Robert J. Lamb

Sex pheromone of orange wheat blossom midge, Sitodiplosis mosellana.

Abstract Pheromone extract of the female orange wheat blossom midge, Sitodiplosis mosellana (Géhin) (SM) (Diptera: Cecidomyiidae), was analyzed by coupled gas chromatographic-electroantennographic detection (GC-EAD) and GC-mass spectrometry (MS), employing fused silica columns coated with DB-5, DB-210, DB-23 or SP-1000. These analyses revealed a single, EAD-active candidate pheromone which was identified as 2,7-nonanediyl dibutyrate. In experiments in wheat fields in Saskatchewan, traps baited with (2S,7S)-2,7-nonanediyl dibutyrate attracted significant numbers of male SM. The presence of other stereoisomers did not adversely affect trap captures. Facile synthesis of stereoisomeric 2,7-nonanediyl dibutyrate will facilitate the development of pheromone-based monitoring or even control of SM populations.

Compensation by Cruciferous Plants is Specific to the Type of Simulated Herbivory

Abstract The specificity of compensation by Brassica napus L. and Sinapis alba L. was investigated for herbivory by three biting and chewing herbivores: a small adult Coleoptera and a small and a large Lepidoptera larva. Phyllotreta cruciferae (Goeze) damaged the apical meristem and its defoliation of cotyledons was highly dispersed; the defoliation of Plutella xylostella L. was moderately dispersed over cotyledons; and Mamestra configurata (Walker) defoliated large contiguous areas of cotyledons. These types of herbivory were simulated in the field, and postdefoliation compensation by the plants was quantified: leaf length, relative growth rate of foliage, and seed production were measured. Plants were unable to compensate completely for meristem defoliation combined with highly dispersed cotyledon defoliation, and compensated better as cotyledon defoliation became less dispersed. Because compensatory responses to artificial defoliation were similar to and usually indistinguishable from those of insect herbivory, we conclude that the specificity of compensation is caused by the type of defoliation. Other interaction-specific processes such as transfer of growth-affecting chemicals from insect to plant need not be invoked. Sinapis alba compensated for defoliation better than B. napus because of inherent differences in compensatory responses, not because insects defoliate the two plant species differently.

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.

Compensation for Herbivory in Cruciferous Plants: Specific Responses to Three Defoliating Insects

Abstract Plant compensation may be specific to the defoliation of a particular insect herbivore, or a generalized response to herbivory. These alternate hypotheses were tested by measuring biomass and seed production of Brassica napus L. and Sinapis alba L. in response to 0, 25, or 75% defoliation of seedling plants. The herbivores were adults of Phyllotreta cruciferae (Goeze), larvae of Plutella xylostella L., and larvae of Mamestra configurata (Walker). Although defoliated to the same extent, both B. napus and S. alba compensated most for defoliation by M. configurata and least for defoliation by P. cruciferae. Both plant species compensated better for 25% than for 75% defoliation, and S. alba compensated better than B. napus. Laboratory and field experiments showed similar patterns of compensatory leaf growth, but recovery was more rapid in a controlled environment. Compensation was associated with changes in root biomass that were correlated with foliage biomass, indicating that root-shoot ratios were maintained. Complete recovery of foliage after defoliation did not assure complete recovery of plant fitness. For these three herbivorous insects, compensation by two plant species for defoliation was specific to the insect defoliator, and not a generalized response to herbivory. The compensatory responses of the two plant species explain, in part, the differential impact the three herbivores have on the crops.

Specific Impacts of Herbivores: Comparing Diverse Insect Species on Young Plants

Abstract A measure of the specific impact for herbivore–plant interactions was used to test whether 7 insect species differ in the severity of their effects on young cruciferous plants. Specific impact was estimated as a biomass conversion ratio (reduction in plant biomass per unit gain in insect biomass) based on dry weights. The armyworm Mamestra configurata (Walker) had a specific impact of 4–5 on both canola, Brassica napus L., and mustard, Sinapis alba L., in the laboratory, and a similar specific impact of 3.4 was estimated for a natural field population. Two herbivore densities showed little if any difference in the specific impact for M. configurata on canola, nor for the flea beetle Phyllotreta striolata (F.), which had impacts of 81 and 96 at the 2 densities tested. The flea beetle Phyllotreta cruciferae (Goeze) had a specific impact of 148 on canola, not significantly different from that of its relative, P. striolata, which feeds in a similar way. The specific impact of the beetle Entomoscelis americana Brown was 3.5, similar to that of M. configurata, and both species had lower specific impacts than another folivore, Plutella xylostella (L.), with an impact of 12. The specific impacts of 2 aphids, Lipaphis erysimi (Kaltenbach) and Myzus persicae (Sulzer), were similar with values of 12 and 16, respectively. Although different herbivores showed substantial differences in their impacts on canola, no consistent pattern of difference was found between herbivores belonging to the leaf-chewing and sap-feeding guilds, nor were there consistent differences between generalist and specialist herbivores.

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.

Variability in life history traits of the aphid, Acyrthosiphon pisum (Harris), from sexual and asexual populations

Many aphid species have shown remarkable adaptability by invading new habitats and agricultural crops, although they are parthenogenetic and might be expected to show limited genetic variation. To determine if the mode of reproduction limits the level of genetic variation in adaptively important traits, we assess variation in 15 life history traits of the pea aphid, Acyrhosiphon pisum (Harris), for five populations sampled along a north-south transect in central North America, and for three traits for three populations from eastern Australia. The traits are developmental times and rates as affected by temperature, body weights as affected by temperature, fecundity, measures of migratory tendency, and photoperiodic responses. The most southerly population from North America is shown to be obligately parthenogenetic, as are the Australian populations, and the four more northerly North American populations are facultatively parthenogenetic with the number of parthenogenetic generations per year increasing from north to south. The broad-sense heritabilities of life history traits varied from 0.36 to 0.71 for nine quantitive traits based on a comparison of within-and between-lineage variances. Using these traits, 7–13 distinct genotypes (i.e. clones) were identified among each of the 18 lines sampled from the North American populations, but the number did not differ significantly among populations. The level of genetic variation differed from trait to trait. For 4 of 12 quantitative traits, the level of variation in the obligately parthenogenetic population from North America was lowest, but significantly lower than all the sexual populations for only 1 trait. The obligately parthenogenetic population had the highest level of genetic variation for two traits, and had intermediate levels for the others. The most northerly population, which was sexual and had relatively few parthenogenetic generations each year, had the lowest level of variation for 5 of 12 traits and the highest level of variation for 2 traits. There was no decline in variability from north to south correlated with the increase in the annual number of parthenogenetic generations. The Australian populations showed no less variation than the North American populations for two of three traits, although the pea aphid was introduced to Australia only 5 years prior to the study, whereas the aphid has been in North America for at least 100 years. The mode of reproduction has not had a substantial impact on the level of genetic variation in life history traits of the pea aphid, but there are population-specific factors that effect the level of variation in certain traits.

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.