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The occurrence and spatial distribution of top kill 1 year following a jack pine budworm (Choristoneura pinus pinus Freeman) outbreak were examined for possible associations with forest stand characteristics derived from resource inventory maps. Associations were computed between top-kill severity and jack pine (Pinus banksiana Lamb.) maturity, stand height, crown closure, and site quality. Three different measures of association with data from a geographic information system (GIS) (Cramer's V, Spearman's nonparametric rank correlation, and Minnick's coefficient of areal determination) were used to assess the strength of these associations. There were no statistical differences among average top-kill lengths, which averaged 2 m, among the light, moderate, and severe top-kill categories. The proportion of trees that experienced top kill ranged from 55% in the light to 61% in the severe top-kill classes. Site quality and stand maturity were more highly associated with top kill than stand height and crown closure. Jack pine areas that sustained moderate and severe top kill were those on poor sites that were overmature (> 85 years), 15-20 m tall with 30-55% crown closure. The GIS approach can be a useful tool for identifying vulnerable stands, and increasingly so as forest inventory classification systems become more detailed.

Nous avons effectue une etude en laboratoire pour mesurer l'efficacite d'assimilation du chlorure de methyl[ 203 Hg]mercure (CH 3 203 HgCl) present dans la nourriture des ecrevisses (Orconectes virilis) et le taux subsequent de depuration du CH 3 203 HgCl assimile. L'efficacite moyenne d'assimilation du CH 3 203 HgCl chez les tetards etait de 96%. Chez la majorite des 50 ecrevisses de l'experience (trois classes de taille de mâles et de femelles designees comme petite, moyenne et grande), on n'a observe sur la periode d'etude de 166 jours aucune disparition significative du CH 3 203 HgCl assimile. Quand une disparition significative du CH 3 203 HgCl etait detectee, les demi-vies observees tombaient generalement entre 1000 et 2000 jours (3-5 ans). La forte efficience d'assimilation et le faible taux de depuration permettent de penser que les ecrevisses peuvent bioamplifier le CH 3 Hg a partir de leur nourriture.

Studies on emergence of insect communities conducted in Mud Creek and Costello Creek in Algonquin Park, Ontario, Canada, over a period of 50 yr were analyzed to assess the changes that have occurred in blackfly populations in streams receiving long-term anthropogenic acidification. The headwaters of Mud Creek are currently experiencing pH depressions to 5.1–4.9 during snowmelt whereas pH depressions downstream in Mud Creek and throughout Costello Creek are less severe (to pH 5.6–5.8) compared with summer and autumn values of about 6.4. Since the time of the earliest collections, the total number of blackflies emerging has increased at the most severely acidified sites at the headwaters of Mud Creek, while emergence remains unchanged at all the less acidified sites. Large fluctuations from year to year in the emergence densities of many individual species were observed at all sites, making it difficult to determine which species may increase or decrease concurrent with elevated acidity.

An investigation to determine the responses of immature blackflies to experimental pulses of acidity simulating snowmelt episodes was conducted. Five species of mature blackfly larvae were subjected to 3- or 4-d experimental pH depressions to approximately 4.0 or 3.5 in stream channels. Differences among species in response to acidity were observed. All species except Simulium vittatum and possibly S. venustum were able to tolerate pH depressions to approximately 4.0, while only Cnephia dacotensis and possibly S. decorum were able to tolerate pH depressions to approximately 3.5. The survival of the larvae (only late instars were tested) was significantly affected by the acidity, while pupal survival generally was not. The rate of pupation, however, was decreased in the acidified channels, resulting in decreased emergence there also. More abnormally formed pupae developed in the acidified channels, suggesting that pupation following snowmelt ensures survival of these species.