Exacerbation of photosynthetic damage through increased heat–light stress resulting from Gargaphia decoris sap-feeding

Exacerbation of photosynthetic damage through increased heat–light stress resulting from Gargaphia decoris sap-feeding 
Blair COWIE, Marcus J. Byrne, Ed Witkowski, Nic Venter
School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Johannesburg, South Africa

Solanum mauritianum (Bugweed) is one of the world’s worst ecological weeds and has been targeted for biological control efforts in South Africa since 1984. One of two promising biological control agents released against Bugweed was the sap-sucking lace bug, Gargaphia decoris (Hemiptera: Tingidae). Few studies have assessed the physiological effects induced by biological control agents feeding damage on their target weeds. Chlorophyll removal by G. decoris feeding caused metabolic impairment which resulted in a reduction of photosynthetic rates of Bugweed leaves, with a greater effect on plants growing in full-sun conditions compared to plants growing in shade. This difference was related to higher leaf temperatures experienced under full-sun conditions. Herbivory caused a 52% reduction in Bugweed transpiration rates, impeding the ability of leaves cool via evaporation. Although G. decoris’s feeding rates were greater on plants in the shade, feeding per unit area was significantly more damaging to plants continually exposed to full sunlight. The increased physiological damage experienced by full-sun plants may be a combination of stresses, particularly the direct effect of chlorophyll removal via herbivory and the indirect effect of accumulated heat–light stress. Given the effectiveness of G. decoris for biological control, factors constraining its current performance in the field need to be identified and addressed.