Norman C. Elliott

Effects of an invading coccinellid on native coccinellids in an agricultural landscape

Seven native coccinellid species inhabited alfalfa, corn, and small grain fields in eastern South Dakota prior to invasion and establishment of Coccinella septempunctata L. Six species occurred in all crops, however, Adalia bipunctata (L.) occurred only in corn. The structure of native coccinellid communities differed significantly for years prior to compared with years after establishment of C. septempunctata in fields of the three agricultural crops. Differences in community structure were accounted for mainly by reduced abundance of two species, C. transversoguttata richardsoni Brown and Adalia bipunctata (L.). Annual abundance of C. transversoguttata richardsoni averaged 20–32 times lower during post-invasion years than in years prior to invasion, depending on crop; while annual abundance of A. bipunctata averaged 20 times lower in corn after invasion. Addition of C. septempunctata to the community did not result in a significant increase in total abundance of coccinellids in the crops. Coccinellid abundance in agricultural crops may be limited by the total abundance of prey or by the availability of other requisites in the landscape as a whole. Therefore, introduction of a new species, while resulting in reductions in native species populations, may not increase total coccinellid abundance, and may therefore have no net effect on biological control of aphid pests.

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.

Functional Responses of an Introduced Parasitoid and an Indigenous Parasitoid on Greenbug at Four Temperatures

Abstract Functional responses and superparasitism by the indigenous parasitoid wasp Lysiphlebus testaceipes Cresson (Hymenoptera: Aphidiidae) and the introduced parasitoid Aphidius colemani Viereck (Hymenoptera: Aphidiidae) on the greenbug, Schizaphis graminum Rondani (Homoptera: Aphididae), were measured at four temperatures (14, 18, 22, and 26°C) during a 24-h period (12:12 L:D). At each temperature, 5–75 greenbugs were exposed to individual wasp mating pairs for 24 h. At all experimental temperatures, functional responses for both wasps most closely fit the type III model. Instantaneous attack rates (a) for A. colemani were not significantly different among experimental temperatures. However, for L. testaceipes, the estimate for a at 14°C was significantly lower than estimates at 22 and 26°C when data were fit to a type II functional response model. When data were fit to a type III functional response model for L. testaceipes, the estimate for a at 14°C was significantly lower than estimates at 18, 22, and 26°C. Superparasitism for both wasps was often less than expected if superparasitism were a random occurrence, suggesting that these parasitoids may be able to sense when the host has been previously parasitized. A. colemani achieved higher parasitism rates than L. testaceipes at lower temperatures. This observations suggests that A. colemani may be an effective addition to the parasitoid guild for biological control of the greenbug during cooler periods in the Southern Great Plains.

Effect of aphids and the surrounding landscape on the abundance of Coccinellidae in cornfields

Abstract We determined how the composition and patchiness of eastern South Dakota agricultural landscapes surrounding cornfields and the abundance of aphids in cornfields affected the abundance of adult coccinellids in cornfields. Four species of Coccinellidae were common in cornfields during the three-year study. Five variables describing the composition and patchiness of the landscape surrounding cornfields (% of pasture/grassland, % of wooded land, % of Conservation Reserve Program land, % of wetland, and boundary density) and abundance of aphids in the cornfields were used as predictor variables to construct stepwise multiple regression models to describe the abundance of each species of coccinellid in the cornfields. Regression models explained for 8–45% of the variation in abundance of adult Coccinellids depending on species. Abundance of aphids entered into regression models for three of the four species. The regression coefficient for aphid abundance was positive in models for two species, but was negative in the model for C. maculata . Each of the five landscape variables was included in a regression model for one or more coccinellid species. Results are discussed in relation to the biology and ecology of the species of Coccinellidae that inhabit cornfields in eastern South Dakota.

Coccinellid Introductions: Potential for and Evaluation of Nontarget Effects

Aphid-feeding Coccinellidae, subfamily Coccinellinae, are found in many terrestrial habitats and have been used extensively for biological control of aphid pest species (Hagen 1962, 1974; Frazer 1988; Hodek and Honek 1996; Obrycki and Kring 1998). Most if not all aphidophagous Coccinellidae feed on several prey items (Hodek 1993, Hodek and Honek 1996); the question is what level of prey specificity is required for the safe introduction of these predators? The selection of coccinellid species for importation is typically based on field observations of predation on the target pest and/or consistent associations of the coccinellid and prey species in their native range. While these observations are a logical first step, observed predation does not necessarily mean that the prey is preferred or suitable for development and reproduction (Strand and Obrycki 1996, Obrycki et al. 1997a).

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.

