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Spatiotemporal distributions of wheat stem sawfly eggs and larvae in dryland wheat fields

Published online by Cambridge University Press:  02 April 2012

Christian Nansen ,
Tulio B. Macedo ,
David K. Weaver  and
Robert K.D. Peterson
Christian Nansen*
Affiliation:
Montana State University, Department of Entomology, 333 Leon Johnson Hall, P.O. Box 173020, Bozeman, Montana 59717, United States of America
Tulio B. Macedo
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
Robert K.D. Peterson
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: weaver@montana.edu).
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Abstract

The wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), is the most serious insect pest of dryland wheat (Triticum aestivum L.; Poaceae) in the southern Canadian Prairies and the northern Great Plains of the United States. We characterized the spatial distribution pattern of C. cinctus eggs and larvae throughout the adult flight period in three dryland winter wheat fields in Montana. Cephus cinctus females laid eggs in wheat stems until a few weeks before wheat maturity, and most wheat stems received only one egg. Wheat stem samples collected along the field edges represented 12%–16% of the samples but 40%–95% of the infested stems. The highest numbers of C. cinctus eggs and larvae found in a single wheat stem were 16 and 4, respectively. On a field scale, both C. cinctus eggs and larvae were significantly aggregated along field edges in most of the sampling events. A comparison of indices of aggregation revealed that C. cinctus larvae were significantly more spatially aggregated than C. cinctus eggs. Based on our spatiotemporal analysis, we speculate that the majority of the eggs are initially laid along field edges, and ovipositing C. cinctus females do not distinguish between previously infested and uninfested wheat stems. Gradually, these eggs hatch and the larvae begin feeding; at the same time, the later emerging C. cinctus females oviposit farther into the wheat fields. One explanation is that ovipositing C. cinctus females avoid stems containing feeding larvae. The analysis of the temporal changes in the spatial distribution patterns of C. cinctus eggs and larvae is discussed in the context of possible integrated pest management strategies.

Résumé

Le cèphe du blé, Cephus cinctus Norton (Hymenoptera: Cephidae), est le ravageur le plus important du blé (Triticum aestivum L.; Poaceae) de zone aride dans le sud des Prairies canadiennes et le nord des Grandes plaines des États-Unis. Nous avons déterminé les patterns de répartition spatiale des oeufs et des larves de C. cinctus durant toute la période de vol des adultes dans trois plantations de blé de zone aride au Montana. Les femelles de C. cinctus pondent leurs oeufs dans les tiges de blé jusqu'à quelques semaines avant la maturité du blé et la plupart des tiges ne reçoivent qu'un seul oeuf. Les échantillons de tiges de blé récoltés sur le pourtour des champs représentent 12 % – 16 % des échantillons, mais ils contiennent 40 % – 95 % des tiges infestées. Les nombres maximaux d'oeufs et de larves de C. cinctus observés dans une même tige de blé sont respectivement 16 et de 4. À l'échelle du champ, tant les oeufs que les larves de C. cinctus ont une répartition contagieuse significative près des lisières dans la plupart des échantillonnages. Une comparaison des indices de contagion montre que les larves de C. cinctus ont une répartition spatiale significativement plus contagieuse que les oeufs. D'après notre analyse spatio-temporelle, nous posons en hypothèse que la majorité des oeufs sont pondus au départ le long des lisières des champs et que les femelles pondeuses de C. cinctus ne font pas de distinction entre les tiges de blé déjà infestées et les tiges non infestées. Les oeufs éclosent graduellement et les larves commencent à se nourrir, alors qu'au même moment, les femelles de C. cinctus à émergence plus tardive pondent plus vers l'intérieur des champs de blé. Une explication du phénomène serait que les femelles pondeuses de C. cinctus évitent les tiges qui contiennent déjà une larve en train de se nourrir. Nous discutons de l'analyse des changements temporels des patterns de répartition des oeufs et des larves de C. cinctus dans le contexte de stratégies possibles de lutte intégrée.

[Traduit par la Rédaction]

Type
Articles
Information
The Canadian Entomologist , Volume 137 , Issue 4 , August 2005 , pp. 428 - 440
Copyright
Copyright © Entomological Society of Canada 2005

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