Prospects for the biological control of the invasive cactus, Opuntia stricta using Dactylopius opuntiae, under conditions of rising atmospheric CO2

Abstract

Opuntia stricta var. stricta (Haworth.) Haworth (Cactaceae) is an important invasive alien plant (IAP) in South Africa, that invades both disturbed and undisturbed habitats mainly within grasslands and savannas. Fortunately, substantial control has been achieved using the biological control agent, Dactylopius opuntiae (Cockerell) (Hemiptera: Dactylopiidae), a sap sucking scale insect. With rising atmospheric CO2 concentrations, many plants alter their growth traits, most notably leaf nutritional quality which in turn affects their arthropod herbivores. Therefore, we tested whether D. opuntiae fitness and its subsequent efficacy as a biological control agent is altered by O. stricta plants grown at elevated CO2. Using walk-in Conviron growth chambers, O. stricta was grown at three CO2 concentrations; 250 (sub-ambient – sCO2). 400 (ambient - aCO2) and 600 ppm (elevated – eCO2). After 70 days of growth, plants were inoculated with 20 first instar D. opuntiae crawlers and we subsequently measured parameters to assess their fitness. Plant traits were measured regularly throughout the 6.5-month experiment. Dactylopius opuntiae fitness index was significantly lowered on eCO2 plants which slowed the rate of herbivory. Dactylopius opuntiae fitness index was negatively correlated to cladode carbon/nitrogen ratios indicating reduced plant quality was a potential driver. However, despite reduced D. opuntiae fitness and slowed rates of damage, O. stricta growth at eCO2 was still inhibited by the insect, and final plant biomass did not differ from aCO2 plants, unlike that observed for the control plants. Furthermore, irrespective of CO2 treatment, all plants succumbed to D. opuntiae herbivory with plants grown at eCO2 taking approximately-three weeks longer than sCO2 and aCO2 plants to die. Despite mortality of O. stricta plants grown at eCO2 due to D. opuntiae feeding, the long-term potential cumulative effects of reduced agent fitness and their subsequent efficacy may hinder the management of this IAP with rising atmospheric CO2 concentrations.