Phytosanitary Treatments Against Bactrocera dorsalis (Diptera: Tephritidae): Current Situation and Future Prospects

Hot Water Treatment for Post-Harvest Disinfestation of Bactrocera dorsalis (Diptera: Tephritidae) and Its Effect on cv. Tommy Atkins Mango

Simple Summary

The oriental fruit fly Bactrocera dorsalis is a major quarantine pest in sub-Saharan Africa that threatens mango production and international trade. In this study, we developed a hot water treatment (HWT) protocol for the post-harvest disinfestation of B. dorsalis and assessed its impact on cv. Tommy Atkins mango quality parameters after treatment. First, we established the rate of development of the immature stages of B. dorsalis in cv. Tommy Atkins mango and then determined their heat tolerance. The third-instar larva was found to be the most heat tolerant of the immature stages. The study demonstrates that a hot water treatment schedule of 46.1 °C for 72.63 min can lead to complete mortality of the most heat-tolerant stage of B. dorsalis in cv. Tommy Atkins mango. Furthermore, we carried out large-scale confirmatory trials to validate our hot water treatment schedule, and none of the 59,120 most heat-tolerant larvae treated survived. Our protocol guarantees effective quarantine security with no adverse effect on the quality of cv. Tommy Atkins mango fruit and can be commercially adopted to promote and increase mango exports to lucrative markets abroad.

Abstract

Mango production and trade in sub-Saharan Africa is hampered by direct damage and the high quarantine status of B. dorsalis and the paucity of effective post-harvest phytosanitary treatments. The current study reports the development of a quarantine treatment protocol using hot water to disinfest B. dorsalis and assess its effect on cv. Tommy Atkins mango quality. We first determined the development of the eggs and all larval stages of B. dorsalis in cv. Tommy Atkins mango and used the information to establish a time–mortality relationship of the immature stages after subjecting infested fruits to a regimen of eight, time instances of hot water at 46.1 °C. Using probit analysis, we estimated the minimum time required to achieve 99.9968% mortality of each stage. Our results indicate that the egg was the least heat tolerant, followed by the first, second, and third instar. The time required to achieve 99.9968% control of the third instar in cv. Tommy Atkins mango (400–600 g) was determined to be 72.63 min (95% Cl: 70.32–74.95). In the confirmatory trials, the hot water treatment schedule of 46.1 °C/72.63 min was validated, and none of the 59,120 most heat-tolerant individuals treated survived. Further, there were no significant differences between hot water-treated and untreated mangoes recorded in weight loss, fruit firmness, pH, total soluble solids, moisture content, and titratable acidity eleven days post-treatment. These findings demonstrate an effectively optimum post-harvest disinfestation treatment against B. dorsalis in cv. Tommy Atkins mango that should be adopted commercially to facilitate access to profitable but strict export markets globally.

Evaluation of Reference Genes for Quantitative Reverse Transcription Polymerase Chain Reaction in Bactrocera dorsalis (Diptera: Tephritidae) Subjected to Various Phytosanitary Treatments

Simple Summary

In this study, seven internal reference genes (G6PDH, GAPDH, RPL-32, Rpl-13, Rps-3, α-tub, and 18S) of Bactroceradorsalis under different quarantine treatments (heat treatment, cold treatment, methyl bromide fumigation, and irradiation) were screened. Finally, the most stable internal reference gene was selected, which laid a foundation for the further study of its resistance mechanisms to some abiotic stresses.

Abstract

Bactrocera dorsalis is a major pest that causes serious damage to many fruits. Although phytosanitary treatment methods have been developed for Bactrocera control, there is a lack of information related to the gene expression pattern of B. dorsalis subjected to phytosanitary treatment conditions. Prior to quantitative reverse transcription polymerase chain reaction analysis of the most stable reference genes in B. dorsalis (Diptera: Tephritidae), B. dorsalis third-instar larvae were exposed to various phytosanitary treatments; seven candidate reference genes (18S, G6PDH, GAPDH, RPL-13, RPL-32, RPS-3, and α-Tub) were amplified and their expression stabilities were evaluated using geNorm, NormFinder, BestKeeper, and RefFinder algorithms. Different reference genes were found under different stress conditions. G6PDH was the most stable gene after heat treatment. After cold treatment, α-Tub exhibited the highest expression stability. G6PDH expression stability was the highest after fumigation with methyl bromide. RPL-32 showed the highest expression stability after irradiation treatment. Collectively, RefFinder analysis results revealed G6PDH and RPL-32 as the most suitable genes for analyzing phytosanitary treatment in B. dorsalis. This study provides an experimental basis for further gene expression analyses in B. dorsalis subjected to various phytosanitary treatments, which can aid in the development of novel phytosanitary treatments against insect pests.

