Observations on the Ecology of Weaver Ants (Oecophylla smaragdina Fabricius) in a Thai Mangrove Ecosystem and Their Effect on Herbivory of Rhizophora mucronata Lam.

Ants of the genus Oecophylla are predators of other insects and are able to protect a variety of terrestrial plants against pest insects; however, observations on the ecology of these ants in mangrove forests are lacking. General observations on the ecology of Oecophylla smaragdina were carried out in a Thai mangrove forest to determine if these ants can protect their host plants in less favorable mangrove habitats. Leaf herbivory and the density of O. smaragdina ants were measured on Rhizophora mucronata trees at two sites. The results showed a negative correlation between ant density and herbivory. At both sites, the mean percent damaged leaf area was more than four times higher on trees without ants compared to “ant‐trees.” A significant negative correlation was found between tree mean percent leaf damage and the density of ants on the tree. Furthermore, on trees with ants, there was less herbivory on leaves close to ant nests compared to other leaves on the tree. Most damage was caused by chrysomelid beetles (62%) and sesarmid crabs (25%) and both types of herbivory were significantly reduced on ant‐trees.     

Density-dependent ant attendance and its effects on the parasitism of a honeydew-producing scale insect,Ceroplastes rubens

The intensity of attendance by a honeydew-foraging ant, Lasuis niger, on the red wax scale insect, Ceroplastes rubens, was estimated at different manipulated densities in the field. The time that individual ants were present and the total attendance time (seconds x number of ants) of ants on scale-infested twigs significantly increased as the density of C. rubens increased, i.e. ant attendance was density dependent. To determine the effects of density dependence of ant attendance on parasitism of C. rubens by Anicetus beneficus, we measured parasitism rates in the field at different density levels of C. rubens both with ant attendance and with ants excluded. Parasitism rates were higher when ants were excluded, at each density level. Although the parasitism rate significantly deceased as scale density increased, whether or not ants attended, the difference in parasitism rate between density levels was strikingly less without ant attendance. Therefore, the density-dependent decrease of parasitism rate was more pronounced with ant attendance. Mortality not due to parasitism showed density dependence in both conditions and did not change when ants were excluded. These results indicate that attending ants reduce parasitism and that, as a consequence of the density dependence of ant attendance, the efficiency of reduction of parasitism by ants is enhanced at higher densities of C. rubens.     

Asian weaver ants,Oecophylla smaragdina,and their repelling of pollinators

The Asian weaver ant, Oecophylla smaragdina, is known to have outstanding predatory power. This ant can protect the host plants from attacks of phytophagous insects and therefore has been used for biological control in the tropics. We present evidence for a possible negative effect of Oecophylla on the performance of host plants. Our observation in a fruit orchard of rambutan in Sumatra suggested that the presence of Oecohylla nests on the trees statistically significantly lowered the flower-visiting rate of flying insects, involving the major pollinator Trigona minangkabau. The visiting rate of Oecophylla workers to each flower shoot of rambutan significantly negatively correlated with the visiting rate of flying insects. Empirical evidence of such an inhibitory effect on flower-visiting of pollinators cased by aggressive ants has been scarce so far.     

Effects of habitat fragmentation on the three-way interaction among ants,aphids and larvae of the giant purple emperor, <Emphasis Type="Italic">Sasakia charonda</Emphasis> (Hewitson), a near-threatened butterfly

Survival rates of both early and middle instar larvae of the nymphalid butterfly, Sasakia charonda, were estimated to be lowest on test trees planted in a meadow (site A), intermediate in a small, narrow secondary deciduous broadleaf forest (small patch, site B) and highest in a large secondary deciduous broadleaf forest (large forest, site C). The larval mortality rates due to predation by tree-climbing predators from the ground (tree climbing predator) such as ants and the larvae of carabids were estimated to be greater at sites A and B than those at site C. The number of predatory ants climbing test trees was significantly greater at sites A and B than at site C, and the ants harvested honeydew from aphids living on tree leaves at those two sites. Aphid densities were significantly higher on trees at sites A and B than at site C, and aphid densities and numbers of predatory ants were significantly and positively correlated at sites A and B. In an experiment controlling aphid density per branch on test trees, the numbers of ants and the mortality rates of S. charonda larvae were greater on branches with high aphid densities than on those with low aphid densities at both sites A and B. These results suggest that the aphid density per host tree was higher in the meadow and the small patch than in the large forest; at both sites these higher aphid densities attracted higher numbers of predatory ants to test trees, and as a result, mortality rates of S. charonda larvae were increased.     

