POPULATION DYNAMICS OF THE WHEAT STEM SAWFLY, CEPHUS CINCTUS (HYMENOPTERA: CEPHIDAE), IN WHEAT

Abstract

Can. Enr. 114: 775-788 (1982) The effects of the major factors on populations of the wheat stem sawfly, Cephus cinctus Norton, were studied in the field at Lethbridge, Alberta between 1970 and1977. Fecundity of the females, which contain an average of 32.7 eggs, is influenced by the condition and variety of host plants in which they develop as larvae. Populations of the adults that emerge from infested crops cut 7-9% by the larvae in one year may infest 7@80% of the stems of adjacent crops in the following year. Increased adult densities, however, fail to increase markedly the percentage of stems' infested because the adults concentrate at field margins when they reinfest the most preferred stems. Because only one larva can mature in each stem, competition within stems that receive more than one egg causes a major drop in the population but not in the percentage of infested stems cut. Another major drop is caused by host plant resistance, which is influenced in both susceptible and resistant wheats by the weather during the growing season.On an average, all of the larvae die in 28% of the infested stems of two susceptible wheats and in 67% of those of a resistant wheat. The mortality in resistant wheats, however, is generally not sufficient to prevent economic infestations in subsequent years. Parasitism, which was a major factor in only 7 of 36 years in southern Alberta, is increased by weather conditions that cause delayed ripening of the wheat crops. The mortality of mature larvae and pupae averaged 21.3%. It is affected by extremes of moisture and temperature and can be ihcreased slightly by shallow tillage of infested stubble. ResumeEntre 1970 et 1977, on a Ctudie sur le terrain, B Lethbridge (Alberta), l'effet des facteurs principaux sur les populations de ctphes du blt (Cephus cinctus Norton). La ftconditk des femelles, qui contiennent en moyenne 32.7 oeufs, dtpend de l'ttat et de la variktk de la plante h6te dans lesquelles elles se sont dtveloppkes au stade larvaire. Les adultes qui tmergent des cultures infesttes, ayant une proportion de 7 i 9% des tiges coupkes par les larves durant l'annee, peuvent infester 70-B 80% des tiges des cultures adjacentes, l'annte suivante. Cependant, une plus grande densitk d'adultes n'a pas pour effet d'accroitre de f a~o n marquke le pourcentage de tiges infestkes, car les insectes se concentrent sur les pourtours des champs ou ils rtinfestent les tiges prtftrtes. Etant donnk qu'une seule larve peut atteindre la maturitt dans chaque tige, la concurrence B l'inttrieur des tiges qui re~oivent plus d'un oeuf entraine une chute marqute de la population, mais non du pourcentage de tiges coupies. Un autre facteur responsable de la diminution de la population est la rksistance de la plante h6te qui dtpend, tant chez les blBs sensibles que rtsistants, des conditions climatiques durant la pkriode de croissance. En moyenne, toutes les larves ont ptri dans 28% des tiges infesttes de deux blts sensibles et dans 67% de celles d'un blt rtsistant. Toutefois la mortalitt dans les bl...

Parasitism and the demography of wheat stem sawfly larvae, Cephus cinctus

“…Adult males generally emerge first and remain near field edges where most mating occurs. However, mating is not necessary for production of viable eggs as females can control the sex of their offspring by laying either a fertilized diploid female egg or an unfertilized haploid male egg (Holmes 1982;. The average lifespan of the adults is 5-8 days depending on climatic conditions and available moisture (Criddle 1923).…”

First record of Bracon lissogaster (Hymenoptera: Braconidae) in Canada – a potentially important parasitoid of Cephus cinctus (Hymenoptera: Cephidae) in the prairies

“…In extensive rearing of parasitoids from C. cinctus from cultivated cereals in southern Alberta and Saskatchewan from the 1940s to 1970s, B. cephi was the only braconid reported (Holmes 1982). However, significant numbers of Bracon lissogaster have been collected near Pendroy MT (Weaver unpublished data), only about 100 km south of the Canadian border, in annual sampling from 1999 to 2011.…”

Developmental instability in a stem-mining sawfly: can fluctuating asymmetry detect plant host stress in a model system?

“…First, adults emerge in June to oviposit in stems of native and cultivated species of grass, which include wheat, Triticum aestivum L (Holmes 1982). Wheat variety does not affect sawfly oviposition choice (Holmes 1979, Cárcamo and Beres, unpublished data) and only one larva will develop within a stem.…”