Interactive effects of light environment and herbivory on growth and productivity of an invasive annual vine, Persicaria perfoliata

Plant populations often exist in spatially heterogeneous environments with varying light levels, which can affect plant growth directly through resource availability or indirectly by altering behavior or success of herbivores. The plant vigor hypothesis predicts that herbivores are more likely to attack vigorously growing plants than those that are suppressed, for example in more shaded conditions. Plant tolerance of herbivory can also vary under contrasting resource availability. Observations suggest that damage by Rhinoncomimus latipes Korotyaev (Coleoptera: Curculionidae), introduced into the United States in 2004 as a biological control agent for mile-a-minute weed (Persicaria perfoliata [L.] H. Gross), is greater in the sun than in shade. We compared weevil densities and plant growth in paired plots in full sun or under shade cloth; a second experiment included insecticide-treated plots in sun and shade, to assess the ability of the plant to compensate for herbivore damage. Greater density of weevils and more node damage (indicating internal larval feeding) were found on P. perfoliata plants growing in sun than on those in shade. Nodes were 14% thicker in the sun, which may have provided better larval habitat. Biomass produced by plants without weevils in the sun was about twice that produced in any other treatment. Herbivory had a greater effect on plant growth in the high-light environment than in the shade, apparently because of movement into the sun and increased feeding there by the monophagous herbivore, R. latipes. Results support the plant vigor hypothesis and suggest that high weevil densities in the sunny habitats favored by P. perfoliata can suppress plant growth, negating the resource advantage to plants growing in the sun.     

Reciprocally beneficial interactions between introduced plants and ants are induced by the presence of a third introduced species

Interspecific interactions play an important role in the success of introduced species. For example, the ‘enemy release’ hypothesis posits that introduced species become invasive because they escape top–down regulation by natural enemies while the ‘invasional meltdown’ hypothesis posits that invasions may be facilitated by synergistic interactions between introduced species. Here, we explore how facilitation and enemy release interact to moderate the potential effect of a large category of positive interactions – protection mutualisms. We use the interactions between an introduced plant (Japanese knotweed Fallopia japonica), an introduced herbivore (Japanese beetle Popillia japonica), an introduced ant (European red ant Myrmica rubra), and native ants and herbivores in riparian zones of the northeastern United States as a model system. Japanese knotweed produces sugary extrafloral nectar that is attractive to ants, and we show that both sugar reward production and ant attendance increase when plants experience a level of leaf damage that is typical in the plants’ native range. Using manipulative experiments at six sites, we demonstrate low levels of ant patrolling, little effect of ants on herbivory rates, and low herbivore pressure during midsummer. Herbivory rates and the capacity of ants to protect plants (as evidenced by effects of ant exclusion) increased significantly when plants were exposed to introduced Japanese beetles that attack plants in the late summer. Beetles were also associated with greater on‐plant foraging by ants, and among‐plant differences in ant‐foraging were correlated with the magnitude of damage inflicted on plants by the beetles. Last, we found that sites occupied by introduced M. rubra ants almost invariably included Japanese knotweed. Thus, underlying variation in the spatiotemporal distribution of the introduced herbivore influences the provision of benefits to the introduced plant and to the introduced ant. More specifically, the presence of the introduced herbivore converts an otherwise weak interaction between two introduced species into a reciprocally beneficial mutualism. Because the prospects for facilitation are linked to the prospects for enemy release in protection mutualisms, species introductions can have complex effects on existing species interactions, between both native and introduced species.     

A Specialist Herbivore Uses Chemical Camouflage to Overcome the Defenses of an Ant-Plant Mutualism

Many plants and ants engage in mutualisms where plants provide food and shelter to the ants in exchange for protection against herbivores and competitors. Although several species of herbivores thwart ant defenses and extract resources from the plants, the mechanisms that allow these herbivores to avoid attack are poorly understood. The specialist insect herbivore, Piezogaster reclusus (Hemiptera: Coreidae), feeds on Neotropical bull-horn acacias (Vachellia collinsii) despite the presence of Pseudomyrmex spinicola ants that nest in and aggressively defend the trees. We tested three hypotheses for how P. reclusus feeds on V. collinsii while avoiding ant attack: (1) chemical camouflage via cuticular surface compounds, (2) chemical deterrence via metathoracic defense glands, and (3) behavioral traits that reduce ant detection or attack. Our results showed that compounds from both P. reclusus cuticles and metathoracic glands reduce the number of ant attacks, but only cuticular compounds appear to be essential in allowing P. reclusus to feed on bull-horn acacia trees undisturbed. In addition, we found that ant attack rates to P. reclusus increased significantly when individuals were transferred between P. spinicola ant colonies. These results are consistent with the hypothesis that chemical mimicry of colony-specific ant or host plant odors plays a key role in allowing P. reclusus to circumvent ant defenses and gain access to important resources, including food and possibly enemy-free space. This interaction between ants, acacias, and their herbivores provides an excellent example of the ability of herbivores to adapt to ant defenses of plants and suggests that herbivores may play an important role in the evolution and maintenance of mutualisms.     

