PhD thesis. Abomey-Calavi University, Benin

Abstract

Mango and cashew are important fruit trees in West Africa, but they are unfortunately subjected to a wide range of insect pests that threaten their production. However, the use of weaver ant Oecophylla longinoda (Hymenoptera: Formicidae), generalist predator, could be efficient to control these pests. In this thesis, we analyzed the use of weaver ant’s technology to bring out their positive and negative impacts. We conducted social survey with actors of mango and cashew value chain, and ran fields experiments.

To identify major barriers in mango and cashew value chain, we conducted a diagnostic study using a value chain analysis. Jointly with stakeholders, we identified the following major barriers: fruit quality problems due to pest attacks, transportation difficulties, insufficient of the production, lack of processing factories and government assistance. The present thesis focused on the use of weaver ants to overcome fruit quality problem.

To assess the effectiveness of the African weaver ant O. longinoda as a biocontrol agent against cashew pests, we conducted field experiments during a two-year study. Yield and quality of nuts were compared among four treatments (with and without weaver ants). Compared with the control (trees without weaver ants), ant’s trees produced more nuts. However, nuts produced by control trees were, on average, of a better quality as they were less damaged by thrips. Though, in absolute numbers, trees in ant treatments produced more nuts of first quality. Moreover, nut count and Kernel Outturn Ratio (KOR) were better in all ant treatments compared with the control. However, ant marks had a negative impact on the kernel. Interviews performed with farmers, mango pickers and consumers showed that mango on ant trees, more precisely mango with scale insects, are sweeter than control mango. This assertion was confirmed by tasters after degustation.

To evaluate some of the important constraints to weaver ant’s implementation, the possible negative effects of O. longinoda on the visiting rate of pollinators and on the number of host trees inflorescences were investigated. At this end, pollinators’ visit on trees with and trees without weaver ants were recorded weekly during flowering season in a two-year study. Findings showed that there was a weak negative relationship between the visiting rate of Oecophylla workers and that of flying insects on mango and cashew flower shoots. However, weaver ant predation on pollinators had been scarce.

The effects of ant densities and nest building on the leaf performance of mango trees were also assessed. Indeed, the total number of leaves, the proportion of leaves used for nest construction, and the tree growth (Diameter at Breast Height) were compared among three groups of trees: trees without ants, trees with low and trees with high ant densities. The result concluded that the costs associated to ant nests were low enough to affect the overall number of leaves per tree, or tree growth.

Finally, to assess the profit of the use of weaver ant technology in cashew and mango orchards, we compared profits among the different treatments. Our results showed that weaver ants alone may work effectively in some cases. Weaver ants used in mango (Gouverneur variety) and cashew orchards seemed to be cost-effective because the profit they provided was positively higher. This positive net gain is due to the low cost associated with the use of weaver ants (alone). However, this technology might be supplemented with additional alternatives control methods like the use of neem (Azadirachta indica) extracts in an IPM packages to ensure an effective pest control and obtain a better fruit quality.