Can. Enr. 119: 371-380 (1987) Temperature effects on development were studied for two Alberta populations of Ostrinia nubilalis (Hiibner), from the South Saskatchewan River valley and the surrounding plains. Lower developmental thresholds for all life stages of both Alberta populations were determined by linear regression. Thresholds for the egg stage were significantly less for plains borers (9.5"C) than for valley borers (10.8"C), and about 2°C lower than for corn borers from the United States. Thresholds in Alberta populations for the 4th (15.3"C) and 5th (14.0°C, plains) instars, and for post-diapause pupation (12.g°C), were much higher than in populations from the United States. Higher temperature thresholds delay development in Alberta populations, thus reducing midsummer pupation. Valley populations developed significantly faster than plains populations during egg development, during the prepupal period of the 5th instar, and during post-diapause pupation. These results explain why valley populations have a partial second generation in some years.On a CtudiC les effets de la tempCrature sur le dkveloppement de deux populations de la pyrale du mais, Ostrinia nubilalis (Hiibner), provenant de la vallCe de la rivi&re Saskatchewan Sud et des plaines environnantes en Alberta. On a dCterminC les seuils thermiques infkrieurs du dCveloppement de tous les stades des deux populations par rkgression 1inCaire. Les seuils du dkveloppement de l'oeuf Ctaient significativement plus bas pour la pyrale des plaines (9,5"C) que celle de la vallCe (10,8"C), et environ 2°C plus bas que celui de pyrale des Etats-Unis. Les seuils des populations albertaines pour les 4ikme (15,3"C) et 5ikme (14,0°C, plaines) stades, et pour lapupaison postdiapause (12,g°C), Ctaient bien supCrieurs a ceux des populations des Etats-Unis. Ces seuils thermiques plus ClevCs des populations albertaines retardent leur dCveloppement, rkduisant ainsi la pupaison mi-estivale. Les populations de la vallCe se sont dCveloppCes significativement plus vite que celles des plaines au stade d'oeuf, durant la p6riode prCpupale du 5ieme stade, et durant la pupaison post-diapause. Ces rksultats expliquent le fait que les populations de la vallCe montrent une seconde gCnCration partielle lors de certaines annCes. THE CANADIAN FNTOMOUX~IST

No abstract

A degree-day model was developed for Alberta populations of Ostrinia nubilalis Hübner. Starting with overwintered fifth-instar larvae, the model calculates the temporal distribution of first- and second-instar larvae which are the stages most vulnerable to chemical suppression. Predictions from three alternative models were compared against field data from southern Alberta. Use of a standard 10°C growth threshold to calculate physiological time scales allowed predictions as accurate as those obtained using either a pooled threshold (11.4°C) calculated specifically from Alberta populations, or a model using two thresholds (12.3°C for fifth-instar larvae to adult and 10.2°C for eggs to second-instar larvae) that incorporated significant differences in growth characteristics observed among life stages. We conclude that standard thresholds are sufficient for degree-day models for northern populations of O. nubilalis. The standard model (t0 = 10°C) predicts that moth emergence will peak at ca. 145 degree-days after median pupation, and that numbers of eggs, and first- and second-instar larvae should peak at 200, 310, and 450 degree-days, respectively. Model predictions can be used to time sampling effort in support of management decisions.

A combination of laboratory and field trials, from 1983 to 1985, were used to determine mortality factors affecting all life stages of Alberta populations of the European corn borer, Ostrinia nubilalis (Hübner), feeding on corn. The resulting life table demonstrated that greatest mortality occurred during the first and second larval instars (62.2%), when mature larvae are in diapause (69.5%), and from moth migration and loss in reproductive potential (70.3%). Stages where mortality was slight included the egg stage (11.7%), non-diapausing third- to fifth-instar larvae (2%), and pupae (10.4%).Laboratory trials showed that greater egg mortality occurred at relative humidities (RH) of 35 and 55% than at a RH of 75%. In the field, egg parasitism was nil, predation of early-instar larvae (5.2%) and diapausing larvae (6%) was low, and parasitoids were scarce (emerging from ca. 2% of pupae). Sustained periods of hot, dry weather with high moisture stress were related to mortality of both eggs and early-instar larvae. Death of early-instar larvae also was related to periods of rainy weather with low evaporation. The 64% non-diapause larval mortality was much lower than that reported elsewhere. The dramatic increase in the corn borer since its recent introduction to Alberta may be due to a lack of natural enemies, and because only non-resistant corn hybrids are grown.

Differences in phenology were observed between two populations of Ostrinia nubilalis (Hbn.) in Alberta. In the South Saskatchewan River valley, borers pupated and emerged 3–6 days earlier than on the plains. This was not simply a function of different microclimates because median pupation occurred at 331 degree-days (DD) for plains populations and at 490 DD for valley populations. First-generation flight extended from the last week in June until the 1st week in August. Flight peaks in 1983 occurred on 8 July in the valley and on 14 July on the plains. Flight was delayed in 1984 by cold nightly temperatures. In 1983 valley populations had a partial second generation, with a flight peak in late August. Mean pupal mass was significantly less for plains populations than valley populations. Transplant experiments demonstrated that these differences reflected mainly environmental influences. During spring 1984, mean larval mass in the two populations did not differ until early June, when both mean wet and dry mass of valley larvae increased significantly but those of plains larvae did not change. This variability of phenotypes contributes to the colonizing ability of the European corn borer.