Light environment and the impacts of foliage quality on herbivorous insect attack and bird predation

Theory predicts that variation in plant traits will modify both the direct interactions between plants and herbivores and the indirect impacts of predators of those herbivores. Light has strong effects on leaf quality, so the impacts of herbivores and predators may differ between plants grown in sun and shade. However, past experiments have often been unable to separate the effects of light environment on plant traits and herbivory from direct effects on herbivores and predators. We first manipulated light availability in an open habitat using a shade cloth pre-treatment to produce oak saplings with different leaf qualities. Leaves on plants exposed to high light were thicker and tougher and had lower nitrogen and water contents, and higher carbon and phenolic contents than leaves on plants under a shade cloth. Then, in the main experiment, we moved all plants to a common shade environment where bird predators were excluded in a factorial design. We measured insect herbivore abundance and leaf damage. Herbivores were significantly more abundant and caused greater leaf damage on sun trees, although these leaf characteristics are usually associated with low-quality food. Bird exclusion did not change herbivore abundance but did increase leaf damage. Contrary to our predictions, the effects of birds did not differ between trees grown in sun and shade conditions. Thus, differences in effects of predators on herbivores and plants between light habitats, when observed, might be due to variation in predator abundance and not bottom-up effects of host plant quality.     

Interspecific variation and ontogenetic change in antiherbivore defense in myrmecophytic Macaranga species

The present study examined whether or not coexisting congeneric plant species have different defense strategies against herbivores, and the intensity of defense changes ontogenetically. We focused on nine myrmecophytic Macaranga species and estimated the intensity of non-biotic and biotic defense by the degree of leaf damage in ant-free and ant-occupied plants, respectively. Ant colonization of myrmecophytic Macaranga species occurred in the early stage of plant development (5–50 cm-tall seedlings). Following the colonization, damage by leaf eaters was minimized and stable during the ontogenetic development of the host plants due to protection by ants. In ant-free trees, however, herbivore damage was immense in seedlings and decreased as trees grew. Interspecific comparison of leaf damage and herbivore fauna supported that coexisting congeneric plants differ in their types of non-biotic (chemical/structural) defense: without ant protection, Macaranga beccariana, for example, was somewhat resistant to leaf eaters but susceptible to gall-makers, Macaranga trachyphylla was heavily infested by generalist leaf eaters, and Macaranga winkleri was exploited by ant-predatory birds. Despite these variations in chemical/structural defense, ant-colonized plants were generally well defended by ants against all kinds of herbivores. This suggests that the individual host-specific ant mutualists are well adapted to deter the chemically or structurally adapted herbivores. These results imply that in the history of diversification in the Macaranga–ant–herbivore system, a sequence of mutual counter adaptation took place not only between plants and herbivores but also between ants and herbivores.     

Mutualism between Maieta guianensis Aubl., a myrmecophytic melastome,and one of its ant inhabitants: ant protection against insect herbivores

Summary The hypothesis that ants (Pheidole minutula) associated with the myrmecophytic melastome Maieta guianensis defend their host-plant against herbivores was investigated in a site near Manaus, Amazonas, Brazil. M. guianensis is a small shrub that produces leaf pouches as ant domatia. Plants whose ants were experimentally removed suffered a significant increase in leaf damage compared with control plants (ants maintained). Ants patrol the young and mature leaves of Maieta with the same intensity, presumably since leaves of both ages are equally susceptible to herbivore attack. The elimination of the associated ant colony, and consequent increase in herbivory, resulted in reduced plant fitness. Fruit production was 45 times greater in plants with ants than in plants without ants 1 year after ant removal.     

