Oviposition in Delia platura (Diptera, Anthomyiidae): The Role of Volatile and Contact Cues of Bean

Odorant Receptor BdorOR49b Mediates Oviposition and Attraction Behavior of Bactrocera dorsalis to Benzothiazole

The ability to recognize a host plant is crucial for insects to meet their nutritional needs and locate suitable sites for laying eggs. Bactrocera dorsalis is a highly destructive pest in fruit crops. Benzothiazole has been found to induce oviposition behavior in the gravid B. dorsalis. However, the ecological roles and the olfactory receptor responsible for benzothiazole are not yet fully understood. In this study, we found that adults were attracted to benzothiazole, which was an effective oviposition stimulant. In vitro experiments showed that BdorOR49b was narrowly tuned to benzothiazole. The electroantennogram results showed that knocking out BdorOR49b significantly reduced the antennal electrophysiological response to benzothiazole. Compared with wild-type flies, the attractiveness of benzothiazole to BdorOR49b knockout adult was significantly attenuated, and mutant females exhibited a severe decrease in oviposition behavior. Altogether, our work provides valuable insights into chemical communications and potential strategies for the control of this pest.

Two odorant receptors regulate 1-octen-3-ol induced oviposition behavior in the oriental fruit fly

The oriental fruit fly Bactrocera dorsalis (Hendel) is a notorious pest of fruit crops. Gravid females locate suitable oviposition sites by detecting host plant volatiles. Here, we demonstrate that 1-octen-3-ol, a volatile from mango, guides the oviposition behavior of female flies. Two odorant receptors (BdorOR7a-6 and BdorOR13a) are identified as key receptors for 1-octen-3-ol perception by qPCR analysis, heterologous expression in Xenopus laevis oocytes and HEK 293 cells followed by in vitro binding assays, as well as CRISPR/Cas9 genome editing in B. dorsalis. Molecular docking and site-directed mutagenesis are used to determine major binding sites for 1-octen-3-ol. Our results demonstrate the potential of 1-octen-3-ol to attract gravid females and molecular mechanism of its perception in B. dorsalis. BdorOR7a-6 and BdorOR13a can therefore be used as molecular targets for the development of female attractants. Furthermore, our site-directed mutagenesis data will facilitate the chemical engineering of 1-octen-3-ol to generate more efficient attractants.

Methyl jasmonate-induced resistance to Delia platura (Diptera: Anthomyiidae) in Lupinus mutabilis

BACKGROUND

Andean lupin (Lupinus mutabilis Sweet) is an important leguminous crop from South America with a high protein content. In Ecuador, lupin yields are severely affected by the infestation of Delia platura larvae on germinating seeds. The application of elicitor molecules with activity against herbivorous insects to control D. platura infestation constitutes an unexplored and promising alternative for chemical insecticides. In this study, methyl jasmonate (MeJA), hexanoic acid, menadione sodium bisulfite, and DL-β-aminobutyric acid were evaluated for their ability to induce resistance against D. platura in three commercial lupin cultivars.

RESULTS

Only seeds pretreated with MeJA significantly impaired insect performance during choice and no-choice assays. Additionally, fitness indicators such as seed germination and growth were not affected by MeJA treatment. To investigate the molecular mechanisms behind the MeJA-mediated resistance, RT-qPCR assays were performed. First, RT-qPCR reference genes were validated, showing that LmUBC was the most stable reference gene. Next, expression analysis over time revealed that MeJA application up-regulated the activity of the jasmonic acid biosynthetic genes LmLOX2 and LmAOS, together with other jasmonate-related defense genes, such as LmTPS1, LmTPS4, LmPI2, LmMBL, LmL/ODC, LmCSD1, and LmPOD.

CONCLUSION

This study indicates that MeJA can be used as an environmentally friendly elicitor molecule to protect Andean lupin from D. platura attack without fitness cost. MeJA application induces plant defense responses to insects in Andean lupin that may be modulated by the onset of terpenoid biosynthesis, proteinase inhibitors, lectins, polyamines, and antioxidative enzymes. © 2021 Society of Chemical Industry.

