutting, WSS infestation (antixenosis), larval mortality (antibiosis), and parasitism (indirect plant defense). Variation in resistance to WSS in barley was compared to observations made for solid-stemmed resistant and hollow-stemmed susceptible wheat lines. Results indicate that both antibiosis and antixenosis are involved in the resistance of barley to the WSS, but antibiosis seems to be more prevalent. Almost all of the barley lines had greater larval mortality than the hollow-stemmed wheat lines, and only a few barley lines had mortality as low as that observed in the solid-stemmed wheat line. Since barley lines lack solid stems, it is apparent that barley has a different form of antibiosis. Our results provide information for use of barley in rotation to control the WSS and may provide a basis for identification of new approaches for improving WSS resistance in wheat.

GC-MS Metabolomics to Evaluate the Composition of Plant Cuticular Waxes for Four Triticum aestivum Cultivars

Wheat (Triticum aestivum L.) is an important food crop, and biotic and abiotic stresses significantly impact grain yield. Wheat leaf and stem surface waxes are associated with traits of biological importance, including stress resistance. Past studies have characterized the composition of wheat cuticular waxes, however protocols can be relatively low-throughput and narrow in the range of metabolites detected. Here, gas chromatography-mass spectrometry (GC-MS) metabolomics methods were utilized to provide a comprehensive characterization of the chemical composition of cuticular waxes in wheat leaves and stems. Further, waxes from four wheat cultivars were assayed to evaluate the potential for GC-MS metabolomics to describe wax composition attributed to differences in wheat genotype. A total of 263 putative compounds were detected and included 58 wax compounds that can be classified (e.g., alkanes and fatty acids). Many of the detected wax metabolites have known associations to important biological functions. Principal component analysis and ANOVA were used to evaluate metabolite distribution, which was attributed to both tissue type (leaf, stem) and cultivar differences. Leaves contained more primary alcohols than stems such as 6-methylheptacosan-1-ol and octacosan-1-ol. The metabolite data were validated using scanning electron microscopy of epicuticular wax crystals which detected wax tubules and platelets. Conan was the only cultivar to display alcohol-associated platelet-shaped crystals on its abaxial leaf surface. Taken together, application of GC-MS metabolomics enabled the characterization of cuticular wax content in wheat tissues and provided relative quantitative comparisons among sample types, thus contributing to the understanding of wax composition associated with important phenotypic traits in a major crop.

Tolerance: the forgotten child of plant resistance

Plant resistance against insect herbivory has greatly focused on antibiosis, whereby the plant has a deleterious effect on the herbivore, and antixenosis, whereby the plant is able to direct the herbivore away from it. Although these two types of resistance may reduce injury and yield loss, they can produce selection pressures on insect herbivores that lead to pest resistance. Tolerance, on the other hand, is a more sustainable pest management strategy because it involves only a plant response and therefore does not cause evolution of resistance in target pest populations. Despite its attractive attributes, tolerance has been poorly studied and understood. In this critical, interpretive review, we discuss tolerance to insect herbivory and the biological and socioeconomic factors that have limited its use in plant resistance and integrated pest management. First, tolerance is difficult to identify, and the mechanisms conferring it are poorly understood. Second, the genetics of tolerance are mostly unknown. Third, several obstacles hinder the establishment of high-throughput phenotyping methods for large-scale screening of tolerance. Fourth, tolerance has received little attention from entomologists because, for most, their primary interest, research training, and funding opportunities are in mechanisms which affect pest biology, not plant biology. Fifth, the efforts of plant resistance are directed at controlling pest populations rather than managing plant stress. We conclude this paper by discussing future research and development activities.

Host Plants of the Wheat Stem Sawfly (Hymenoptera: Cephidae)

Wheat stem sawfly (Cephus cinctus Norton) is a pest of economic importance across much of the wheat (Triticum aestivum L.)-growing areas of the western Great Plains of North America as well as an ecologically important insect owing to its wide range of grass hosts. Little research has been published regarding the noncultivated native and invasive grasses attacked by this insect. Knowledge of the complete host range of C. cinctus can inform future research about potential new sources of genetic resistance, improve understanding of the biology and spread of natural enemies, and better define this insect's role in grassland and agricultural systems. The aim of this review is to compile a checklist of reported host plants of C. cinctus and present data from an extensive survey of noncultivated hosts used by C. cinctus.

