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

Abstract: A caged field experiment was used to determine how Centaurea stoebe L. phenostage (rosette, single-stem, multiple-stem) and ploidy level (diploid=2× and tetraploid=4×) influence oviposition and feeding of two biological control agents, Agapeta zoegana (Lep.: Cochylidae) and Cyphocleonus achates (Col.: Curculionidae). Ploidy level did not influence oviposition patterns of A. zoegana but rosette and one-stem plants had significantly more eggs than multiple-stem (4×) plants. Differences in oviposition levels did not translate into differences in larval densities, but 2× plants (particularly large one-stem plants) had significantly more larvae than 4× plants. There was a significant positive correlation between numbers of larvae and root diameter. Ploidy level and phenostage both had a significant effect on C. achates feeding damage, with adults feeding more frequently on multiple-stem plants. No C. achates larvae were observed when the roots were dissected. Furthermore, the generalist herbivore Arion lusitanicus, naturally present in the garden plots, was predominantly associated with young rosette plants, a stage at which survival rate is acknowledged to be the most important determinant of knapweed density. These results indicate that the combined damage caused by A. zoegana and C. achates, superimposed on damage caused by generalist herbivores in the local community, could provide effective control for C. stoebe. [Copyright &y& Elsevier]