Thomas O. Holtzer

Habitat and Season in Structuring Ground-Dwelling Spider (Araneae) Communities in a Shortgrass Steppe Ecosystem

Abstract We assessed habitat differences in ground-dwelling spider communities that stem from fine-scale differences in plant cover types and height in a shortgrass steppe ecosystem. The number of spiders captured in pitfall traps was used to determine habitat and temporal dynamics in the spider communities. Two habitat types were sampled, grass and mixed-grass/shrub during May, June, July, and August 1995 and 1996. Spiders in the Lycosidae (56%) and Gnaphosidae (29%) were the numerically dominant spiders collected. The Lycosidae was represented by three species and significantly more abundant in grass habitats in 1995 only. The Gnaphosidae was the most speciose family collected (15 spp). The most abundant species of Gnaphosidae exhibited consistent habitat and seasonal patterns of species turnover and abundance during the 2-yr study. Results from detrended correspondence analysis revealed significant species-specific differences in the spider communities between the two habitats. Of the 10 most abundant species, Schizocosa mccooki (Montgomery), S. mimula (Gertsch), Gnaphosa clara (Keyserling), and G. brumalis (Thorell), were more abundant in grass sites. The most abundant spiders in the mixed-grass/shrub habitat were Schizocosa spp. (immatures), Gnaphosa spp. (immatures ), G. sericata (Koch), Haplodrassus signifer (Koch), Habronattus altanus (Chamberlin), and Xysticus nigromaculatus (Keyserling). Overall, the spider communities differed between grass and mixed-grass/shrub habitats in their response to seasonal change, and these differences in community composition were similar both years of the study. Our results underscore the importance of habitat selection at fine-scales and patterns of seasonal activity that translate to population and community patterns at larger spatial and temporal scales.

Abundance and effects of predators and parasitoids on the Russian wheat aphid (Homoptera: Aphididae) under organic farming conditions in Colorado.

Abstract Diuraphis noxia (Mordvilko) is an exotic, economically important pest in North American cereal crops. A survey of insect natural enemies of D. noxia was made from 1991 to 1994 on a farm using organic farming methods. Crested wheatgrass, Agropyron cristatum L., had fewer D. noxia and natural enemies than barley, Hordeum vulgare L., or wheat, Triticum aestivum L., but on all plants D. noxia was the most abundant aphid. We observed 41 species of natural enemies: 15 carabids, 12 coccinellids, six spiders, five syrphids, two nabids, and two chrysopids. The most consistently abundant were the coccinellids and nabids. Hippodamia convergens (Guerin) and Nabis alternatus Parsh were the most common species in each family, respectively. Diaeretiella rapae M’Intosh was the only primary parasitoid found in the 4-yr study, and D. noxia parasitism rates were generally <5%. Four hyperparasitoids, found in association with D. rapae, combined to make a 29% average hyperparasitism rate. Predator and parasitoid exclusion studies using cages showed aphid populations to be between 2.6 and 11.2 times higher in cages compared with wheat plants exposed to natural enemies. Four predatory species were released—Eupeodes nuda (F.), Hippodamia variegata (Goeze), Leucopis ninae Tanasijtshuk, and Propylea quatuordecimpunctata (L.). Of these, only a small colony of eight L. ninae and only one P. quatuordecimpunctata were later observed. Four parasitoids species were released—Aphelinus asychis Walker, Aphelinus varipes Forester, Aphelinus matricariae Haliday, and Diaerietilla rapae. Only A. asychis and D. rapae were observed after the release date, although D. rapae were already present.

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.

Patch and prey utilization behaviors by Aphelinus albipodus and Diaeretiella rapae (Hymenoptera: Aphelinidae and Aphidiidae) on Russian wheat aphid (Homoptera: Aphididae)

Abstract The patch and host utilization behaviors of the parasitoids Diaeretiella rapae (Kurdjumov) and Aphelinus albipodus (Hayat and Fatima) in relation to the Russian wheat aphid, Diuraphis noxia (Mordvilko), were examined on wheat plants. Individual female parasitoids were observed for 1 h after arriving on a wheat plant with varying D. noxia densities. The total amount of time spent on the leaf and the number of occasions a parasitoid left the leaf were dependent on aphid densities for D. rapae . In contrast, A. albipodus remained on a plant for almost the entire hour irrespective of aphid densities. After 1 h on the wheat plants, single D. rapae females produced up to 31 progeny from 40 aphids, while single A. albipodus produced a maximum of six progeny. These results can be explained by the differences between the parasitoids in prey handling times, searching, and host feeding behaviors, and parasitoid physiology. The mean oviposition time for A. albipodus was 119 s compared to 1 or 2 s for D. rapae . The time between attacks was also much greater for A. albipodus at all prey densities, partially a result of this parasitoid feeding on aphids. Our results agree with the observation that A. albipodus has many fewer eggs ready for oviposition compared to D. rapae . Therefore, it is not surprising that A. albipodus remained for extended periods on wheat plants with many aphids, allowing development of additional eggs and parasitization of more aphids. The patch and prey utilization rates by A. albipodus seem likely to limit its rate of spatial spread during a growing season relative to D. rapae .

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.).

Validating spatiotemporal predictions of an important pest of small grains

BACKGROUND Arthropod pests are typically managed using tactics applied uniformly to the whole field. Precision pest management applies tactics under the assumption that within-field pest pressure differences exist. This approach allows for more precise and judicious use of scouting resources and management tactics. For example, a portion of a field delineated as attractive to pests may be selected to receive extra monitoring attention. Likely because of the high variability in pest dynamics, little attention has been given to developing precision pest prediction models. Here, multimodel synthesis was used to develop a spatiotemporal model predicting the density of a key pest of wheat, the Russian wheat aphid, Diuraphis noxia (Kurdjumov). RESULTS Spatially implicit and spatially explicit models were synthesized to generate spatiotemporal pest pressure predictions. Cross-validation and field validation were used to confirm model efficacy. A strong within-field signal depicting aphid density was confirmed with low prediction errors. CONCLUSION Results show that the within-field model predictions will provide higher-quality information than would be provided by traditional field scouting. With improvements to the broad-scale model component, the model synthesis approach and resulting tool could improve pest management strategy and provide a template for the development of spatially explicit pest pressure models.