Phenotypic and Haplotype Diversity among Tetraploid and Hexaploid Wheat Accessions with Potentially Novel Insect Resistance Genes for Wheat Stem Sawfly

Genetic diversity in breeding programs can be impaired by fixation of alleles derived from a limited number of founder lines. This is demonstrated with the use of a solid-stem trait derived from the Portuguese landrace 'S-615' over 70 yrs ago that is widely used to resist the wheat stem sawfly ( Norton, WSS) in North America. The objective of this study was to evaluate haplotype diversity underlying the quantitative trait locus (QTL) that controls the majority of the S-615 derived solid-stem genetic variation using single-nucleotide polymorphism (SNP) assays in a diverse set of 228 solid-stem tetraploid and hexaploid wheat accessions originating from areas of the world infested with various species of WSS. Haplotype analysis showed all WSS-resistant hexaploid wheat varieties in North America, except 'Conan', evaluated in this study contain a haplotype associated with the S-615 solid-stem allele. In total, 26 haplotypes were identified among the hexaploid and tetraploid accessions at . Prevalence of most haplotypes were skewed toward either the hexaploid or tetraploid wheat accessions. The haplotype found in the S-615- hexaploid wheat landrace was not found in the solid-stem tetraploid landrace accessions evaluated in this study. Haplotype analysis revealed several new haplotypes that have potential to contain novel alleles for solid-stems at , which may form the basis for introducing genetic diversity into breeding programs aimed at WSS resistance.

Tea green leafhopper, Empoasca vitis, chooses suitable host plants by detecting the emission level of (3Z)-hexenyl acetate

Green leaf volatiles (GLVs) have been reported to play an important role in the host-locating behavior of several folivores that feed on angiosperms. However, next to nothing is known about how the green leafhopper, Empoasca vitis, chooses suitable host plants and whether it detects differing emission levels of GLV components among genetically different tea varieties. Here we found that the constitutive transcript level of the tea hydroperoxide lyase (HPL) gene CsiHPL1, and the amounts of (Z)-3-hexenyl acetate and of total GLV components are significantly higher in tea varieties that are susceptible to E. vitis (Enbiao (EB) and Banzhuyuan (BZY)) than in varieties that are resistant to E. vitis (Changxingzisun (CX) and Juyan (JY)). Moreover, the results of a Y-tube olfactometer bioassay and an oviposition preference assay suggest that (Z)-3-hexenyl acetate and (Z)-3-hexenol offer host and oviposition cues for E. vitis female adults. Taken together, the two GLV components, (Z)-3-hexenol and especially (Z)-3-hexenyl acetate, provide a plausible mechanism by which tea green leafhoppers distinguish among resistant and susceptible varieties. Future research should be carried out to obtain the threshold of the above indices and then assess their reasonableness. The development of practical detection indices would greatly improve our ability to screen and develop tea varieties that are resistant to E. vitis.

Host plant quantitative trait loci affect specific behavioral sequences in oviposition by a stem-mining insect

Genetic diversity in quantitative loci associated with plant traits used by insects as cues for host selection can influence oviposition behavior and maternal choice. Host plant selection for oviposition is an important determinant of progeny performance and survival for phytophagous insects. Specific cues from the plant influence insect oviposition behavior; but, to date, no set of host plant quantitative trait loci (QTLs) have been shown to have an effect on behavioral sequences leading to oviposition. Three QTLs in wheat (Triticum aestivum L.) have been identified as influencing resistance to the wheat stem sawfly (WSS) (Cephus cinctus Norton). Wheat near-isogenic lines (NILs) for each of the three QTLs were used to test whether foraging WSS were able to discriminate variation in plant cues resulting from allelic changes. A QTL on chromosome 3B (Qss-msub-3BL) previously associated with stem solidness and larval antibiosis was shown to affect WSS oviposition behavior, host preference, and field infestation. Decreased preference for oviposition was also related to a QTL allele on chromosome 2D (Qwss.msub-2D). A QTL on chromosome 4A (Qwss.msub-4A.1) affected host plant attractiveness to foraging females, but did not change oviposition preference after females landed on the stem. These findings show that oviposition decisions regarding potential plant hosts require WSS females to discriminate signals from the plant associated with allelic variation at host plant quantitative loci. Allele types in a host plant QTL associated with differential survival of immature progeny can affect maternal choices for oviposition. The multidisciplinary approach used here may lead to the identification of plant genes with important community consequences, and may complement the use of antibiosis due to solid stems to control the wheat stem sawfly in agroecosystems.

