Alan C. Gange

Insect herbivory below ground.

Publisher Summary Insect herbivores can attack both the vegetative and reproductive structures of plants in a variety of ways such as defoliating, sap feeding, stem boring, leaf mining, gall forming, flower and seed feeding, and root feeding. This chapter discusses insect herbivory below ground. The organisms feeding on below-ground plant structures are either different species, or different stages in the life-history of species, to those feeding above ground. Below-ground herbivory is mainly found in the rodents, nematodes, molluscs, and insects. Of these groups, herbivory by nematodes is most widely documented. Nematodes exhibit very similar adaptations to insects for life in the soil and there are parallels among them in terms of their effects on plant physiology and growth. The paucity of ecological studies of below-ground herbivory most likely stems from difficulties with sampling and taxonomy of the causal agents and in assessing the extent of damage. Below-ground herbivory can influence the full complement of parameters affected by the feeding activities of herbivores above ground.

Impacts of Plant Symbiotic Fungi on Insect Herbivores: Mutualism in a Multitrophic Context

We consider how fungi that form symbiotic associations with plants interact with insect herbivores attacking the same plants. Both endophytes and mycorrhizae have significant impacts on herbivores with which they are in relatively intimate contact, but weaker effects on those from which they are spatially separated. Generalist insects are usually adversely affected by the presence of endophytes and mycorrhizae, whereas specialist insects may often benefit. Effects on feeding guilds vary according to type of fungi; for example, aphids are often negatively affected by endophytes but respond positively to mycorrhizae, and leaf-chewers are usually negatively affected by both types of fungi. There is a strong taxonomic bias in the literature and many interactions remain little studied; laboratory studies predominate over field studies. Although some patterns emerge, there is a large amount of specificity and context dependency in the outcome of interactions, reflecting the influence of fungal and host genotype, fungal, host, and insect species, and environmental factors. Whereas some of the mechanisms underpinning these interactions are relatively well characterized, others remain unclear and await elucidation by molecular and metabolomic techniques.

Effects of mycorrhizal fungi on insect herbivores: a meta‐analysis

Mycorrhizal status of the host plant is often ignored in studies on plant-herbivore interactions, but mycorrhizal colonization is known to induce many morphological, physiological, and biochemical changes in host plants, which in turn may alter plant quality as a host for insect herbivores. Both positive and negative effects of mycorrhizal colonization of the host plant on performance and density of insect herbivores have been reported in previous studies. We have conducted a meta-analysis of 34 published and unpublished studies on this topic in order to find out the sources of variation in mycorrhizae effects on insect herbivores. Effects of mycorrhizae on chewing insects depended upon the parameter measured and the degree of herbivore feeding specialization. Density and consumption of chewing insects were higher on mycorrhizal plants, but this did not lead to greater plant damage, presumably because herbivore survival tended to be lower on mycorrhizal plants. Mono- and oligophagous chewers benefited from mycorrhizal colonization of their host plants, whereas performance of polyphagous chewers was reduced on mycorrhizal plants. Among sucking insects, phloem feeders benefited from mycorrhizal infection, but performance of mesophyll feeders was lower on mycorrhizal plants. The type of mycorrhiza was not important for chewing insects, but performance of sucking insects was increased more by arbuscular mycorrhizal fungi (AM) than by ectomycorrhizae (ECM). Among AM inoculation studies, the most commonly used fungal species, Glomus intraradices, tended to have a negative effect on chewer performance, whereas all other fungal species tended to have a positive effect. There was no significant difference in results between studies using inoculation and fungicides, field and laboratory studies, and published and unpublished studies. Mycorrhizal status of the host plant thus influences insect herbivore performance, but the magnitude and direction of the effect depend upon the feeding mode and diet breadth of the insect and the identity of fungi.

