Scott C. Merrill

Modeling spatial variation of Russian wheat aphid overwintering population densities in Colorado winter wheat.

ABSTRACT The Russian wheat aphid, Diuraphis noxia (Kurdjumov), is a pest of small grain crops that has caused hundreds of millions of dollars of damage since it was first reported in the United States in 1986. Much is known about D. noxia population dynamics during the spring and early summer when most of the crop damage occurs, whereas little is known about the system during the overwintering period. Using a spatially explicit model developed from field observations in a wheat/fallow agroecosystem, we sought for predictable variation in overwintering success of D. noxia based on environmental factors such as topography and soil type. Successful modeling of densities of D. noxia would facilitate early control efforts targeting locations where D. noxia successfully overwintered. D. noxia density data were collected over 3 yr at two sites in eastern Colorado. The model incorporates georeferenced data from soil surveys, topography, and satellite imagery as predictor variables. Our approach links an information theoretic approach for model inference and model selection to landscape ecology, allowing for the examination of multiple candidate models and variables within each of the candidate models. Results were used to create trend surface models for D. noxia density in winter wheat agroecosystems. The model has the potential for use in site specific pesticide applications. Using site specific pesticide applications, pesticide inputs could be reduced by an estimated 30%, reducing input costs to the producer, increasing natural enemy refuges, reducing environmental contamination, augmenting pesticide resistance management practices, and reducing exposure of agricultural workers.

Diuraphis noxia Reproduction and Development With a Comparison of Intrinsic Rates of Increase to Other Important Small Grain Aphids: A Meta-Analysis

ABSTRACT The Russian wheat aphid, Diuraphis noxia (Kurdjumov), is a significant pest of small grains in the United States and worldwide. There is an increasing need for quality population dynamic models to aid in development of integrated pest management strategies. Unfortunately, there exists high variability in published data regarding basic life history traits that frequently direct model parameterization. Metadata were analyzed to develop relationships between temperature and reproductive and developmental traits of D. noxia. Specifically, functions were developed between temperature and the following traits: lifespan, fecundity, fecundity rate, pre-nymphipositional period, reproductive period, and intrinsic rate of increase. Lower and upper temperature reproductive thresholds were calculated as 0.6 and 36.9°C, respectively. The lower temperature developmental threshold was calculated as -0.69°C. Modeled longevity reached its maximum at ≈80 d. Meta-analysis indicates maximum fecundity at ≈18.5°C, with a maximum fecundity rate of ≈2.1 nymphs per day over the nymphipositional period. The calculated maximum total fecundity was ≈55 nymphs per female. The maximum reproductive period was calculated to be 29.9 d. Compared with other aphid species, as temperature increased, the intrinsic rate of increase of D. noxia increased more slowly relative to Schizaphis graminum (Rondani) and Bhopalosiphum padi L., but at a similar rate to Sitobian avenae (F.).

Reproductive Rates of Russian Wheat Aphid (Hemiptera: Aphididae) Biotypes 1 and 2 on a Susceptible and a Resistant Wheat at Three Temperature Regimes

Abstract The reproductive rates of Russian wheat aphid, Diuraphis noxia (Kurdjumov) (Hemiptera: Aphididae), Biotype 1 (RWA 1) and Biotype 2 (RWA 2) were compared in the laboratory at three temperature regimes on a Russian wheat aphid resistant cultivar (‘Prairie Red’) and a susceptible cultivar (‘TAM 107′). The objective of this study was to expose RWA 1 and RWA 2 to three temperature regimes and two levels of resistance to find whether there were reproductive differences that may occur within each biotype as well as differences in reproduction between biotypes. In addition, temperature effects of the Dn4 gene on biotype reproduction were noted. Differences in reproductive rates between the two biotypes seem to be driven by temperature. For both biotypes, longevity and reproductive rate parameters, except for intrinsic rate of increase, were lower at the 24–29°C temperature regime than the 13–18°C and 18–24°C temperature regimes. The intrinsic rate of increase was higher for both biotypes at the 18–24°C and 24–29°C temperature regimes than at the 13–18°C temperature regime. Reproductive rates between biotypes were similar at the two higher temperature regimes, but the fecundity for RWA 1 was less than RWA 2 at the 13–18°C temperature. The change in fecundity rates between RWA 1 and RWA 2 at lower temperatures could have ecological and geographical implications for RWA 2.