Variation in the effectiveness of biotic defence: the case of an opportunistic ant-plant protection mutualism

Benefits to plants in facultative ant protection mutualisms are highly variable. This allows examination of the sources of this variation and the mechanisms by which ants protect plants. We studied opportunistic interactions between ants and an extrafloral nectary-bearing vine, Dioscorea praehensilis, during 3 different years. Variation in plant protection among years was striking. Several factors affected the effectiveness of the biotic defence. Stems recently emerged from the underground tuber were self-supporting, contacting no other plants and encountering few foraging ants. Stems then became lianescent, and contact with supporting plants greatly increased ant recruitment. Both species and number of ant workers influenced the effect of ants on the major herbivore, the chrysomelid beetle Lilioceris latipennis. Protective actions included limitation of oviposition (reduction in the number of eggs laid on the plant) and predation, leading to increased larval mortality. The probability of successful predation was strongly dependent on larval size. If temporarily low ant-patrolling activity allows larvae to grow beyond a critical size, their mechanical (thick integument) or chemical (plant-derived compounds in a fecal shield) defences become more effective against ants. Secondary metabolites derived from the host plant thus appear to be important for the anti-predator mechanisms of this beetle, being necessary for its survival and reproduction on a host plant that actively recruits ants as a biotic defence against herbivores.     

The effect of disturbance on an ant–plant mutualism

Protective ant–plant mutualisms—where plants provide food or shelter to ants and ants protect the plants from herbivores—are a common feature in many ecological communities, but few studies have examined the effect of disturbance on these interactions. Disturbance may affect the relationship between plants and their associated ant mutualists by increasing the plants’ susceptibility to herbivores, changing the amount of reward provided for the ants, and altering the abundance of ants and other predators. Pruning was used to simulate the damage to buttonwood mangrove (Conocarpus erectus) caused by hurricanes. Pruned plants grew faster than unpruned plants, produced lower levels of physical anti-herbivore defenses (trichomes, toughness), and higher levels of chemical defenses (tannins) and extrafloral nectaries. Thus, simulated hurricane damage increased plant growth and the amount of reward provided to ant mutualists, but did not have consistent effects on other anti-herbivore defenses. Both herbivores and ants increased in abundance on pruned plants, indicating that the effects of simulated hurricane damage on plant traits were propagated to higher trophic levels. Ant-exclusion led to higher leaf damage on both pruned and upruned plants. The effect of ant-exclusion did not differ between pruned and unpruned plants, despite the fact that pruned plants had higher ant and herbivore densities, produced more extrafloral nectaries, and had fewer physical defenses. Another common predator, clubionid spiders, increased in abundance on pruned plants from which ants had been excluded. I suggest that compensatory predation by these spiders diminished the effect of ant-exclusion on pruned plants.     

Extrafloral nectary-bearing plant Mallotus japonicus uses different types of extrafloral nectaries to establish effective defense by ants

Extrafloral nectary (EFN)-bearing plants attract ants to gain protection against herbivores. Some EFN-bearing plants possess different types of EFNs, which might have different effects on ants on the plants. Mallotus japonicus (Thunb.) Muell. Arg. (Euphorbiaceae) bears two types of EFNs, including a pair of large EFNs at the leaf base and many small EFNs along the leaf edge. This study aimed to determine the different roles of the two types of EFNs in biotic defense by ants. A field experiment was conducted to investigate the effect of leaf damage on EFN production and on the distribution pattern of ants. After leaf damage, the number of leaf edge EFNs increased in the leaves first-produced. The number of ants on the leaves also increased, and the foraging area of ants extended from the leaf base to the leaf tip. An EFN-covering field experiment revealed that leaf edge EFNs had a greater effect than leaf base EFNs on ant dispersal on leaves. The extended foraging area of ants resulted in an increase of encounter or attack rate against an experimentally placed herbivore, Spodoptera litura. These results suggest that M. japonicus plants control the foraging area of ants on their leaves using different types of EFNs in response to leaf damage, thus achieving a very effective biotic defense against herbivores by ants.

