The impact of nutritional quality and gut bacteria on the fitness of Bactrocera minax (Diptera: Tephritidae)

Role of gut symbionts of insect pests: A novel target for insect-pest control

Insects possess beneficial and nuisance values in the context of the agricultural sector and human life around them. An ensemble of gut symbionts assists insects to adapt to diverse and extreme environments and to occupy every available niche on earth. Microbial symbiosis helps host insects by supplementing necessary diet elements, providing protection from predators and parasitoids through camouflage, modulation of signaling pathway to attain homeostasis and to trigger immunity against pathogens, hijacking plant pathways to circumvent plant defence, acquiring the capability to degrade chemical pesticides, and degradation of harmful pesticides. Therefore, a microbial protection strategy can lead to overpopulation of insect pests, which can drastically reduce crop yield. Some studies have demonstrated increased insect mortality via the destruction of insect gut symbionts; through the use of antibiotics. The review summarizes various roles played by the gut microbiota of insect pests and some studies that have been conducted on pest control by targeting the symbionts. Manipulation or exploitation of the gut symbionts alters the growth and population of the host insects and is consequently a potential target for the development of better pest control strategies. Methods such as modulation of gut symbionts via CRISPR/Cas9, RNAi and the combining of IIT and SIT to increase the insect mortality are further discussed. In the ongoing insect pest management scenario, gut symbionts are proving to be the reliable, eco-friendly and novel approach in the integrated pest management.

Gut bacteria induce oviposition preference through ovipositor recognition in fruit fly

Gut bacteria play important roles in insect life cycle, and various routes can be used by insects to effectively transmit their gut bacteria. However, it is unclear if the gut bacteria can spread by actively attracting their insect hosts, and the recognition mechanisms of host insects are poorly understood. Here, we explore chemical interactions between Bactrocera dorsalis and its gut bacterium Citrobacter sp. (CF-BD). We found that CF-BD could affect the development of host ovaries and could be vertically transmitted via host oviposition. CF-BD could attract B. dorsalis to lay eggs by producing 3-hexenyl acetate (3-HA) in fruits that were hosts of B. dorsalis. Furthermore, we found that B. dorsalis could directly recognize CF-BD in fruits with their ovipositors in which olfactory genes were expressed to bind 3-HA. This work reports an important mechanism concerning the active spread of gut bacteria in their host insects.

Antibiotic treatment reduces fecundity and nutrient content in females of Anastrepha fraterculus (Diptera: Tephritidae) in a diet dependent way

Insect microbiota, particularly, gut bacteria has recently gained especial attention in Tephritidae fruit flies, being Enterobacteriaceae the predominant bacterial group. This bacterial group has been postulated to contribute to the fitness of fruit flies through several life-history traits. Particularly in Anastrepha fraterculus, removal of Enterobacteria from male gut via antibiotic treatment impaired their mating behavior. Because the impact of gut bacteria on female reproduction was not yet addressed, we here analysed the effect of antibiotic treatment on female fecundity and nutritional status, and further explored the role of bacteria under different dietary regimes. The removal of culturable Enterobacteria from the gut of females was associated to a reduction in fecundity as well as in the protein and lipid reserves. However, fecundity reduction depended on the dietary regime; being more pronounced when females fed a poor diet. Our results suggest that nutrient reserves of females are determined, at least to some extent, by intestinal bacteria (particularly Enterobacteria). The effect of antibiotics on fecundity could be explained, thus, as a consequence of a poorer nutritional status in antibiotic-treated females compared to control females. Our results contribute to understand the interaction between gut bacteria and Tephritidae fruit flies. Considering the relevance of this insect as fruit pest and the widespread use of the sterile insect technique to control them, these findings may lead to practical applications, such as development of efficient mass rearing protocols of A. fraterculus that supplement the adult diet with probiotics.

Effects of symbiotic population impairment on microbiome composition and longevity of Bactrocera dorsalis

In order to understand the role of symbionts for their insect hosts, it is customary to treat them with antibiotics or to sterilize eggs (treatments), resulting in aposymbiotic and axenic insects, respectively. Such axenic insects can then be compared with untreated controls. Fruit flies often bear complex communities which are greatly reduced by such treatments. However, the bacterial community is not completely eliminated. Here, we examine the effect of these procedures on the structure of the remaining bacterial communities in Bactrocera dorsalis (Diptera: Tephritidae) and on the insect longevity. The antibiotics (Norfloxacin and Ceftazedime) were administered to 1-day-old adult flies through sugar meal for 7 days, and eggs were surface sterilized and dechorionated to produce axenic lines. The flies were starved of protein before they were offered full diets or diets containing non-essential amino acids only. Antibiotic and egg disinfection treatments resulted in a significant reduction of the vast majority of gut bacterial populations, especially Proteobacteria, Firmicutes and Bacteroidetes. On the other hand, antibiotic allowed the persistence of Actinobacteria, Cyanobacteria and Acidobacteria populations. In untreated control flies, longevity was extended irrespective of diet quality in comparison to treated flies. Conversely, when gut bacteria were largely reduced (aposymbiotic and axenic flies), longevity was reduced in the non-essential amino acids diet treatment versus slightly improved in the presence of a protein diet. We discuss these results in an ecological-life-history perspective.

