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.

Plant vigor metrics determine spatio-temporal distribution dynamics of Oulema melanopus (Coleoptera: Chrysomelidae) and its larval parasitoid, Tetrastichus julis (Hymenoptera: Eulophidae)

The cereal leaf beetle, Oulema melanopus (L.) (Coleoptera: Chrysomelidae), is a new invasive insect pest of oat, wheat, and barley in western Canada. Biological control with its principal larval parasitoid, Tetrastichus julis Walker (Hymenoptera: Eulophidae), is the primary management strategy. However, to implement control successfully, a thorough understanding of the spatio-temporal dynamics of the interactions between these two species is important. We examined the nature of spatial associations and distribution dynamics of O. melanopus and T. julis with reference to host plant nutrients and plant vigor traits using Spatial Analysis by Distance Indices. A grid design was used to understand spatial associations between O. melanopus and T. julis. Distributions of O. melanopus and T. julis indicated the presence of significant patches and gaps. Plant nutrient availability and plant vigor varied across the grid in all study years. On a spatial scale, O. melanopus and T. julis represented a tightly coupled system demonstrating the strong density-dependent nature of parasitoid dispersal. Among the factors examined, plant vigor traits significantly influenced field distributions of both O. melanopus and T. julis. Areas across grids with high plant density, greater plant height, and high availability of plant leaves indicated higher establishment of O. melanopus larvae, consequently exhibiting bottom-up effects on T. julis distributions. Maintenance of uniform plant vigor can be a critical aspect in mitigating yield losses from O. melanopus infestation.

Antixenosis resistance to Oulema melanopus (Coleoptera: Chrysomelidae) in central Asian wheat germplasm

ABSTRACT The cereal leaf beetle, Oulema melanopus (L.) (Coleoptera: Chrysomelidae), which is Eurasian in origin, is an emerging pest of agriculturally important cereal crops in western Canada, including wheat, oat, and barley. Among these crops, wheat (Triticum aestivum L.) has the highest number of resistant genotypes to O. melanopus infestation. Here we explored six genotypes from central Asia with putative resistance to the beetle in comparison with a widely grown Canadian wheat variety. We noted the highest oviposition on the susceptible control genotype, CDC GO, and the lowest on one of the central Asian genotypes (NN-100) in both choice and no-choice tests. Two of the other Asian genotypes (NN-78 and NN-105) were also less attractive for oviposition than the CDC GO check. Feeding trials also indicated less damage on some of the Asian genotypes (NN-100, NN-105, and NN-78) relative to CDC GO, but other Asian genotypes (NN-41, NN-45, and NN-27) were highly attractive for feeding and oviposition and did not exhibit stronger resistance to O. melanopus infestation than moderately resistant central Asian genotypes (NN-103, NN-78, or NN-105). There were no significant differences in the amount of feeding by overwintered and teneral adults. The results indicated that some wheat genotypes developed in central Asia have categories of nonpreference for feeding and oviposition worth considering in further development of host-plant resistance and integrated management of this pest.

Response of Pisum sativum (Fabales: Fabaceae) to Sitona lineatus (Coleoptera: Curculionidae) infestation: effect of adult weevil density on damage, larval population, and yield loss

Sitona lineatus L. (Coleoptera: Curculionidae) is an invasive pest in North America and its geographical range is currently expanding across the Canadian prairies. Adults and larvae of S. lineatus feed upon the foliage and root nodules, respectively, of field pea, Pisum sativum L. (Fabales: Fabaceae), and may contribute to economic losses when population densities are high. Integrated pest management (IPM) programs that incorporate economic thresholds should be used to manage S. lineatus populations in a sustainable manner. The impact of nitrogen fertilizer on field pea yield and the relationships between adult weevil density and above- and below-ground damage and yield were investigated in southern Alberta, Canada using exclusion cages on field pea plots. In each cage, 32 field pea plants were exposed to weevil densities ranging from zero to one adult weevil per plant. Nitrogen-fertilized plants yielded 16% more than unfertilized plants. Nitrogen-fertilized plants had fewer root nodules than unfertilized plants, but fertilizer had no effect on foliar feeding by S. lineatus. Adult density affected foliar feeding damage, with increases in above-ground damage associated with increases in S. lineatus density. Adult density did not affect root nodule damage, larval density, foliar biomass or seed weight. Overall, these results indicate that terminal leaf damage may be used to estimate adult weevil density but cannot be used to predict larval density or yield loss. Further research is required to better understand the impact of larval damage on yield and determine if economic thresholds can be developed using data from large-scale production systems.

