Research Article: At Lunch with a Killer: The Effect of Weaver Ants on Host-Parasitoid Interactions on Mango

Date Published: February 1, 2017

Publisher: Public Library of Science

Author(s): Valentina Migani, Sunday Ekesi, Katharina Merkel, Thomas Hoffmeister, Joseph Clifton Dickens.


Predator-prey interactions can affect the behaviour of the species involved, with consequences for population distribution and competitive interactions. Under predation pressure, potential prey may adopt evasive strategies. These responses can be costly and could impact population growth. As some prey species may be more affected than others, predation pressure could also alter the dynamics among species within communities. In field cages and small observation cages, we studied the interactions between a generalist predator, the African weaver ant, Oecophylla longinoda, two species of fruit flies that are primary pests of mango fruits, Ceratitis cosyra and Bactrocera dorsalis, and their two exotic parasitoids, Fopius arisanus and Diachasmimorpha longicaudata. In all experiments, either a single individual (observation cage experiments) or groups of individuals (field cage experiments) of a single species were exposed to foraging in the presence or absence of weaver ants. Weaver ant presence reduced the number of eggs laid by 75 and 50 percent in B. dorsalis and C. cosyra respectively. Similarly, parasitoid reproductive success was negatively affected by ant presence, with success of parasitism reduced by around 50 percent for both F. arisanus and D. longicaudata. The negative effect of weaver ants on both flies and parasitoids was mainly due to indirect predation effects. Encounters with weaver ant workers increased the leaving tendency in flies and parasitoids, thus reduced the time spent foraging on mango fruits. Parasitoids were impacted more strongly than fruit flies. We discuss how weaver ant predation pressure may affect the population dynamics of the fruit flies, and, in turn, how the alteration of host dynamics could impact parasitoid foraging behaviour and success.

Partial Text

A major challenge in ecology is to understand the complex interactions that shape ecological communities, and to conceptualize the involved processes. Consumption (i.e. predation) and competition are considered to be two main forces structuring ecological communities [1–3]. Trophic relations are particularly interesting since communities can be represented as food chains of interacting resources, consumers, and predators. A growing literature focuses on predator-prey dynamics, disentangling important mechanisms underlying these interactions [4,5]. The main focus of this research has been on the effects of direct consumption by a predator on its prey. However, the impact of predation is not limited to lethal effects of predators upon their prey. In fact, the mere presence of a predator can induce changes in the expressed phenotype of the prey in order to avoid or reduce predation pressure. These changes include, amongst others, increased patch leaving probability [6], reduced foraging activity [7], enhanced movement to predator-free areas [8], and an increased tendency by the forager to search in low quality patches [9–11]. When compared to direct consumption itself, the costs associated with such changes can have an equal, or even higher impact upon the prey population growth [4, 12–14]. Moreover, they might affect not only the prey, but also the organisms directly linked to it, such as resources or competitors [15, 16].

We carried out two sets of experiments at the International Centre of Insect Physiology and Ecology (icipe), Nairobi, Kenya, hence forth named as “field cage-” and “observation cage experiment”. The field cage experiment aimed at elucidating if the presence of weaver ants would impact mango infestation caused by the fruit flies and the foraging efficiency of the parasitoids on infested mangoes under semi-field conditions. Since we could not study the nature of the interaction, i.e., direct with lethal encounters or indirect across the time period that cohorts of a single species of flies or parasitoids were exposed, we used experiments in small observation cages to observe behavioural changes of individual fruit flies or parasitoids in the presence and absence of ant workers. All experiments were carried out under ambient light and temperature conditions. Replicates for different treatments were interspersed and arranged such that time and day effects could not contribute systematically to the differences between treatments.




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