FOSSIL EXTRAFLORAL NECTARIES,EVIDENCE FOR THE ANT-GUARD ANTIHERBIVORE DEFENSE IN AN OLIGOCENE POPULUS

Extrafloral nectaries are secretory glands, usually found on leaves, that have been shown to promote ant defense against the insect herbivores of many modem day plants. Extrafloral nectaries were found on the 35-million-year-old fossil leaves of the extinct Populus crassa from Florissant, Colorado. Extinct ant species (belonging to five still extant genera that have modem ant-guard species), and other predators and parasitoids (whose modem relatives frequent extrafloral nectaries) also lived at Florissant. The extrafloral nectaries of P. crassa (and perhaps other plants) probably operated to attract ants and/or other arthropod defenders as early as the Oligocene.     

Incidence of Extrafloral Nectaries and Their Relationship with Growth and Survival of Lowland Tropical Rain Forest Trees

Mutualistic relationships between organisms have long captivated biologists, and extrafloral nectaries, or nectar‐producing glands, found on many plants are a good example. The nectar produced from these glands provides food for ants, which may defend the plant from potential herbivores in turn. However, relatively little is known about their impact on the long‐term growth and survival of plants that produce them. To better understand the ecological significance of extrafloral nectaries, we examined their incidence on lowland tropical rain forest trees in Yasuní National Park in Amazonian Ecuador, and collated data from two other tropical lowland forest sites (Barro Colorado Island, Panamá and Pasoh Forest Reserve, Malaysia). At Yasuní, extrafloral nectaries were found on 137 of 1123 species censused (12.2%), widely distributed among different angiosperm families. This rate of incidence is high but consistent with other tropical locations. Furthermore, this study adds 18 new genera and two new families (Urticaceae and Caricaceae) to the list of taxa exhibiting extrafloral nectaries. Using demographic data from long‐term forest dynamics plots at each site, we compared the growth and mortality rates of species with extrafloral nectaries to those without. After controlling for phylogeny, no general relationship between extrafloral nectary presence and demographic rates could be detected, suggesting little demographic signal from any community‐wide ecological effects.     

Testing for mycorrhizal fungi-plant-ant indirect effects

Abstract

Recent work has demonstrated indirect effects between mycorrhizal fungi and insect herbivores and pollinators. The existence of indirect effects between mycorrhizal fungi and protection-for-food mutualists, such as extrafloral nectar-foraging ‘bodyguard ants’, is unknown. In this study, we examined the potential for indirect effects of arbuscular mycorrhizal fungi on aggressive ant bodyguards, mediated by changes in the expression of extrafloral nectaries of a shared host plant. We found that mycorrhizal plants grew larger and produced more extrafloral nectaries compared to their non-mycorrhizal counterparts. The difference in the number of nectaries between mycorrhizal and non-mycorrhizal plants, however, was too small to elicit differences in ant attendance. In spite of the lack of a significant indirect effect of mycorrhizal fungi on ant attendance, mycorrhizal plants suffered damage to a significantly greater proportion of their leaves compared to non-mycorrhizal plants. This result likely stems from other (non-ant-mediated) indirect effects of mycorrhizal fungi on herbivores.     

Establishment and dominance of an introduced herbivore has limited impact on native host-parasitoid food webs

The gypsy moth is considered one of the most harmful invasive forest insects in North America. It has been suggested that gypsy moth may indirectly impact native caterpillar communities via shared parasitoids. However, the impact of gypsy moth on forest insect food webs in general remains unstudied. Here we assess such potential impacts by surveying forest insect food webs in Ontario, Canada. We systematically collected caterpillars using burlap bands at sites with and without histories of gypsy moth outbreak, and then reared these caterpillars until potential parasitoid emergence. This procedure allowed us to generate quantitative food webs describing caterpillar-parasitoid interactions. We estimated the degree of parasitoid sharing between gypsy moth and native caterpillars. We also statistically modeled the effect of gypsy moth outbreak history and current gypsy moth abundance on standard indices of quantitative food web structure and the diversity of parasitoid communities. Rates of gypsy moth parasitism were very low and gypsy moth shared very few parasitoids with native caterpillars, suggesting limited potential for indirect interactions. We did not detect any significant effects of gypsy moth on either food web structure or parasitoid diversity, and the small amount of parasitoid sharing strongly implies that this lack of significance is not merely due to low statistical power. Our study suggests that gypsy moth has limited impact on native host-parasitoid food webs, at least for species that use burlap bands. Our results emphasize that extrapolations of theoretical and experimental conclusions on the impacts of invasive species should be tested in natural settings.     

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.     

