Host specificity and comparative foraging behaviour of Aenasius vexans andAcerophagus coccois, two endo-parasitoids of the cassava mealybug

Two encyrtid parasitoids, Aenasius vexans Kerrich (Hymenoptera: Encyrtidae) and Acerophagus coccois Smith (Hymenoptera: Encyrtidae), were compared for their degree of dietary specialisation and the impact this has on their fo raging strategies. Both parasitoid species are significant for biological c ontrol of the cassava mealybug, Phenacoccus herreni, Cox & Williams (Homopt era: Sternorrhyncha) a major Latin American pest of cassava, Manihot escule nta Crantz, an important root crop. Host acceptance and parasitism were ana lysed in seven mealybug species (with different levels of polyphagy) occurr ing in and around cassava fields. Results demonstrate that, in this ecosyst em, An. vexans is a specialist for P. herreni while Ac. coccois is a genera list on the first and second trophic level. Of the seven mealybug species, P. herreni and P. madeirensis Green were the most acceptable hosts for Ac. coccois, followed by Ferrisia virgata Cockerell. Ac. coccois did not accept the other four mealybug species. The foraging and oviposition behaviour of individual parasitoids was observed in bioassays with cassava leaves infes ted by P. herreni. The two species used different strategies to locate thei r host. Aenasius vexans spent significantly more time walking and standing on an infested leaf and examined a host longer than did Ac. coccois. Acerop hagus coccois, in contrast, spent more time for oviposition. As a consequen ce An. vexans parasitised more hosts in a given time than did Ac. coccois. Because the rate of offspring production of the two species did not differ, we conclude that the gregarious Ac. coccois's strategy to deposit several eggs at once might compensate for its relatively low number of ovipostition s, compared with the solitary An. vexans. These findings suggest that, give n the advantages and limitations of each species, a multi-species approach to biological control of P. herreni may yield best results.