Grain yield loss caused by bird cherry‐oat aphid (Rhopalosiphum padi L.) infestation and barley yellow dwarf virus (BYDV) infection may result from direct damage to the winter wheat (Triticum aestivum L.) crop as well as from reduced crop tolerance to stress environments. This greenhouse study measured the effects of R. padi infestation, BYDV infection, or a combination of R. padi plus BYDV on plant height, date of anthesis, yield, and yield components of four winter wheat varieties (‘Roughrider’, ‘Norstar’, ‘TAM 107’, and ‘Vona’) in the absence of additional environmental stresses. Treatments were applied at the two‐leaf growth stage. Early in the pre‐vernalization growth period, R. padi treatment (alone or in combination with BYDV) reduced plant height to about 55 to 60% of the control plant height while BYDY treated plants were about 90% of control. During the post‐vernalization growth period, plant heights attained about 90% of control in the R. padi treatment, to about 80% of control in the BYDV treatment and to about 70% of control in the R. padi+ BYDV treatment. Dates of anthesis were later in the R. padi+BYDV treatments than in the R. padi treatments for Norstar, Roughrider, and Vona but not for TAM 107. Individual kernel weights in the BYDV and R. padi+BYDV treatments were less than control or R. padi treatments for Norstar, TAM 107, and Vona but not for Roughrider. Control or R. padi‐treated plants had a greater number of fertile heads than plants given the BYDV or R. padi+BYDV treatments. Grain yield was strongly associated with kernel number per plant. The number of kernels per plant was reduced 19% by the R. padi treatment, 36% by the BYDV treatment, and 50% by the R. padi+BYDV treatment. Grain yield was reduced 21% by the R. padi treatment, 46% by the BYDV treatment, and 58% by the R. padi+BYDV treatment. With the exception of date of anthesis and individual kernel weight, there were no significant treatment by variety interactions for plant height, grain yield, and yield components. We conclude that R. padi infestation and BYDV infection caused significant yield reductions and that the varieties tested had little difference in their responses to these treatments in the absence of additional environmental stress.

No abstract

Insect‐tolerant plants provide farmers with alternatives to insecticide use for limiting crop losses. This study was conducted to evaluate western corn rootworm (Diabrotica virgifera virgifera LeConte) larval feeding tolerance of single‐cross maize (Zea mays L.) hybrids representing three successive 10‐yr eras. A 2‐yr study was conducted as a split‐split plot with plant density (37, 50, or 74 thousand kernels ha−1) as main plot treatments, rootworm infestation level (0, 2200, or 6600 eggs m−1 of row) as subplot treatments, and plant genotype (11 genotypes from the 1980, 1970, and 1960 eras) as sub‐subplot treatments. There was no significant interaction between genotype and infestation level for root damage ratings, suggesting that genotypes used in our study did not exhibit antibiosis to rootworm larvae. Vertical root pull resistance values for 1980‐ and 1970‐era genotypes were significantly greater than those seen for 1960‐era genotypes. At the low plant density, the lodging percentage of all genotypes gradually became greater in response to increased rootworm infestation level. At higher plant densities, however, the 1960‐era genotypes suffered significantly more lodging than the 1970‐ and 1980‐era genotypes in the presence of moderate rootworm damage. Our results suggest that large root system size and decreased plant lodging are the rootwormtolerance traits present in 1980 maize genotypes. Even with this level of tolerance in 1980‐era hybrids, however, substantial grain yield loss occurred when plants sustained moderate to heavy rootworm larval damage when these genotypes were grown at low and moderate plant densities.

Aggregate stability is a measure of the ability of soil to withstand the disrupting forces of rain and nonsymmetrical wetting. Erosion may influence aggregate stability by exposing soil with properties different than the original soil surface. This study was conducted to determine: (i) the influence of antecedent soil water content on wet aggregate stability using aggregates prehumidified to near saturation; (ii) the effects of organic C and clay content on the soil water content‐aggregate stability relationship; and (iii) the effect of topsoil removal on aggregate stability. Seventy‐eight soil samples were taken from six profiles of Beadle clay loam (fine, montmorillonitic, mesic, Typic Argiustoll). These samples were analyzed for pH, organic C, texture, CaCO3, antecedent soil water content, cation‐exchange capacity (CEC), and aggregate stability. Isolated test plots were also saturated with an overhead sprinkling infiltrometer. Samples were taken periodically throughout a 4‐d period for aggregate‐stability determination. A modified wet sieving technique, which included pre‐humidifying of the sample to near saturation, was used to determine aggregate stability. A negative linear relationship was found between aggregate stability of prehumidified aggregates and antecedent soil water content. Prediction equations were developed that explain up to 77% of variability for the aggregate stability. Organic C was the most important of the independent variables, followed by antecedent soil water content in the multiple regression analysis. Topsoil removal by desurfacing and erosion reduced aggregate stability. Aggregate stabilities for soils with high organic C and clay content were less affected by antecedent soil water content.

A 3-yr investigation was conducted in commercial corn, Zea mays (L.), fields in eastern South Dakota to determine how reduced application rates of planting-time soil insecticides would influence temporal emergence patterns and survival of northern and western corn rootworms, Diabrotica barberi Smith and Lawrence, and D. virgifera virgifera LeConte, respectively. Beetle emergence was monitored at 2-d intervals throughout the entire adult emergence period of three growing seasons from corn plots treated with planting-time applications of labeled (1X) and reduced (0.5 and 0.75X) application rates of terbufos, tefluthrin, and chlorethoxyfos. No consistent insecticide- or rate-related impacts on mean total emergence per trap were recorded for any of the compounds investigated. However, terbufos applications resulted in a 52% reduction in the number of beetles captured per trap, 53% reduction in maximum rate of adult emergence, and a 59% reduction in overall rate of emergence over time for male D. virgifera during 1994. Terbufos also significantly extended the time required for emergence to peak and linear emergence of female D. virgifera to end in 1994. Tefluthrin applications delayed onset, end, and time of maximum emergence of female D. barberi by 9.9, 14.1, and 12 d, respectively, during 1993. Tefluthrin also reduced emergence rates over time for male (38%) and female (46%) D. barberi during 1994. Overall, application rate was inconsequential regarding total emergence, seasonal emergence pattern, or level of plant protection provided for all insecticides we tested in this 3-yr investigation. Our findings demonstrate that, if properly applied, the reduced application rates used in this study provide adequate root protection and will not significantly impact the biology of these pest species.