Host stage preference and sex allocation in Aenasius vexans, an encyrtid parasitoid of the cassava mealybug

The solitary endoparasitoid Aenasius vexans Kerrich (Hymenoptera: Encyrtida e) is used for augmentative releases against the cassava mealybug, Phenacoc cus herreni Cox & Williams (Sternorrhyncha: Pseudococcidae), an important p est on cassava in South America. In light of the need for large numbers of high quality females, experiments were conducted on host stage suitability and sex allocation. In choice and no-choice experiments, individual female wasps were offered second and third instar, as well as adult, hosts. During the first five days after emergence, the wasps showed a steady increase in the number of hosts they successfully parasitised per day, but the respect ive secondary sex ratio for each instar remained constant. Parasitism was h ighest for third instar hosts in no-choice tests, while in choice tests par asitism was highest in both third instars and adults. The later the develop mental stage of the host at oviposition, the faster the parasitoids develop ed and emerged, and for each host stage, the development time of males was shorter than for females. The sex ratio of the wasps emerging from hosts th at were parasitised as second instars was strongly male-biased, while the a pparently preferred later stages yielded significantly more females than ma les. Female and male A. vexans emerging from hosts parasitised at the third instar were significantly larger than for the other stages. This may expla in the preference for the third instar as well as the female-biased sex rat io, as size is usually positively correlated with higher fitness, especiall y in females. The results suggest that third instar hosts are the most suit able for rearing high numbers of large females.