G. J. Michels

Predator abundance in alfalfa fields in relation to aphids, within-field vegetation, and landscape matrix

Abstract We used multiple regression modeling to investigate the numerical response by the predatory insects Hippodamia convergens Guérin-Méneville, H. parenthesis (Say), and C. septempunctata L. (Coleoptera: Coccinellidae), Chrysoperla plorabunda (Fitch) (Neuroptera: Chrysopidae), and Nabis americoferus Carayon (Hemiptera: Nabidae) to aphids during 5 yr in three geographically separated alfalfa fields in eastern South Dakota. Regression models for abundance of adults of all species were significant. Regression models for immature H. convergens, H. parenthesis, and C. septempunctata were significant, but regression models for immature C. plorabunda and N. americoferus were not significant. Regression parameters differed among the three fields for most predator species, indicating that the numerical response was dependent on geographical location. To obtain insight into why the numerical response by predators differed among fields we determined how the abundance of predators in alfalfa fields was influenced by the landscape surrounding a field and the vegetation in it. Variables describing the complexity of the landscape surrounding alfalfa fields and the plant community in the fields entered into regression models for predator abundance and explained a greater proportion of the variance in predator abundance than aphid abundance did. We conclude that the structure of the landscape matrix plays an important role in determining the abundance of aphid predators in alfalfa fields, as does the plant community in a field. These effects can sometimes overshadow the direct numerical response by predators to aphids.

Hyperspectral Spectrometry as a Means to Differentiate Uninfested and Infested Winter Wheat by Greenbug (Hemiptera: Aphididae)

Although spectral remote sensing techniques have been used to study many ecological variables and biotic and abiotic stresses to agricultural crops over decades, the potential use of these techniques for greenbug, Schizaphis graminum (Rondani) (Hemiptera: Aphididae) infestations and damage to wheat, Triticum aestivum L., under field conditions is unknown. Hence, this research was conducted to investigate: 1) the applicability and feasibility of using a portable narrow-banded (hyperspectral) remote sensing instrument to identify and discern differences in spectral reflection patterns (spectral signatures) of winter wheat canopies with and without greenbug damage; and 2) the relationship between miscellaneous spectral vegetation indices and greenbug density in wheat canopies growing in two fields and under greenhouse conditions. Both greenbug and reflectance data were collected from 0.25-, 0.37-, and 1-m2 plots in one of the fields, greenhouse, and the other field, respectively. Regardless of the growth conditions, greenbug-damaged wheat canopies had higher reflectance in the visible range and less in the near infrared regions of the spectrum when compared with undamaged canopies. In addition to percentage of reflectance comparison, a large number of spectral vegetation indices drawn from the literature were calculated and correlated with greenbug density. Linear regression analyses revealed high relationships (R2 ranged from 0.62 to 0.85) between greenbug density and spectral vegetation indices. These results indicate that hyperspectral remotely sensed data with an appropriate pixel size have the potential to portray greenbug density and discriminate its damage to wheat with repeated accuracy and precision.

Cross-species transferability of microsatellite markers from six aphid (Hemiptera: Aphididae) species and their use for evaluating biotypic diversity in two cereal aphids.

The abundance and distribution of microsatellites, or simple sequence repeats (SSRs) were explored in the expressed sequence tag (EST) and genomic sequences of the pea aphid, Acyrthosiphon pisum (Harris), and the green peach aphid, Myzus persicae (Sulzer). A total of 108 newly developed, together with 40 published, SSR markers were investigated for their cross‐species transferability among six aphid species. Genetic diversity among six greenbug, Schizaphis graminum (Rondani) and two Russian wheat aphid, Diuraphis noxia (Kurdjumov) biotypes was further examined with 67 transferable SSRs. It was found that the pea aphid genome is abundant in SSRs with a unique frequency and distribution of SSR motifs. Cross‐species transferability of EST‐derived SSRs is dependent on phylogenetic closeness between SSR donor and target species, but is higher than that of genomic SSRs. Neighbor‐joining analysis of SSR data revealed host‐adapted genetic divergence as well as regional differentiation of greenbug biotypes. The two Russian wheat aphid biotypes are genetically as diverse as the greenbug ones although it was introduced into the USA only 20 years ago. This is the first report of large‐scale development of SSR markers in aphids, which are expected to have wide applications in aphid genetic, ecological and evolutionary studies.