     

Ants suppressing plant pathogens: a review

Ant–plant mutualisms are usually regarded as driven by ants defending plants against herbivores in return for plant‐produced food rewards and housing. However, ants may provide additional services. In a review of published studies on ant–pathogen–plant interactions, we investigated whether ants’ extensive hygiene measures, including the use of ant‐produced antibiotics, extend to their host plants and reduce plant pathogen loads. From 30 reported species combinations, we found that the presence of ants lead to reduced pathogen levels in 18 combinations and to increased levels in 6. On average, ants significantly reduced pathogen incidence with 59%. This effect size did not differ significantly from effect sizes reported from meta‐analyses on herbivore protection. Thus, pathogen and herbivore protection could be of equal importance in ant–plant mutualisms. Considering the abundance of these interactions, ecological impacts are potentially high. Furthermore, awareness of this service may stimulate the development of new measures to control plant diseases in agriculture. It should be noted, though, that studies were biased toward tropical ant–plant symbioses and that the literature in the field is limited at present. Future research on plant pathogens is needed to enhance our understanding of ant–plant mutualisms and their evolution.     

Echinacea angustifolia and its specialist ant‐tended aphid: a multi‐year study of manipulated and naturally‐occurring aphid infestation

1. The impact of herbivores on plant fitness depends on multiple ecological mechanisms, including interactions between herbivore guilds. 2. This study assessed the effects of a specialist aphid (Aphis echinaceae) on performance and foliar herbivore damage of a long‐lived perennial plant (Echinacea angustifolia) native to the North American tallgrass prairie. A 2‐year field experiment manipulating aphid infestation on 100 plants was compared with concurrent and past observations of unmanipulated plants in the same outdoor experimental plot. Because ants co‐occur with aphids, the experiment tested the combined effects of aphids and ants. 3. Neither manipulated nor naturally‐occurring aphid infestations led to measurable declines in plant performance. Results for foliar herbivore damage differed between experimental and observational studies: the occurrence of foliar herbivore damage decreased with aphid infestation in the first year of the experiment and increased with aphid infestation over 5 years in unmanipulated plants. 4. While the experimental results concur with other experiments of ant–hemipteran–herbivore relationships, the observational results suggest that ant–aphid interactions do not naturally play a major role in determining patterns of foliar herbivory in this system. This result demonstrates the value of using field observations to interpret the relevance of experimental results.     

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.     

Invasive interactions: can Argentine ants indirectly increase the reproductive output of a weed?

The direct and indirect interactions of invasive ants with plants, insect herbivores, and Hemiptera are complex. While ant and Hemiptera interactions with native plants have been well studied, the effects of invasive ant–scale insect mutualisms on the reproductive output of invasive weeds have not. The study system consisted of Argentine ants (Linepithema humile), boneseed (Chrysanthemoides monilifera monilifera), and sap-sucking scale insects (Hemiptera: Saissetia oleae and Parasaissetia nigra), all of which are invasive in New Zealand. We examined the direct and indirect effects of Argentine ants on scale insects and other invertebrates (especially herbivores) and on plant reproductive output. Argentine ants spent one-third of their time specifically associated with scale insects in tending behaviours. The invertebrate community was significantly different between uninfested and infested plants, with fewer predators and herbivores on ant-infested plants. Herbivore damage was significantly reduced on plants with Argentine ants, but sooty mould colonisation was greater where ants were present. Herbivore damage increased when ants were excluded from plants. Boneseed plants infested with Argentine ants produced significantly more fruits than plants without ants. The increase in reproductive output in the presence of ants may be due to increased pollination as the result of pollinators being forced to relocate frequently to avoid attack by ants, resulting in an increase in pollen transfer and higher fruit/seed set. The consequences of Argentine ant invasion can be varied; not only does their invasion have consequences for maintaining biodiversity, ant invasion may also affect weed and pest management strategies.     

Ant Species Identity has a Greater Effect than Fire on the Outcome of an Ant Protection System in Brazilian Cerrado

Although fire‐ and ant–plant interactions influence the community structure and dynamics of Neotropical savannas, no previous studies have considered their simultaneous effects on target host plants. We monitored the effect of ant exclusion for 3 years on leaf area loss to leaf chewing insects, thrips abundance, and reproductive output of the extrafloral nectary‐bearing shrub, Peixotoa tomentosa (Malpighiaceae). We predicted that the impact of ants on herbivores and plants would depend on the ant species, and that fire would reduce the effect of ants. We deliberately chose control plants that differed in their occupant ant species. Fire occurred in the second year of the study, allowing us to determine its effect on the benefit afforded by ants. Ants reduced leaf area loss and thrips abundance, and increased fruit and seed production in all 3 years. Some ant species were more effective than others, while plants with multiple ant species suffered higher leaf area loss than plants with a single ant species. In the year following the fire, leaf damage was greater than in the other years, regardless of the ant species, and the proportional effect of ants in reducing damage was less. Interactions affecting thrips abundance did not change following fire, nor was the benefit to the plant proportionally reduced. Overall, the identity of the ant species had a greater effect than did the occurrence of fire on the ant–herbivore–plant interaction: the identity of the ant species influenced leaf area loss, thrips numbers, and bud and seed production, while fire only modified the impact of ants on the amount of leaf area consumed by insect herbivores.     

