Winter wheat fields are typically interspersed in a mosaic of habitats in other uses, and we hypothesized that the spatial and temporal composition and configuration of landscape elements, which contribute to agroecosystem diversity also influence biological control of common aphid pests. The parasitoid Lysiphlebus testaceipes Cresson; Hymenoptera: Aphidiinae is highly effective at reducing aphid populations in wheat in Oklahoma, and though a great deal is known about the biology and ecology of L. Our objective was to determine the influence of landscape structure on parasitism of cereal aphids by L. Seventy fields were studied during the three growing seasons. Significant correlations between parasitism by L. Correlations between parasitism and landscape variables were generally greatest at a 3.
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Winter wheat fields are typically interspersed in a mosaic of habitats in other uses, and we hypothesized that the spatial and temporal composition and configuration of landscape elements, which contribute to agroecosystem diversity also influence biological control of common aphid pests. The parasitoid Lysiphlebus testaceipes Cresson; Hymenoptera: Aphidiinae is highly effective at reducing aphid populations in wheat in Oklahoma, and though a great deal is known about the biology and ecology of L.
Our objective was to determine the influence of landscape structure on parasitism of cereal aphids by L. Seventy fields were studied during the three growing seasons. Significant correlations between parasitism by L.
Correlations between parasitism and landscape variables were generally greatest at a 3. Correlations between parasitism and landscape variables that would be expected to increase with increasing landscape diversity were usually positive. Subsequent regression models for L. Overall, results indicate that L. Colonization of wheat fields by L. Winter wheat in Oklahoma is planted during September and October, grows from fall through spring, and is harvested in June.
Several aphid species infest wheat fields in this region, the most common and important being greenbug, Schizaphis graminum Rondani ; bird cherry-oat aphid, Rhopalosiphum padi L. Heteroptera: Aphididae. Apart from the use of insecticidal seed treatments, which are used preventively in Oklahoma by an increasing number of producers, insecticide applications are infrequent, and primarily used to control insects such as fall armyworm in autumn, and cereal aphids, primarily greenbug, in autumn or spring Royer et al.
During the wheat growing season, winter wheat fields are typically interspersed in a mosaic of lands in other uses. Grasslands pasture and rangeland with varying levels of management are the most abundant land use type. Grasslands range from semi-natural lands that have never been cultivated and have high plant species diversity to highly managed lands planted to a single grass species. Fallow fields will mostly be planted to summer crops soybean, corn, sorghum, and cotton in spring, and fields may also be planted to other winter crops, such as canola and barley.
Riparian areas and other semi-natural lands are also present in the landscape. Based on previous studies that investigated pest suppression in agricultural landscapes, the spatial and temporal configuration of landscape elements, which contribute to agroecosystem diversity are likely to influence populations of natural enemies, and possibly, biological control of insect pests by determining the availability of resources for beneficial insects Rusch et al.
In Oklahoma the parasitoid Lysiphlebus testaceipes Cresson; Hymenoptera: Aphidiinae is highly effective at reducing aphid populations in winter wheat Webster and Phillips , Giles et al. Even though a great deal is known about the biology and ecology of the species, there are gaps in knowledge that limit understanding of the L. The structure of the landscape surrounding a particular agricultural field has been shown to influence populations of predatory insects in wheat fields Elliott et al.
However, only one study has specifically addressed effects of landscape context on L. Brewer et al. Our objective was to determine the relative influence of landscape structure on parasitism of cereal aphids by L. Our hypothesis was that the landscape context within which a wheat field is embedded influences the level of parasitism in a wheat field through its presumed effect on resource availability to L.
Landscape context would be predicted to have less influence later in the growing season spring when aphid and parasitoid population dynamics within wheat fields predominates over colonization in determining abundance and parasitism. Materials and Methods Field Study We utilized aphid-infested sentinel barley plants in pots to quantify aphid parasitism within wheat fields in north central Oklahoma over a 3-yr period Fig.
Potted wheat plants grown in a greenhouse and infested with parasitoid free bird cherry-oat aphids were set out in commercial wheat fields during autumn and spring. The technique has been successfully used to compare relative parasitism rates within and between wheat fields Brewer et al. Each year, approximately 24 wheat fields were selected to achieve broad coverage of the area with randomization partially restricted by the availability of cooperating farmers and that study fields were separated by a minimum of 5 km.