Reproduction and Development of Russian Wheat Aphid Biotype 2 on Crested Wheatgrass, Intermediate Wheatgrass, and Susceptible and Resistant Wheat

Abstract The Russian wheat aphid, Diuraphis noxia (Kurdjumov), is an economically important pest of small grains. Since its introduction into North America in 2003, Russian wheat aphid Biotype 2 has been found to be virulent to all commercially available winter wheat, Triticum aestivum L., cultivars. Our goal was to examine differences in Russian wheat aphid reproduction and development on a variety of plant hosts to gain information about 1) potential alternate host refuges, 2) selective host pressures on Russian wheat aphid genetic variation, and 3) general population dynamics of Russian wheat aphid Biotype 2. We studied host quality of two wheatgrasses (crested wheatgrass, Agropyron cristatum [L.] Gaertn., and intermediate wheatgrass, Agropyron intermedium [Host] Beauvoir) and two types of winter wheat (T. aestivum, one Biotype 2 susceptible wheat, ‘Custer’ and one biotype 2 resistant wheat, STARS02RWA2414-11). The susceptible wheat had the highest intrinsic rate of increase, greatest longevity and greatest fecundity of the four host studied. Crested wheatgrass and the resistant wheat showed similar growth rates. Intermediate wheatgrass had the lowest intrinsic rate of increase and lowest fecundity of all tested hosts.

Quantifying Russian Wheat Aphid Pest Intensity Across the Great Plains

ABSTRACT n Wheat, the most important cereal crop in the Northern Hemisphere, is at-risk for an approximate 10% reduction in worldwide production because of animal pests. The potential economic impact of cereal crop pests has resulted in substantial research efforts into the understanding of pest agroecosystems and development of pest management strategy. Management strategy is informed frequently by models that describe the population dynamics of important crop pests and because of the economic impact of these pests, many models have been developed. Yet, limited effort has ensued to compare and contrast models for their strategic applicability and quality. One of the most damaging pests of wheat in North America is the Russian wheat aphid, Diuraphis noxia (Kurdjumov). Eighteen D. noxia population dynamic models were developed from the literature to describe pest intensity. The strongest models quantified the negative effects of fall and spring precipitation on aphid intensity, and the positive effects associated with alternate food source availability. Population dynamic models were transformed into spatially explicit models and combined to form a spatially explicit, model averaged result. Our findings were used to delineate pest intensity on winter wheat across much of the Great Plains and will help improve D. noxia management strategy.

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

ABSTRACT n 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.

The distribution of European corn borer (Lepidoptera: Crambidae) moths in pivot-irrigated corn.

ABSTRACT n The European corn borer, Ostrinia nubilalis (Hübner), is a damaging pest of numerous crops including corn, potato, and cotton. An understanding of the interaction between O. nubilalis and its spatial environment may aid in developing pest management strategy. Over a 2-yr period, ≈8,000 pheromone trap catches of O. nubilalis were recorded on pivot-irrigated corn in northeastern Colorado. The highest weekly moth capture per pivot-irrigated field occurred on the week of 15 July 1997 at 1,803 moths captured. The lowest peak moth capture per pivot-irrigated field was recorded on the week of 4 June 1998 at 220 moths captured. Average trap catch per field ranged from ≈1.6 moths captured per trap per week in 1997 to ≈0.3 moths captured per trap per week in 1998. Using pheromone trap moth capture data, we developed a quantified understanding of the spatial distribution of adult male moths. Our findings suggest strong correlations between moth density and adjacent corn crops, prevailing wind direction, and an edge effect. In addition, directional component effects suggest that more moths were attracted to the southwestern portion of the crop, which has the greatest insolation potential. In addition to the tested predictor variables, we found a strong spatial autocorrelation signal indicating positive aggregations of these moths and that males from both inside and outside of the field are being attracted to within-field pheromone traps, which has implications for refuge strategy management.

