Reducing losses inflicted by insect pests on cashew,using weaver ants as a biological control agent

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The effect of weaver ants Oecophylla smaragdina on the shoot borer Hypsipyla robusta on African mahoganies in Australia

• 1 African mahogany Khaya senegalensis is a high‐value timber tree species widely grown in central Africa, south‐east Asia and northern Australia. Pilot plantings show that the tree grows well in the wet‐dry tropical areas of northern Australia, and the shoot borer Hypsipyla robusta (Lepidoptera: Pyralidae) is a potential pest of the tree. The weaver ant Oecophylla smaragdina is an efficient biocontrol agent in some horticulture crops. To investigate whether the ants control shoot borers, field experiments were conducted at two sites near Darwin, Australia from April 2006 to January 2009.
• 2 In the weaver ant treatments, the overall percentage of trees damaged by shoot borers was 0–2.7% at Berrimah Farm and 0–4.2% at Howard Springs, and the damaged trees were attacked once only. In the treatments without weaver ants, however, 9.9–52.1% trees were damaged at Berrimah Farm, and 6.3–64.6% at Howard Springs, and the damaged trees were generally attacked more than once.
• 3 At both sites, significantly fewer trees on each monitoring occasion were damaged in weaver ant treatments than in treatments without weaver ants.
• 4 The mean percentage of overall flushing shoots damaged by the pest at both sites was significantly lower in weaver ant treatments compared with treatments without weaver ants.
• 5 Fewer shoots were damaged per damaged tree in weaver ant treatments compared with treatments without weaver ants.
• 6 The data obtained suggest that weaver ants were effective biological control agents of the shoot borer, and that the ants can be used to manage the pest on African mahogany trees.

     

Protection against herbivores of the myrmecophyte Leonardoxa africana (Baill.) Aubrèv. T3 by its principal ant inhabitant Aphomomyrmex afer Emery

Leonardoxa africana T3 is a myrmecophyte, a plant with specialized structures (domatia) that shelter ants. Adult trees are essentially all occupied by the ant Aphomomyrmex afer. One tree possesses one ant colony. Ants tend homopterans inside the domatia. The plant provides ants with nest sites and food via production of extrafloral nectar and via honeydew produced by homopterans. Workers patrol the young leaves, although their nectaries are not yet functional. This study was conducted to investigate the nature of the relationship between the plant and its ants. In order to determine whether ants protect the plant against herbivorous insects, we placed microlepidopteran larvae on young leaves of several trees, and measured the time until discovery of the larvae by the workers. We then studied the responses of workers as a function of insect size. We showed that workers patrolled the young leaves of the majority of trees. There was, however, inter-colony variability in intensity of patrolling. Workers attacked every larva they found, killing and eating the smaller ones, and chasing larger ones off the young leaf. Most of the phytophagous insects attacking young leaves of L. africana T3 were inventoried in this study. We showed that the larvae of microlepidopterans, one of the most important herbivores of this species, form part of the diet of A. afer. The function of the stereotyped behaviour of ant patrolling on young leaves may be in part to obtain insect protein to complement carbohydrate-rich nectar and honeydew, and in part to protect the host and thus increase its production of resources for ants. Our study shows that ants protect the tree against herbivores, and that even if this protection is less pronounced and more variable than that demonstrated for their sister species L. africana sensu stricto and Petalomyrmex phylax, the association between L. africana T3 and A. afer is a mutualism.     

Trait‐mediated indirect interactions: Moose browsing increases sawfly fecundity through plant‐induced responses

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Response of the foraging behaviour of red wood ants (Formica rufa group) to exclusion from trees