Experimental evaluation of the impacts of two ant species on banana weevil in Uganda

The banana weevil, Cosmopolites sordidus (Germar), is an important pest of bananas. Predatory ants are increasingly being viewed as possible biological control agents of this pest because they are capable of entering banana plants and soil in search of prey. We studied ant predation on banana weevil in Uganda in crop residues and live plants in both laboratory and field experiments. Field studies with live plants used chemical ant exclusion in some plots and ant enhancement via colony transfer in others to measure effects of Pheidole sp. 2 and Odontomachus troglodytes Santschi on plant damage and densities of immature banana weevils.In crop residues, an important pest breeding site, twice as many larvae were removed from ant-enhanced plots as in control plots. In young (2 month) potted suckers held in shade houses, ant ability to reduce densities of banana weevil life stages varied with the weevil inoculation rate. At the lowest density (2 female weevils per pot), densities of eggs, larvae, and pupae were reduced by ants. At higher rates there was no effect. In older suckers (5–11 months) grown in larger containers, banana weevil densities were not affected by ants, but damage levels were reduced. In a field trial lasting a full crop cycle (30 months), we found that the ants tested reduced the density of banana weevil eggs in suckers during the crop, but did not affect larval densities in the sampled suckers. However, most larvae occur in the main banana plants, rather than associated suckers. Nevertheless, levels of damage in mature plants at harvest did not differ between Amdro-treated and ant-enhanced plots, suggesting the ant species studied were not able to provide economic control of banana weevil under our test conditions.     

Effect of supplementary feeding of Oecophylla longinoda on their abundance and predatory activities against cashew insect pests

Many studies have shown the efficiency of using weaver ants (Oecophylla species) as natural biocontrol agents against agricultural pests. Supplementary feeding could promote fast growth of this ant's population and discourage them from moving away. However, such artificial feeding might slow down ants search rates and in this way make them less efficient bio-agents. The experiments were conducted for two consecutive seasons at Naliendele Research Station. Cashew trees planted at a spacing of 12 m × 12 m in 2002 were used to investigate whether supplementary feeding could enhance foraging behaviour of Oecophylla longinoda. Fed O. longinoda colonies fed weaver ants (FWA) were supplemented with a 30% sugar solution and approximately 22 g of finely ground fish meat at two-week intervals while the unfed colonies unfed weaver ants (UWA) had access to only naturally occurring food sources. Weaver ant densities and pest damage were monitored fortnightly on newly damaged shoots, panicles and fruits and nut yields assessed after each harvest season. The results revealed that there was a significant difference (P < 0.05) with higher weaver ant densities in the FWA compared to UWA colonies and significantly lower (P < 0.05) pest damage levels were recorded on weaver ant treatments compared to plots without weaver ants. No significant differences (P > 0.05) in yields and mean damage levels were recorded between the two weaver ant treatments. Highest nut yield (4.22 ± 0.30 kg/tree and 5.37 ± 0.27 kg/tree) was recorded in the fed colonies, followed by non-fed colonies (4.20 ± 0.30 kg/tree and 4.88 ± 0.24 kg/tree) and the least (2.66 ± 0.19 kg/tree and 2.99 ± 0.19 kg/tree) was recorded from the untreated controls in 2012/2013 and 2013/2014, respectively. The studies indicated that supplementary feeding could boost weaver ants to higher population levels without reducing their effectiveness as biocontrol agents.     

Bionomics ofPhyllonorycter (Lepidoptera,Gracillariidae) onQuercus. II. Effects of ants

The effects of ants on a population ofPhyllonorycter leaf miners were observed in an oak chaparral on the Ishikari Coast. The density of ant nests was higher at the chaparral's edge than in the interior, while the density of mines was lower at the edge than in the interior. The effects of ant predation were examined in a comparison between treated ant-free trees and untreated ant-rich trees. Percentages of the torn mines observed were higher on the ant-rich trees than on the ant-free trees, whereas the mortality due to parasitoids was higher on the ant-free trees than on the ant-rich trees. This shows that ants greatly contributed to the mortality of leaf miners.     

