The influence of propagule size on incipient colony establishment of invasive Argentine ant, Linepithema humile Mayr (Hymenoptera: Formicidae)

Abstract

Propagule size is an important factor in determining establishment success. For the invasive Argentine ant, Linepithema humile Mayr, there is limited information about the role of propagule size though it is one of the very successful invasive ants. Here, in a laboratory and natural environments, I manipulated number of workers to investigate the effect of propagule size on the survival and establishment of L. humile propagules. In the laboratory study, I introduced replicates of four different sized L. humile propagules (10,100, 200 and 1000 workers plus two queens per worker category) into foraging arena in the presence of 200 workers of the New Zealand native ant species, Monomorium antarcticum Smith, and also varied food densities. I found that at colony size of 10,100 and 200 workers, L humile only survived for a short time, but at colony size of 1000 workers, L. humile successfully resisted M. antarcticum colonies consisting of 200 workers and raised brood. I also examined the effect of propagule size and varying amounts of food on the aggressive behaviour of L. humile and M. antarcticum. Increasing colony size caused L. humile to display less benign interactions and more aggressive behaviours. On the other hand, M. antarcticum maintained high aggressive behaviour at colony size of 10,100 and 200 workers, but the proportion of aggressive behaviours decreased at a colony size of 1000 workers. Food played an insignificant role on the survival and growth of L. humile. I replicated the laboratory study in the field using colonies of 10,100 & 1000 workers and one queen per worker category to test for the effect of propagule size, temperature and resident ant abundance on the survival of L. humile colonies. I also examined interspecific competition of the L. humile with resident ant species over sugar and tuna baits and measured their foraging distances. Contrary to the laboratory study, I found that none of the measured factors, including propagule size had a significant effect on the survival of the L. humile. Over half of the colonies in propagule sizes of 100 and 1000 workers survived longer than propagule size of 10 workers, but the mean survival times of all the propagule sizes were not significantly different. In all the propagule sizes, workers died first leaving the queens to survive alone about the same amount of time as with workers. Consequently, the mean survival time of colonies (queens with workers) was longer than mean survival time of workers in all the propagule sizes. Also, all the propagule sizes did not excel in exploitative or interference competitions. Linepithema humile in propagule size of 100 and 1000 workers discovered the baits in about the same amount of time (20-90 minutes) as native ant species, M. antarcticum but recruited very poorly to the baits. Consequently, L. humile never dominated the baits. Increasing L. humile propagule size increased foraging distance with propagule size of 1000 workers foraging the furthest distance. The results of laboratory and field studies provide first direct experimental evidence of the importance of propagule size during incipient colony establishment of L. humile. Propagule size of 1000 workers increased the ability of L. humile to resist M. antarcticum in the laboratory but failed to do so in the field condition. If propagule sizes of more than 1000 workers were used in the field a significant result would have been possible. However, the strong aggressive behaviour shown by M. antarcticum and failure of increasing propagule size to influence survival of L. humile in the field suggest that native habitats are difficult to invade. Therefore, incipient L. humile colonies that entered New Zealand may have done so in large colonies, perhaps containing more than 1000 workers.