Abstract 1 Because of the large numbers within a colony and their aggressive nature, red wood ants (Formica rufa group) have a potential to greatly influence the cold‐temperate forest ecosystem. Wood ants are omnivorous and hunt in trees as well as on the forest floor. 2 A field experiment in a mixed forest in central Sweden was carried out to examine (i) the foraging behaviour of wood ants on the forest floor and (ii) the impact of increased numbers of wood ants on the soil fauna. The foraging behaviour of wood ants was manipulated by excluding the ants from their food resources in the tree canopy, with the intention to increase ant activity on the forest floor. To estimate this activity, the number of trees with foraging ants, the numbers of ants going to and from their nests and the prey carried by home‐running wood ants were determined during the summer period. Pitfall traps were placed in the soil to determine effects on mobile soil invertebrates. 3 When excluded from local trees, wood ants searched other trees further away from the nests rather than searching more intensively for prey on the forest floor. By contrast to the initial hypothesis, more soil‐living prey were caught by ants in the control plots than in the plots where the local trees were not accessible to the wood ants. The proportion of soil‐living to tree‐living prey tended to be greater in the control plots. 4 In the treated plots (no access to the trees), wood ants had a negative effect on the activity of Linyphiidae spiders. There was little effect of wood ants on other soil invertebrates. 5 This study suggests that the role of wood ants as top predators in the forest soil food‐web in central Sweden is limited.     

Sources of variation in the incidence of ant–aphid mutualism in boreal forests

• 1 The mutualism between wood ants of the Formica rufa group and aphids living in the canopy of trees is a widespread phenomenon in boreal forests, and it can affect tree growth. However, not all trees in the forest are involved in this interaction.
• 2 To assess the incidence of host trees involved in this ant–aphid mutualism and its spatial distribution in boreal forests, we inventoried sample plots with a radius of 10–15 m around wood ant mounds in 12 forest stands of two age classes (5–12‐year‐old sapling stands and 30–45‐year‐old pole stands) and two dominant tree species (Scots pine and silver birch) in Eastern Finland from 2007 to 2009.
• 3 The proportion of trees visited by ants out of all trees on the individual study plots were in the range 4–62%, and 1.5–39% of the trees on the plots were consistently visited by ants during all 3 years. The percentage of host trees increased with the ant mound base area on the plots. Trees visited by ants were larger and closer to the mound than trees not visited by ants. Within the group of visited trees, more ants were found on bigger trees and on trees close to the ant mounds.
• 4 Extrapolated from plot to stand level, we estimated that 0.5–6.6% of the trees were host trees in at least one of the three study years, and that only 0.01–2.3% of all the trees were consistently visited by ants during all 3 years. It is concluded that ant–aphid mutualism is a minor occurrence at the stand level.

     

Biological control of the fruit‐spotting bug Amblypelta lutescens using weaver ants Oecophylla smaragdina on African mahoganies in Australia

• 1 African mahogany Khaya senegalensis is a high‐value timber tree. Pilot plantings showed that the fruit‐spotting bug Amblypelta lutescens causes severe damage of the tree in the wet–dry tropics of northern Australia. The weaver ant Oecophylla smaragdina is an efficient biocontrol agent in some horticulture crops. To investigate whether the ants control this pest, field experiments were conducted from April 2006 to January 2009 at two study sites in the Darwin area, Australia. A laboratory experiment was carried out in March 2007 at Berrimah Farm.
• 2 During the experimental period, in the weaver ant treatments, the overall percentage of trees damaged by the pest was 0–8% at both sites, and the damaged trees were attacked once only. In the treatments without weaver ants, however, the damage level was > 80% at Berrimah Farm and 31–100% at Howard Springs, and the damaged trees were attacked more than once.
• 3 The mean percentage of trees damaged per monitoring occasion was 0–2.6% in the weaver ant treatments at both sites, whereas, in the treatments without the ants, the damage percentages were 14.2–27.0% at Howard Springs and 28.2–48.6% at Berrimah Farm.
• 4 Extrafloral nectar of African mahoganies is attractive to weaver ants. Fruit‐spotting bugs only damage the tender parts of flushing shoots and growing tips. Weaver ants live on sugar solution and meat, and they frequently harvest extrafloral nectar on growing shoots, on which they catch nymphs of the pest for their meat supply. The aggressive behaviour of the ants also repels the pest away from flushing shoots.
• 5 The data suggest that weaver ants were effective biocontrol agents of fruit‐spotting bugs, and the ants can be used to manage the pest on African mahoganies.
• 6 The present study demonstrates that the introduced African mahogany comprises another major host of the fruit‐spotting bug.

     

Abundance and mortality of leaf miners on artificially shaded Emory oak

ABSTRACT.

