Previous work by Kinghorn and Chapman (1959) has shown that in coastal British Columbia, the ambrosia beetle Trypodendron linmtum hibernates in the forest litter or duff at various distances within forest edges in the vicinitv of D J brood logs. I t was pointed out that further study was required t o determine the characteristics of the optimum hibernation sites. It was mentioned that stand density, shade, aspect, slope, and the nature of the duff might be factors controlling the selection of the place of hibernation. T h e base of trees and the relatively deeper duff, characteristic of this situation, was reported to be the location of the larger populations of hibernating beetles although the level of population was found to vary greatly both from tree to tree and in depth within different forest edges.T h e present study describes several aspects of hibernation. T h e movement of the beetles from brood-logs to hibernation within the edge of an adjacent forest was studied. T h e nature of the hibernation sites, in the duff and in the bark of trees, was investigated to determine whether the depth of rhe duff or the thickness and roughness of the tree bark were factors influencing the number of beetles which select these sites. Habitats within the forest at the base of trees, between trees, under salal (Gaultheria shallon Pursh.), and in open areas were studied to measure their relationship to hibernation of Trypodendron. In habitats having similar characteristics, large variations in the number of beetles were found both at different distances from the forest edge and at different parts of the forest at the same distance from the edge of the stand. These variations in number of beetles were studied in relation to the amount of shelter and incident light penetrating to the ground through the forest canopy.T h e forest stand in which these studies were carried out was mature and had marked variation in the density of the main crown canopy. It consisted predominantly of Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) with a few western hemlock (Truga keterophylh (Rafn.) Sarg.), western red cedar (Thuja plicata Don) and western white pine (Pinus monticola Dougl.). Small patches of second growth hemlock and cedar provided a dense canopy contrasting with the lighter shade beneath the large, high crowns of the clear-boled mature trees. Low shade occurred under scattered small trees growing as understory below the main canopy. The stand was about 1,300 feet long and 300 feet wide, bounded by an open roadway on one side and cleared areas on the other three sides (Figure 1).
Prepupae of Neodiprion sertifer (Geoff.) held in darkness at 21 °C undergo a short normal diapause if exposed as feeding larvae to long photoperiods, but have no diapause if reared in the larval stage under short photoperiods. If cocoons from short-day rearings are exposed to 10 °C instead of 21 °C, a small proportion of the prepupal larvae undergo the normal or short diapause, but the majority enter an intense or prolonged type of diapause which results in three clearly defined periods of adult emergence within the range of 160 to 800 days after cocoon spinning. Mortality increases directly with the prolonged cocoon periods associated with intense diapause at 10 °C and there is a progressive decrease in reproductive potential of the survivors. If cocoons are held at 29 °C normal diapause and morphogenesis are prolonged but there is no evidence that a state of intense diapause is induced. Mortality is extreme, however, and the reproductive potential of the survivors is greatly reduced.The implication of these findings is discussed in relation to prolonged diapause in nature, a factor critical to the assessment of the seasonal history and population dynamics of the insect.
Measurements of the supercooling points of eggs from different latitudes suggest that the cold-hardiness of N. sertifer is increased by the selective pressure of low winter temperatures. In Ontario, the insect is only beginning to experience temperatures capable of influencing population trends, but the selection of a more cold-hardy population may be in progress. It is predicted that this insect will eventually be capable of extendng its range into natural stands of jack pine in the northwestern portion of the province.Overwintering eggs are inherently able to supercool to about −26 °C., but exposure to certain non-lethal conditioning temperatures increases their cold-hardiness. Increased cold-hardiness apparently is associated with the appearance of glycerol in the eggs. Mortality curves based on supercooling points of eggs exposed to optimal conditioning temperatures are useful in predicting overwintering mortality of eggs that are not protected by snow cover.
Overwintering eggs of Porthetria dispar increase their capacity to supercool during exposure to non-lethal conditioning temperatures. Measurements of supercooling points of eggs collected in Quebec and Massachusetts indicate that the gradual movement of the insect north and west from the initial point of introduction into North America has not been accompanied by natural selection for a more cold-hardy population. The protection afforded eggs by cold-hardening is, however, greatly enhanced by the placement of egg masses, typically at or near ground level, where they are subject to maximum insulation by snow cover.Examination of climatic regimes within the current area of distribution of the gypsy moth in Eurasia indicates that temperature may not be a key factor limiting its northward spread. In central Canada, in the presence of abundant snow cover, the insect may spread as far north as suitable food plants exist. This would favour extension of the range of the gypsy moth throughout the Great Lakes – St. Lawrence Forest Region and much of the Boreal Forest Region of Ontario and Quebec south of James Bay.
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