Testing Western North American Fir Species for Genetic Tolerance and Resistance to Balsam Woolly Adelgid

Abstract

Balsam woolly adelgid (BWA), Adelges piceae (Ratzeburg) (Hemiptera: Adelgidae), is an invasive insect in North America that is native to Europe. Throughout its introduced range in North America, it affects true fir (Abies sp.) species via canopy and stem infestations. In this study, I tested 7 different fir species ranging in their presumed level of susceptibility and tolerance to BWA by inoculating seedlings using two approaches: a new modified passive inoculation and manual inoculation. I also aimed to reconstruct the BWA infestation history on the Gifford Pinchot National Forest, Washington, using dendrochronology. The new modified passive method was successful in inoculating seedlings; for example, 141 of 159 seedlings were successfully inoculated using this approach, which suggests this can be used in future efforts that seek to test true fir species in their resistance and tolerance to BWA. All North American species were highly susceptible to BWA with the Eastern species having higher BWA densities, regardless of seedling size. Both the modified passive and manual inoculation methods revealed that two European species, Nordmann fir (Abies nordmanniana) and Turkish fir (Abies bornmuelleriana), which are the evolutionarily-native hosts of BWA, developed fewer number of gouts and produced less new growth in response to BWA feeding than the Western (grand fir, Abies grandis, and subalpine fir, Abies lasiocarpa) and Eastern (Balsam fir, Abies balsamea; Fraser fir, Abies fraseri; and Canaan fir, Abies balsamea var. phanerolepis) North American species tested. When using dendrochronological approaches on tree cores collected from the Gifford Pinchot National Forest, the residuals from a tree growth model showed an increase in tree ring widths in the 1950s, which is a previously reported tree growth response to BWA feeding and one that coincides with prior reports of the initial establishment of BWA in this National Forest. The results of this research reinforces the known range of susceptibility of the 7 fir species and furthers our knowledge about BWA in the PNW.