BackgroundSymbiotic bacteria affect insect physiology and ecology. They may also mediate insecticide resistance within their hosts and thereby impact pest and vector control practices. Here, we document a novel mechanism of insecticide resistance in which a gut symbiont of the tephritid pest fruit fly Bactrocera dorsalis enhances resistance to the organophosphate insecticide trichlorphon.ResultsWe demonstrated that the gut symbiont Citrobacter sp. (CF-BD) plays a key role in the degradation of trichlorphon. Based on a comparative genomics analysis with other Citrobacter species, phosphatase hydrolase genes were identified in CF-BD. These CF-BD genes had higher expression when trichlorphon was present. Bactrocera dorsalis inoculated with isolated CF-BD obtained higher trichlorphon resistance, while antibiotic-treated flies were less resistant confirming the key role of CF-BD in insecticide resistance.ConclusionsOur findings suggest that symbiont-mediated insecticide resistance can readily develop in B. dorsalis and may represent a more widely relevant insecticide resistance mechanism than previously recognized.Electronic supplementary materialThe online version of this article (doi:10.1186/s40168-017-0236-z) contains supplementary material, which is available to authorized users.

The data provided a comprehensive survey of insecticide resistance in Bactrocera dorsalis in mainland China. All results suggested that early resistance management programmes should be established for restoring the efficacy of pesticide-based control measures.

Diaphorina citri Kuwayama (Hemiptera: Sternorrhyncha: Psyllidae) is a vector of huanglongbing, a disease of citrus that in Asia is caused by ‘Candidatus Liberibacter asiaticus’ (α‐Proteobacteria) (Las). Acquisition of Las by D. citri appears to be variable, and this variability may be due to the suitability of the host plants and their tissues for acquisition. Therefore, this study aimed to determine the effect of symptom severity of the disease on the feeding behaviour of D. citri. Use of an electrical penetration graph showed that the pathway phase of D. citri consisted of four waveforms, A, B, C, and D; waveforms A and B have not been reported for D. citri before. The remaining waveforms, E1, E2, and G, conform to those described before for D. citri. The duration of the non‐penetration period did not differ between healthy or infected plants. However, in moderately and severely symptomatic plants, the duration of the pathway phase increased, whereas the phloem phase was shorter. In all diseased plants, the times to first and sustained salivation in the phloem were longer than those in control plants, with the times being related to symptom severity. As symptom expression increased, the percentage of time spent by psyllids salivating during the phloem phase increased; however, the percentage of time spent in phloem activities reduced gradually from ca. 74% in the control plants to ca. 8% in the severely symptomatic plants. In contrast, the percentage of time spent on xylem activities increased, as did the proportion of psyllids feeding from xylem. The differences in the durations of the E waveforms on plants showing different levels of symptom expression may account for differences in acquisition found amongst studies; therefore, future work on the acquisition and transmission of Las needs to carefully document symptom expression.

To accurately assess gene expression levels, it is essential to normalize real-time quantitative PCR (RT-qPCR) data with suitable internal reference genes. For the red imported fire ant, Solenopsis invicta, reliable reference genes to assess the transcript expression levels of the target genes have not been previously investigated. In this study, we examined the expression levels of five candidate reference genes (rpl18, ef1-beta, act, GAPDH, and tbp) in different developmental stages, castes and tissues of S. invicta. To evaluate the suitability of these genes as endogenous controls, three software-based approaches (geNorm, BestKeeper and NormFinder) and one web-based comprehensive tool (RefFinder) were used to analyze and rank the tested genes. Furthermore, the optimal number of reference gene(s) was determined by the pairwise variation value. Our data showed that two of the five candidate genes, rpl18 and ef1-beta, were the most suitable reference genes because they have the most stable expression among different developmental stages, castes and tissues in S. invicta. Although widely used as reference gene in other species, in S. invicta the act gene has high variation in expression and was consequently excluded as a reliable reference gene. The two validated reference genes, rpl18 and ef1-beta, can be widely used for quantification of target gene expression with RT-qPCR technology in S. invicta.

Although mutualism between ants and honeydew-producing hemipterans has been extensively recognized in ecosystem biology, however few attempts to test the hypothesis that mutualism between two alien species leads to the facilitation of the invasion process. To address this problem, we focus on the conditional mutualism between S. invicta and P. solenopsis by field investigations and indoor experiments. In the laboratory, ant colony growth increased significantly when ants had access to P. solenopsis and animal-based food. Honeydew produced by P. solenopsis also improved the survival of ant workers. In the field, colony density of P. solenopsis was significantly greater on plots with ants than on plots without ants. The number of mealybug mummies on plants without fire ants was almost three times that of plants with fire ants, indicating a strong effect of fire ants on mealybug survival. In addition, the presence of S. invicta successfully contributed to the spread of P. solenopsis . The quantity of honeydew consumption by S. invicta was significantly greater than that of a presumptive native ant, Tapinoma melanocephalum . When compared with the case without ant tending, mealybugs tended by ants matured earlier and their lifespan and reproduction increased. T. melanocephalum workers arrived at honeydew more quickly than S. invicta workers, while the number of foraging S. invicta workers on plants steadily increased, eventually exceeding that number of T. melanocephalum foragers. Overall, these results suggest that the conditional mutualism between S. invicta and P. solenopsis facilitates population growth and fitness of both species. S. invicta tends to acquire much more honeydew and drive away native ants, promoting their predominance. These results suggest that the higher foraging tempo of S. invicta may provide more effective protection of P. solenopsis than native ants. Thus mutualism between these two alien species may facilitate the invasion success of both species.