Estimation of worker population size-density by nest counting in the Asian weaver ant, Oecophylla smaragdina (Formicidae: Formicinae) and its dynamic in oil palm plantations industry

Abstract

Oecophylla smaragdina, arboreal ants in nature, is of economic interest in the Asian tropics for its potential as a low-cost biological control agent in commercial crops (cashew-nuts and recently oil palm trees). Population density-dynamics is fundamental in the study of control agents. In ants, such empirical data are scarce, considering their underground life where excavation destroys the colonies. Moreover, in the course of estimating the worker relative population size and density in a fixed area by nests counting, a population of O. smaragdina was investigated in oil palm plantations, to implement integrated pest management for controlling dominant bagworm defoliators. To minimize the estimation bias caused by the difference in nest characteristics, we analyzed the correlation of those variables to the nest contents. The models of the nest counting method found that the number of nests, the mean of total workers, and the number of trees (correlated with the occupied area) became the most significant variables in estimating the population density. No external nest characteristics was statistically significantly correlated with the number of workers. Hence, the total average worker number per nest, 6333+2907SE (52 undamaged nests, average mean total major-minor: 6332.55) multiplied by the total nest count is appropriate to estimate the relative population size and density of workers (over few million per colony in a fixed occupied area). Four absolute population-wide estimation of the worker density in four plantations was ensued. This study demonstrated the population dynamic stability of O. smaragdina with sufficient numerical resident ants by a long-term monitoring of the occurrence of incipient colonies. Controlling Metisa plana by such IPM might minimize the usage of broad-spectrum long-range contact life cycle disruptive chemicals on natural enemies’ balance in agro-ecosystems.

Distribution and Nest Occupancy Patterns of <em>Oecophylla smaragdina</em> (Hymenoptera: Formicidae) Colonies in Southeast Asia Oil Palm Plantations

“…The distribution of nests was similar to other studies in various fruit plants [53] such as in mango [54] in coconut [55] in Citrus spp [31]. They are polydomous in nature [12] and a single tree may harbour many nests [7,11]. Ref.…”

“…An average of four colonies fall within a 500 m wide transect set up along a path in each plantation block. The sampling was performed according to [7]. Figure 2 shows the random sample point collection used.…”

“…While O. smaragdina predilection for dwelling in taller palms is unmatched, absence is still rare in shorter palms. In one case only circumstantially due to the black ants D. thoracicus presence [7], then weaver ants manage to take dominance status the following years (Teluk Intan, Perak plantations) [11]. Conversely, in a southern plantation in Johor, their abundance was higher in a younger plantation with the first fresh fruit bunches yet to emerge soon.…”

“…By putting brood nests at higher height from the ground, it is to prevent the colony overall individuals exposure to danger related to the maintenance activity during fronds pruning. The barrack nests are positioned at much lower ground level with only bigger size major workers securing the canopies territory and representing the first line of defence [7]. The barrack nests are functioning as a different level of defence line for each occupied palm tree to secure the colony from any intruders [67,18,68].…”

“…O. smaragdina it is also a component of integrated pest management (IPM) program to control the fruit fly, Bactrocera jarvisi, a major pest of mango in Australia [4]. Considering the potential application to keep the invasive bagworms, Metisa plana, below the economic threshold level (ETL) in oil palm plantations [5,6], O. smaragdina distribution need to be verified to ensure easy access and the availability of its nests in the oil palms [7] for population manipulations [8].…”