Weaver Ants Suppress Pests and Increase Yields in Tropical Tree Crops

JH Thurman

Weaver Ants naturally occur throughout tropical regions of the world with (Oecophylla smaragdina) throughout Australia, India and South-East Asia and (O. smaragdina) in Sub-Saharan Africa.These ants are known for their arboreal nature and the large nests they weave out of leaves in the canopy. Like most ants, Weaver Ants are generalist predators which feed on various arthropods and plants, and harvest honeydew producers.

When farmers of various tree crops noticed these ants moving into their orchards, they were left asking themselves the same question: Are these ants harmful or beneficial to my crop?

A series of studies were conducted to answer that question in a quantitative way and we compiled the data from 34 studies on cacao, cashew, citrus, coconut palm, mahoghany, mango, palm oil and pongamia crops to determine if these ants provide a global trend of ecosystem services in tropical tree crops.

After compiling the data from 34 studies, we found a general trend of significantly increased yields when ants were present. This is likely to be explained by the pest suppression which the ants supplied. When ants were present, pest damage and density was significantly decreased (Figure 1). While the ants were shown to suppress most pests, honeydew producer populations increased when ants were present (Figure 2B).

Figure 1. Percentage of pest density (A) and pest damage (B) per tree for several tropical crops for trees with and without Oecophylla spp. ants. Ant treatments had significantly fewer pests and less pest damage than treatments without ants. Outliers are shown as white dots, while the solid black line indicates the median and the box shows the distribution of the dataset compiled from 25 studies (Fig. 2 in Thurman et al. 2019).

By harvesting honeydew producers, Weaver Ants colonies were sustained to population densities that were necessary to provide pest suppression, however. And overall, these honeydew producers are not considered to be primary pests.

Further review of case studies on how these ants interact with the rest of the arthropod community revealed positive interactions with other groups of beneficial arthropods like pollinators and predators (Figure 2, A and C). The ants also had a nutrient exchange with their host tree, suggesting a mutualistic relationship.

Figure 2. Weaver ants (Oecophylla spp.) interact with predators and parasitoids (A), pests (B), pollinators (C), and a host tree. Overall impacts of Oecophylla spp. on these groups of arthropods and their host tree can be positive, negative, or both where certain groups of predators, parasitoids or pollinators benefit from ant presence, while others are deterred. Information on these interactions was gathered from a global meta-analysis on both O. smaragdina and O. longinoda as biological control agents in mango, cashew, and other tropical crops (Fig. 5 in Thurman et al. 2019).

Overall, Weaver Ants can supply ecosystem services in a variety of tree crops throughout their native distribution. These services may be the result of a complex suite of interactions with predators, pollinators, pests, and their host tree (Figure 2). We suggest research into potential biological control agents as they occur throughout their native distribution as this allows the agents to be present with their own natural enemies (Figure 3). Finally, we suggest the usage of Weaver Ants as a method of conservative biological control, which has the potential to reduce pesticide sprays in tropical tree crops.

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