I.L. Wise

An interspersed refuge for Sitodiplosis mosellana (Diptera: Cecidomyiidae) and a biocontrol agent Macroglenes penetrans (Hymenoptera: Pteromalidae) to manage crop resistance in wheat.

An interspersed refuge of susceptible plants in a resistant, spring-sown wheat crop was tested as a strategy to protect crop resistance against evolution of virulence by the wheat midge Sitodiplosis mosellana (Géhin), and also to conserve a biocontrol agent Macroglenes penetrans(Kirby). Eight replicated field experiments were conducted using seed mixtures of 0, 5, 10, 15 and 100% or 0, 5 and 100% susceptible wheat with an agronomically similar wheat expressing the antibiotic resistance gene Sm1. The frequencies of eggs, mature larvae and parasitized larvae in susceptible and resistant wheat spikes, and midge-affected seeds in the harvest, were recorded for each plot. In susceptible wheat, insect densities and seed damage were typical of those in commercial wheat. In resistant wheat, few larvae completed development, 2% or less compared with about 80% in susceptible wheat, when larvae were sampled at maturity. This resistant wheat also deterred midge oviposition, reducing egg densities by 65% compared with susceptible wheat. The wheat midge and its parasitoid oviposited throughout the plots, and parasitism was density independent. The densities of mature midge larvae and parasitoids were in proportion to the size of the refuge. A 5% susceptible refuge produced about 41 mature larvae for each mature larva from the resistant wheat, and provided effective control of damage. An interspersed refuge of susceptible plants in resistant wheat is a promising strategy for sustaining resistance conferred by Sm1 and biocontrol of the wheat midge.

Oviposition Preference and Offspring Performance of a Wheat Midge Sitodiplosis mosellana (Géhin) (Diptera: Cecidomyiidae) on Defended and Less Defended Wheat Plants

Abstract Oviposition preferences of a herbivore, the wheat midge Sitodiplosis mosellana (Géhin) (Diptera: Cecidomyiidae), on wheat were investigated in relation to two hypotheses: female preferences are adapted to offspring performance; plants may evolve independent defenses that both deter oviposition and reduce offspring performance. Variation in egg density and larval performance were measured for three plant genotypes: a preferred, susceptible wheat; a less preferred, susceptible wheat; a less preferred wheat defended against larval feeding. Oviposition peaked 10–11 d after emergence of the inflorescences and then declined sharply on all three wheat genotypes, although the inflorescences of the genotypes developed at different rates. On the preferred, susceptible wheat, larval performance was high for oviposition that occurred until pollination and low later. On a less preferred wheat, larval performance was high when eggs were laid before or after pollination. On a defended wheat, larval performance was always low. Oviposition preference was associated with larval performance that varied with plant developmental stage, but imperfectly, possibly because females do not detect cues for seed development. Females deposited eggs further from larval feeding sites when ovipositing on less preferred wheats, regardless of whether larval performance on the wheat was high or low. A low preference in combination with a shift in oviposition site supports the hypothesis that some wheats have evolved a defense that deters oviposition. This defense against oviposition is independent of a defense that reduces larval performance, which causes an apparent failure in the expected preference-performance relationship.

Sex ratios of Sitodiplosis mosellana (Diptera: Cecidomyiidae): implications for pest management in wheat (Poaceae)

Sex ratios of populations of the wheat midge Sitodiplosis mosellana Gehin, developing on wheat Triticum aestivum L., were determined at reproduction, adult emergence, and dispersal. The patterns of sex ratio through the life cycle of S. mosellana result from: (i) a genetic mechanism that causes all or nearly all of the progeny of individual females to be a single sex, with an overall sex ratio that is slightly biased at 54-57% females; (ii) a differential mortality during diapause that increases the sex ratio to 60-65% females; (iii) mating which occurs near the emergence site followed by female dispersal which causes the post-dispersal sex ratio to rise to nearly 100% females; and (iv) oviposition which spreads eggs among different plants and assures that the next generation has a local sex ratio close to the population average. These changes in sex ratio through the life cycle have implications for using crop resistance or pheromones to manage S. mosellana, because mating takes place quickly near emergence sites, and because mated females but not males disperse from emergence sites to oviposition sites. Crop refuges used to protect resistance genes against the evolution of virulence by S. mosellana must be interspersed to prevent assortative mating that would occur in separate blocks of resistant and susceptible plants. Monitoring or mating disruption using a pheromone would be ineffective when wheat is grown in rotation with a non-host crop.

Survival of Sitodiplosis mosellana (Diptera: Cecidomyiidae) on wheat (Poaceae) with antibiosis resistance: implication for the evolution of virulence

Small numbers of larval wheat midge, Sitodiplosis mosellana Géhin, survived and matured in each of five field seasons in a plot of spring wheat carrying the Sm1 gene for antibiosis resistance against this insect. Wheat midge developing on resistant wheat had higher survival in the laboratory than in the field, but survival was always very low compared with that of larvae developing on susceptible wheat. The mass of these larvae and their survival during diapause were approximately half those of larvae developing on susceptible wheat in both the laboratory and the field. The survival of some wheat midge larvae on resistant wheat, and their reduced mass, is consistent with the hypothesis that a virulence allele allowing adaptation to Sm1 is present in the population. Assuming this to be the case, the frequency of the allele in the population was estimated to be between 0.8 × 10−4 and 1.6 × 10−2, if surviving larvae are heterozygous for the allele. Although rare, a virulence allele occurring at this frequency would likely allow the wheat midge to overcome the resistance gene Sm1 once resistant wheat is grown over a wide area.

