Response of the invasive pompom weed,Campuloclinium macrocephalum (Asteraceae), to feeding by the foliage-deforming thrips Liothrips tractabilis (Phlaeothripidae) under outdoor conditions in South Africa

The foliage-deforming thrips Liothrips tractabilis Mound and Pereyra (Thysanoptera: Phlaeothripidae) was recently released in South Africa as the first classical biological control agent against the invasive South American herb, Campuloclinium macrocephalum (Less.) DC. (Asteraceae). The impact of the thrips on young plants and regrowth of C. macrocephalum was assessed under outdoor conditions, and the results were compared to those of earlier laboratory trials that were conducted while the agent was still under investigation in quarantine. The outdoor trials revealed that feeding by L. tractabilis reduced the growth and biomass accumulation of C. macrocephalum, particularly in young plants and to a lesser extent in regrowth, which was largely consistent with the results of the earlier laboratory trials. Thrips-infested young plants were significantly shorter, produced fewer leaves and displayed significantly lower wet root masses and lower below- and above-ground dry masses, compared to the control plants. Although thrips-infested regrowth were significantly shorter, produced fewer leaves and displayed significantly lower above-ground dry masses than the control plants, the differences in wet root masses and below-ground dry masses were not significant. Although laboratory-based impact assessments are not necessarily an accurate reflection of field impact, in this instance the results were largely validated by the outdoor assessments. The results suggest that L. tractabilis will have a negative impact on C. macrocephalum populations in the field and thus contribute towards the weed’s biological control. The establishment of L. tractabilis has recently been confirmed in South Africa, providing the opportunity to verify these predictions.