Resistance to Russian wheat aphid biotype 2 in CIMMYT synthetic hexaploid wheat lines.

The Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae), devastates wheat and barley production on all continents except Australia. Although D. noxia-resistant cultivars exist, virulent D. noxia populations exist in Asia, North America, and South America that have the ability to overcome resistance. In this study, synthetic hexaploid wheat genotypes created at the International Maize and Wheat Improvement Center (CIMMYT) were evaluated and characterized for resistance to North American biotype 2 (RWA2). Fewer RWA2 were produced on several genotypes, and D. noxia-induced leaf rolling and chlorosis were reduced on these genotypes as well. Aphid numbers were positively correlated with chlorosis and with leaf rolling. However, some genotypes were highly resistant to leaf rolling and chlorosis while supporting large RWA2 populations. There were negative correlations between leaf chlorosis and leaf dry weight (r = -0.267, df = 106, P = 0.006) and between aphid numbers and leaf dry weight (r = -0.297, df = 105, P = 0.002). These results indicate that chlorosis and aphid number individually explain at least 27% of the changes observed in leaf dry weights. Interestingly, there was no correlation between leaf rolling and leaf dry weight. The RWA2-resistant lines identified, which are also resistant to D. noxia populations in Mexico and to greenbug (Schizaphis graminum Rondani) biotype G, are strong candidates for use in improving the genetic diversity in bread wheat for resistance to different biotypes of both S. graminum and D. noxia.

Effect of corn hybrids expressing the coleopteran-specific cry3Bb1 protein for corn rootworm control on aboveground insect predators.

Field and laboratory studies were conducted to determine the effect of transgenic Bacillus thuringiensis (Bt) corn, Zea mays L. (YieldGard Rootworm), expressing the Cry3Bb1 protein on aboveground nontarget insect predators (minute pirate bug, ladybird beetles, and carabids). Visual counts of adult and immature Orius insidiosus (Say), Coleomegilla maculata (DeGeer), Hippodamia convergens Gurin-Meneville, and Scymnus spp. occurring in Bt corn and its non-Bt isoline were made at Manhattan, KS, in 2002 and at Manhattan and Scandia, KS, in 2003. No significant differences were found between the Bt corn and non-Bt isoline plots in the abundance (number per plant) of O. insidiosus, C. maculata, H. convergens, and Scymnus spp. Field predation on Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae) egg masses was also observed during the silking stage of corn at Manhattan and Scandia in 2003. No significant differences were observed among treatments in predation rate for predators with chewing versus sucking mouthparts. Two laboratory studies determined the effect of Cry3Bb1 protein expressed in Bt corn pollen on C. maculata and carabids. The larvae of C. maculata were reared on Bt pollen, non-Bt pollen, or greenbugs, Schizaphis graminum (Rondani). The duration of larval and pupal stages, developmental time from egg hatch to adult emergence, percentage of survival, and elytra length were compared among treatments. There were no significant differences in developmental time of larvae fed pollen or greenbugs during their first two instars. However, significantly prolonged development of the third (1 d) and fourth instars (2 d) was observed for larvae fed greenbugs only. Total time for larval development was significantly longer for larvae that fed on greenbugs versus larvae fed on pollen. No significant differences were observed among treatments in the percentage of larvae that pupated or pupal stage duration. Larvae that fed on greenbugs had higher pupal and adult weights compared with pollen-fed larvae. However, pupal and adult weights did not vary between the Bt and non-Bt pollen treatments. No significant differences occurred in longevity and elytra length of beetles among all treatments. Two carabid species, Harpalus caliginosus F. and Harpalus pensylvanicus DeGeer, were reared on moistened dog food sprinkled with Bt or non-Bt corn pollen. No significant differences in mortality of H. caliginosus and H. pensylvanicus were detected among any of the treatments. There was no significant effect of Bt pollen on fecundity and egg viability of H. caliginosus. Our studies showed that YieldGard Rootworm had no effect on the selected coleopteran predators; therefore, this Bt corn hybrid could be used in an integrated pest management system.

Categories and inheritance of resistance to Russian wheat aphid (Homoptera: Aphididae) biotype 2 in a selection from wheat cereal introduction 2401.

The Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Homoptera: Aphididae), is one of the most devastating insect pests of wheat (Triticum spp.) and barley (Hordeum spp.) in the world. Yield losses and control costs are valued at several hundred million dollars each year. The use of D. noxia-resistant cultivars is an ecologically, economically, and biologically sound method of managing this pest. Several D. noxia resistance (Dn) genes from wheat have been used to develop cultivars resistant to D. noxia. However, a new U.S. D. noxia biotype (biotype 2) in Colorado is virulent to all known Dn genes except the Dn7 gene from rye (Secale spp.). Hence, there is an immediate need to identify and characterize unique sources of D. noxia resistance to biotypes. In this article, we report resistance to D. noxia biotype 2, identified in a selection from wheat cereal introduction (CItr) 2401, that is controlled by two dominant genes. CItr2401 has a strong antibiosis effect that is exhibited as a reduced intrinsic rate of increase of D. noxia biotype 2. CItr2401 plants also exhibit tolerance to leaf rolling and chlorosis. No antixenosis was detected in CItr2401.

