WINTER SURVIVAL AND OUTBREAKS OF BERTHA ARMYWORM, MAMESTRA CONFIGURATA (LEPIDOPTERA: NOCTUIDAE), ON CANOLA

Abstract

AbstractThe winter survival of artificial and natural infestations of pupae of the bertha armyworm, Mamestra configurata Walker, are assessed in relation to soil temperature and snow depth. The results are discussed in relation to the hypothesis that winter mortality affects the spatial distribution and timing of outbreaks of this pest. Pupae did not survive a Manitoba winter in snow-free field plots, but 55% survived in plots with 5 or 10 cm of snow. Pupal survival in the plots was estimated accurately from daily soil temperatures using a computer simulation model, confirming that natural soil-temperature regimes can affect pupal survival. Based on the model, an equation was derived to relate pupal survival to the number of winter days with soil temperatures of −10°C or less. For natural populations in canola fields, the model, in conjunction with a model that estimated soil temperatures from standard meteorological data, could estimate the observed survivals. However, the estimates were sensitive to estimated snow cover and the measurements of snow cover in the fields were incomplete. There was a significant negative relationship between mortality and snow depth measured at the end of January up to about 20 cm. The sensitivity of soil temperature and therefore pupal survival to small changes in snow depth makes it difficult to predict pupal survival for large areas. Although the distribution of outbreaks of bertha armyworm coincided with the areas of the canola-growing region where winter soil temperatures were highest, the summers when outbreaks occurred were not preceded by winters with high soil temperatures, nor were summers without outbreaks associated with low soil temperatures during the preceding winters. Winter soil temperatures apparently affected the spatial distribution of outbreaks, but variation in pupal survival due to winter soil temperatures did not, by itself, initiate or terminate bertha-army worm outbreaks.