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

440 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 Cana- dian Prairies and the northern Great Plains of the United States. We characterized the spatial dis- tribution 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 in- fested 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 aggrega- tion 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 ini- tially laid along field edges, and ovipositing C. cinctus females do not distinguish between previ- ously 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 lar- vae. 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. Resume—Le cephe du ble, Cephus cinctus Norton (Hymenoptera : Cephidae), est le ravageur le plus important du ble (Triticum aestivum L.; Poaceae) de zone aride dans le sud des Prairies ca- nadiennes et le nord des Grandes plaines des Etats-Unis. Nous avons determine les patterns de repartition spatiale des oeufs et des larves de C. cinctus durant toute la periode de vol des adultes dans trois plantations de ble de zone aride au Montana. Les femelles de C. cinctus pondent leurs oeufs dans les tiges de ble jusqu'a quelques semaines avant la maturite du ble et la plupart des ti- ges ne recoivent qu'un seul oeuf. Les echantillons de tiges de ble recoltes sur le pourtour des champs representent 1 2%-1 6 %d esechantillons, mais ils contiennent 4 0%-9 5 %d es tiges infestees. Les nombres maximaux d'oeufs et de larves de C. cinctus observes dans une meme tige de ble sont respectivement 16 et de 4. A l'echelle du champ, tant les oeufs que les larves de C. cinctus ont une repartition contagieuse significative pres des lisieres dans la plupart des echan- tillonnages. Une comparaison des indices de contagion montre que les larves de C. cinctus ont une repartition spatiale significativement plus contagieuse que les oeufs. D'apres notre analyse spatio-temporelle, nous posons en hypothese que la majorite des oeufs sont pondus au depart le long des lisieres des champs et que les femelles pondeuses de C. cinctus ne font pas de distinc- tion entre les tiges de ble deja infestees et les tiges non infestees. Les oeufs eclosent graduelle- ment et les larves commencent a se nourrir, alors qu'au meme moment, les femelles de C. cinctus a emergence plus tardive pondent plus vers l'interieur des champs de ble. Une explication du phenomene serait que les femelles pondeuses de C. cinctus evitent les tiges qui contiennent deja une larve en train de se nourrir. Nous discutons de l'analyse des changements temporels des pat- terns de repartition des oeufs et des larves de C. cinctus dans le contexte de strategies possibles de lutte integree. (Traduit par la Redaction)