Extrafloral nectaries: ants,herbivores and fecundity in Cassia fasciculata

Summary Extrafloral nectaries of Cassia fasciculata attract nectar feeding ants which protect the plant against leaf herbivores. High ant visitation in late July coincided with high herbivore densities at two sites in east central Iowa. The highest level of leaf herbivory occurred during the time of flowering and early fruit filling, just after the peak of herbivore and ant activity. Results of ant exclusion experiments at the two sites showed that ant visitation resulted in decreased herbivore numbers, decreased leaf area loss, increased growth, and at one site decreased plant mortality. However, this reduction in leaf area loss and increase in growth did not translate into seed set differences between plants with and without ants at either site. Initial plant size was more important than the presence or absence of ants in determining fecundity for this temperate annual during a year of summer drought.     

Ants on plants: a meta-analysis of the role of ants as plant biotic defenses

We reviewed the evidence on the role of ants as plant biotic defenses, by conducting meta-analyses for the effects of experimental removal of ants on plant herbivory and fitness with data pooled from 81 studies. Effects reviewed were plant herbivory, herbivore abundance, hemipteran abundance, predator abundance, plant biomass and reproduction in studies where ants were experimentally removed (n = 273 independent comparisons). Ant removal exhibited strong effects on herbivory rates, as plants without ants suffered almost twice as much damage and exhibited 50% more herbivores than plants with ants. Ants also influenced several parameters of plant fitness, as plants without ants suffered a reduction in biomass (−23.7%), leaf production (−51.8%), and reproduction (−24.3%). Effects were much stronger in tropical regions compared to temperate ones. Tropical plants suffered almost threefold higher herbivore damage than plants from temperate regions and exhibited three times more herbivores. Ant removal in tropical plants resulted in a decrease in plant fitness of about 59%, whereas in temperate plants this reduction was not statistically significant. Ant removal effects were also more important in obligate ant–plants (=myrmecophytes) compared to plants exhibiting facultative relationships with hemiptera or those plants with extrafloral nectaries and food bodies. When only tropical plants were considered and the strength of the association between ants and plants taken into account, plants with obligate association with ants exhibited almost four times higher herbivory compared to plants with facultative associations with ants, but similar reductions in plant reproduction. The removal of a single ant species increased plant herbivory by almost three times compared to the removal of several ant species. Altogether, these results suggest that ants do act as plant biotic defenses, but the effects of their presence are more pronounced in tropical systems, especially in myrmecophytic plants. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. N. P. de U. Barbosa, L. Diniz, Y. Oki and F. Pezzini contributed equally to this work and are listed in alphabetical order.     

Herbivory on Temperate Rainforest Seedlings in Sun and Shade: Resistance,Tolerance and Habitat Distribution

Differential herbivory and/or differential plant resistance or tolerance in sun and shade environments may influence plant distribution along the light gradient. Embothrium coccineum is one of the few light-demanding tree species in the temperate rainforest of southern South America, and seedlings are frequently attacked by insects and snails. Herbivory may contribute to the exclusion of E. coccineum from the shade if 1) herbivory pressure is greater in the shade, which in turn can result from shade plants being less resistant or from habitat preferences of herbivores, and/or 2) consequences of damage are more detrimental in the shade, i.e., shade plants are less tolerant. We tested this in a field study with naturally established seedlings in treefall gaps (sun) and forest understory (shade) in a temperate rainforest of southern Chile. Seedlings growing in the sun sustained nearly 40% more herbivore damage and displayed half of the specific leaf area than those growing in the shade. A palatability test showed that a generalist snail consumed ten times more leaf area when fed on shade leaves compared to sun leaves, i.e., plant resistance was greater in sun-grown seedlings. Herbivore abundance (total biomass) was two-fold greater in treefall gaps compared to the forest understory. Undamaged seedlings survived better and showed a slightly higher growth rate in the sun. Whereas simulated herbivory in the shade decreased seedling survival and growth by 34% and 19%, respectively, damaged and undamaged seedlings showed similar survival and growth in the sun. Leaf tissue lost to herbivores in the shade appears to be too expensive to replace under the limiting light conditions of forest understory. Following evaluations of herbivore abundance and plant resistance and tolerance in contrasting light environments, we have shown how herbivory on a light-demanding tree species may contribute to its exclusion from shade sites. Thus, in the shaded forest understory, where the seedlings of some tree species are close to their physiological tolerance limit, herbivory could play an important role in plant establishment.     