Russian wheat aphid (Hemiptera: Aphididae) reproduction and development on five noncultivated grass hosts

The Russian wheat aphid, Diuraphis noxia (Kurdjumov), is a small grains pest of worldwide economic importance. The Russian wheat aphid is polyphagous and may encounter differential selective pressures from noncultivated grass hosts. Aphid biotypic diversity can disrupt the progress of plant breeding programs, leading to a decreased ability to manage this pest. The goal of this research was to quantify Russian wheat aphid biotype 2 (RWA2) reproductive and development rates on five common noncultivated grass hosts to gain information about host quality, potential refuges, and sources of selection pressure. First, RWA2 reproduction was compared on crested wheatgrass (Agropyron cristatum, (L.) Gaertn.), intermediate wheatgrass (Elytrigia intermedia, (Host) Nevski), slender wheatgrass (Elymus trachycaulus, (Link) Gould ex Shinners), western wheatgrass (Pascopyrum smithi, (Rydb.) A. Löve), and foxtail barley (Hordeum jubatum, (L.) Tesky) at 18–24°C. Second, RWA2 reproduction was compared on intermediate and crested wheatgrass at three temperature regimes 13–18°C, 18–24°C, and 24–29°C. At moderate temperatures (18–24°C), the intrinsic rate of increase values for all five hosts ranged from 0.141 to 0.199, indicating the possibility for strong population sources on all tested hosts. Aphids feeding on crested and intermediate wheatgrass at the 13–18°C temperature had lower fecundity, less nymph production days, longer generational times, and lower intrinsic rate of increase than aphids feeding at the 18–24°C temperature regime. Aphids feeding at 24–29°C did not survive long enough to reproduce. The positive intrinsic rates of increase in Russian wheat aphid on the wheatgrasses suggest that these grasses can support aphid populations at moderate to low temperatures.

Spatial Variability of Western Bean Cutworm (Lepidoptera: Noctuidae) Pheromone Trap Captures in Sprinkler Irrigated Corn in Eastern Colorado

ABSTRACT n Strategies for controlling pests are an integral part of any agricultural management plan. Most field crops, such as wheat (Triticum spp.) and corn (Zea mays L.) are managed as if they are homogeneous units. However, pests within fields are rarely homogenous. Development of plans that use targeted pest control tactics requires knowledge of the ecological drivers of the pest species. That is, by understanding the spatio-temporal factors influencing pest populations, we can develop management strategy to prevent or reduce pest damage. This study was conducted to quantify variables influencing the spatial variability of adult male western bean cutworm, Striacosta albicosta (Smith). Striacosta albicosta moths were collected in pheromone traps in two center pivot, irrigated corn fields near Wiggins, CO. We hypothesized that moth abundance would be influenced by the distance from the edge of the field, distance to nearest alternative corn crop and affected by anisotropic effects, such as prevailing wind direction. Greater trap catches of S. albicosta in each of the fields were found with increased proximity to the edge of the field, if the nearest neighboring crop was corn. Prevailing wind direction and directional effects were found to influence abundance. Results serve as a first step toward building a precision pest management system for controlling S. albicosta.

Nonlinear Degree-Day Models for Postdiapause Development of the Sunflower Stem Weevil (Coleoptera: Curculionidae)

ABSTRACT The sunflower stem weevil, Cylindrocopturus adspersus (LeConte) (Coleoptera: Curculionidae), has caused yield losses across much of the western Great Plains, Little is known about the field biology of this pest. Simple prediction models, such as degree-day models, are an integral tool for development of C. adspersus management strategies. Using data collected in Colorado, Kansas, and Nebraska, we sought for predictable variation between C. adspersus pupation, adult eclosion, and emergence and accumulated degree-days Celsius (DD) by using a temperature threshold of 5°C. Accurate phenological models can be used to time scouting efforts and pesticide applications. The relationship between phenological data and accumulated DD fit nonlinear, Gaussian distributions better than uniform distributions. Phenological models were developed to describe these distributions for pupation, adult presence within the stalk and adult emergence, The pupation model predicts 50% pupation at 197 DD and 90% at 307 DD. Model results predict that 50% of adult eclosion within the stalks will have transpired at 396 DD and 90% at 529 DD. A model-averaged result from two data sets predicts 5% adult emergence from stalks at 262 DD, 50% emergence at 540 DD, 75% emergence at 657 DD, and 90% at 777 DD. Scouting for adults thus can be initiated at 262 DD. Current chemical controls target adults to prevent oviposition. Thus, applications therefore should not be made before this point.