• 1 We measured solar radiation reaching ten Quercus emoryi Torr. trees and recorded densities of four leaf-mining insect species on these trees from June until September 1982.
• 2 The measurements showed that densities of two leaf miner species were negatively correlated with solar radiation.
• 3 In a field experiment, polypropylene shade fabric was suspended 1.3 m above four experimental trees to test for effects of reduced sunlight.
• 4 Leaves of experimentally-shaded trees were heavier and contained lower percentages of proteins and gallotannins than leaves of control, sun trees, while per cent foliar monomeric, polymeric, and total phenols, and water content did not differ between sun and shaded trees.
• 5 Two of four leaf miner species had greater densities on experimentally-shaded trees than sun trees. One of these two species experienced lower survivorship on sun trees owing to high levels of death from unknown causes on sun trees.
• 6 Two leaf miner species had greater densities on sun trees. One of these species had higher survivorship on shaded trees owing to high levels of parasitism on sun trees.
• 7 We conclude that even subtle differences in shading influences leaf miner density and mortality; however, the effects of shading vary from positive to negative among leaf miner species.

     

Assessment of trends in predation pressure on insects across temperate forest microhabitats

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Azteca protection of Cecropia: ant occupation benefits juvenile trees

Summary In this 15 month investigation I experimentally demonstrated that sapling Cecropia aff. obtusifolia in lowland western Ecuador grow more vigorously when occupied by the ant Azteca constructor than when the ants have been removed. Thus the interaction is directly beneficial to Cecropia juveniles. The difference in growth is associated with differences in herbivory and vine cover. Removal of ants significantly increases nocturnal Coleoptera herbivory on unoccupied plants. In contrast to the influence on beetle numbers, Azteca are ineffective against Homoptera and cecidomyiid gall flies. Although ant-occupied saplings had less chewing herbivore damage throughout the study, the ants were more effective protectors in the dry season than in the rainy season, when herbivore pressure increased. In addition to reducing herbivory, Azteca efficiently remove vines from occupied saplings.     

Intraspecific variation in beech scale populations and in susceptibility of their host Fagus sylvatica

ABSTRACT.

• 1 Within a seed orchard in southern England, beech trees (Fagus sylvatica L.) belonging to three clones were artificially infested by introducing beech scale larvae (Cryptococcus fagisuga Lind. Homoptera: Coccidae) into small cages attached to the bark.
• 2 Some larvae developed to fecund adults on trees of two susceptible clones but all failed to develop on a third, resistant clone.
• 3 Within susceptible clones, survival of larvae on individual trees was positively related to their degree of natural infestation.
• 4 Larvae deriving from several separate trees differed significantly in their ability to survive when inoculated onto trees of susceptible clones.
• 5 Five forest trees which acted as both donors of larvae and as hosts for artificial inoculation were each inoculated with larvae from all five trees.
• 6 There was significant variation in survival of inoculated larvae both between the host trees and between sources of larvae on each host.
• 7 Survival of larvae reinoculated onto their original host was significantly higher than that of larvae originating from other trees.
• 8 Fecundity of adults on the forest trees was positively correlated with the probability of inoculated larvae surviving to the adult stage.

     

Ants protect conifer seedlings from feeding damage by the pine weevil Hylobius abietis

• 1 Ants that protect food resources on plants may prey on (or deter) herbivores and thereby reduce damage. Red wood ants (of the Formica rufa group) are dominant ants in boreal forests of Eurasia and affect the local abundance of several herbivorous species.
• 2 The pine weevil Hylobius abietis (L.) is a herbivore that causes severe damage by feeding on the bark of coniferous seedlings within areas of forest regeneration.
• 3 We investigated whether ants can protect conifer seedlings from pine weevil feeding. In a manipulative experiment, ants were attracted to sugar baits attached to spruce seedlings and the damage caused by pine weevils was compared with control seedlings without ant‐baits.
• 4 The feeding‐scar area was approximately one‐third lower on the seedlings with ant‐baits compared with the controls. Besides red wood ants, Myrmica ants were also attracted in high numbers to the ant baits and the relative effects of these species are discussed.
• 5 The results obtained in the present study support the trophic cascade hypothesis (i.e. damage to herbivores is suppressed in the presence of predators). The decreased pine weevil feeding on the baited seedlings was probably a result of nonconsumptive interactions [i.e. the presence of (or harassment by) ants distracting pine weevils from feeding].
• 6 Understanding the role of ants may have important implications for future strategies aiming to control pine weevil damage. For example, maintaining suitable conditions for ants after harvesting stands may be an environmentally friendly but currently unexploited method of for decreasing weevil damage.

