Physiological response of Parthenium hysterophorus to defoliation by the leaf-feeding beetle Zygogramma bicolorata.
The annual shrub, Parthenium hysterophorus L. (Asteraceae), is a severe noxious weed of global significance, having been the target of control programmes in several affected countries. Biological control efforts in South Africa have seen a total of four agents released to date, one of the most promising of which is the leaf-feeding beetle, Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae), first released in 2013. Detailed study of the ecophysiological interactions of insect herbivores and their target weeds are under-represented in the biological control literature. Extensive leaf-feeding by Z. bicolorata adults metabolically impaired P. hysterophorus leaves, resulting in a ∼36% reduction to photosynthesis in remaining leaf tissue. However, in response to Z. bicolorata herbivory, adjacent undamaged leaves showed an up-regulation in photosynthesis of ∼11%. Photosynthetic reduction was attributed to the removal of leaf tissue, physiological impairment and increased microbial abundance resulting from Z. bicolorata. Zygogramma bicolorata was found to increase the number of bacterial and fungal microbes present on P. hysterophorus leaves. Despite the ability to photosynthetically up-regulate in response to herbivory, P. hysterophorus is unlikely to overcome the physiological implications associated with continued Z. bicolorata feeding and the increase in microbes. Given the potential effectiveness of Z. bicolorata as a biological control agent, efforts should be made to maximise incidence of the beetle in the field.