Plant-ants use symbiotic fungi as a food source: new insight into the nutritional ecology of ant-plant interactions

Usually studied as pairwise interactions, mutualisms often involve networks of interacting species. Numerous tropical arboreal ants are specialist inhabitants of myrmecophytes (plants bearing domatia, i.e. hollow structures specialized to host ants) and are thought to rely almost exclusively on resources derived from the host plant. Recent studies, following up on century-old reports, have shown that fungi of the ascomycete order Chaetothyriales live in symbiosis with plant-ants within domatia. We tested the hypothesis that ants use domatia-inhabiting fungi as food in three ant-plant symbioses: Petalomyrmex phylax/Leonardoxa africana, Tetraponera aethiops/Barteria fistulosa and Pseudomyrmex penetrator/Tachigali sp. Labelling domatia fungal patches in the field with either a fluorescent dye or (15)N showed that larvae ingested domatia fungi. Furthermore, when the natural fungal patch was replaced with a piece of a (15)N-labelled pure culture of either of two Chaetothyriales strains isolated from T. aethiops colonies, these fungi were also consumed. These two fungi often co-occur in the same ant colony. Interestingly, T. aethiops workers and larvae ingested preferentially one of the two strains. Our results add a new piece in the puzzle of the nutritional ecology of plant-ants.     

Non-protein amino acids in plant defense against insect herbivores: representative cases and opportunities for further functional analysis

Chemical defense against herbivores is of utmost importance for plants. Primary and secondary metabolites, including non-protein amino acids, have been implicated in plant defense against insect pests. High levels of non-protein amino acids have been identified in certain plant families, including legumes and grasses, where they have been associated with resistance to insect herbivory. Non-protein amino acids can have direct toxic effects via several mechanisms, including misincorporation into proteins, obstruction of primary metabolism, and mimicking and interfering with insect neurological processes. Additionally, certain non-protein amino acids allow nitrogen to be stored in a form that is metabolically inaccessible to herbivores and, in some cases, may act as signals for further plant defense responses. Specialized insect herbivores often possess specific mechanisms to avoid or detoxify non-protein amino acids from their host plants. Although hundreds of non-protein amino acids have been found in nature, biosynthetic pathways and defensive functions have been elucidated in only a few cases. Next-generation sequencing technologies and the development of additional plant and insect model species will facilitate further research on the production of non-protein amino acids, a widespread but relatively uninvestigated plant defense mechanism.     

Compound effects of induced plant responses on insect herbivores and parasitoids: implications for tritrophic interactions

1. Induced plant responses can affect herbivores either directly, by reducing herbivore development, or indirectly, by affecting the performance of natural enemies. Both the direct and indirect impacts of induction on herbivore and parasitoid success were evaluated in a common experimental system, using clonal poplar trees Populus nigra (Salicales: Salicaceae), the gypsy moth Lymantria dispar (L.) (Lepidoptera: Lymantriidae), and the gregarious parasitoid Glyptapanteles flavicoxis (Marsh) (Hymenoptera: Braconidae). 2. Female parasitoids were attracted to leaf odours from both damaged and undamaged trees, however herbivore‐damaged leaves were three times more attractive to wasps than undamaged leaves. Parasitoids were also attracted to herbivore larvae reared on foliage and to larval frass, but they were not attracted to larvae reared on artificial diet. 3. Prior gypsy moth feeding elicited a systemic plant response that retarded the growth rate, feeding, and survival of gypsy moth larvae, however induction also reduced the developmental success of the parasitoid. 4. The mean number of parasitoid progeny emerging from hosts fed foliage from induced trees was 40% less than from uninduced trees. In addition, the proportion of parasitised larvae that survived long enough to issue any parasitoids was lower on foliage from induced trees. 5. A conceptual and analytical model is provided to describe the net impacts of induced plant responses on parasitoids, and implications for tritrophic interactions and biological control of insect pests are discussed.     