Host plant quantitative trait loci affect specific behavioral sequences in oviposition by a stem-mining insect

KEY MESSAGE

Genetic diversity in quantitative loci associated with plant traits used by insects as cues for host selection can influence oviposition behavior and maternal choice. Host plant selection for oviposition is an important determinant of progeny performance and survival for phytophagous insects. Specific cues from the plant influence insect oviposition behavior; but, to date, no set of host plant quantitative trait loci (QTLs) have been shown to have an effect on behavioral sequences leading to oviposition. Three QTLs in wheat (Triticum aestivum L.) have been identified as influencing resistance to the wheat stem sawfly (WSS) (Cephus cinctus Norton). Wheat near-isogenic lines (NILs) for each of the three QTLs were used to test whether foraging WSS were able to discriminate variation in plant cues resulting from allelic changes. A QTL on chromosome 3B (Qss-msub-3BL) previously associated with stem solidness and larval antibiosis was shown to affect WSS oviposition behavior, host preference, and field infestation. Decreased preference for oviposition was also related to a QTL allele on chromosome 2D (Qwss.msub-2D). A QTL on chromosome 4A (Qwss.msub-4A.1) affected host plant attractiveness to foraging females, but did not change oviposition preference after females landed on the stem. These findings show that oviposition decisions regarding potential plant hosts require WSS females to discriminate signals from the plant associated with allelic variation at host plant quantitative loci. Allele types in a host plant QTL associated with differential survival of immature progeny can affect maternal choices for oviposition. The multidisciplinary approach used here may lead to the identification of plant genes with important community consequences, and may complement the use of antibiosis due to solid stems to control the wheat stem sawfly in agroecosystems.

Effect of Wheat Cultivars and Blends on the Oviposition and Larval Mortality of Cephus cinctus (Hymenoptera: Cephidae) and Parasitism by Bracon cephi (Hymenoptera: Braconidae)

The wheat stem sawfly (Cephus cinctus Norton) is a major historical pest of wheat in the northern Great Plains of North America. The insect spends most of its life as a larva protected inside grass stems so that its management has relied on strategies other than insecticides. We conducted a study in southern Alberta from 2006-2009 to assess the effects of wheat species, cultivar, seeding rate, and blending a resistant and a vulnerable cultivar, on oviposition, larval infestation, and cutting damage. The mortality caused by its primary parasitoid, Bracon cephi (Gahan), was also assessed to investigate the potential benefit of cultivar blends to enhance sawfly biological control. Sawfly laid fewer eggs on plants of the durum cultivar 'AC Avonlea' and on those of the solid-stemmed cultivar 'Lillian' compared to plants of the hollow-stemmed cultivar 'CDC Go.' Larval establishments (infestation) followed a similar pattern to that of oviposition. At these locations there was low cutting damage in most years and to a large extent this was due to mortality inflicted by the parasitoid Bracon cephi (40-60%). However, the remaining mortality was attributed to other factors and host, particularly the inclusion of the solid-stemmed cultivar. Direct and indirect factors likely affected the success of the parasitoid in the crop monocultures and blends, and these mechanisms require further research.

A Multiple Decrement Life Table Reveals That Host Plant Resistance and Parasitism Are Major Causes of Mortality for the Wheat Stem Sawfly

This study investigated the dynamics of parasitism, host plant resistance, pathogens, and predation on the demography of wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), developing in susceptible (hollow stem) and resistant (solid stem) wheat hosts. This study is also the first to investigate the prevalence and impact of cannibalism on wheat stem sawfly mortality. Wheat stem sawflies were sampled in two commercial wheat fields over 4 yr from the egg stage through adult emergence, and multiple decrement life tables were constructed and analyzed. Cannibalism, host plant resistance, or unknown factors were the most prevalent factors causing egg mortality. Summer mortality of prediapause larvae ranged from 28 to 84%, mainly due to parasitism by Bracon cephi (Gahan) and Bracon lissogaster Muesebeck, cannibalism, and host plant resistance. Winter mortality ranged from 6 to 54% of the overwintering larvae, mainly due to unknown factors or pathogens. Cannibalism is a major cause of irreplaceable mortality because it is absolute, with only a single survivor in every multiple infested stem. Subsequent to obligate cannibalism, mortality of feeding larvae due to host plant resistance was lower in hollow stem wheat than in solid stem wheat. Mortality from host plant resistance was largely irreplaceable. Irreplaceable mortality due to parasitoids was greater in hollow stem wheat than in solid stem wheat. Host plant resistance due to stem solidness and parasitism in hollow stems cause substantial mortality in populations of actively feeding larvae responsible for all crop losses. Therefore, enhancing these mortality factors is vital to effective integrated pest management of wheat stem sawfly.