A saturated SNP linkage map for the orange wheat blossom midge resistance gene Sm1

Key messageSNP markers were developed for the OWBM resistance geneSm1that will be useful for MAS. The wheatSm1region is collinear with an inverted syntenic interval inB. distachyon.AbstractOrange wheat blossom midge (OWBM, Sitodiplosis mosellana Géhin) is an important insect pest of wheat (Triticum aestivum) in many growing regions. Sm1 is the only described OWBM resistance gene and is the foundation of managing OWBM through host genetics. Sm1 was previously mapped to wheat chromosome arm 2BS relative to simple sequence repeat (SSR) markers and the dominant, sequence characterized amplified region (SCAR) marker WM1. The objectives of this research were to saturate the Sm1 region with markers, develop improved markers for marker-assisted selection (MAS), and examine the synteny between wheat, Brachypodium distachyon, and rice (Oryza sativa) in the Sm1 region. The present study mapped Sm1 in four populations relative to single nucleotide polymorphisms (SNPs), SSRs, Diversity Array Technology (DArT) markers, single strand conformation polymorphisms (SSCPs), and the SCAR WM1. Numerous high quality SNP assays were designed that mapped near Sm1. BLAST delineated the syntenic intervals in B. distachyon and rice using gene-based SNPs as query sequences. The Sm1 region in wheat was inverted relative to B. distachyon and rice, which suggests a chromosomal rearrangement within the Triticeae lineage. Seven SNPs were tested on a collection of wheat lines known to carry Sm1 and not to carry Sm1. Sm1-flanking SNPs were identified that were useful for predicting the presence or absence of Sm1 based upon haplotype. These SNPs will be a major improvement for MAS of Sm1 in wheat breeding programs.

Gene Characterization of Two Digestive Serine Proteases in Sitodiplosis mosellana: Implications for Alternative Control Strategies

Abstract Two full-length cDNA sequences encoding digestive serine proteases (designated as SmPROT-1 and SmPROT-2) were recovered from the midgut of the orange wheat blossom midge, Sitodiplosis mosellana (Géhin) (Diptera: Cecidomyiidae), in an ongoing expressed sequence tag project. The deduced amino acid sequences shared homology with digestive serine proteases from insect and non-insect species, including conserved regions such as the catalytic triad, active pocket, and conserved structural motifs. Secretory signal peptides in both proteases at the N-terminals indicate that these proteins could function as midgut digestive serine proteases. A phylogenetic analysis grouped SmPROT-1 and SmPROT-2 with trypsin-like and chymotrysin-like serine proteases, respectively. Quantitative real-time PCR analysis showed that SmPROT-1 and SmPROT-2 were expressed predominantly in the midgut rather than in other tissues (fat body and salivary glands). Expression analyses revealed high mRNA levels for the feeding instars (1st- and 2nd-instar larvae) compared with other stages (neonate, 3rd instar, pupa, and adult). These results provide new insights into the biology of S. mosellana and are discussed in the context of developing alternative control strategies.

Sequential decision plan for controlling Mamestra configurata in spring canola

Abstract A sequential decision plan was developed for controlling larvae of the bertha armyworm, Mamestra configurata Walker (Lepidoptera: Noctuidae), in canola (Brassica napus L. and B. rapa L., Brassicaceae), using 0.25 and 0.5 m2 sampling units. Fields in Manitoba were sampled from 1980 to 1994 to determine minimum sample sizes and upper and lower cumulative larval counts at three economic thresholds. Taylors power law described most of the variation between mean larval density and variance for 0.25 m2 (r 2 = 0.926) and 0.5 m2 (r2 = 0.924) samples. Larvae were found to have a moderately clumped distribution in canola (b = 1.42). Levels of precision (D0) varying from 0.15 to 0.25 caused minimum sample sizes to vary between 6 and 21 for the 0.5 m2 samples to between 9 and 31 for the 0.25 m2 samples, for an economic threshold of 16–24 larvae/m2 (P = 0.20). Mean sampling times ranged from 40–108 for the 0.25 m2 samples to 49–126 min for the 0.5 m2 samples. The sampling plan for the 0.25 m2 samples was verified in 18 fields in 2006 and 2007. A correct decision was made in 87% (D0 = 0.25), 91% (A) = 0.20), and 94% (D0 = 0.15) of the fields when the recommendation was to spray if a decision could not be reached after a second sampling. The mean number of samples needed for making a decision was 14 (D0 = 0.25), 19 (D0 = 0.20), and 32 (D0 = 0.15). We recommend that growers use a precision level of 0.20 to minimize error rates and sampling effort. In most years, the minimum number of 0.25 m2 samples per field that growers would need to take is 14–17.