No abstract

The overall distribution of Linepithema humile, introduced about a hundred years ago and now widespread in western and southern Portugal, has not notably changed in the last forty years. The ant is particularly common in sand and in clay loam soils but, inexplicably and despite otherwise favourable conditions, is largely absent in adjoining easterly mostly sandy loam soils associated with metamorphic rocks. In these circumstances there was a striking change in transition areas from exclusive dominance by L. humile to exclusive dominance by the competing native species, Crematogaster scutellaris and Pheidole pallidula, which was directly associated with the change in soil type. In a site with a particular sandy loam soil type (Vt) comprising adjoining patches colonized either by L. humile or native ants, the former occurred where there was a greater sand content. In oak planted sandy and Vt soils L. humile was present in cultivated or grazed ground but was replaced by C. scutellaris when there was well established scrub and ground vegetation. There was intense fighting at a transition line in one place with uniform tree and ground vegetation that supported large populations of L. humile and of native dominants on opposite sides of the transition. Here, the former had spread about 30 m per year between 1993 and 1996 compared with an estimated 800 m per year in the first 60 years after introduction. It is concluded that the dominant native C. scutellaris and P. pallidula in their favoured plantation habitats, and T. nigerrimum and T. hispanicum in some arable and pasture habitats can delay or prevent spread of L. humile in soils and vegetation conditions that also suit it. Although L. humile was present in most valley floors, its presence and abundance were not significantly related to valley conditions, and it was sometimes abundant at the tops of adjoining hillsides and in seasonally water-deficient sandy areas. Its association with some built-up areas was strikingly displayed by abundance in a town on inimical metamorphic-based soils that were uncolonized in immediately adjoining vegetationally favourable plantation and agricultural land. The direct and indirect effects of soil type on L. humile are analysed and discussed. Three habitats largely free of human interference -sand dunes with native trees and bare areas, a similarly patchily vegetated rocky river gorge and an eroded arête -may exemplify natural conditions exploited by L. humile in its native South America.

SummaryIn central Portugal, 28 species of ants were found in oak, 25 in pine, 35 in eucalyptus and 12 in arable habitats, a total of 43 species. They include five numerically dominant species, the exotic Argentine ant. Linepithema (Iridomyrmex) humile and four native species, Crematogaster scutellaris, Pheidole pallidula, Tapinoma nigerrimum and Tetramorium hispanicum.L. humile occurred in 34 % of sampled sites. When present, it was abundant in all three arboreal habitats, where it was associated with strikingly decreased ant species richness and equitability. Such diversity was greater in L. humile -colonised arable habitats, where the ant was much less abundant than in the arboreal habitats.Where L. humile was absent, C. scutellaris and P pallidula were usually co-dominants in oak and pine, and T. nigerrimum and T. hispanicum in the arable habitat, but all were absent or rare in eucalyptus. Ants were numerically least abundant in eucalyptus but, in the absence of L. humile, species richness was greater and the community more equitable than in the other habitats. L. humile -absent arable habitats supported the poorest and least equitable ant community.

Summary 1 Defoliation by larvae of the pine processionary moth, Thaumetopoea pityocampa, is negligible in stands of Pinus pinaster colonized by the Argentine ant Linepithema humile which preys fiercely on the young moth larvae. In contrast, such damage is widespread where pine plantations are colonized by native ants, predominantly Tapinoma nigerrimum and Lasius niger, which seemingly disregard the larvae. 2 Where L. humile‐ and native ant‐occupied sectors adjoin, there is a 20–50 m overlap in the transition area between L. humile‐ and native ant‐occupied pines. This was most evident in a > 500 ha plantation where there was severe or very severe T. pityocampa attack in native ant sectors contrasting with none in adjoining L. humile sectors. 3 Predation by L. humile is no doubt enhanced by its existence as super‐colonies over very large areas, by its foraging activity and recruitment on trees throughout the time when T. pityocampa and other prey are present, and by honeydew‐ producing Homoptera which help retain foraging L. humile workers in pine tree crowns. 4 The role of L. humile could be enhanced by cultivations that disturb the soil and restrain ground vegetation.

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.