     

Colony structure in a plant-ant: behavioural,chemical and genetic study of polydomy in<Emphasis Type="Italic"> Cataulacus mckeyi</Emphasis> (Myrmicinae)

Social organisation of colonies of obligate plant-ants can affect their interaction with myrmecophyte hosts and with other ants competing for the resources they offer. An important parameter of social organisation is whether nest sites of a colony include one or several host individuals. We determined colony boundaries in a plant-ant associated with the rainforest understorey tree Leonardoxa africana subsp. africana, found in coastal forests of Cameroon (Central Africa). This myrmecophyte is strictly associated with two ants, Petalomyrmex phylax and Cataulacus mckeyi. Plants provide food and nesting sites for P. phylax, which protects young leaves against insect herbivores. This mutualism is often parasitised by C. mckeyi, which uses but does not protect the host. The presence of C. mckeyi on a tree excludes the mutualistic ant. Because Petalomyrmex -occupied trees are better protected, their growth and survival are superior to those of Cataulacus -occupied trees, giving P. phylax an advantage in occupation of nest sites. C. mckeyi often colonises trees that have lost their initial associate P. phylax, as a result of injury to the tree caused by disturbance. Polydomy may allow C. mckeyi to occupy small clumps of trees, without the necessity of claustral colony foundation in each tree. Investigating both the proximate (behavioural repertoire, colony odour) and the ultimate factors (genetic structure) that may influence colony closure, we precisely defined colony boundaries. We show that colonies of C. mckeyi are monogynous and facultatively polydomous, i.e. a colony occupies one to several Leonardoxa trees. Workers do not produce males. Thus, the hypothesis that polydomy allows workers in queenless nests to evade queen control for their reproduction is not supported in this instance. This particular colony structure may confer on C. mckeyi an advantage in short-distance dispersal, and this could help explain its persistence within the dynamic Leonardoxa system.     

Root disturbance of common ash, Fraxinus excelsior (Oleaceae), leads to reduced foliar toughness and increased feeding by a folivorous weevil, Stereonychus fraxini (Coleoptera, Curculionidae)

Abstract.

• 1 Bioassays were carried out to examine differences in the feeding of a folivorous weevil, Stereonychus fraxini De Geer, on leaves from ash saplings with undamaged roots, and those with damaged roots. Beetles ate significantly more of the leaves of root-damaged saplings in a choice experiment.
• 2 A separate study of the effect of root damage on leaf toughness was carried out on a second group of ash saplings. Saplings with damaged roots had less tough leaves than control saplings. Decreased leaf toughness in hypothesized as a mechanism to explain the preference of weevils for leaves from damaged trees.
• 3 Both chemical and physical changes may occur in the tissues of ash trees in response to environmental stress such as drought and root damage.
• 4 Root damage caused by agricultural disturbance is hypothesized as a mechanism making mature ash trees in hedgerow ecosystems more susceptible to insect herbivores.

     

Positive effects of ants on host trees are critical in years of low reproduction and not influenced by liana presence

