Intersectional cottonwood hybrids are particularly susceptible to the poplar bud gall mite

Abstract

The occurrence of hybrid breakdown, the poor performance of certain hybrids, was investigated in cottonwoods (Populus) with respect to susceptibility to the poplar bud gall mite, Aceria parapopuli. The abundance of mite galls was investigated for all 352 cottonwoods on an island at Lethbridge, Alberta, where a natural trispecific hybrid swarm involves the narrowleaf cottonwood (Populus angustifolia) and the balsam poplar (Populus balsamifera) of section Tacamahaca and the prairie cottonwood (Populus deltides) of section Aigeiros. There, 78% of the intersectional hybrids (i.e., P. deltoides × P. angustifolia or P. balsamifera), 56% of the intrasectional (P. angustifolia × P. balsamifera) hybrids, and 36% of the single-species trees displayed some galls. Nine percent were heavily infested, and these were almost all intersectional hybrids. At four other riparian sites in British Columbia and Alberta with only single species or the two Tacamahaca cottonwoods, very few galls were observed and no trees were severely infested. Conversely, artificially planted intersectional hybrids near all sites displayed severe infestations, confirming the occurrence of mites throughout the region and the susceptibility of intersectional hybrids. The susceptibility to pests probably hinders growth and reproduction and thus disfavors the intersectional hybrids. Since hybrid poplars are widely planted for horticultural or silvicultural applications, this study suggests that future selections should consider the vulnerability of hybrids and especially intersectional hybrids to insect pests. Key words: Populus, insect pests, cottonwoods, galls, hybrids, riparian vegetation.