The Effect of Erwinia amylovora Infection in Apple Saplings and Fruit on the Behavior of Delia platura (Diptera: Anthomyiidae)

The Vector Manipulation Hypothesis (VMH) posits that phytopathogens develop strategies to enhance dissemination by mediating behavior change in insect vectors. The VMH is poorly studied in phytopathogenic bacteria, especially in systems with numerous, occasional vectors. Erwinia amylovora is a bacterial pathogen of pome fruit that produces a bacterial ooze and is mechanically vectored by insects after they feed on ooze. The blossom blight phase of the disease exhibits manipulation of honeybees, leading to enhanced transmission, but whether the same occurs during the shoot blight phase of the disease is unknown. The goal of this study was to evaluate the effect of E. amylovora on the behavior of Delia platura, a fly with a worldwide endemic presence that may transmit E. amylovora. We show that D. platura prefer infected, oozing fruit to uninfected fruit in choice tests and that preference subsides when bacterial ooze is removed from the infected fruit. Flies did not exhibit a preference between infected saplings and uninfected saplings. The volatiles of infected fruit did not attract D. platura, indicating that diseased fruit odor is not responsible for the observed preference for infected fruit. Flies did not differentiate between sapling odors until infected trees had died, at which point they preferred uninfected tree odors. This study supports previous hypotheses suggesting that E. amylovora takes advantage of existing plant-insect interactions, though it is not fully understood how significantly behavioral changes affect transmission. Additional pathosystems with occasional, nonspecific vectors should be studied to further understanding of the VMH.

Can soil microbial diversity influence plant metabolites and life history traits of a rhizophagous insect? A demonstration in oilseed rape

Interactions between plants and phytophagous insects play an important part in shaping the biochemical composition of plants. Reciprocally plant metabolites can influence major life history traits in these insects and largely contribute to their fitness. Plant rhizospheric microorganisms are an important biotic factor modulating plant metabolites and adaptation to stress. While plant-insects or plant-microorganisms interactions and their consequences on the plant metabolite signature are well-documented, the impact of soil microbial communities on plant defenses against phytophagous insects remains poorly known. In this study, we used oilseed rape (Brassica napus) and the cabbage root fly (Delia radicum) as biological models to tackle this question. Even though D. radicum is a belowground herbivore as a larva, its adult life history traits depend on aboveground signals. We therefore tested whether soil microbial diversity influenced emergence rate and fitness but also fly oviposition behavior, and tried to link possible effects to modifications in leaf and root metabolites. Through a removal-recolonization experiment, 3 soil microbial modalities ("high," "medium," "low") were established and assessed through amplicon sequencing of 16S and 18S ribosomal RNA genes. The "medium" modality in the rhizosphere significantly improved insect development traits. Plant-microorganism interactions were marginally associated to modulations of root metabolites profiles, which could partly explain these results. We highlighted the potential role of plant-microbial interaction in plant defenses against Delia radicum. Rhizospheric microbial communities must be taken into account when analyzing plant defenses against herbivores, being either below or aboveground.

Larval Performance and Adult Attraction of Delia platura (Diptera: Anthomyiidae) in a Native and an Introduced Crop