Response by coccinellids to spatial variation in cereal aphid density

Abstract The objectives of this study were to determine if coccinellids adjusted their distribution within spring wheat fields in response to spatial variation cereal aphid density in the fields and to describe the patterns of cereal aphid population growth that resulted. Field experiments were completed in which the physical dimensions of patches infested with cereal aphids, cereal aphid density, and access to patches by coccinellids were varied. Aphid infestations consisted of naturally occurring densities (natural patches) and much greater densities created by supplementing patches with aphids (supplemented patches). Coccinellids were denied access to some supplemented patches (exclusion patches) but allowed unlimited access to others. Densities of adult Hippodamia convergens and Coccinella septempunctata were correlated with aphid density in patches whereas density of Coleomegilla maculata was not. Aggregation by coccinellids was independent of patch area. The realized aphid population growth rate (r) was lower in supplemented than natural patches in all four trials but was significantly lower in only one trial. The lower r in supplemented patches was not exclusively caused by coccinellid predation, and emigration of aphids from patches probably also contributed. r was significantly greater in exclusion patches than supplemented and natural patches, indicating that coccinellids markedly reduced aphid numbers in patches even when aphid density was extremely high.

Effects of Riparian and Grassland Habitats on Ground Beetle (Coleoptera: Carabidae) Assemblages in Adjacent Wheat Fields

Abstract Natural habitats surrounding agricultural fields provide a source of natural enemies to assist in pest control. The boundaries among landscape elements filter some organisms attempting to cross them, resulting in differing communities within the landscape elements. Ground beetles are numerous and generally disperse by walking. These qualities make them excellent organisms for the study of boundary dynamics. Our goal was to determine if natural habitats adjacent to wheat fields affected the species composition of ground beetles within the wheat fields. We captured ground beetles from autumn through spring 1996–1997 at two sites using directional pitfall traps placed in wheat fields and adjacent grasslands and riparian zones. Ground beetle abundance reached two peaks, one in autumn and the other in spring. Species composition was most strongly related to these seasons. Axis 1 of a canonical correspondence analysis separated spring active beetles from autumn active beetles. Axis 2 separated winter active beetles. With the effects of season and sites removed, axes 1 and 2 of a partial canonical correspondence analysis separated beetles with respect to habitat. Axis 1 separated beetles into wheat and natural habitat assemblages. Axis 2 further distinguished assemblages in wheat fields as those adjacent to grasslands and those adjacent to riparian habitats. Axis 2 also separated grassland, grassland edge, and riparian edge assemblages from riparian assemblages. Net dispersal of beetles across the boundaries showed no consistent pattern during autumn, winter, or spring. However, mark–recapture studies showed that several species routinely cross boundaries, which resulted in different community structures and an increase in abundance of beetles in the wheat interiors during spring.

Development and Validation of a Binomial Sequential Sampling Plan for the Greenbug (Homoptera: Aphididae) Infesting Winter Wheat in the Southern Plains

Abstract From 1997 to 1999, Schizaphis graminum (Rondani), intensity (number per tiller) was estimated on 115 occasions from hard red winter wheat fields located throughout the major wheat growing regions of Oklahoma. A total of 32 and 83 fields was sampled during the fall and spring, respectively. The parameters of linear regressions relating the mean number of greenbugs per tiller (m) and the proportion of infested tillers (PT) differed significantly between fall and spring infestations. The PT - m linear model provided a good fit for data on S. graminum for fall and spring infestations at tally thresholds of 0, 1, 2, and 3. A tally threshold (T) represents the number of greenbugs present on a tiller before the tiller is classified as infested by >T greenbugs. A regression model with a tally threshold of 2 was the most precise for classifying S. graminum populations during fall growth of winter wheat because it explained a greater amount of the variation in the PT - m relationship (97%) than models with other tally thresholds. A separate spring model with a tally threshold of 1 was the most precise for classifying S. graminum populations during spring growth of winter wheat. Sequential sampling stop lines based on sequential probability ratio tests were calculated for economic thresholds of 3 or 6 greenbugs per tiller for fall infestations and 6 or 9 greenbugs per tiller for spring infestations. With the newly developed parameters, the average sample number required to classify greenbug populations near economic thresholds (as above or below the economic threshold) varied from 69 to 207. We expect that the sampling plans for greenbugs in winter wheat developed during this study will be efficient and useful tools for consultants and producers in the southern plains.

Influence of adult diet on the reproductive biology and survival of the western corn rootworm, Diabrotica virgifera virgifera

Adult corn rootworm beetles, Diabrotica virgifera virgifera LeConte, were maintained on three dietary regimes which mimicked the progression of corn tissues that would be available in the field for beetles eclosing when corn was in the following growth stages: (A) after tassels had emerged but prior to silking and pollen shed; (B) while plants were silking and shedding pollen; and (C) after pollination was complete and silks had turned brown. A fourth regime (D) was established in which green corn silks, pollen, and leaves were provided throughout the study. The mean number of eggs laid per female over the 12‐week duration of the study was 125, 235, 179, and 441 for regimes A, B, C, and D, respectively. Median length of life was 7.2, 7.2, 6.7, and 8.8 weeks for regimes A, B, C, and D respectively. Beetles in regime A laid a greater proportion of their eggs at an older age than did beetles from regimes B and C. To further investigate the influences on survival of changes in food quality of corn as plants and beetles aged, newly‐eclosed beetles and beetles that had been maintained in caged plots of corn growing in a greenhouse for various lengths of time were caged on corn at different stages of growth, and the proportion of beetles surviving for 48 h was determined. Survival decreased as plants aged for both groups of beetles, but decreased at a faster rate for old than for young beetles.