Distribution and morphology of extrafloral nectaries in some Cucurbitaceae

A study of extrafloral nectaries has been made in the Cucurbitaceae to ascertain their structure and assess their taxonomic potential. Nineteen species representing nine Old World genera and one New World genus were examined. These included Telfairia occidentalis, Telfairia pedata, Momordica charantia, Lagenaria siceraria, Citrullus lanatus, Luffa aegyptiaca, Cucurbita moschata and Trichosanthes cucumerina , which are of economic importance and cultivated in Nigeria for their leaves and/or fruits.
Observation of the regularity of ant and insect-visitors, along with tests for glucose and β-glucosidase enzymes, revealed the presence of extrafloral nectaries in nine species. Considerable variation exists in the distribution and morphology of nectaries between genera, especially in the tribe Benincaseae. The nutritional and ecological significance of the occurrence of extrafloral nectaries in Telfairia occidentalis is discussed.     

A novel indirect defence in Brassicaceae: Structure and function of extrafloral nectaries in Brassica juncea

While nectaries are commonly found in flowers, some plants also form extrafloral nectaries on stems or leaves. For the first time in the family Brassicaceae, here we report extrafloral nectaries in Brassica juncea. The extrafloral nectar (EFN) was secreted from previously amorphic sites on stems, flowering stalks and leaf axils from the onset of flowering until silique formation. Transverse sections at the point of nectar secretion revealed a pocket‐like structure whose opening was surrounded by modified stomatal guard cells. The EFN droplets were viscous and up to 50% of the total weight was sugars, 97% of which was sucrose in the five varieties of B. juncea examined. Threonine, glutamine, arginine and glutamate were the most abundant amino acids. EFN droplets also contained glucosinolates, mainly gluconapin and sinigrin. Nectar secretion was increased when the plants were damaged by chewing above‐ and belowground herbivores and sap‐sucking aphids. Parasitoids of each herbivore species were tested for their preference, of which three parasitoids preferred EFN and sucrose solutions over water. Moreover, the survival and fecundity of parasitoids were positively affected by feeding on EFN. We conclude that EFN production in B. juncea may contribute to the indirect defence of this plant species.     

Loss of extrafloral nectary on an oceanic island plant and its consequences for herbivory

Two Hibiscus (Malvaceae) species coexist on the oceanic Bonin (Ogasawara) Islands: Hibiscus glaber (an endemic species) and H. tiliaceus (the ancestral non-endemic species). Hibiscus tiliaceus produces extrafloral nectar from the sepals, while H. glaber does not. To clarify the effects of extrafloral nectar loss on Hibiscus-insect relationships, we examined herbivory and insect communities on flower buds of H. glaber and H. tiliaceus. Larvae of the endemic moth Rehimena variegata (Lepidoptera: Pyralidae) attacked 20% of the flower buds on H. glaber, while less than 0.2% of buds on H. tiliaceus were attacked. Introduced species of ants frequently visited the flower buds of H. tiliaceus to collect extrafloral nectar from the sepal, while they rarely visited those of H. glaber. Therefore, extrafloral nectar on H. tiliaceus sepals may function as a facultative defense against flower bud herbivory. The loss of extrafloral nectaries of H. glaber sepals may be related to the original paucity of native herbivores and ants on the Bonin Islands.     

MORPHOLOGY AND ANATOMY OF FLORAL AND EXTRAFLORAL NECTARIES IN CAMPSIS (BIGNONIACEAE)

The genus Campsis (Bignoniaceae), with one New World and one Old World species, is unusual among temperate plants in having five distinct nectary sites. Multiple nectaries occur at all four of the extrafloral sites (petiole, calyx, corolla, fruit), representing an advanced strategy for ant attraction. The morphology and anatomy of the extrafloral nectaries in both species are uniform for the petioles, calyces, and young fruits; those on the outer corolla lobes are of slightly different forms. The generalized structure consists of one layer of basal cells, and a one- to two-layered secretory cup. Because of their small size, there is no vascular tissue in them. The large, vascularized (phloem only) floral nectary is an annular structure subtending the ovary.     

THE EXTRAFLORAL NECTARIES OF IPOMOEA LEPTOPHYLLA (CONVOLVULACEAE)

Ipomoea leptophylla Torr. (Convolvulaceae) is a sprawling dry-site morning glory with two types of extrafloral nectaries: foliar nectaries and nectaries on the outside of the sepals. Both are shown to greatly increase insect visitation to the plant. Ants visiting sepal-surface nectaries significantly decrease flower damage caused by grasshoppers and seed losses caused by bruchids. These results are similar to those for I. carnea and other plants whose extrafloral nectary-ant interactions have been studied, but differ in detail. This is the first demonstration of antiherbivore defense of a prairie plant by nectary visitors.     

