Richa Dubey, B. P. Katlam, Gajendra Chandrakar, Anusuiya Panda

Introduction
Insects comprise a hugely diverse group of organisms and constitute a key component in the ecosystem. Some insects are harmful to humans, for example, causing damages to agriculture, forestry, stored products, or human health, and are therefore considered pest species. Like all other organisms, insects live in close association with microorganisms, which profoundly influence their ecology and evolution. Microbes, such as bacteria, archaea, fungi, protozoa, viruses, may be associated with their insect host permanently or transiently, and such associations may be beneficial or harmful to the insect’s fitness. For instance, endo-symbionts (i.e., microbes that live inside host cells or tissues) tend to be dependent on the insect hosts for obtaining nutrients, whereas they can provide fitness advantages in terms of nutritional provisioning, overcoming host defenses, and protection from pathogens, parasites, or other environmental stressors. However, microbes might also be pathogenic, reducing viability and causing morbidity. Furthermore, harboring endo-symbionts can incur physiological costs.

Bacteria
Bacterial microbes can endow their hosts with nutritional benefits, but also help them coping with temperature stress and provide protection against natural enemies.

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Example

Function

1.

Phloem-feeding aphids

Aphids  require the essential amino acid tryptophan that is not present in the phloem sap. These are provided by the bacterium Buchnera aphidicola , a primary symbiont of aphids.

2.

Tephritid olive fruit fly, Bactrocera oleae (Rossi)

The fly lay eggs in unripe olives that are nutritionally restricted in amino acids and are rich in secondary metabolites like phenolics. This olive fruit fly has an obligate symbiont, the bacterium Candidatus Erwinia dacicola, that furnishes essential amino acids to adult flies from nitrogen sources of proteinogenic origin (like bird droppings), increasing their reproductive output.

3.

Protective function of  aphids

Bacteria Buchnera endosymbionts recurrently carry a mutation that governs thermal tolerance of their pea aphid hosts, affecting their fitness positively under lower temperatures.

4.

Honeybee

Specific bacterial taxa like Lactobacillus, Bifidobacterium, Snodgrassella, Apibacter, Frischella and Gilliamella exhibit probiotic, antimicrobial and symbiotic properties that safeguard bee gut homeostasis. 

Fungi

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Example

Function

1.

Bark beetles belonging to the genus Dendroctonus

Beetles feed on phloem tissues and rely on fungi (species of molds belonging to the genera Aspergillus, Beauveria, Metarhizium, Cordyceps, Isaria, and Pandora) for nutrients throughout their life cycle.

2.

Plant-hoppers of the genus Nilaparvata 

These symbionts recycle the excretory material uric acid of the plant-hopper into reusable nitrogenous products. Additionally, they enhance the reproductive investment by the adults and shorten the developmental time of the various stages of the progenies.

 


Viruses
Viruses belonging to the Baculoviridae, Parvoviridae, Flaviviridae, Ascoviridae, Togaviridae, Bunyavirales, and Rhabdoviridae have most commonly been associated with insects.

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Example

Function

1.

Western flower thrips, Frankliniella occidentalis (Pergande)

Thrips showed that the virus had a positive effect on thrips development, with more offspring and shorter developmental time, when the thrips fed on virus-infected compared to uninfected plants.

2.

Aphid

For instance, aphids that harbor H. defensa with APSE (Acyrthosiphon pisum secondary endosymbiont) phages tend to have a greater defense capacity against the attack of parasitoids than the ones without APSE. The phage infects its bacterial host; the bacteria then defend the aphid against its natural enemies

Archaea
In insects, methanogenic and non-methanogenic archaea belonging to the phylum Euryarchaeota, have been reported in beetles, cockroaches, termites, and millipedes. The methanogenic archaea are usually present in the hindguts, an environment with limited oxygen availability. Presence of Crenarchaeota in the larval gut of beetles also reported.

Protozoa
The best-studied association in this respect is between Anopheles spp. mosquitoes and the malaria parasite Plasmodium falciparum Welch. Other examples include the protozoan species Trypanosoma brucei Plimmer & Bradford that induces sleeping sickness and is vectored by the tsetse flies, and the Leishmania protozoa that are spread by sandflies. Similar as with the viruses, however, non-parasitic protozoa may provide benefits to their insect hosts in mutualistic interactions, as shown for the termites.

The taxonomic diversity of hemolymph microorganisms
The most widely-reported hemolymph microorganisms are bacteria of the genus Spiroplasma. These bacteria are members of the family Mollicutes (phylum Tenericutes).


The hemolymph microbiome and insect nutrition
The hemolymph is both the principal route for nutrient translocation around the insect body and a major site for nutrient storage. Consequently, it is a nutrient-rich medium for microorganisms. The principal hemolymph solutes are carbohydrates, usually dominated by the disaccharide trehalose at 5- > 50 mM but also containing other sugars (especially glucose) and sugar alcohols, such as mannitol and sorbitol, in some insects.


Insects, like humans, have their guts colonized by diverse communities of microbes known as gut microbiota. These microbes significantly influence insect physiology, affecting behaviour, immunity, growth, development, and reproduction. Insect gut microbiome focuses on pest control, industrial applications (like cellulose degradation), neuroactive compounds that affect behaviour, antimicrobial peptides, nitrogen-fixing bacterial symbionts, and adaptations to overcome plant defences, such as lignin, phenolics, and crystalline cellulose. Despite growing interest in insect gut microbiomes, the properties of symbiotic microbial species and their enzymes as biocontrol agents remain largely unexplored.

Given figure depicts the various roles played by gut symbionts in the insect host body.

The relationship between insect growth environments and intestinal flora requires further study. Understanding how to regulate these microbes based on living conditions, diets, and physiological changes will enhance our knowledge of their functions and provide new opportunities for improving the health of beneficial insects and managing harmful ones. Insects' ability to distinguish between beneficial pathogens and non-pathogenic, mutualistic intestinal microbes will probably be the subject of much more research in the coming years. These discoveries will support attempts to modify the gut microbes of insects in order to prevent harmful insect species from emerging or to preserve good ones, such as pollinators. They can provide new insights into the growth and diversity of insect gut microbial communities and play a significant role in the bioremediation, development of various industrial applications and environmentally friendly wealth-generating technologies, such as the production of biofuels.