Vesicular-Arbuscular Mycorrhizal Fungi: A Determinant of Plant Community Structure in Early Succession

1. Infection of plants by vesicular-arbuscular mycorrhizas (VAM) was reduced by application of the contact fungicide iprodione in two early successional plant communities. 2. One community was initiated in 1988 and monitored for four years, the other started in 1990 and monitored for 18 months. 3. In both communities reduction in fungal infection resulted in a lower plant species richness, with a number of species occurring in untreated (control) plots but absent in fungicide-treated plots. These were mostly perennial forbs. 4. It is suggested that VAM fungi are important in structuring early successional communities and that this is brought about by enhanced seedling establishment of forbs when fungi are present

Biosynthesis of antimicrobial silver nanoparticles by the endophytic fungus Aspergillus clavatus

AIM To induce the biosynthesis of silver nanoparticles (AgNPs) using Aspergillus clavatus and evaluate their antimicrobial potential. MATERIALS & METHODS Aspergillus clavatus (AzS-275), an endophytic fungus isolated from sterilized stem tissues of Azadirachta indica A. Juss., was challenged with 1 mM AgNO(3) solution. The characterization of the AgNPs was carried out by x-ray diffraction spectrometry, transmission-electron microscopy and atomic force microscopy. RESULTS & DISCUSSION The synthesized AgNPs were found to be extracellular, polydispersed spherical or hexagonal particles ranging from 10 to 25 nm in size. Antimicrobial activity was performed using a disc-diffusion method against Candida albicans, Pseudomonas fluorescens and Escherichia coli. The results showed an average minimum inhibitory concentration of 5.83 microg ml(-1) and minimum fungicidal concentration of 9.7 microg ml(-1) against C. albicans. CONCLUSIONS AgNPs can be mycosynthesized extracellularly using A. clavatus as the fungal system, which is highly advantageous over chemical synthesis not only because it can be synthesized on a large scale, but because of the ease of downstream processing and its biomedical application in antimicrobial activity.

Differential Effects of Above- and Below-Ground Insect Herbivory during Early Plant Succession

The effects of aboveand below-ground insect herbivory on a natural plant community colonising bare ground were determined by manipulative field experiments. A foliar insecticide (Dimethoate-40) and a soil insecticide (Dursban 5G) were applied separately and in combination in a factorial experimental design over a two-year period. Characteristics of the developing vegetation were assessed in insecticidetreated and control plots. The insecticides had no independent effect on plant growth. Plant species richness and diversity were increased by the application of soil insecticide and, by the second season, depressed by foliar insecticide. Vegetation frequency and particularly cover abundance were enhanced by both compounds, with the soil insecticide having a greater effect in the second year. Vegetation height, assessed by a weighted mean height index, was only increased by the application of soil insecticide in the first year. Three major life-history groupings (annual and perennial herbs and perennial grasses) responded differently to herbivory. Annual herbs were promoted by foliar and soil insecticides in the first season, but only by the latter in the second. Within this general pattern, common species varied in response to the two compounds. Perennial grasses were strongly promoted by foliar insecticide and although few perennial herbs were present these increased in number of species and cover when below-ground herbivory was reduced. The different responses of life-history groupings and individual species to the two types of herbivory have a considerable effect on the pattern of early succession. The role of herbivory in relation to competition and seedling mortality in the developing plant community are discussed. The latter is relevant to the creation of species-rich communities of potential conservation value.

Arbuscular mycorrhizal fungi, Collembola and plant growth

Arbuscular mycorrhizal fungi are ubiquitous in field soils, as are mycophagous animals such as Collembola. It has been suggested that these animals reduce the functioning of the mycorrhiza and are thus detrimental to plant growth. However, recent choice experiments suggest that Collembola preferentially feed on nonmycorrhizal fungi in the rhizosphere. If these preferences also occur in field soils, then Collembola might indirectly benefit plants through an enhancement of mycorrhizal functioning and indirect multitrophic links to foliar-feeding insect herbivores.