Interacting agricultural pests and their effect on crop yield: application of a Bayesian decision theory approach to the joint management of Bromus tectorum and Cephus cinctus

Worldwide, the landscape homogeneity of extensive monocultures that characterizes conventional agriculture has resulted in the development of specialized and interacting multitrophic pest complexes. While integrated pest management emphasizes the need to consider the ecological context where multiple species coexist, management recommendations are often based on single-species tactics. This approach may not provide satisfactory solutions when confronted with the complex interactions occurring between organisms at the same or different trophic levels. Replacement of the single-species management model with more sophisticated, multi-species programs requires an understanding of the direct and indirect interactions occurring between the crop and all categories of pests. We evaluated a modeling framework to make multi-pest management decisions taking into account direct and indirect interactions among species belonging to different trophic levels. We adopted a Bayesian decision theory approach in combination with path analysis to evaluate interactions between Bromus tectorum (downy brome, cheatgrass) and Cephus cinctus (wheat stem sawfly) in wheat (Triticum aestivum) systems. We assessed their joint responses to weed management tactics, seeding rates, and cultivar tolerance to insect stem boring or competition. Our results indicated that C. cinctus oviposition behavior varied as a function of B. tectorum pressure. Crop responses were more readily explained by the joint effects of management tactics on both categories of pests and their interactions than just by the direct impact of any particular management scheme on yield. In accordance, a C. cinctus tolerant variety should be planted at a low seeding rate under high insect pressure. However as B. tectorum levels increase, the C. cinctus tolerant variety should be replaced by a competitive and drought tolerant cultivar at high seeding rates despite C. cinctus infestation. This study exemplifies the necessity of accounting for direct and indirect biological interactions occurring within agroecosystems and propagating this information from the statistical analysis stage to the management stage.

Airborne signals of communication in sagebrush: a pharmacological approach

When plants receive volatiles from a damaged plant, the receivers become more resistant to herbivory. This phenomenon has been reported in many plant species and called plant-plant communication. Lab experiments have suggested that several compounds may be functioning as airborne signals. The objective of this study is to identify potential airborne signals used in communication between sagebrush (Artemisia tridentata) individuals in the field. We collected volatiles of one branch from each of 99 sagebrush individual plants. Eighteen different volatiles were detected by GC-MS analysis. Among these, 4 compounds; 1.8-cineol, β-caryophyllene, α-pinene and borneol, were investigated as signals of communication under natural conditions. The branches which received either 1,8-cineol or β-caryophyllene tended to get less damage than controls. These results suggested that 1,8-cineol and β-caryophyllene should be considered further as possible candidates for generalized airborne signals in sagebrush.

Odorant receptors of a primitive hymenopteran pest, the wheat stem sawfly

The wheat stem sawfly, Cephus cinctus, is an herbivorous hymenopteran that feeds exclusively on members of the Graminae family. Synanthropically, it has become one of the most important insect pests of wheat grown in the northern Great Plains region of the USA and Canada. Insecticides are generally ineffective because of the wheat stem sawfly's extended adult flight period and its inaccessible larval stage, during which it feeds within the wheat stems, making it virtually intractable to most pest management strategies. While research towards integrated pest management strategies based on insect olfaction has proved promising, nothing is known about the molecular basis of olfaction in this important pest species. In this study we identified 28 unique odorant receptor (Or) transcripts from an antennal transcriptome. A phylogenetic analysis with the predicted Ors from the honey bee and jewel wasp genomes revealed at least four clades conserved amongst all three Hymenoptera species. Antennal expression levels were analysed using quantitative real-time PCR, and one male-biased and five female-biased Ors were identified. This study provides the basis for future functional analyses to identify behaviourally active odours that can be used to help develop olfactory-mediated pest management tools.

Terpenoid variations within and among half-sibling avocado trees, Persea americana Mill. (Lauraceae)

Chemical analyses were conducted to determine the qualitative and quantitative differences in monoterpenes and sesquiterpenes in plant material from avocado trees, Persea americana Mill. (Lauraceae). The initial study analyzed plant material sampled from the trunk to the leaves through different branch diameters to quantify proximo-distal spatial differences within a tree. All trees were seedlings initiated from a single maternal tree. Two-way analysis of variance was conducted on 34 chemicals that comprised at least 3% of the total chemical content of at least one tree and/or location within a tree. There were significant interactions between genotype and location sampled for most chemicals. Parentage analysis using microsatellite molecular markers (SSR's) determined that the four trees had three fathers and that they represented two full-siblings and two half-sibling trees. Descriptive discriminant analysis found that both genotype and location within a tree could be separated based on chemical content, and that the chemical content from full-siblings tended to be more similar than chemical content from half-siblings. To further explore the relationship between genetic background and chemical content, samples were analyzed from leaf material from 20 trees that included two sets of full-sibling seedling trees, the maternal tree and the surviving paternal tree. Descriptive discriminant analysis found good separation between the two full-sibling groups, and that the separation was associated with chemistry of the parental trees. Six groups of chemicals were identified that explained the variation among the trees. We discuss the results in relation to the discrimination process used by wood-boring insects for site-selection on host trees, for tree selection among potential host trees, and the potential use of terpenoid chemical content in chemotaxonomy of avocado trees.