CLIMATE AND THE ECOLOGY OF THE BALSAM WOOLLY APHID

Abstract

The balsam woolly aphid was introduced over 60 years ago from Europe into the Maritime Provinces of Canada where it established on Abies balsamea (L.) Mill. Ecological characteristics of the insect that are important to the survival and spread of populations in the new environment are feeding habits, cold-hardiness, and dispersal capacity. The aphid feeds on the bark cortex of all parts of the tree from the stem to the crown. ’Stem attack’ refers to the population established on the main stem while ’twig attack’ refers to the population established in the crown where the aphid may settle on numerous sites but mainly at the base of young shoots. Freezing is fatal and the probability of freezing increases as temperatures fall below −5°F. There are no survivors at −35°F no matter how brief the exposure. The adults are wingless and populations are spread mainly through wind dispersal of crawlers and eggs.The climate of the Maritime Provinces changes markedly from the coast inland from maritime, through transitional, to continental. Infestations in maritime and continental climatic regions follow particular patterns while in the transitional region the course of infestation is irregular. Climate is unimportant in determining population trend in maritime climatic regions. The probability of experiencing temperatures below −15°F in a winter is 0.10 and populations survive on the stem and in the crown. Crown populations are dispersed freely and the aphid and aphid damage is general. In the continental climatic region, the probability of −30°F occurring in a winter is 0.46 and populations are restricted to the base of the tree where snow cover provides protection. Wind velocities below the canopy are light and infestations remain isolated and small with damage from stem attack accumulating at a slow rate. In the transitional climatic region, the severity of winter is irregular with periols of mild winters being broken by occasional extremely cold winters. Tree mortality from twig and stem attack has been heavy in some stands while infestations in adjacent and equally susceptible stands have been arrested by severe winters.