Approximate boundaries of the geographic area where field studies were conducted during the —, —, and — wheat growing seasons. Open in new tab Download slide Approximate boundaries of the geographic area where field studies were conducted during the —, —, and — wheat growing seasons. To determine if landscape effects on parasitism existed during autumn and spring, the field study described below was repeated in autumn of three consecutive years, —, and in spring of two consecutive years, and Studies were initiated in October, soon after wheat plants emerged from the soil, and during mid-March when aphid and parasitoid activity is common Giles et al.
Approximately 10 barley seeds variety Eight Twelve were planted in a mixture of peat moss and fritted clay in Each cage was vented in the side and top with fine muslin cloth and pressed about 2.
The double caging was done to ensure that aphids on sentinel plants remained parasitoid free until they were uncaged in the field.
Two-week old sentinel plants in cages were infested with ca. Approximately 10 d later, when aphid counts in cages averaged ca. One of the 15 sentinel plants from each cage total of 12 plants was retained in the greenhouse as a check to ensure that parasitoids had not entered cages and parasitized aphids while they were being grown in the greenhouse.
In each wheat field, seven uncaged sentinel plants were placed into the soil and arranged 25 m apart in a T-shaped pattern. Sentinel plants were placed 5 m from a field edge, and every 25 m along a transect perpendicular to the field edge until five sentinel plants had been placed.
Sentinel plants were also placed 25 m to the right and left of the fourth plant at degree angles. After 7 d the barley plants from each pot were cut, separately placed in an emergence canister, and held for an additional 7 d to allow parasitoids to emerge as adults. Adult parasitoids in each emergence canister were counted and identified to species. In wheat fields in Oklahoma, L. During our study L. Thus, the number of adult L.
Check plants that remained under greenhouse conditions were processed identically to the experimental sentinel plants. No parasitoids were recovered from check plants during the 3-yr study indicating that it was very unlikely that contamination of aphid infested sentinel plants by parasitoids occurred in the greenhouse.
We recorded several attributes for each wheat field. We recorded whether insecticidal seed treatment was used on the seed planted in a field, whether the field was in a no-till or conventional till system, whether the field was in a crop rotational system or in continuous wheat, the wheat plant growth stage, and aphid abundance in the field at the time sentinel plants were deployed.
Wheat plant growth stage for each field was estimated using a 0—10 scale Zadoks et al. Sampling by D-Vac has been shown to provide useful estimates of aphid abundance in cereals Hand A sample from each field consisting of three subsamples was taken within the area where the sentinel plants were deployed by walking three equally spaced linear transects.
The first transect was situated parallel to and about 5 m to the left or right of the transect along which five of the seven sentinel plants had been stationed. The other two transects were parallel to and approximately 25 m to the right and left of the first transect.
Approximately every 5 m the D-vac collecting unit was placed straight down over growing wheat plants to just above the soil surface until 20 such placements had been made.
Each time the collecting unit was placed down it was held in position slightly above the soil surface for 5 s. After 20 stops, the sampling bag was removed from the D-vac and all arthropods in it were transferred to a labeled plastic bag.
Bags were brought to the laboratory and placed in a freezer. Aphids were counted at a later date. Frozen aphids are difficult to identify to species so we did not distinguish among cereal aphid species. Landscape Data Landscape context for each field was quantified for each of three circular areas centered on the focal wheat field with radii of 0. The Cropland Data Layer was acquired for the year in which winter wheat was present in the layer for the study fields for that year, and was used to quantify the amount and distribution of all land use types.
Distances probably encompassed the range of potential dispersal ability of L. In addition to NASS data, we collected fine spatial scale ground survey data of the grass species in the wheat field boundaries.
All grass species within one m transect at an arbitrary location within each of two accessible field edges e. One of the edges sampled was the one adjacent to where sentinel plants were deployed in the field. In addition, percent cover by Johnson grass, Sorghum halepense L. The cropland data layer was re-classified to retain eight land use categories: wheat, summer crops, winter crops other than wheat, fallow, grassland pasture and rangeland , wooded, built areas and roads, and water.
