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

Authors: LE, T.T. - Plant Protection Research Institute - Vietnam; GRAZIOSI, I. - University Of Kentucky; CIRA, T. - University Of Minnesota; Gates, Michael; WYCKHUYS, K.A. - University Of Agriculture - Vietnam

Submitted to: Biocontrol Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/11/2018
Publication Date: 6/15/2018
Citation: Le, T.N., Graziosi, I., Cira, T.M., Gates, M.W., Wyckhuys, K.G. 2018. Landscape context does not constrain biological control of Phenacoccus manihoti in intensified cassava systems of southern Vietnam. Biocontrol Science and Technology. 121:129-139.

Interpretive Summary: Mealybugs cause millions of dollars in crop losses and control measures annually. The notorious cassava mealybug devastated cassava production in the 1970s and recently spread to southeast Asia production (2008). Parasitic wasps attack this mealybug, providing excellent control. This paper reports on parasitic wasps associated with cassava mealybug in low and high diversity landscapes in Vietnam. This information will be used by farmers, biocontrol workers, and entomologists.

Technical Abstract: Following its continent-wide spread in Africa during the 1970s and early 1980s, the cassava mealybug, Phenacoccus manihoti (Homoptera: Pseudococcidae) was inadvertently introduced to Southeast (SE) Asia in late 2008. In both regions P. manihoti can inflict severe damage on cassava, impact food security, and affect (smallholder) farming families and rural agro-industries alike. The endophagous parasitoid Anagyrus lopezi (Hymenoptera: Encyrtidae) was introduced to Africa in 1981 for P. manihoti management and the subsequent continent-wide biological control program received international acclaim. In 2009, A. lopezi was also released in Southeast Asia, though surveys to assess its local establishment, spatial spread and impact on invasive mealybug populations have yet to be reported. 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 province, Vietnam. Across years and landscape types we found 24.8 ± 17.7% (mean ± SD) plants infected with at least one P. manihoti and on average 5.6 ± 5.3 P. manihoti per cassava tip. High parasitism levels were attained across both years, with season-long averages of 49.9 and 52.1% in low- and high-diversity settings. Hyperparasitism levels were on average 2.8 ± 5.4%, with a maximum of 26.4%. While the primary parasitoid community was entirely comprised of A. lopezi, a total of three (potential) hyperparasitoid species were found: Chartocerus sp. near walkeri (Signiphoridae), Promuscidea unfasciativentris (Eriaporidae) and Prochiloneurus sp. (Encyrtidae). Field age was a significant predictor of P. manihoti incidence (proportion of plants infested), abundance (number of mealybugs per plant), 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 both individual cassava tips and entire fields, Anagyrus lopezi expressed a strong density-dependent response to P. manihoti during the early season. Density-dependency was more pronounced in low-diversity settings, and likely relates to dispersal modalities and other ecological traits of the parasitoid. This work constitutes the first, comprehensive assessment of A. lopezi establishment, parasitism rates, and parasitoid × host dynamics from a key cassava-growing region in SE Asia. Our study also 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.