Interference competition,not predation,explains the negative association between wood ants (Formica rufa) and abundance of ground beetles (Coleoptera: Carabidae)

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Look before leaping: foraging selectivity of capuchin monkeys on acacia trees in Costa Rica

Acacia trees in Costa Rica have an obligate mutualism with three species of Pseudomyrmex ants, which vigorously defend their host tree from insect and mammalian herbivores. Depending on the size and species of ant colony, individual acacia trees may be differentially protected. For animals able to discern between weakly and highly aggressive ant colonies, costs of ant stings from less active colonies might be offset by nutritional value acquired from feeding on acacia fruit or ant larvae in swollen thorns. We examined foraging selectivity of capuchin monkeys on acacia trees in Santa Rosa National Park, Costa Rica. We measured four characteristics of the acacia trees from which capuchins fed and of acacias immediately adjacent to those in which the monkeys fed: diameter at breast height (DBH), accessibility, species of closest tree and ant species present. We found that capuchins prefer to forage in acacias that are large and accessible. We also made two measurements of ant colony activity on each tree, one before and one after disturbing the ant colony. We found that the three species of mutualistic ants differ in baseline activity levels and that mutualistic ants are more active than non-mutualistic ant species found in acacia trees. We also found that capuchins foraged more frequently in trees colonized by non-mutualistic ants, but the explanatory value (r ²) of this model was low. Furthermore, monkeys did not discriminate between acacias on the basis of baseline ant activity or the ant colony’s response to disturbance. We conclude that these monkeys select acacia trees in which to forage based on characteristics of the trees rather than the ants. In addition, our study suggests that white-faced capuchins act as predators on the acacia ants but they probably benefit the dispersal and reproductive success of acacia trees. Capuchins may in fact function as an additional mutualistic partner for acacia trees via seed dispersal, but they must overcome the ants’ defense of the trees to do so.     

When are ant-attractant devices a worthwhile investment? Vicia faba extrafloral nectaries and Lasius niger ants

Most studies aiming to determine the beneficial effect of ants on plants simply consider the effects of the presence or exclusion of ants on plant yield. This approach is often inadequate, however, as ants interact with both non-tended herbivores and tended Homoptera. Moreover, the interaction with these groups of organisms is dependent on ant density, and these functional relationships are likely to be non-linear. A model is presented here that segregates plant herbivores into two categories depending on the sign of their numerical response to ants (myrmecophiles increase with ants, non-tended herbivores decline). The changes in these two components of herbivores with increasing ant density and the resulting implications for ant-plant mutualisms are considered. It emerges that a wide range of ant densities needs to be considered as the interaction sign (mutualism or parasitism) and strength is likely to change with ant density. The model is used to interpret the results of an experimental study that varied levels of Aphis fabae infestation and Lasius niger ant attendance on Vicia faba bean plants. Increasing ant density consistently reduced plant fitness and thus, in this location, the interaction between the ants and the plant can be considered parasitic. In the Vicia faba system, these costs of ants are unlikely to be offset by other beneficial agents (e.g., parasitoids), which also visit extrafloral nectaries.     

Ant foraging on extrafloral nectaries of Qualea grandiflora (Vochysiaceae) in cerrado vegetation: ants as potential antiherbivore agents

Summary Qualea grandiflora is a typical tree of Brazilian cerrados (savanna-like vegetation) that bears paired extrafloral nectaries (EFNs) along its stems. Results show that possession of EFNs increases ant density on Q. grandiflora shrubs over that of neighbouring non-nectariferous plants. Frequency of ant occupancy and mean number of ants per plant were much higher on Qualea than on plants lacking EFNs. These differences resulted in many more live termitebaits being attacked by foraging ants on Qualea than on neighbours without EFNs. Termites were attacked in equal numbers and with equal speeds on different-aged leaves of Qualea. The greatest potential for herbivore deterrence was presented by Camponotus ants (C. crassus, C. rufipes and C. aff. blandus), which together attacked significantly more termites than nine other ant species grouped. EFNs are regarded as important promoters of ant activity on cerado plants.     

Ant predation ofEriocrania miners in a polluted area

We tested the hypothesis that air pollution may affect population densities of birch-feeding leafminers via changes in ant predation. Foraging activity of three ant species (Formica rufa, F. fusca andF. lemani), predation rates and population densities of both solitary and gregariousEriocrania (Lepidoptera: Eriocraniidae) miners were investigated at 13 sites around the Harjavalta copper-nickel smelter, SW Finland. Ant species differed in their distribution patterns relative to pollution. However, the total percentage of birch trees foraged by ants (all species combined) showed no correlation with the distance from the factory complex. As a result, no clear trends in predation rates were apparent in relation to the distance from the pollution source for either solitary or gregariousEriocrania species. Densities of the solitaryEriocrania species tended to increase with the distance from the pollution source whereas densities of the gregariousE. haworthi peaked close to the factory complex. No corresponding differences in predation rates between solitary and gregarious miners were found. Ant predation, thus, did not explain density patterns ofEriocrania miners in the polluted area.     

