Influence of plant phenostage and ploidy level on oviposition and feeding of two specialist herbivores of spotted knapweed, Centaurea stoebe

► Agapeta zoegana oviposited significantly more on rosette and one-stem plants. ► 2× plants had significantly greater A. zoegana larval density than 4× plants. ► Significant correlation between the number of larvae infesting plant roots and Centaurea stoebe root diameter. ► Cyphocleonus achates fed more frequently on multiple-stem 4× plants. ► Arion lusitanicus had a strong feeding preference for younger rosette plants.