Ants (Hymenoptera: Formicidae) as egg predators of coconut pests, especially in relation to biological control of the coconut caterpillar, Opisina arenosella Walker (Lepidoptera: Xyloryctidae), in Sri Lanka

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Abstract
A study was made of ants in Sri Lankan coconut plantations, especially of species foraging and nesting in palm crowns. At least 11 species were found nesting in the spadices, and a special study was made of interactions between some likely to be insect egg predators or to interfere with predation. Oecophylla smaragdina (F.) and Paratrechina longicornis (Latreille) were mutually exclusive, but some non-dominant species such as Monomorium spp. commonly nested and foraged in palms occupied by large populations of dominants such as O. smaragdina and Crematogaster sp. Eggs of the coconut pest Opisina arenosella Walker (and of Corcyra cephalonica (Stainton), which were used to simulate the former on palm fronds) were rapidly found and removed by certain ant species, notably M. floricola (Jerdan) and Crematogaster sp. Oecophylla smaragdina and P. longicornis seemed erratic as egg predators. Surveys of ants in areas where Opisina arenosella does, and does not, develop to outbreak populations showed that Monomorium spp. in particular were notably more common in the non-outbreak areas. However, other ants, notably Technomyrmex albipes (F. Smith), were sometimes common in outbreak areas and were seemingly good egg predators. The presence of thriving Oecophylla smaragdina colonies on currently attacked palms shows that this otherwise well-recognized predator has little or no effect on Opisina arenosella. It is concluded that several species of ants which nest in coconut palm crowns probably contribute importantly to the mortality of eggs of O. arenosella and help keep it in its rare endemic state in most places most of the time, even in outbreak areas.