Evaluation of alternative planting strategies to reduce wheat stem sawfly (Hymenoptera: Cephidae) damage to spring wheat in the northern Great Plains

Wheat, Triticum aestivum L., producers are often reluctant to use solid-stemmed wheat cultivars resistant to wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), due to concerns regarding yield, efficacy or market opportunities. We evaluated the impact of several planting strategies on wheat yield and quality and wheat stem sawfly infestation at two locations over a three-year period. Experimental units consisted of large plots (50 by 200 m) located on commercial farms adjacent to wheat stem sawfly-infested fields. Compared with a monoculture of a hollow-stemmed cultivar ('AC Barrie'), planting a monoculture of a solid-stemmed cultivar ('AC Eatonia') increased yield by an average of 16% (0.4 mg ha(-1)) and increased the grade of wheat by one unit at the two most heavily infested site-years. Planting a 1:1 blend of AC Eatonia and AC Barrie increased yield by an average of 11%, whereas planting 20- or 40-m plot margins to AC Eatonia increased yield by an average of 8%. High wheat stem sawfly pressure limited the effectiveness of using resistant cultivars in field margins because plants were often infested beyond the plot margin, with uniform infestation down the length of the plots at the two most heavily infested site-years. The effectiveness of AC Eatonia to reduce wheat stem sawfly survivorship was modest in this study, probably due to weather-related factors influencing pith expression and to the high abundance of wheat stem sawfly. Greater benefits from planting field margins to resistant cultivars or planting a blend of resistant and susceptible cultivars might be achievable under lower wheat stem sawfly pressure.

Carabid assemblages (Coleoptera: Carabidae) in a rotation of three different crops in southern Alberta, Canada: a comparison of sustainable and conventional farming

Carabids were sampled in 2000 (pretreatment year) and 2003-2005 in experimental plots in southern Alberta, Canada, after a rotation of beans, wheat, and potato under sustainable and conventional farming practices. Each phase of the rotation was present in every year. Crop type had a stronger effect than sustainable treatment on carabid-expected species richness, diversity, and species composition. However, carabid activity density was consistently higher in plots under sustainable treatments than those maintained conventionally. Potato plots, which were sprayed with insecticide for pest control, showed a significantly lower carabid activity density than the other crops. These results support other studies showing the beneficial effect of sustainable farming on activity density of carabid beetles.

Response of ground beetle (Coleoptera: Carabidae) field populations to four years of Lepidoptera-specific Bt corn production

Pitfall traps were used to monitor populations of ground beetles (Coleoptera: Carabidae) in plots of corn grown in continuous cultivation during a 4-yr period (2000-2003). Treatments included transgenic corn expressing a Bt Cry protein with efficacy specific against Lepidoptera (Bt), conventional corn grown with insecticide application (I), and the same conventional cultivar grown without insecticide application (NI). Mixed-model analyses of variance were performed on pitfall captures of beetles combined across weeks to give seasonal sums. Effects of corn treatment were not detected (P > 0.05) on total beetle abundance or species richness in any year. Effects of corn treatment on individual taxa were detected (P < 0.05) for 3 of the 39 species-by-year combinations examined. Effects of near significance (P < 0.08) were detected for an additional two species. In 2001, captures of Amara farcta Leconte and Harpalus amputatus Say were lower in Bt plots than in I or NI plots. In 2003, captures of Amara apricaria (Paykull) and Amara carinata (Leconte) were higher in Bt plots than in I or NI plots. Also in 2003, captures of Poecilus scitulus Leconte were higher in I plots than in Bt or NI plots. These patterns were not repeated among years. Results of this study indicate that cultivation of Lepidoptera-specific Bt corn in southern Alberta does not appreciably affect ground beetle populations.

Effect of wheat stem sawfly damage on yield and quality of selected Canadian spring wheat

The wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), has reached outbreak status at most locations in the southern Canadian prairies. Solid-stemmed wheat, Triticum aestivum L., cultivars, which are less susceptible to damage, remain the primary management option. This article quantifies the effect of wheat stem sawfly damage on grain yield and quality at harvest and determines how cultivar selection affects harvest losses. Solid-stemmed cultivars were compared with hollow-stemmed cultivars and with blends of a 1:1 ratio of each. The hollow-stemmed cultivars with the exception of'McKenzie', which had intermediate levels of stem cutting, were all significantly more susceptible to stem cutting than solid-stemmed cultivars. Cultivar blends had lower damage but were still significantly higher than the solid-stemmed cultivars. The solid-stemmed 'AC Eatonia' and 'AC Abbey' had the lowest levels of stem cutting and ranked second and third overall for yield in 2001 and 2002. McKenzie ranked first, which reflects its yield potential in combination with its partial resistance to stem cutting. Lower cutting in AC Eatonia, AC Abbey, McKenzie, and the blend of AC Abbey/ McKenzie was significantly correlated with lower grain losses. Grain lost at harvest has major economic implications if sawfly pressure is moderate to high and susceptible cultivars predominate.