Alteration of gut microbiota with a broad-spectrum antibiotic does not impair maternal care in the European earwig

The microbes residing within the gut of an animal host often increase their own fitness by modifying their host's physiological, reproductive and behavioural functions. Whereas recent studies suggest that they may also shape host sociality and therefore have critical effects on animal social evolution, the impact of the gut microbiota on maternal care remains unexplored. This is surprising, as this behaviour is widespread among animals, often determines the fitness of both juveniles and parents, and is essential in the evolution of complex animal societies. Here, we tested whether life-long alterations of the gut microbiota with rifampicin-a broad-spectrum antibiotic-impair pre- and post-hatching maternal care in the European earwig. Our results first confirm that rifampicin altered the mothers' gut microbial communities and indicate that the composition of the gut microbiota differs before and after egg care. Contrary to our predictions, however, the rifampicin-induced alterations of the gut microbiota did not modify pre- or post-hatching care. Independent of maternal care, rifampicin increased the females' faeces production and resulted in lighter eggs and juveniles. By contrast, rifampicin altered none of the other 21 physiological, reproductive and longevity traits measured over the 300 days of a female's lifetime. Overall, these findings reveal that altering the gut microbiota with a large spectrum antibiotic such as rifampicin does not necessarily affect host sociality. They also emphasize that not all animals have evolved a co-dependence with their microbiota and call for caution when generalizing the central role of gut microbes in host biology.

Diversity of bacteria in different life stages and their impact on the development and reproduction of Zeugodacus tau (Diptera: Tephritidae)

Fruit flies usually harbor diverse communities of bacteria in their digestive systems, which are known to play a significant role in their fitness. However, little information is available on Zeugodacus tau, a polyphagous pest worldwide. This study reports the first extensive analysis of bacterial communities in different life stages and their effect on the development and reproduction of laboratory-reared Z. tau. Cultured bacteria were identified using the conventional method, and all bacteria were identified by high-throughput technologies (16S ribosomal RNA gene sequencing of V3-V4 region). A total of six bacterial phyla were identified in larvae, pupae, and male and female adult flies, which were distributed into 14 classes, 32 orders, 58 families and 96 genera. Proteobacteria was the most represented phylum in all the stages except larvae. Enterobacter, Klebsiella, Providencia, and Pseudomonas were identified by conventional and next-generation sequencing analysis in both male and female adult flies, and Enterobacter was found to be the main genus. After being fed with antibiotics from the first instar larvae, bacterial diversity changed markedly in the adult stage. Untreated flies laid eggs and needed 20 days before oviposition while the treated flies showed ovary development inhibited and were not able to lay eggs, probably due to the alteration of the microbiota. These findings provide the cornerstone for unexplored research on bacterial function in Z. tau, which will help to develop an environmentally friendly management technique for this kind of harmful insect.

The Use of Dietary Antifungal Agent Terbinafine in Artificial Diet and Its Effects on Some Biological and Biochemical Parameters of the Model Organism Galleria mellonella (Lepidoptera: Pyralidae)

Diet quality widely affects the survival, development, fecundity, longevity, and hatchability of insects. We used the greater wax moth Galleria mellonella (Linnaeus) to determine the effects of the antifungal, antibiotic terbinafine on some of its' biological parameters. The effects of terbinafine on malondialdehyde (MDA) and protein carbonyl (PCO) contents and the activity of the detoxification enzyme, glutathione S-transferase (GST), in the midgut of seventh-instar larvae of G. mellonella were also investigated. The insects were reared on an artificial diet containing terbinafine at concentrations of 0.001, 0.01, 0.1, and 1 g. The survival rates at all development stages of G. mellonella were significantly decreased at all terbinafine concentrations. The females from a control diet produced 82.9 ± 18.1 eggs; however, this number was significantly reduced to 51.4 ± 9.6 in females given a 0.1 g terbinafine diet. The highest concentration of terbinafine (1 g) completely inhibited egg laying. Terbinafine significantly increased MDA content and GST activity in the midgut tissue of seventh-instar larvae in a dose-dependent manner. Relative to controls, these low dietary concentrations of terbinafine significantly increased midgut PCO content; a 0.1 g terbinafine concentration raised PCO content from 155.19 ± 21.8 to 737.17 ± 36.4 nmol/mg protein. This study shows concentration-dependent effects on the biological traits of the greater wax moth G. mellonella, including the oxidative status and detoxification capacity of the midgut. Low terbinafine concentrations may be possible for use as an antifungal agent in insect-rearing diets.