Delia platura Meigen is an important pest in crops around the world. Its host range includes almost 50 species, and it can develop in soil organic matter. In Ecuador, D. platura is a serious problem for the crop, Lupinus mutabilis Sweet (Chocho), and it also attacks broccoli (Brassica oleracea L.). After broccoli is harvested, crop residue is mixed with soil or collected and stored close to Chocho fields. The objectives of this study were to determine the adaptive responses of larvae reared on different hosts and whether D. platura females are preferentially attracted to germinating L. mutabilis seeds or broccoli residue. Accordingly, larval performance and attraction of female D. platura reared on broccoli residue and L. mutabilis seeds were evaluated. The number of larvae, pupae, and adults were higher when reared on broccoli. Conversely, pupal weight was higher and time from larva to pupa, pupa to adult, and total life cycle were longer in flies reared on L. mutabilis. Although D. platura developed more quickly on broccoli, L. mutabilis was also a good host since pupae were heavier compared with flies reared on broccoli. Delia platura females reared on broccoli preferred broccoli residue to L. mutabilis in an olfactometer. Volatiles from broccoli residue in soil may attract D. platura females and stimulate oviposition on L. mutabilis seeds. Environmentally benign production of L. mutabilis crops with minimal insecticide applications may require the elimination of fresh broccoli residue as fertilizer in soils where L. mutabilis is cultivated.

Evolutionarily conserved odorant receptor function questions ecological context of octenol role in mosquitoes

Olfaction is a key insect adaptation to a wide range of habitats. In the last thirty years, the detection of octenol by blood-feeding insects has been primarily understood in the context of animal host-seeking. The recent discovery of a conserved octenol receptor gene in the strictly nectar-feeding elephant mosquito Toxorhynchites amboinensis (TaOr8) suggests a different biological role. Here, we show that TaOR8 is a functional ortholog of its counterparts in blood-feeding mosquitoes displaying selectivity towards the (R)-enantiomer of octenol and susceptibility to the insect repellent DEET. These findings suggest that while the function of OR8 has been maintained throughout mosquito evolution, the context in which this receptor is operating has diverged in blood and nectar-feeding mosquitoes.

Identification of odorant-binding protein genes from antennal expressed sequence tags of the onion fly, Delia antiqua

Insect odorant-binding proteins (OBPs) are thought to play a crucial role in the chemosensation of hydrophobic molecules such as pheromones and host chemicals. The onion fly, Delia antiqua, is a specialist feeder of Allium plants, and utilizes a host odorant n-dipropyl disulfide as a cue for its oviposition. Because n-dipropyl disulfide is a highly hydrophobic compound, some OBPs might be indispensable for perception of it. However, no OBP gene has been identified in D. antiqua. Here, to obtain the DNA sequences of D. antiqua OBPs, we performed an analysis of antennal expressed sequence tags (ESTs). Among 288 EST clones, eight D. antiqua OBP genes were identified for the first time. Phylogenetic analysis revealed that each D. antiqua OBP gene is more closely related to its Drosophila orthologs than to the other D. antiqua OBP genes, suggesting that these OBP genes had emerged before the divergence of Delia and Drosophila species. All of the eight D. antiqua OBPs are expressed not only in the antennae but also in the legs, suggesting additional roles in the taste perception of non-volatile compounds. These findings serve as an important basis for understanding the molecular mechanisms underlying the host adaptations of D. antiqua.

Role of odour compounds in the attraction of gamete vectors in endophytic Epichloë fungi

Grass-infecting Epichloë endophytes (Ascomycota, Calvicipitaceae) depend on Botanophila flies for gamete transfer, while fly larvae feed and develop on the fertilized fungal fruiting structures. Flies are known to be attracted by volatile signals, but the exact mechanisms of chemical communication and the degree of specialization are unknown. Headspace samples collected from five different Epichloë species were analysed with respect to physiologically active substances using Botanophila flies. In field bioassays using synthetic compounds, their attractiveness and the specificity of the Epichloë-Botanophila attraction were investigated. The identification of a new natural product, methyl (Z)-3-methyldodec-2-enoate, attracting Botanophila flies is reported here, and chokol K is confirmed as an attractive compound. Different blends of the two compounds attracted Botanophila flies under field conditions, but the three fly taxa present at the study site showed no preference for specific blends of volatiles. Chemical communication in the Epichloë-Botanophila system relies on a few specific compounds, known as a communication system with 'private channels'. Although ratios of emitted compounds vary in different Epichloë species, this seems not to lead to specialized attraction of Botanophila flies. Low selective pressure for specialization may have maintained a more generalist interaction between fungi and flies.