Tropical tree-climbing lianas form paths that benefit foraging of dominant ants which might protect the host tree against herbivores. In contrast, lianas are often associated with negative effects on growth and reproduction of host trees due to light obstruction, structural stress and other negative effects. It is unclear if dominant ants could mitigate the negative effects of lianas on host plants. We investigated how lianas and carton nest ants (Azteca chartifex) affected herbivory and reproductive structures of the host tree Byrsonima sericea. Considering 68 trees, almost half of them were naturally colonized by A. chartifex nests (32 trees). We removed lianas from half of the trees (34), establishing a factorial sampling design between A. chartifex and liana presence. We sampled ants and leaf herbivory before and after removing lianas, and measured plant fitness in two consecutive years after removing lianas. Liana removal had no effect on A. chartifex foraging, on leaf herbivory and flower-fruit conversion of host plants. However, A. chartifex decreased leaf herbivory and increased B. sericea flower-fruit conversion irrespective of lianas presence. A noticeable positive effect of ants was detected only in the second year of the experiment, consistently on all plants at each experimental level. The reproductive conditions of the first year resulted in most plants with more than 75% flower-to-fruit conversion success, regardless of the presence of A. chartifex, a success sustained only on those ant-colonized plants in the second year. Our results contribute to understanding multi-trophic interactions in tropical forest canopies as we demonstrated i) that dominant arboreal ants can benefit plants even in a non-obligatory interaction and ii) that the influence of lianas on its host tree is context-dependent, presenting even neutral effects depending on habitat type and species involved.     

Enhanced seed dispersal of Prunus africana in fragmented and disturbed forests?

Forest destruction and disturbance can have long-term consequences for species diversity and ecosystem processes such as seed dispersal. Understanding these consequences is a crucial component of conserving vulnerable ecosystems. In the heavily fragmented and disturbed Kakamega Forest, western Kenya, we studied seed dispersal of Prunus africana (Rosaceae). In the main forest, five forest fragments, and differently disturbed sites, we quantified the overall frugivore community as an indicator for species diversity. Furthermore, we determined the frugivores on 28 fruiting P. africana trees, estimated seed dispersal, crop size and the general fruit availability of surrounding trees. During the overall frugivore census we recorded 49 frugivorous species; 36 of them were observed visiting P. africana trees and feeding on their fruits. Although overall frugivore species richness was 1.1 times lower in fragments than in main forest sites and 1.02 times higher in highly disturbed than in less disturbed sites, P. africana experienced 1.1 times higher numbers of frugivores in fragments than in main forest sites and 1.5 times higher numbers of frugivores in highly disturbed than in less disturbed sites. Correspondingly, seed dispersal was 1.5 times higher in fragments than in main forest sites and 1.5 times higher in more disturbed than less disturbed sites. Fruit availability of surrounding trees and crop size influenced the number of visitors to some degree. Thus, the number of dispersed seeds seemed to be slightly higher in fragmented and highly disturbed sites. This indicates that loss of single species does not necessarily lead to a decrease of ecosystem services. However, loss of diversity could be a problem in the long term, as a multitude of species might act as buffer against future environmental change.     

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.     

Impact of an invasive alien ant,Wasmannia auropunctata Roger., on a specialised plant–ant mutualism,Barteria fistulosa Mast. and Tetraponera aethiops F. Smith., in a Gabon forest

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Stand type is more important than red wood ant abundance for the structure of ground‐dwelling arthropod assemblages in managed boreal forests

• 1 The relationships between red wood ants (Formica rufa group) and other ground‐dwelling arthropods were studied in young managed forests stands in Eastern Finland. The main objectives were: (i) to test the influence of stand type (dominant tree species; age: sapling versus pole stage) and numbers of red wood ants on the occurrence of other ground‐dwelling arthropods and (ii) to study the occurrence of red wood ants versus other arthropods on a distance gradient from ant mounds. We used pitfall traps set in 5–14‐year‐old sapling stands and 30–45‐year‐old pole‐stage stands of Scots pine (Pinus sylvestris L.) and birch (Betula spp.) forests.
• 2 Pitfall trap catches of red wood ants did not vary significantly between the forest stand types, although some groups of other arthropods showed clear responses to stand type (e.g. catches of other Formicinae and Gnaphosidae were higher in sapling stands than in pole‐stage stands). The number of red wood ants clearly explained less of the variation in assemblages of other ground‐dwelling arthropods than the forest stand type.
• 3 Red wood ant numbers decreased significantly with distance from the mounds, but the other ground‐dwelling arthropods were insensitive to this gradient or even showed a preference for proximity to ant mounds and high ant activity.
• 4 The results obtained in the present study suggest that wood ants do not have strong effects on several other ground‐dwelling arthropod groups in young managed forests other than in the immediate vicinity of their mounds.

     

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.