Arbuscular mycorrhizal fungi reduce the construction of extrafloral nectaries in <Emphasis Type="Italic">Vicia faba</Emphasis>

Arbuscular mycorrhizal fungi (AMF) can alter the physiology and morphology of their host plant, and therefore may have indirect effects on insect herbivores and pollinators. We conducted this study to test the hypothesis that AMF can also affect insects involved in protection-for-food mutualisms. We examined the constitutive and inducible production of food rewards [extrafloral (EF) nectaries] in Vicia faba plants by manipulating the presence/absence of AMF and by simulating various levels of herbivory. Plants inoculated with AMF produced significantly fewer EF nectaries than uninoculated plants, even after accounting for differences in plant growth. In contrast to earlier studies, EF nectaries were not inducible: damaged plants produced significantly fewer EF nectaries than undamaged plants. Moreover, the effects of mycorrhizal and damage status on EF nectary production were additive. The reduction in EF nectaries in mycorrhizal plants potentially represents a mechanism for indirect effects of AMF on the protective insects that exploit EF nectaries as a food source (e.g., ants). Reduced reward size should result in reduced protection by ants, and could therefore be a previously unappreciated cost of the mycorrhizal symbiosis to host plants. However, the overall effect of AMF will depend upon the extent to which the reduction of EF nectaries affects the number and activity of ants and the extent to which AMF alter other aspects of host plant physiology. Our results emphasize the complexity of multitrophic interactions, particularly those that span belowground and aboveground ecology.     

Foraging behavior of Brachygastra lecheguana (Hymenoptera: Vespidae) on Banisteriopsis malifolia (Malpighiaceae): Extrafloral nectar consumption and herbivore predation in a tending ant system

In the Brazilian savanna many plant species bear regular associations with patrolling ants that are aggressive towards insect herbivores. However, not only ants but also several species of predatory wasps are attracted to plants due to the extrafloral nectaries (EFNs). Such wasps feed on both herbivores and plant exudates. In this study we describe the foraging behavior of the social Polistinae wasp Brachygastra lecheguana in the extrafloral nectaried shrub Banisteriopsis malifolia, and investigated the influence of patrolling ants Camponotus blandus on the activity of the wasp. Brachygastra lecheguana fed on the endophytic larvae of Anthonomus (Curculionidae) beetles that developed inside flower buds. The wasp lacerated the bud layers to reach the beetle larvae located at the bud core. The wasp visits to Ba. malifolia were statistically related to the abundance of flower buds and beetles. Ant exclusion experiments revealed that the hunting behavior of B. lecheguana on beetles was not related to the absence of C. blandus. However we found that wasps spent more time consuming extrafloral nectar on branches where ants were excluded. This is the first study reporting extrafloral nectar consumption by B. lecheguana, as well as the predation on herbivores in natural areas. In cerrado vegetation, ants benefit the plant by reducing insect herbivores, and our study provides evidence that the B. lecheguana – Ba. malifolia system represents a potential interaction where the wasp may also benefit the host plant. The value of this wasp species as a plant‐guard is discussed.     

Ant protection of the nectaried fern Polypodium plebeium in central Mexico

Nectaries on fronds of Polypodium spp. have been studied previously only in cultivated specimens. We conducted field observations in middle-elevation forests in Mexico and found five ant species associated with nectaries of Polypodium plebeium and P. lepidotrichum. To investigate whether nectaries promote protection against herbivores, we performed ant-exclusion experiments with nectary-bearing ferns (P. plebeium) and other ferns without nectaries (Polypodium plesiosorum, P. furfuraceum, and Phlebodium pseudoaureum). When ants were excluded from the developing fronds of Polypodium plebeium, damage from foliage-feeding sawfly and lepidopteran caterpillars was significantly greater than in control fronds. Ferns without nectaries did not show a difference in damage between ant-excluded and control fronds. Our results demonstrate that fern nectaries can support ant defense of the plant body as do the extrafloral nectaries of many angiosperms.     