High spectral and spatial resolution hyperspectral imagery for quantifying Russian wheat aphid infestation in wheat using the constrained energy minimization classifier

Abstract. The effects of insect infestation in agricultural crops are of major ecological and economic interest because of reduced yield, increased cost of pest control and increased risk of environmental contamination from insecticide application. The Russian wheat aphid (RWA, Diuraphis noxia) is an insect pest that causes damage to wheat (Triticum aestivum L.). We proposed that concentrated RWA feeding areas, referred to as “hot spots,” could be identified and isolated from uninfested areas within a field for site specific aphid management using remotely sensed data. Our objectives were to (1) investigate the reflectance characteristics of infested and uninfested wheat by RWA and (2) evaluate utility of airborne hyperspectral imagery with 1-m spatial resolution for detecting, quantifying, and mapping RWA infested areas in commercial winter wheat fields using the constrained energy minimization classifier. Percent surface reflectance from uninfested wheat was lower in the visible and higher in the near infrared portions of the spectrum when compared with RWA-infested wheat. The overall classification accuracies of > 89 % for damage detection were achieved. These results indicate that hyperspectral imagery can be effectively used for accurate detection and quantification of RWA infestation in wheat for site-specific aphid management.

Effect of biotype and temperature on fitness of greenbug (Hemiptera: Aphididae) on sorghum.

ABSTRACT The greenbug, Schizaphis graminum (Rondani) (Hemiptera: Aphididae), is a major aphid pest of small grains and sorghum, Sorghum bicolor (L.) Moench. Greenbugs extract juice and inject toxin, killing seedlings or limiting the yield of older plants. Understanding greenbug biology and how biotypes develop is important for evaluating and developing sorghum with durable resistance. Prereproductive period, fecundity, and longevity of greenbug biotypes E and I were assessed on susceptible ‘RTx430’ sorghum at four cycling temperatures of 10–23, 14–27, 18–31, and 22–35°C in an incubator. A photoperiod of 14:10 (L:D) h corresponded with daily warm and cool temperatures. Greenbug fitness was more affected by temperature than biotype. Greenbug prereproductive period, total fecundity, and longevity did not differ among temperature regimes except at the warmest regime (22–35°C), at which all parameters were greater for biotype E than biotype I. The prereproductive period of greenbug biotypes E and I combined was more than twice as long at the coolest temperature of 10–23°C as at 22–35°C. Greenbugs produced a maximum average of 3.3 more nymphs per day at warmer than cooler temperature regimes. Average total fecundity was greatest at 10–23°C, with fewest nymphs being produced at 22–35°C. Longevity of greenbug biotypes E and I combined was 6 times longer at 10–23°C than at 22–35°C. This study provides information on optimal temperatures under which to evaluate damage to sorghum being developed for resistance to different biotypes of greenbug.

Spatial and Temporal Distribution of Induced Resistance to Greenbug (Homoptera: Aphididae) Herbivory in Preconditioned Resistant and Susceptible near Isogenic Plants of Wheat

Abstract Interactions between biotype E greenbugs, Schizaphis graminum (Rodani), and two near isogenic lines of the greenbug resistance gene Gb3 of wheat, Triticum aestivum L., were examined for 62 d after infestation. By comparing aphid performance and host responses on control and greenbug-preconditioned plants, we demonstrated that systemic resistance to greenbug herbivory was inducible in the resistant genotype with varying intensities and effectiveness in different parts of the plants. Preconditioning of susceptible plants resulted in modification of within-plant aphid distribution and reduction of cumulative greenbug densities, but it showed no effect on reducing greenbug feeding damage to host plant. Preconditioning of resistant plants altered greenbug population dynamics by reducing the size and buffering the fluctuation of the aphid population. Preconditioning in the first (oldest) leaf of the resistant plant had no phenotypically detectable effect in the stem and induced susceptibility locally in the first leaf within the first 2 d after infestation. The preconditioning-induced resistance reduced greenbug density, delayed aphid density peaks and extended the life of younger leaves in resistant plants. Expression of induced resistance was spatially and temporally dynamic within the plant, which occurred more rapidly, was longer in duration, and stronger in intensity in younger leaves. Host resistance gene-mediated induced resistance was effective in lowering greenbug performance and reducing damage from greenbug herbivory in host plants. Results from this study supported the optimal defense theory regarding within-plant defense allocation.

Airborne Remote Sensing to Detect Greenbug1 Stress to Wheat

Abstract. Vegetation indices calculated from the quantity of electromagnetic radiation reflected from plants measured using multi-spectral imaging systems have been used to quantify stress levels in plants. Greenbugs, Schizaphis graminum (Rondani), cause stress to wheat, Triticum aestivum L., plants and therefore multispectral remote sensing may be useful for detecting greenbug-infested wheat fields. The objectives of this study were to assess whether variation in light reflectance from plants infested with varying densities of greenbugs could be detected and quantified using airborne imagery obtained with a multi-spectral digital camera mounted in a fixed-wing aircraft. In a replicated experiment where greenbug density was manipulated in 1-m2 plots of two winter wheat varieties (‘Jagger’ and ‘OK 101’) planted in a 1 ha field, we found that wheat infested with greenbugs exhibited a similar reflectance response for the two varieties. Both varieties showed a reduction in the normalized differenced vegetation index (NDVI) as greenbug density increased, as indicated by a negative slope for the regression of NDVI on greenbug density. Neither slopes nor intercepts differed significantly between the two varieties. A second vegetation index, Green NDVI, was not consistently correlated with greenbug density in the experiment. In a second experiment in large plots in four production winter wheat fields we found significant negative correlations between greenbug density and the vegetation indices for three of the four fields. The results indicated that airborne multi-spectral imaging can be used to detect stress caused by greenbug infestation of wheat fields.