Preferential Attraction of Oviposition-Ready Oriental Fruit Flies to Host Fruit Odor over Protein Food Odor

Simple Summary

Oriental fruit fly, Bactrocera dorsalis, is one of the most destructive invasive pests of tropical fruit and vegetable crops worldwide. Current oriental fruit fly quarantine programs focus heavily on the control and surveillance of male flies, which is less effective for mitigating the impact caused by female populations. We investigated the relationship between olfactory preference and oviposition outcome of oriental fruit flies. In laboratory bioassays using similarly aged (14–16 day old) cohorts of mated females, some females preferred host fruit odors over protein food odor (torula yeast), while some preferred protein odor. The females that preferred host fruit odor had 2.1 times greater egg load and laid 2.4 times more eggs than females that preferred protein odor. Our results suggest that mated female oriental fruit flies with a preference for host fruit odor are likely to be in an imminent oviposition-ready physiological status, while mated females that prefer torula yeast odor are likely more protein-hungry and need more protein to produce the critical egg load necessary for oviposition.

Abstract

Olfaction plays a key role in the location of food and oviposition resources by tephritid fruit flies. Adult females, including oriental fruit fly, Bactrocera dorsalis, can sustain egg production throughout their lives provided they obtain sufficient protein. Thus, preferential attraction to food or oviposition sites (host fruit) will depend on a fly’s particular physiological state. In this study, laboratory bioassays were conducted with mature, mated B. dorsalis (provisioned protein and sugar ad libitum) to evaluate attraction to traps baited with torula yeast versus six host fruit sources (guava, guava juice, mango, orange, Surinam cherry, or white sapote). Females that preferred fruit laid a significant number of eggs around the trap entrance (average 405 eggs/fly), while almost no eggs were laid by females that preferred yeast (0.5 and 1.3 eggs/fly on two occasions). Similar results were observed in a bioassay using headspace extracts of guava juice and torula yeast, supporting olfactory-mediated responses. When individual females were allowed to oviposit in guava juice traps 0–24 h after a choice test, 45.8% of females that chose guava juice laid eggs (average 14.7 eggs/fly), compared with 27.5% that chose yeast (average 6.5 eggs/fly). Dissections indicated that females with a preference for guava juice had an egg load 2.4 times greater than females that preferred yeast. These results suggest there is an olfactory-based behavioral switch in preference from protein to host odors when female B. dorsalis are oviposition-ready. We discuss the implications of our findings for practical behavioral management and detection programs for B. dorsalis.

Scientific opinion on the import of Musa fruits as a pathway for the entry of non-EU Tephritidae into the EU territory

Following a request from the European Commission, the EFSA Panel on Plant Health examined evidence as to whether the import of fruits of Musa (bananas and plantains) could provide a pathway into the EU for Bactrocera dorsalis (Hendel) (Diptera: Tephritidae) or other non-EU Tephritidae for which Musa is a host. Relevant scientific and technical information, including unpublished information provided to the EFSA Panel on Plant Health by the European Commission from research conducted in Cabo Verde, were taken into account. The majority of EU imports of Musa fruit comes from Ecuador, Colombia and Costa Rica where B. dorsalis does not occur. Commercial Musa fruits are harvested at 'green stage one' before they begin to ripen naturally. Postharvest processes are designed to ensure that only high quality, unripe fruit are exported. Green stage one fruit are transported to the EU in controlled conditions and stimulated to ripen when exposed to exogenous ethylene in ripening rooms in the EU. There is no evidence that any Tephritidae can naturally infest commercial varieties of Musa fruit at green stage one or earlier. When experimentally infested with eggs of Tephritidae, larvae fail to develop in green stage one fruit. Physical and chemical changes that occur during fruit ripening enable B. dorsalis and 11 other species of Tephritidae to oviposit and develop in Musa at later stages of fruit development. Reports of B. dorsalis or other Tephritidae infesting bunches of Musa fruit are a consequence of the fruit being left to develop beyond green stage one in the field. There is no evidence that commercially grown fruits of Musa, for export to the EU, provide a pathway for the entry of non-EU Tephritidae. Passengers bringing Musa fruit from countries where Tephritidae can infest ripened Musa fruit do however provide a potential pathway for the entry of non-EU Tephritidae into the EU territory.

Influence of inoculated gut bacteria on the development of Bactrocera dorsalis and on its susceptibility to the entomopathogenic fungus, Metarhizium anisopliae

Background

Symbiotic interactions between insects and bacteria have been associated with a vast variety of physiological, ecological and evolutionary consequences for the host. A wide range of bacterial communities have been found in association with the oriental fruit fly, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae), an important pest of cultivated fruit in most regions of the world. We evaluated the diversity of gut bacteria in B. dorsalis specimens from several populations in Kenya and investigated the roles of individual bacterial isolates in the development of axenic (germ-free) B. dorsalis fly lines and their responses to the entomopathogenic fungus, Metarhizium anisopliae.

