Plant utilization patterns of the gall midge Jaapiella ivannikovi on its host, the exotic Russian knapweed (Rhaponticum repens)

The gall midge Jaapiella ivannikovi is a classical biological control agent of Russian knapweed in North America.  A greenhouse-based rearing program has produced galls for field release in the western US from 2011 through 2014.  We have collected data on Russian knapweed growth characteristics that could influence gall production.  In general, the characteristic most strongly associated with galling is the relative abundance of lateral meristems (i.e. 'branches') on Russian knapweed shoots.  The absolute number of shoots per plant has a positive, though weaker, relationship with gall abundance.  The numbers of non-bolting rosettes and flowers or flower buds are not related to gall production, and these are seldom utilized by J. ivannikovi.