Production of food bodies on the reproductive organs of myrmecophytic Macaranga species (Euphorbiaceae): effects on interactions with herbivores and pollinators

In protective ant–plant mutualisms, plants offer ants food (such as extrafloral nectar and/or food bodies) and ants protect plants from herbivores. However, ants often negatively affect plant reproduction by deterring pollinators. The aggressive protection that mutualistic ants provide to some myrmecophytes may enhance this negative effect in comparison to plant species that are facultatively protected by ants. Because little is known about the processes by which myrmecophytes are pollinated in the presence of ant guards, we examined ant interactions with herbivores and pollinators on plant reproductive organs. We examined eight myrmecophytic and three nonmyrmecophytic Macaranga species in Borneo. Most of the species studied are pollinated by thrips breeding in the inflorescences. Seven of eight myrmecophytic species produced food bodies on young inflorescences and/or immature fruits. Food body production was associated with increased ant abundance on inflorescences of the three species observed. The exclusion of ants from inflorescences of one species without food rewards resulted in increased herbivory damage. In contrast, ant exclusion had no effect on the number of pollinator thrips. The absence of thrips pollinator deterrence by ants may be due to the presence of protective bracteoles that limit ants, but not pollinators, from accessing flowers. This unique mechanism may account for simultaneous thrips pollination and ant defense of inflorescences.     

Ants (Hymenoptera: Formicidae) as potential egg predators of lepidopteran pests of sweet potato, <Emphasis Type="Italic">Ipomoea batatas</Emphasis> (Solanales: Convolvulaceae), in Japan

The sweet potato Ipomoea batatas (L.) Lam., which has extrafloral nectaries, was visited by nine indigenous ant species in south Kyushu, Japan. These ant species were evaluated for their ability to remove the eggs of two lepidopteran species—Aedia leucomelas (Linnaeus) and Spodoptera litura (Fabricius)—through ant-exclusion experiments. The results showed that Tetramorium bicarinatum (Nylander), one of the dominant ant species, is an efficient predator of lepidopteran eggs. This ant species removed A. leucomelas eggs from ant-visited plants in six trials conducted from mid-July to mid-October, with high removal rates (90–100%) occurring from mid-September onward. On ant-excluded plants, only 0–18% of exposed eggs were removed by ants that accidentally invaded the plants. Tetramorium bicarinatum also removed egg masses and the hatchlings of S. litura. The mean percent area of egg-laden leaves eaten by S. litura larvae was lower on ant-visited plants (0.9%) than on ant-excluded plants (57.3%). In contrast, Formica japonica Motschulsky, another dominant ant species, was not observed removing the eggs or larvae of the lepidopteran species. These results show that certain indigenous ant species are efficient as biocontrol agents. Thus, individual ant species need to be investigated for their ability to control insect pests in various crops.     

Ants impact sawfly oviposition on bracken fern in southern California

Although it has been speculated that ant visits to extrafloral nectaries of bracken fern may convey a fitness benefit for the plant, this has never been demonstrated with native herbivores and natural insect densities. We tested the hypothesis that ants attracted to extrafloral nectaries of bracken fern provide a mutualistic benefit by protecting fronds from herbivore damage in a field manipulation experiment in southern California. We examined densities of sawfly eggs and larvae on bracken fronds with and without ant exclusion. Because bracken fern in this region is also impacted by nitrogenous air pollution, we included an N addition treatment. We found that sawfly egg abundance was significantly higher for fern plants when ants were excluded, regardless of N treatment. Ants tended to have higher abundance on fertilized plants, but there was no interaction between N additions and ant exclusion. Bracken fern may derive a fitness benefit from attracting ants during the early phases of plant growth, through decreased herbivore oviposition, rather than through the deterrence of feeding larvae.