Landscape context does not constrain biological control of Phenacoccus manihoti in intensified cassava systems of southern Vietnam

In 2008, the cassava mealybug, Phenacoccus manihoti (Homoptera: Pseudococcidae) was inadvertently introduced to Southeast (SE) Asia, where it inflicted severe damage on cassava crops, and impacted countless farming families and rural agro-industries. The host-specific endophagous parasitoid Anagyrus lopezi (Hymenoptera: Encyrtidae) was imported into Thailand in 2009, and subsequently released in neighboring countries. At present, nothing is known about the degree to which A. lopezi establishment, in-field colonization and impact on P. manihoti populations is shaped by local agro-landscape context. In this study, we contrasted temporal population fluctuations of P. manihoti, A. lopezi, and associated hyperparasitoids within low- and high-diversity landscapes in Tay Ninh (Vietnam). Across years and landscape types we found 24.8 ± 17.7% (mean ± SD) plants infected with P. manihoti and on average 5.6 ± 5.3 mealybugs per cassava tip. High parasitism levels were attained across both years, with season-long averages of 49.9-52.1% in either low- or high-diversity settings. Hyperparasitism levels were on average 2.8 ± 5.4%, and a total of three hyperparasitoid species were recorded. Cassava age was a significant predictor of P. manihoti incidence, abundance, parasitism rate and hyperparasitism rate. Landscape type significantly affected P. manihoti incidence and hyperparasitism rate (at particular ages), but not P. manihoti abundance or parasitism rate. At the scale of individual cassava tips and entire fields, A. lopezi exhibited a strong density-dependent response to P. manihoti during the early season. This work constitutes the first, comprehensive assessment of A. lopezi establishment, parasitism rates, and parasitoid x host dynamics from a key cassava-growing region in SE Asia. Our study underlines how this exotic parasitic wasp effectively suppresses a globally-important insect pest in its newly invaded range, thus providing cost-free, environmentally-sound and lasting control across the developing-world tropics.