New Insights Into Culturable and Unculturable Bacteria Across the Life History of Medicinal Maggots Lucilia sericata (Meigen) (Diptera: Calliphoridae)

Because of the nutritional ecology of dung- and carrion-feeding, bacteria are the integral part of Lucilia sericata life cycle. Nevertheless, the disinfected larvae of the blowfly are applied to treat human chronic wounds in a biosurgery named maggot debridement therapy (MDT). To realize the effects of location/diet on the gut bacteria, to infer the role of bacteria in the blowfly ecology plus in the MDT process, and to disclose bacteria circulating horizontally in and vertically between generations, bacterial communities associated with L. sericata specimens from various sources were investigated using culture-based and culture-independent methods. In total, 265 bacteria, including 20 families, 28 genera, and 40 species, were identified in many sources of the L. sericata. Culture-dependent method identified a number of 144 bacterial isolates, including 21 species, in flies reared in an insectary; specimens were collected from the field, and third-instar larvae retrieved from chronic wounds of patients. Metagenetic approach exposed the occurrences of 121 operational taxonomic units comprising of 32 bacterial species from immature and adult stages of L. sericata. Gammaproteobacteria was distinguished as the dominant class of bacteria by both methods. Bacteria came into the life cycle of L. sericata over the foods and transovarially infected eggs. Enterococcus faecalis, Myroides phaeus, Proteus species, Providencia vermicola, and Serratia marcescens were exchanged among individuals via transstadial transmission. Factors, including diets, feeding status, identification tool, gut compartment, and life stage, governed the bacteria species. Herein, we reemphasized that L. sericata is thoroughly connected to the bacteria both in numerous gut compartments and in different life stages. Among all, transstadially transmitted bacteria are underlined, indicating the lack of antagonistic effect of the larval excretions/secretions on these resident bacteria. While the culture-dependent method generated useful data on the viable aerobic gut bacteria, metagenomic method enabled us to identify bacteria directly from the tissues without any need for cultivation and to facilitate the identification of anaerobic and unculturable bacteria. These findings are planned to pave the way for further research to determine the role of each bacterial species/strain in the insect ecology, as well as in antimicrobial, antibiofilm, anti-inflammatory, and wound healing activities.

Symbiotic bacteria motivate the foraging decision and promote fecundity and survival of Bactrocera dorsalis (Diptera: Tephritidae)

Background

The gut bacteria of tephritid fruit flies play prominent roles in nutrition, reproduction, maintenance and ecological adaptations of the host. Here, we adopted an approach based on direct observation of symbiotic or axenic flies feeding on dishes seeded with drops of full diet (containing all amino acids) or full diet supplemented with bacteria at similar concentrations to explore the effects of intestinal bacteria on foraging decision and fitness of Bactrocera dorsalis.

Results

The results show that intestinal probiotics elicit beneficial foraging decision and enhance the female reproduction fitness and survival of B. dorsalis (symbiotic and axenic), yet preferences for probiotic diets were significantly higher in axenic flies to which they responded faster compared to full diet. Moreover, females fed diet supplemented with Pantoea dispersa and Enterobacter cloacae laid more eggs but had shorter lifespan while female fed Enterococcus faecalis and Klebsiella oxytoca enriched diets lived longer but had lower fecundity compared to the positive control. Conversely, flies fed sugar diet (negative control) were not able to produce eggs, but lived longer than those from the positive control.

Conclusions

These results suggest that intestinal bacteria can drive the foraging decision in a way which promotes the reproduction and survival of B. dorsalis. Our data highlight the potentials of gut bacterial isolates to control the foraging behavior of the fly and empower the sterile insect technique (SIT) program through the mass rearing.

Symbiosis in Sustainable Agriculture: Can Olive Fruit Fly Bacterial Microbiome Be Useful in Pest Management?

The applied importance of symbiosis has been gaining recognition. The relevance of symbiosis has been increasing in agriculture, in developing sustainable practices, including pest management. Insect symbiotic microorganisms' taxonomical and functional diversity is high, and so is the potential of manipulation of these microbial partners in suppressing pest populations. These strategies, which rely on functional organisms inhabiting the insect, are intrinsically less susceptible to external environmental variations and hence likely to overcome some of the challenges posed by climate change. Rates of climate change in the Mediterranean Basin are expected to exceed global trends for most variables, and this warming will also affect olive production and impact the interactions of olives and their main pest, the obligate olive fruit fly (Bactrocera oleae). This work summarizes the current knowledge on olive fly symbiotic bacteria towards the potential development of symbiosis-based strategies for olive fruit fly control. Particular emphasis is given to Candidatus Erwinia dacicola, an obligate, vertically transmitted endosymbiont that allows the insect to cope with the olive-plant produced defensive compound oleuropein, as a most promising target for a symbiosis disruption approach.