Aggregating land uses into fewer categories than represented in the original NASS data was desirable for calculating meaningful landscape metrics because many categories would have been represented by very small areas and metrics would be subject to high variability. These four metrics have straight-forward interpretations, and the latter three were found by Ritters et al. For the central Oklahoma landscapes in our study the three metrics were correlated among themselves, and were also correlated with patch density.
Patch density measures the number of patches per km2 and indicates average patch size for a landscape. The perimeter to area fractal dimension is dimensionless and increases with increasing patch boundary curvilinearity. Contagion, measures the amount of clumping of patch types within a landscape as a percentage of the maximum. Maximum contagion for a given landscape would be achieved when each landscape element type occurred as a single contiguous patch.
High contagion indicates highly aggregated and poorly interspersed patches. The number of patch types present did not vary much among landscapes in our study because most patch types were present. Measured attributes of vegetation in field edges and landscape metrics calculated using Fragstats Version 4 were used to quantify landscape structure. Fragstats derived variables were calculated at each of the three hierarchically increasing spatial extents 0.
Data Analysis Correlation was used to evaluate pairwise relationships among variables. When one or both variables were categorical, such as tillage type, which was coded numerically as zero for no-till and one for conventional tillage, spearman rank correlation coefficients were calculated.
Pearson correlation coefficients were calculated when both variables were continuous. The magnitude of correlation coefficients was used to determine the spatial extent to account for the greatest amount of variation in parasitism. Many of the landscape variables were correlated so principal components analysis was used to derive a set of linearly independent regressors for use in regression modeling.
Principal components were rotated using varimax rotation Dillon and Goldstein The number of rotated standardized principal components retained for use as independent variables in regressions was determined by the scree method.
The scree method involves plotting the eigenvalue associated with each principal component in successive order and determining the point beyond which the smaller eigenvalues form an approximately straight line.
The components retained are those associated with eigenvalues that fall above the straight line formed by the smaller eigenvalues Dillon and Goldstein The components that were retained were used as regressors in models relating parasitism to landscape context.
Only linear first order terms were included in models. Standardized components were interpreted based on magnitudes of component loadings on the original variables. Stepwise multiple regression was used to construct models using the components as regressors.
Component loadings were entered as independent variables in stepwise multiple regression models with the mean number of L. Results General Patterns Seventy-one fields were studied in autumn during the three growing seasons, however data for one field was not used because all but one sentinel plant were destroyed by wildlife.
Grozil Wasps disperse by flying, or by being carried inside winged aphids which may undergo long migration flights. Lysipylebus the target audience includes soybean farmers, but only indirectly and in the longer term. Both no-choice and choice assays were conducted. Our objective was to determine the influence of landscape structure on parasitism of cereal aphids by L. In addition, the aphid is known to transmit several viruses that are pathogens of soybeans. There are two reasons parasitoids rather than predators, the other type of natural enemy often used in biological control efforts were targeted for importation and establishment against the soybean aphid: Impacts 1 Soybean aphid populations were at historic lows in the fields we monitored, with peak numbers less than 50 per plant.
LYSIPHLEBUS TESTACEIPES PDF
Lysiphlebus testaceipes Hymenoptera: Aphidiinae Aphidiid wasps attack only aphids. The conspicuous sign of aphidiid activity is the presence of aphid "mummies" - swollen, dead aphids that have been tanned and hardened to form a protective case for the developing wasp pupa. Appearance L. However, the distinctive aphid mummies which remain on leaves after the parasitoid has killed the greenbug can easily be detected. The mummy consists of the outer skin of the greenbug which becomes modified into a tough protective shell after the developing wasp kills the greenbug by its internal feeding. Greenbugs parasitized by L.
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Survival of L. Open in new tab Discussion Honek and Kocourek showed that insect development data appeared to be correlated with average annual temperatures that occurred in the region of origin for each species. They hypothesized that species from warmer climates generally require higher developmental thresholds and fewer degree-days for development compared with insects originating in cooler climates. Yu reported that temperature and photoperiod affected diapause development among geographic isolates of Aphelinus varipes Foerster when grown on Russian wheat aphid Diuraphis noxia Mordvilko. Weber et al. Sota and Mogi observed clinal variation in adult size and expression of autogeny among strains of Aedes togoi Diptera: Culicidae , but no apparent differences in overall developmental rate.