Insights into mealybug-parasitoid food webs in Vietnamese cassava fields affected by Phenacoccus manihoti

The cassava mealybug Phenacoccus manihoti Matile-Ferrero is one of the most destructive pests of cassava worldwide, and recently made its arrival in Southeast Asia. In 2008, P. manihoti was recorded in eastern Thailand and has since spread to Laos, Cambodia, Vietnam, Malaysia and Indonesia. Subsequently, the encyrtid wasp Anagyrus lopezi was released throughout SE Asia for control of this mealybug pest.  Up till present, no formal assessment has been made of A. lopezi parasitism levels in cassava fields. In the meantime, little is known about arthropod food webs in SE Asian cassava fields and (potential) interference of A. lopezi with locally-occurring (hyper-) parasitoids. In this study, we documented temporal fluctuations in P. manihoti parasitism levels in cassava fields of differing size and complexity in southern Vietnam. The exotic A. lopezi reached parasitism levels up to 59.4% in small-scale cassava plots, while parasitism levels in large cassava plantations reached 29.9%. Parasitism levels were highest during the dry season, when P. manihoti equally reached peak abundance. A number of other natural enemies were recorded from local cassava fields. Some of these species possibly may affect P. manihoti biological control, either in a direct or indirect fashion. Our report of high parasitism levels by A. lopezi in a range of cassava cropping systems carries considerable promise for long-term management of the invasive cassava mealybug in SE Asia. In the meantime, the presence of a number of natural enemies (and hyperparasitoids), some of which could interfere with P. manihoti biological control, points at an urgent need for ecological studies.