Deterrent effect of a guild of extrafloral nectary-visiting ant species on Raphidopalpa foveicollis, a major insect pest of sponge gourd, Luffa cylindrica

Among the 10 ant species visiting the extrafloral nectaries of sponge gourd plants [ Luffa cylindrica L. (Cucurbitaceae)], Camponotus compressus (Fabricius) (Hymenoptera: Formicidae) appears to be the ecologically dominant species, due to its abundance and the high frequency of deterrent encounters with the major insect herbivore, Raphidopalpa foveicollis Lucas (Coleoptera: Chrysomelidae). A significant positive linear correlation was found between the number of extrafloral nectaries per leaf, calyx, bract, and bracteole and the patrolling time of C. compressus , Camponotus paria (Emery), Pheidole spec., and Tetramorium spec. (all Hymenoptera: Formicidae) at the corresponding plant parts. Many one-to-one deterrent interactions between the ant visitors and adult R. foveicollis were recorded, with C. compressus being involved in most encounters on the calyx and bracts, Pheidole spec. on the leaves, and Pachycondyla tesserinoda (Emery) (Hymenoptera: Formicidae) on the bracteoles. Tapinoma melanocephalum (Fabr.) (Hymenoptera: Formicidae) exhibited significant deterrent effects on adult herbivores on the corolla when in groups larger than 15 ants. Ant exclusion and inclusion experiments revealed a significant reduction in the residence time, as well as dose- and time-dependent reduction in the frequency of visits of the herbivore on crop plants with C. compressus , C. paria , Pheidole spec., and T. melanocephalum ants, compared to the control. Results support the optimal defence hypothesis, as the plant structures most closely linked to plant fitness bear the extrafloral nectaries and are defended by the visiting ant species. This is a valuable, indirect plant-protection strategy of an annual crop plant.     

Insect visitors to extrafloral nectaries of Byttneria aculeata (Sterculiaceae): relative importance and roles

Abstract. 1. Ants, parasitoids and flies are about equally frequent at foliar nectaries of Byttneria aculeata (Sterculiaceae) in lowland Costa Rica during the dry season, a pattern previously unreported but also observed at other plants in the area.
2. Species of Ectatomma, Crematogaster and Camponotus were the most frequent of twenty-four ant species on Byttneria, eight of which nested in the hollow stems. Ants spent most time at nectaries and little in patrolling.
3. Collections at nectaries yielded large numbers of species of parasitoid Hymenoptera with few individuals of each. Rearing studies of leaf-feeding herbivores yielded several species of parasitoids, including one species taken at a nectary and two others congeneric or closely related.
4. Flies appear to be nectar thieves, in the same sense as non-pollinating floral visitors, despite close association with Byttneria.
5. Ant-plants may be poor models for the larger number of species of less specialized plants with extrafloral nectaries. Plants which have only extrafloral nectaries may better suit the needs of parasitoids than ants, and plants such as Byttneria may benefit as much from parasitoids as from ants.     

The occurrence and abundance of plants with extrafloral nectaries, the basis for antiherbivore defensive mutualisms, along a latitudinal gradient in east Asia

Abstract. The occurrence and abundance of indigenous plants with extrafloral nectaries was evaluated within local communities and regional floras along a north to south gradient from tundra in northeastern Russia (64–70°N) through temperate types in eastern Russia and Korea to subtropical vegetation in the Bonin Islands (26–27°N) south of Japan. Moving from tundra to subtropical vegetation, there is a pattern of increasing abundance of extrafloral bearing plants as a function of total plant cover (from 10.25 to 40.18%), number of species per sampled area (from 0.11 to 1.13/100 m), and proportion of species within regional floras (from 0.32 to 7.46%). There were some plants with extrafloral nectaries in all communities but their abundance varied greatly, c. 1–25% in the four northern latitudes and c. 7–70% in the subtropical region. Ants, the primary mutualists associated with plants bearing extrafloral nectaries, have a similar pattern of increasing abundance (species richness, nest density, and colony size) along the same north–south latitudinal gradient.     

Contrasting exotic Solenopsis invicta and native Forelius pruinosus ants as mutualists with Catalpa bignonioides, a native plant

Abstract. 1. The suitability of the red imported fire ant Solenopsis invicta Buren and a native ant Forelius pruinosus (Roger) as participants in a food-for-protection mutualism with a native nectaried tree Catalpa bignonioides Walter was compared.
2. The mean mortality of folivore larvae of Ceratomia catalpae Boisduval was similar for S. invicta and F. pruinosus although S. invicta attacked fewer caterpillar aggregations and was a devastating pupal predator. Solenopsis invicta also differed from the native ant in that it attacked the parasitoid Cotesia congregata Say, another plant mutualist, and visited extrafloral nectaries less frequently.
3. Habitats invaded by S. invicta are characterised by a scarcity of both herbivores and of beneficial insects that visit extrafloral nectaries. The plants do not require protection, and extrafloral nectaries are visited rarely. Although plants are defended incidentally by S. invicta , the insect-plant mutualism therein is greatly simplified or defunct.     