Examining the Competitive Advantage of Diuraphis noxia (Hemiptera: Aphididae) Biotype 2 Over Biotype 1

ABSTRACT The Russian wheat aphid, Diuraphis noxia (Kurdjumov) is a serious pest of small grains, such as wheat and barley. High population growth rates and a broad gramineae host range have allowed this aphid to successfully establish and become pestiferous across much of North America since its invasion in the mid-1980s. Resistant wheat cultivars were developed and provided control of D. noxia until 2003, when a new biotype (designated RWA2, as contrasted with the original biotype’s designation, RWA1) emerged and rapidly spread through dryland winter wheat-growing regions. RWA2 displaced RWA1 more quickly than expected, based on RWA2’s advantage in RWA1-resistant wheat cultivars. Previous research suggested that RWA2 may out-compete RWA1 in cooler temperatures. Thus, we sought to determine if RWA2 had a competitive advantage over RWA1 during the overwintering period. We placed a known distribution of RWA1 and RWA2 aphids in the field for the winter at three sites across a latitudinal gradient (from northern Colorado to Texas) to test for a competitive advantage between these biotypes. We found overwhelming support for an overwintering competitive advantage by RWA2 over RWA1, with evidence suggesting a >10-fold advantage even at our Texas site (i.e., the site with the mildest winter). This substantial overwintering advantage helps explain the quick dispersion and displacement of RWA1 by RWA2.

A Predictive Degree Day Model for the Development of Bactericera cockerelli (Hemiptera: Triozidae) Infesting Solanum tuberosum

ABSTRACT Bactericera cockerelli (Sulc) (Hemiptera: Triozidae) is a pest of potato (Solanum tuberosum L.) that vectors the bacterium that putatively causes zebra chip disease in potatoes, ‘Candidatus Liberibacter solanacearum.’ Zebra chip disease is managed by controlling populations of B. cockerelli in commercial potato fields. Lacking an integrated pest management strategy, growers have resorted to an intensive chemical control program that may be leading to insecticide-resistant B. cockerelli populations in south Texas and Mexico. To initiate the development of an integrated approach of controlling B. cockerelli, we used constant temperature studies, nonlinear and linear modeling, and field sampling data to determine and validate the degree day parameters for development of B. cockerelli infesting potato. Degree day model predictions for three different B. cockerelli life stages were tested against data collected from pesticide-free plots. The model was most accurate at predicting egg-to-egg and nymphto- nymph peaks, with less accuracy in predicting adult-to-adult peaks. It is impractical to predict first occurrence of B. cockerelli in potato plantings as adults are present as soon cotyledons break through the soil. Therefore, we suggest integrating the degree day model into current B. cockerelli management practices using a two-phase method. Phase 1 occurs from potato planting through to the first peak in a B. cockerelli field population, which is managed using current practices. Phase 2 begins with the first B. cockerelli population peak and the degree day model is initiated to predict the subsequent population peaks, thus providing growers a tool to proactively manage this pest.

Effect of Greenbug (Hemiptera: Aphididae) from Resistant Sorghum on Developmental Rates of Convergent Lady Beetle (Coleoptera: Coccinellidae)

Understanding tritrophic effects of resistant crops on natural enemies feeding on pests is important for developing resistant plants while conserving natural enemies. The effect on convergent lady beetles, Hippodamia convergens Guerin-Meneville, fed greenbugs, Schizaphis graminum (Rondani), reared on resistant sorghum, Sorghum bicolor (L.) Moench, was assessed. Convergent lady beetles from sorghum fields were paired and allowed to mate. F1 progeny were reared at a photoperiod of 14:10 light:dark hours at 23 or 30°C and fed daily with a known number of biotype I greenbugs from resistant PI550607 or susceptible RTx430 sorghum. Lady beetle eggs hatched 1 day later at 23°C (3.0 days) than at 30°C. Larvae completed development in 0.75 more day and pupae developed in 6.0 versus 3.0 days at 23 than at 30°C. Lady beetles consumed similar numbers of greenbugs from either genotype of sorghum, but larvae and adults consumed 1.7 and 2.5 times more greenbugs at 23 than at 30°C, respectively. Each lady beetle adult at 23°C consumed an average of 17,124 greenbugs. Adult lady beetles were paired in four combinations depending on the sorghum source of greenbugs from which they had been fed. Most eggs were produced (2,893 eggs) and hatched (2,657 or 91.0%) per lady beetle fed greenbugs from susceptible sorghum at 23°C.