An analysis of mutualistic interactions between exotic ants and honeydew producers in the Yanbaru district of Okinawa Island,Japan

Invasive exotic ants often have a mutualistic relationship with other insects excreting honeydew, and this is considered to play a key role in their invasion success. We investigated the multispecies association patterns between ants and hemipteran insects in the Yanbaru forests, Okinawa, Japan, an Asian biodiversity hotspot. We especially focused on roadside environments, which are the frontlines of invasion for exotic ants. We found that only a small number of herbaceous and pioneer plants were predominant on the roadsides. Four honeydew producers, Melanaphis formosana, Dysmicoccus sp. A, Heteropsylla cubana, and Sogata hakonensis, living on these roadside plants accounted for 94.9% of the total honeydew-producer aggregations observed. Only a few exotic ants, such as Technomyrmex brunneus and Anoplolepis gracilipes, were observed with these honeydew-producer aggregations, and densities of these ants and honeydew producers were often positively correlated. An ant exclusion experiment showed that exotic ant occurrence improved the survival of some of the hemipteran colonies. Interestingly, the abundance of native ants was not correlated with the abundance of honeydew producers, and the local density of Pheidole noda was negatively correlated with that of M. formosana. These findings, i.e., only a few ants, all exotic, tended to hemipteran honeydew producers despite the existence of many native ants, and the abundances of those exotic ants and those hemipteran insects had positive correlations, provide some insights into the mechanism of biological invasion and provide information for the management of exotic ants.