Impact of non‐pathogenic bacteria on insect disease resistance: importance of ecological context

1. The aerial surface of plants is a habitat for large and diverse microbial communities; termed the phyllosphere. These microbes are unavoidably consumed by herbivores, and while the entomopathogens are well studied, the impact of non‐pathogenic bacteria on herbivore life history is less clear. 2. Previous work has suggested that consumption of non‐entomopathogenic bacteria induces a costly immune response that might decrease the risk of infection. However, we hypothesised that insect herbivores should be selective in how they respond to commonly encountered non‐pathogenic bacteria on their host plants to avoid unnecessary and costly immune responses. 3. An ecologically realistic scenario was used in which we fed cabbage looper, Trichoplusia ni Hübner, larvae on cabbage or cucumber leaves treated with the common non‐entomopathogenic phyllosphere bacteria, Pseudomonas fluorescens and P. syringae. Their constitutive immunity and resistance to a pathogenic bacterium (Bacillus thuringiensis; Bt) and a baculovirus (T. ni single nucleopolyhedrovirus) were then examined. 4. While feeding on bacteria‐treated leaves reduced the growth rate and condition of T. ni, there was no effect on immunity (haemolymph antibacterial and phenoloxidase activities and haemocyte numbers). Phyllosphere bacteria weakly affected the resistance of T. ni to Bt but the direction of this effect was concentration dependent; resistance to the virus was unaffected. Host plant had an impact, with cucumber‐fed larvae being more susceptible to Bt. 5. The lack of evidence for a costly immune response to non‐entomopathogenic bacteria suggests that T. ni are probably adapted to consuming common phyllosphere bacteria, and highlights the importance of the evolutionary history of participants in multi‐trophic interactions.     

Effect of generalist insect herbivores on introduced Lepidium draba (Brassicaceae): implications for the enemy release hypothesis

The enemy release hypothesis (ERH) states that decreased regulation by natural enemies allows plants to increase in distribution, abundance and vigour following their introduction into an exotic range. Invasive plants rarely escape herbivory entirely, and for hoary cress [ Lepidium draba L. (Brassicaceae)] it has been demonstrated that generalist insect abundance is greater in its introduced North American range than in the native European range. We assessed the role of increased generalist herbivory on hoary cress using representatives of four important herbivore niches commonly found in the introduced range. We experimentally examined the density dependent impact of these herbivores individually and in combination on hoary cress in a series of greenhouse experiments. We found that defoliation of the oligophagous diamondback moth Plutella xylostella (L.) (Lep., Plutellidae) had the strongest and most consistent impact, while damage by the stem-mining weevil Ceutorhynchus americanus Buchanan (Col., Curculionidae) tended to have the highest per capita effect. Plant response to feeding by the oligophagous crucifer flea beetle Phyllotreta cruciferae (Goeze) (Col., Chrysomelidae) was minor despite obvious feeding damage, and the impact of the polyphagous tarnished plant bug Lygus hesperus Knight (Het., Miridae) was negligible. In multiple-species experiments, herbivore impacts were usually additive. In general, we found that hoary cress can tolerate high densities of oligophagous insect herbivory and effectively resisted attack by the polyphagous L. hesperus, but also the oligophagous C. americanus . Our results indicate that a combination of plant resistance and tolerance allows hoary cress to withstand increased generalist herbivore load in its introduced range, consistent with the predictions of the ERH.     

Evaluation of some plant products for the management of Prostephanus truncatus (Horn) (Coleoptera: Bostrichidae) in stored maize (Zea mays) and their effect on nutritional composition of the kernels

The bioactivity of powder from the leaves of Aristolochia ringens, Dalbergia saxatilis, Dioclea reflexa, Ocimum gratissimum, Chrysophyllum albidum, Moringa oleifera, Citrus limon and Newbouldia laevis was tested against the larger grain borer (LGB) in maize kernels and the effect of the powder on the nutritional composition of the kernels was determined. A total of 72 glass bottles containing 100?g maize kernels was divided into four groups. In the first group of 32 glass bottles, 5?g powder of the experimental plants was admixed uniformly with the kernels (5%?w/w). Ten 1–5-day-old LGB adults were introduced into each of the glass bottles. A second group of 32 glass bottles contains 100?g maize kernels admixed with the plant powder as described above, but LGB were not added. In the third group of four glass bottles, 100?g maize kernels received insects only, while in the fourth group of four glass bottles 100?g maize kernels were neither mixed with plant products nor infested with LGB. The treatments were arranged on worktables using completely randomised design and left for three months. The plant products-treated samples (insect free) were passed through a 30?mesh sieve to sieve out the powder and the clean kernels were analysed for proximate composition. The untreated maize kernel had a significantly (p?<?0.05) higher % of grain weight loss (17.33), grain damage % (70.96), number (122.0) of adult LGB, weight (14.46?g) of grain dust and lower % of grain germination. The adsorption and absorption of some of the plant powder by the kernels had significant effect on the kernel composition of protein, fibre and carbohydrate, but not on moisture, dry matter and fat and ash content. Control of LGB with the powder of the test plants could be used as green insecticide for the management of LGB.