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Within-field spatial distribution of Cephus cinctus (Hymenoptera: Cephidae) larvae in Montana wheat fields

Published online by Cambridge University Press:  02 April 2012

Christian Nansen ,
David K. Weaver ,
Sharlene E. Sing ,
Justin B. Runyon ,
Wendell L. Morrill ,
Matthew J. Grieshop ,
Courtney L. Shannon  and
Megan L. Johnson
Christian Nansen*
Affiliation:
Montana State University, Department of Entomology, 333 Leon Johnson Hall, P.O. Box 173020, Bozeman, Montana 59717, United States of America
David K. Weaver
Affiliation:
Montana State University, Department of Entomology, 333 Leon Johnson Hall, P.O. Box 173020, Bozeman, Montana 59717, United States of America
Sharlene E. Sing
Affiliation:
Montana State University, Department of Entomology, 333 Leon Johnson Hall, P.O. Box 173020, Bozeman, Montana 59717, United States of America
Justin B. Runyon
Affiliation:
Montana State University, Department of Entomology, 333 Leon Johnson Hall, P.O. Box 173020, Bozeman, Montana 59717, United States of America
Wendell L. Morrill
Affiliation:
Montana State University, Department of Entomology, 333 Leon Johnson Hall, P.O. Box 173020, Bozeman, Montana 59717, United States of America
Matthew J. Grieshop
Affiliation:
Montana State University, Department of Entomology, 333 Leon Johnson Hall, P.O. Box 173020, Bozeman, Montana 59717, United States of America
Courtney L. Shannon
Affiliation:
Montana State University, Department of Entomology, 333 Leon Johnson Hall, P.O. Box 173020, Bozeman, Montana 59717, United States of America
Megan L. Johnson
Affiliation:
Montana State University, Department of Entomology, 333 Leon Johnson Hall, P.O. Box 173020, Bozeman, Montana 59717, United States of America
*
1Corresponding author (e-mail: christian.nansen@steritech.com).
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Abstract

The wheat stem sawfly, Cephus cinctus Norton, is a major insect pest in dryland wheat (Triticum L. spp.; Poaceae) fields in the northern Great Plains of the United States and in southern regions of the prairie provinces of Canada. Field infestations by this pest commonly show a distinct “edge effect”, with infestation levels highest at the field edge and declining inwards. In this study, we characterized the edge effect of C. cinctus infested wheat fields in Montana at four locations in two separate years. "Infestation level" refers to the average proportion of wheat stems infested with C. cinctus in samples collected in a given wheat field. The gradual decrease in C. cinctus infestation towards the centre of wheat fields is referred to as the “infestation slope”, and it was presented as a proportion of infestation level at field edges. We showed that (i) for most of the fields bordering grassland or fallow wheat, a simple exponential decline curve provided a significant fit to the infestation slope; (ii) the infestation slopes in different fields and in different directions within fields were similar and appeared to be independent of the overall infestation level; and (iii) the relationship between infestation level and the proportion of samples infested followed an asymptotic curve, which indicates a high level of dispersal of ovipositing C. cinctus females. The general perception has been that the edge effect of C. cinctus infestation is a result of ovipositing C. cinctus females being both weak flyers and short-lived, but our results do not fully corroborate this perception. Currently, the only reliable way to detect C. cinctus infestations is to dissect individual stems and look for developing instars. However, this type of sampling is extremely time consuming and labor intensive and therefore impractical for wheat growers. Increasing the understanding of the spatial patterns in C. cinctus infestations is a first step towards development of an optimal sampling plan for this important field pest. Thus, the results presented are relevant for researchers involved in sampling designs and integrated pest management procedures for field pests. We discuss behavioral and evolutionary factors that may contribute to the edge effect of C. cinctus infestations.

Résumé

Le cèphe du blé, Cephus cinctus Norton, est un insecte ravageur important du blé (Triticum L. spp., Poaceae) dans les champs arides des Grandes plaines du nord des États-Unis et des régions du sud des provinces canadiennes des prairies. Les infestations des champs par ce ravageur montrent souvent un « effet de lisière », les infestations étant plus fortes en bordure des champs et moindres vers le centre. Notre étude caractérise cet effet de lisière dans des champs de blé infestés par C. cinctus à quatre sites du Montana sur deux années différentes. Le « niveau d'infestation » représente la proportion moyenne des tiges de blé infestées par C. cinctus dans les échantillons prélevés dans un champ de blé donné. Le déclin graduel de l'infestation par C. cinctus vers le centre du champ est la « pente de l'infestation » et est représenté par le pourcentage du niveau de l'infestation par comparaison à celui en bordure du champ. Nous montrons que (i) dans la plupart des champs adjacents à des prairies ou des champs de blé en jachère, une simple courbe exponentielle décroissante s'ajuste de façon significative à la pente de l'infestation, (ii) les pentes d'infestation dans les différents champs et dans les différentes directions dans un même champ sont semblables et semblent être indépendantes du niveau global d'infestation et (iii) la relation entre le niveau d'infestation et le pourcentage d'échantillons infestés suit une courbe asymptotique, ce qui indique un fort niveau de dispersion des femelles de C. cinctus lors de la ponte. On explique généralement l'effet de lisière dans les infestations de C. cinctus par le fait que les femelles pondeuses ont un vol faible et qu'elles ne vivent pas longtemps, mais nos résultats n'appuient pas complètement ces explications. Actuellement, la seule méthode fiable pour détecter les infestations de C. cinctus est la dissection de tiges individuelles à la recherche de larves de divers stades en développement. Ce type d'échantillonnage exige beaucoup de temps et coûte très cher; il est donc peu pratique pour les cultivateurs de blé. La compréhension accrue de la structure spatiale des infestions de C. cinctus est une première étape dans la mise au point d'un plan d'échantillonnage optimal pour cet important ravageur des champs. Nos résultats sont donc d'intérêt pour les chercheurs impliqués dans la planification d'échantillonnage et les méthodologies de lutte intégrée contre les ravageurs des champs. Les facteurs comportementaux et évolutifs qui peuvent contribuer à l'effet de lisière des infestations de C. cinctus font l'objet d'une discussion.

[Traduit par la Rédaction]

Type
Articles
Information
The Canadian Entomologist , Volume 137 , Issue 2 , April 2005 , pp. 202 - 214
Copyright
Copyright © Entomological Society of Canada 2005

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