Argentine ant infestation affects the abundance of the native myrmecophagic jumping spider <Emphasis Type="Italic">Siler cupreus</Emphasis> Simon in Japan

The effect of an Argentine ant invasion on the abundance of the myrmecophagic jumping spider Siler cupreus Simon was investigated in the Hiroshima Prefecture, southwestern Japan. The frequency and density of S. cupreus were significantly higher in sites infested with Argentine ants than in ant-free sites. S. cupreus actually preyed on the adult ants and the brood. The dominance of Argentine ants possibly provides an abundant food source for S. cupreus, thus causing an increase in the number of spiders. Received 16 May 2007; revised 27 December 2007; accepted 15 January 2008.     

Using insect traps to increase weaver ant (Oecophylla longinoda) prey capture

Weaver ants (Oecophylla spp.) are managed in plantations to control insect pests and are sometimes harvested as a protein‐rich food source. In both cases, the amount of insect prey caught by the ants is imperative for returns, as more prey leads to more effective biocontrol and to a higher production of ants. Malaise‐like traps placed in trees may catch flying insects without catching ants, as ants may use pheromone trails to navigate in and out of the traps. Thus, ants may increase their prey intake if they are able to extract insects caught in traps. In a mango plantation in Tanzania, we estimated the amount of insects caught by simple traps (cost per trap = 3.9 USD), and whether Oecophylla longinoda was able to collect insects from them. On average, a trap caught 110 insects per month without catching any weaver ants. The number of insects found in traps with ant access was 25% lower than in control traps (ants excluded), showing that ants were able to gather prey from the traps. Ant activity in traps increased over time, showing that prey extraction efficiency may increase as ants customize to the traps. The prey removed from traps by ants constituted 5% of the number of prey items collected by O. longinoda under natural conditions (without traps), potentially increasing to 14% if ants learn to extract all insects. Thus, prey intake may be increased with 5–14% per 3.9 USD invested in traps. These numbers increased to 38 and 78%, respectively, when light was used to attract insects during night time. Combining ant predation with insect trapping is a new approach potentially building increased returns to ant biocontrol and to ant entomophagy.     

<Emphasis Type="Italic">Myrmica</Emphasis> host-ants limit the density of the ant-predatory large blue <Emphasis Type="Italic">Maculinea nausithous</Emphasis>

Butterflies of the highly endangered genus Maculinea are parasites of red Myrmica ants. Prior to the adoption by Myrmica worker ants Maculinea caterpillars feed on a specific host plant. This field study aims to answer the question whether the density and distribution of the host plant Sanguisorba officinalis or the density of the host ant M. rubra limit the density of M. nausithous egg, larval and adult stage. We found that the density of M. nausithous egg stage and adult stage increased with the density of the host ant. The density of M. nausithous caterpillars was not associated with ant density or plant density. This study suggests that the density of M. nausithous is limited by the density of the host ant M. rubra. We conclude that habitat management for M. nausithous should focus on the maintenance of habitats that hold both resources, but that enable high densities of M. rubra. In addition, it is discussed why high densities of host ants might be more important in predatory than in cuckoo-feeding Maculinea.     

Effects of ants on the foraging of birds in spruce trees

I experimentally excluded ants from randomly selected spruce trees Picea abies near colonies of the wood ant Formica aquilonia. Foraging activity of birds in these trees was then compared to the foraging activity of birds in neighboring spruce trees, where ants were allowed to continue foraging. Birds which foraged in the foliage showed the effects of competition with ants: they visited the trees without ants more frequently, and for longer periods. In addition, the insects and spiders that they utilized as food were more abundant in the foliage of trees without ants. Cone-foraging birds, however, which fed on seeds in cones at the tops of the trees, did not show a preference for trees without ants. The differences of tree usage between foliage-gleaning and coneforaging birds can be explained by alteration of the birds' food supply by wood ants: ants did not feed on seeds in cones, and so did not compete with cone-foraging birds. However, foraging wood ants did feed on arthropods living in the foliage, thus reducing the amount of food available to birds there.     