Effects of root herbivory by an insect on a foliar-feeding species, mediated through changes in the host plant

SummaryThe effects of root herbivory by larvae of the scarabaeid, Phyllopertha horticola, on the growth of Capsella bursa-pastoris were examined. Individuals of Aphis fabae were reared on the leaves to determine what effect, if any, root feeding has on the performance of this insect. The experiment was conducted under two watering regimes (‘low’ and ‘high’). Low watering and root feeding caused water stress in the plants and this was reflected in a reduction in vegetative biomass and an increase in the proportion of material allocated to reproduction. Supplying plants with ample water in the ‘high’ treatment enabled the water stress caused by root herbivory to be offset, but not completely overcome. Low watering and root feeding caused an increase in aphid weight and growth rate, while root feeding also increased fecundity and adult longevity. These effects are attributed to an improvement in food quality, measured by total soluble nitrogen, and caused by amino acid mobilization due to the water stress. The implications of these results in agricultural and ecological situations are discussed.

Secondary plant succession: how is it modified by insect herbivory?

The effects of foliar- and root-feeding insects on the dynamics of an early successional plant community, representing the first four years of colonisation, were examined. Subterranean insect herbivores were found to increase in density with increasing successional age of the plant community. In early succession, chewing insects mainly Coleoptera (Scarabaeidae) and Diptera (Tipulidae) were dominant. This was in direct contrast to the foliar-feeding insects, which were dominated by sap-feeders (mainly Auchenorrhynchan Hemiptera). Reduction of both foliar- and root-feeding insects with appropriate insecticides had different, but dramatic, consequences for the plant community. Reducing foliar herbivory resulted in large increases in perennial grass growth, with plant species richness being reduced as the grasses outcompeted the forbs. Reducing subterranean herbivory prolonged the persistence of annual forbs, greatly increased perennial forb colonisation and, as a consequence, plant species richness. Foliar-feeding insects thus act to delay succession by slowing grass colonisation. In contrast, root-feeding insects accelerate succession by reducing forb persistence and colonisation. The structure of early successional plant communities is therefore modified by the two modes of herbivory.

ENDOPHYTIC ACTINOMYCETES FROM AZADIRACHTA INDICA A. JUSS.: ISOLATION, DIVERSITY, AND ANTI-MICROBIAL ACTIVITY

Endophytic actinomycetes from Azadirachta indica A. Juss. were screened and evaluated for their anti-microbial activity against an array of pathogenic fungi and bacteria. A total of 55 separate isolates were obtained from 20 plants, and 60% of these showed inhibitory activity against one or more pathogenic fungi and bacteria. Actinomycetes were most commonly recovered from roots (54.5% of all isolates), followed by stems (23.6%), and leaves (21.8%). The dominant genus was Streptomyces (49.09% of all isolates), while Streptosporangium (14.5%), Microbispora (10.9%), Streptoverticillium (5.5%), Sacchromonospora sp. (5.5%), and Nocardia (3.6%) were also recovered. Streptomyces isolates AzR 006, 011, and 031 (all from roots) had acute activity against Pseudomonas fluorescens, while AzR027, 032, and 051 (also all from roots) showed activity against Escherichia coli. Meanwhile, an isolate of Nocardia sp. from leaves (AzL025) showed antagonism against Bacillus subtilis. Overall, 32 of the 55 were found to have broad spectrum significant antimicrobial activity, while about 4% of them showed strong and acute inhibition to pathogenic fungi and bacteria. Isolates of Streptomyces AzR031, 008, and 047, Nocardia sp. AzL025, and Streptosporangium sp. AzR 021 and 048 are of particular interest because they showed significant antagonistic activity against root pathogens, including Pythium and Phytophthora sp. Thus, many of the isolates recovered from A. indica in this study may be used in developing potential bio-control agents against a range of pathogenic fungi and bacteria and in the production of novel natural antimicrobial compounds. These results not only further our understanding of plant–microbe interactions but also indicate that there is an untapped resource of endophytic microorganisms that could be exploited in the biotechnological, medicinal, and agricultural industries.