Biological control of citrus blackfly in Texas

Abstract

Studies of citrus blackfly (CBF), Aleurocanthus woglumi Ashby, in the Lower Rio Grande Valley of Texas (1977-82) indicated a widespread current distribution of the exotic parasite Encarsia opulenta Silvestri, established during 1974-5. Competitive displacement of the exotic parasites E. clypealis Silvestri and Amitus hesperidum Silvestri has apparently occurred without detrimental impact on biological control by parasites. Phenological studies demonstrated the capability of E. opulenta to rapidly suppress high-density CBF infestations within a relatively short time interval, and to maintain a stable interaction with its host at low equilibrium densities (0.001-0.005 CBF/leaf). No periodic seasonal disruptions in the host-parasite interaction were detected during the study period. The regulative impact of E. opulenta and a native predator complex was investigated by a combination of life table analysis and experimental check methods. Predation was poorly correlated with intergeneration trends in CBF mortality and appeared to be of minor importance in low-density regulation. Parasitism by E. opulenta was significantly correlated with intergeneration trends in CBF mortality and comprised 71.2-97.2% of the total indispensable mortality due to natural enemies occurring in experimental CBF cohorts. A regulative capability of E. opulenta was demonstrated by a strong direct density-dependent response by individual female parasites to the host, and a delayed density-dependent response by the parasite population. Experimental exclusion of E. opulenta resulted in significant decreases in generation mortality of CBF, and significant increases in replacement rates of the latter. Complete biological control of CBF in Texas is due primarily to effective regulation by E. opulenta during all seasonal periods. Attributes correlated with this high degree of effectiveness include a high rate of increase relative to that of the host, tolerance by the parasite to a wide range of environmental conditions, density-dependent response, high searching capacity, and related attributes of considerable ecological significance.