Territories of the African Weaver Ant (Oecophylla longinoda [Latreille]); A Field Study

The African weaver ant (Oecophylla longinoda) is known to be highly aggressive toward conspecific aliens. In the study area Shimba Hills Reserve (Kenya) individual territories sometimes covered an area of up to approximately 1600 m², comprising 17 major trees. The territorial defense is organized by an elaborate defense recruitment system, which can also be aimed against several other ant species, which are potential competitors of Oecophylla for essential resources. This selective “enemy identification” seems to be the major behavioral mechanism by which the mosaic distribution of ecologically dominant ants is regulated.     

Spatio-temporal patterns of ground-dwelling ant assemblages in a lowland rainforest in southeast Cameroon

We studied the spatio-temporal patterns of ground-dwelling and -foraging ant assemblages in a continuous mixed secondary lowland rainforest on the northern periphery of the Dja Biosphere Reserve in southeast Cameroon. The effect of season and of the interaction of season with vegetation type on ant species density, activity and composition were investigated. We also checked for a possible impact of army ant activity on the ant assemblage. Ants were collected by pitfall trapping in seven vegetation types in each of three seasons during 12 months in 2003 and 2004–2005. Season significantly influenced the ground-dwelling ant assemblage. The late wet season had the lowest ant species density and a different species composition than the early wet and the dry season. Also inter-annual variation in ant species density and composition was detected and could partly be explained by an influence of Dorylus activity. In general, vegetation type and inter-annual variation had a larger influence on the ground-ant assemblage than season. Significant effects of interactions between season, year, vegetation type and army ants on the ant assemblage confirm that also in African rainforests ant assemblages are extremely dynamic and depend on a complex combination of availability of food resources, nest sites and predators. Received 4 October 2006; revised 4 June 2007; accepted 2 July 2007.     

Symbiotic ants as an alternative defense against giraffe herbivory in spinescent Acacia drepanolobium

Summary We explore here the occurrence of aggressive ants in an apparently symbiotic relationship with the savanna tree Acacia drepanolobium and their effects on giraffe herbivory on the Athi-Kapiti Plains, Kenya. Trees taller than 1.3 m were more likely to be occupied by aggressive ants in the genus Crematogaster than were shorter trees. Ants wereconcentrated on shoot tips, the plant parts preferred by giraffes. Trees with relatively more foliage had more swarming ants than did trees with less foliage. The feeding behavior of individual freeranging giraffes on Acacia drepanolobium was studied. Giraffe calves exhibited a strong sensitivity to Crematogaster ants inhabiting A. drepanolobium, feeding for significantly shorter periods on trees with a greater number of aggressive ants. Older giraffes were apparently less sensitive to ants, and did not feed for shorter periods on trees with fuller foliage, despite significantly greater ant activity on these plants. The thorns of A. drepanolobium are significantly shorter than are the thorns of A. seyal, a species without symbiotic ants, a pattern that may indicate a trade-off between ants and thorns as defenses.     

Plant defense, herbivory, and the growth of Cordia alliodora trees and their symbiotic Azteca ant colonies

The effects of herbivory on plant fitness are integrated over a plant??s lifetime, mediated by ontogenetic changes in plant defense, tolerance, and herbivore pressure. In symbiotic ant?Cplant mutualisms, plants provide nesting space and food for ants, and ants defend plants against herbivores. The benefit to the plant of sustaining the growth of symbiotic ant colonies depends on whether defense by the growing ant colony outpaces the plant??s growth in defendable area and associated herbivore pressure. These relationships were investigated in the symbiotic mutualism between Cordia alliodora trees and Azteca pittieri ants in a Mexican tropical dry forest. As ant colonies grew, worker production remained constant relative to ant-colony size. As trees grew, leaf production increased relative to tree size. Moreover, larger trees hosted lower densities of ants, suggesting that ant-colony growth did not keep pace with tree growth. On leaves with ants experimentally excluded, herbivory per unit leaf area increased exponentially with tree size, indicating that larger trees experienced higher herbivore pressure per leaf area than smaller trees. Even with ant defense, herbivory increased with tree size. Therefore, although larger trees had larger ant colonies, ant density was lower in larger trees, and the ant colonies did not provide sufficient defense to compensate for the higher herbivore pressure in larger trees. These results suggest that in this system the tree can decrease herbivory by promoting ant-colony growth, i.e., sustaining space and food investment in ants, as long as the tree continues to grow.