Molecular mapping of sorghum genes expressing tolerance to damage by greenbug (Homoptera: Aphididae).

Genetic linkage maps are fundamental for the localization of genes conferring tolerance to greenbug, Schizaphis graminum (Rondani), feeding damage in sorghum, Sorghum bicolor (L.) Moench. Thirteen linkage groups (LGs) containing 60 simple sequence repeat (SSR) loci were mapped by using a set of sorghum recombinant inbred lines (RILs) obtained from the cross '96-4121' (greenbug-tolerant parent) x Redlan (greenbug-susceptible parent). The LG spanned a distance of 603.5 cM, with the number of loci per LG varying from 2 to 14. Seventeen additional SSR loci were unlinked at a log of odds value of 3.0. Based on chlorophyll loss occurring after greenbug feeding, visual damage ratings, and soil plant analysis development (SPAD), chlorophyll-loss indices were recorded for each RIL and for the parents used in the cross. Composite-interval mapping identified three quantitative trait loci (QTLs) associated with biotype I and five QTLs associated with biotype K. The amount of phenotypic variation explained by these QTLs ranged from 9 to 19.6%. The identification of QTLs that influence greenbug tolerance will not only facilitate the use of marker-assisted selection in sorghum breeding programs but also will provide a solid foundation for detailed characterization of individual loci implicated in greenbug tolerance in sorghum.

Characterization of two genes expressed in the salivary glands of the Hessian fly, Mayetiola destructor (Say).

Two genes, SSGP-11A1 and SSGP-12A1, have been isolated that encodes proteins with a secretion signal peptide at theN-terminals from the Hessian fly (Mayetiola destructor (Say)). The SSGP-11A1 gene contains one small intron (89 bp) and encodes a putative protein with 79 amino acids. The first 18 amino acids constitute a putative secretion signal peptide. The SSGP-12A1 gene contains three small introns and encodes a putative protein with 234 amino acids. The first 19 amino acids constitute a putative secretion signal peptide. Northern blot analysis revealed that both of the genes are primarily expressed in the salivary glands of Hessian fly larvae, the feeding stage of the insect. These observations are consistent with the possibility that the proteins encoded by them are secreted into host plants during feeding. Even though both genes are exclusively expressed in Hessian fly larvae, the expression profiles between them were quite different in insects at different instars. The SSGP-11A1 gene was expressed in all instars of larvae while the SSGP-12A1 gene was almost exclusively expressed in the first instar larvae. The differential expression suggests that the proteins encoded by these two genes may perform different functions. In situ hybridization revealed that the SSGP-11A1 gene is located on the short arm of chromosome A1 while SSGP-12A1 gene is on the long arm of chromosome A2.

Evaluation of thiamethoxam and imidacloprid as seed treatments to control European corn borer and Indianmeal moth (Lepidoptera: Pyralidae) larvae.

Efficacy of thiamethoxam (Cruiser) and imidacloprid (Gaucho) were evaluated as seed treatments for controlling European corn borer, Ostrinia nubilalis (Hübner) and Indianmeal moth, Plodia interpunctella (Hübner) larvae in stored grain. At approximately 22-26 degrees C, all fifth instar European corn borers died after two or 4 d of exposure to corn treated with 250 and 500 ppm thiamethoxam, respectively, while mortality of larvae exposed for two and 4 d on corn treated with 6.3-937.5 ppm imidacloprid did not exceed 48% at any concentration. At 29 degrees C, all nondiapausing fifth instars were killed after 3, 4, and 6-d exposure to 400, 300 and 200-ppm thiamethoxam, respectively, while survival increased at successively lower concentrations of 100, 50, 25, and 12.5 ppm. At 29 degrees C, the LC50 decreased from 85.9 to 7.2 ppm as the duration of exposure on treated corn increased from 2 to 6 d. All second and third instar Indianmeal moth larvae died after a 5 d exposure period to corn grain treated with thiamethoxam at 50 ppm or higher, but as the larvae aged, higher concentrations and longer exposure periods were required to give 100% mortality of each larval instar. Similar results were obtained when larval Indianmeal moths were exposed on corn treated with imidacloprid, or on sorghum treated with thiamethoxam. Mature wandering phase fifth instars were the most tolerant larval stage of the Indianmeal moth.