Metabolism of poplar salicinoids by the generalist herbivore Lymantria dispar (Lepidoptera)

The survival of insect herbivores on chemically defended plants may often depend on their ability to metabolize these defense compounds. However, only little knowledge is available on how insects actually process most plant defense compounds. We investigated the metabolism of salicinoids, a major group of phenolic glycosides in poplar and willow species, by a generalist herbivore, the gypsy moth (Lymantria dispar). Seven salicinoid metabolites identified in gypsy moth caterpillar feces were mostly conjugates with glucose, cysteine or glycine. Two of the glucosides were phosphorylated, a feature not previously reported for insect metabolites of plant defense compounds. The origins of these metabolites were traced to specific moieties of three major poplar salicinoids ingested, salicin, salicortin and tremulacin. Based on the observed metabolite patterns we were able to deduce the initial steps of salicinoid breakdown in L. dispar guts, which involves cleavage of ester bonds. The conjugated molecules were effectively eliminated within 24 h after ingestion. Some of the initial breakdown products (salicin and catechol) demonstrated negative effects on insect growth and survival in bioassays on artificial diets. Gypsy moth caterpillars with prior feeding experience on salicinoid-containing poplar foliage converted salicinoids to the identified metabolites more efficiently than caterpillars pre-fed an artificial diet. The majority of the metabolites we identified were also produced by other common poplar-feeding insects. The conversion of plant defenses like salicinoids to a variety of water-soluble sugar, phosphate and amino acid conjugates and their subsequent excretion fits the general detoxification strategy found in insect herbivores and other animals.     

Spatiotemporal dynamics and plant-part preference patterns of the plant-visiting ants and the insect herbivores of sponge gourd plants

Field investigations revealed a synchrony between daily and seasonal activity patterns of two Raphidopalpa pests on the extrafloral nectary (EFN)-bearing sponge gourd, Luffa cylindrica (L.) plants. A similar pattern was revealed in six of the most abundant plant-visiting ant species. The spatial activity patterns of the herbivores R. foveicollis and R. intermedia revealed a high frequency on the leaves (except, newly unfolded leaves) and the corolla, but not on the bracts, bracteoles and the calyces of male/female buds and flowers. Young expanding leaves (leaf age class 2) had a significantly higher density of extrafloral nectaries (EFNs), as well as minimum leaf damage compared with the other leaf age classes. The density of EFNs was highest on the bracteoles and lowest on the leaves. In behavioural assays, ants demonstrated significantly greater preference for the 2-day old buds/newly opened leaves than the 2-day old intact unisexual flowers/corolla. Also, young, newly expanding leaves were preferred over young (unopened) and mature leaves. Thus, the higher density of EFNs at vulnerable plant sites, along with the visits of ants to such sites, may influence the spatiotemporal activity patterns of the insect herbivores. These results provide support for the optimal defense hypothesis since they show that younger tissues and reproductive tissues may be getting better protection by the ant species.     

Seed predators are undeterred by nectar-feeding ants on Chamaecrista nictitans (Caesalpineaceae)

There are many examples of mutualistic interactions between ants and plants bearing extrafloral nectaries (EFN). The annual legume Chamaecrista nictitans (Caesalpineaceae) secretes nectar from EFN, specialized structures that attract ants, spiders, and other arthropods. The effects of manipulated C. nictitans patch size and location on plant-ant interactions were tested. Defense from herbivores was not detected; plants with ants did not set significantly more fruit or seed than plants with ants excluded. On the contrary, in one year, plants without ants set more fruit and seed than C. nictitans with ants. The cause of this was not determined. Furthermore, insect herbivore damage was low during three years of observations. Sennius cruentatus (Bruchidae), a specialist seed predator beetle, escaped ant defense despite the presence of numerous ants. Beetle progeny are protected during development by living inside maturing C. nictitans fruit and preventing fruits from dehiscing before emerging as adults. Although ants reduced percent of infestation in 1995, the total number of S. cruentatus per plant was not affected by ants in years of infestation. Overall, larger experimental C. nictitans patches attracted more ants, parasitoid wasps, and percent infestation by S. cruentatus while insect herbivores declined with increasing patch size. Location of patches within fields, however, did not affect numbers of arthropod visitors. Similar to manipulated populations, very little insect herbivory occurred in four reference populations. In contrast to the experimental populations, no S. cruentatus were recovered in reference populations of C. nictitans. Herbivory by insects may not always depress seed set by C. nictitans or may not exceed a threshold level. Thus, herbivory-reduction by ants may not have been detectable in these results. Seed predation may be more influential on C. nictitans reproduction. This revised version was published online in August 2006 with corrections to the Cover Date.