Effects of a mixed species infestation on the cassava mealybug and its encyrtid parasitoids

The two encyrtid endo-parasitoids, Aenasius vexans Kerrich and Acerophagus coccois Smith, are biological control agents of the cassava mealybug, Phenacoccus herreni Cox and Willliams, in Latin America. Parasitoids used as biological control agents are released in agro-ecosystems in which plants are typically attacked by various herbivore species. We studied the effects of a mixed species infestation by cassava mealybug and cassava green mite, Mononychellus tanajoa Bondar, or whitefly, Aleurotrachelus socialis Bondar, on the mealybug pest and its encyrtid parasitoids. Development, distribution and reproduction of the cassava mealybug as well as the post-alighting behavior and reproduction of the two parasitoid species were analyzed in a mixed species infestation as compared to an infestation by cassava mealybug only. Results show that developmental time of cassava mealybug females was altered when an additional herbivore species was feeding on the same plant. The duration of the third larval instar was significantly shorter when cassava green mite was present. In contrast, the duration of this instar was longer when whitefly was present. Reproduction of cassava mealybug was not affected, nor was the ratio of its distribution on the adaxial and the abaxial surface of the cassava leaf. Our findings on altered developmental time of phloem feeding cassava mealybug in mixed species infestations suggest that the desiccation of leaves caused by the cell sap feeding cassava green mite accelerated mealybug development. In contrast, the interaction with the similarly phloem feeding whitefly retarded it. The post-alighting behavior of the specialist parasitoid A. vexans differed between the two mixed species infestations as compared to the infestation with the cassava mealybug only. The preference of females for the adaxial leaf surface observed in the infestation with cassava mealybug only was not found in either of the mixed species infestations. In contrast, the post-alighting behavior of the generalist parasitoid A. coccois was similar in the mixed species infestations as in the infestation by cassava mealybug only. These results indicate a more robust behavior of the generalist, but the altered behavior of the specialist remained without consequences on parasitism rate under the conditions of this study.