Investigations of Balsam Woolly Aphid - Fraser Fir Interaction: Feeding Site Characteristics and Wound Response

Young, vigorously growing Fraser firs have exhibited a lower degree of susceptibility to balsam woolly aphid infestations than mature, mixed stands. This study investigated the relationship between balsam woolly aphid feeding site preference and bark morphological features, plus how these morphological features varied with stem size, age, growth rate, percent live crown and stand structure characteristics. Investigations were also conducted on the wound healing processes within the bark of Fraser fir as well as on how the aphid might interfere with these processes. Sampling was performed in the stands considered to be representative of anticipated conditions for the next generation of Fraser fir following the death of existing mature Fraser fir.

For successful feeding the balsam woolly aphid required modification of the tight, smooth, gray bark characteristics of young vigorously growing Fraser fir trees. These modifications were in the form of lenticels and splitting of the bark. Fir trees with slow growth rates associated with high stand densities had rougher bark and more lenticels per unit area than trees growing in open, less competitive conditions.

The wound healing processes within the bark were studied by observing the rate of formation on non-suberized impervious tissue and necrophylactic periderm following mechanical wounding. Sampling was conducted along an elevational gradient and a stand density gradient. Open-grown trees at low elevations showed the fastest rates of healing (17 days), whereas trees growing at the highest elevations required 26 days to heal. At the same elevation, open-grown trees formed necrophylactic periderm an average of 5 days sooner than forest-grown trees. Wounding combined with injection of selected plant growth substances (some of which are suspected of being secreted by the aphid while feeding) was also investigated. All treatments utilizing auxin-like compounds (indole-3-acetic acid and naphthaleneacetic acid) required 11 more days for periderm formation than for the control of mechanical wounding only. Treatments utilizing a gibberellin and a cytokinin formed necrophylactic periderm at the same rate as the control. Additionally, histological examination of aphid feeding sites failed to reveal a single case of necrophylactic periderm formation around the feeding zone. Thus, the balsam woolly aphid was able to inhibit the normal defense mechanism of Fraser firs following penetration of the living bark tissue by the insect's stylet.