Results

We sequenced 16S rRNA to evaluate microbiomes and coupled this with bacterial culturing. Bacterial isolates were mono-associated with axenic B. dorsalis embryos. The shortest embryonic development period was recorded in flies with an intact gut microbiome while the longest period was recorded in axenic fly lines. Similarly, larval development was shortest in flies with an intact gut microbiome, in addition to flies inoculated with Providencia alcalifaciens. Adult B. dorsalis flies emerging from embryos that had been mono-associated with a strain of Lactococcus lactis had decreased survival when challenged with a standard dosage of M. anisopliae ICIPE69 conidia. However, there were no differences in survival between the germ-free lines and flies with an intact microbiome.

Conclusions

These findings will contribute to the selection of probiotics used in artificial diets for B. dorsalis rearing and the development of improved integrated pest management strategies based on entomopathogenic fungi.

Supplementary information

Supplementary information accompanies this paper at 10.1186/s12866-020-02015-y.

Effect of host fruit, temperature and Wolbachia infection on survival and development of Ceratitis capitata immature stages

The Mediterranean fruit fly, Ceratitis capitata (Diptera: Tephritidae), holds an impressive record of successful invasions promoted by the growth and development of international fruit trade. Hence, survival of immatures within infested fruit that are subjected to various conditions during transportation seems to be a crucial feature that promotes invasion success. Wolbachia pipientis is a common endosymbiont of insects and other arthropods generating several biological effects on its hosts. Existing information report the influence of Wolbachia on the fitness traits of insect host species, including the Mediterranean fruit fly. However, little is known regarding effects of Wolbachia infection on immature development in different host fruits and temperatures. This study was conducted to determine the development and survival of immature stages of four different Mediterranean fruit fly populations, either infected or uninfected with Wolbachia, in two hosts (apples, bitter oranges) under three constant temperatures (15, 25 and 30°C), constant relative humidity (45-55 ± 5%), and a photoperiod of 14L:10D. Our findings demonstrate both differential response of two fruit fly lines to Wolbachia infection and differential effects of the two Wolbachia strains on the same Mediterranean fruit fly line. Larva-to-pupa and larva-to-adult survival followed similar patterns and varied a lot among the four medfly populations, the two host fruits and the different temperatures. Pupation rates and larval developmental time were higher for larvae implanted in apples compared to bitter oranges. The survival rates of wildish medflies were higher than those of the laboratory adapted ones, particularly in bitter oranges. The Wolbachia infected medflies, expressed lower survival rates and higher developmental times, especially the wCer4 infected line. High temperatures constrained immature development and were lethal for the Wolbachia infected wCer4 medfly line. Lower temperatures inferred longer developmental times to immature stages of all medfly populations tested, in both host fruits. Implications on the ecology and survival of the fly in nature are discussed.

The Effects of a Cold Disinfestation on Bactrocera dorsalis Survival and Navel Orange Quality

Citrus sinensis (L.) Osbeck is an important economic product in South China, but the presence of quarantine pests in this product proposes the potential threat to international trade security. To find a proper phytosanitary cold treatment for Bactrocera dorsalis (Hendel) (Diptera: Tephritidae), commonly called oriental fruit fly, one of the most serious quarantine insects in navel orange, eggs in petri dish and larvae in navel orange fruits were exposed to a 1.7 °C cold chamber for 0–11 days to compare the tolerance to cold treatment. The 2nd instar larva (4 days) is the most tolerant stage, and the estimated time for 99.9968% mortality at the 95% confidence level is 11.3 (9.5, 14.6) days. Then 15 days was selected as the target time for the confirmatory tests, resulting in no survivors from 37,792 treated larvae with the efficacy of 99.9921% mortality at the 95% confidence level. The quality assessments were conducted to compare the effect on the navel orange fruit between cold treatment and the conventional cold storage. Results indicated that the cold treatment did not negatively affect the fruit quality. Therefore, this cold treatment showed potential as a commercial quarantine treatment for navel orange in international trade.

Unexpected Diversity of Wolbachia Associated with Bactrocera dorsalis (Diptera: Tephritidae) in Africa

Bactrocera dorsalis (Hendel) is an important pest of fruit-bearing plants in many countries worldwide. In Africa, this pest has spread rapidly and has become widely established since the first invasion report in 2003. Wolbachia is a vertically transmitted endosymbiont that can significantly influence aspects of the biology and, in particular, the reproduction of its host. In this study, we screened B. dorsalis specimens collected from several locations in Africa between 2005 and 2017 for Wolbachia using a PCR-based assay to target the Wolbachia surface protein wsp. Of the 357 individuals tested, 10 were positive for Wolbachia using the wsp assay. We identified four strains of Wolbachia infecting two B. dorsalis mitochondrial haplotypes. We found no strict association between the infecting strain and host haplotype, with one strain being present in two different host haplotypes. All the detected strains belonged to Super Group B Wolbachia and did not match any strains reported previously in B. dorsalis in Asia. These findings indicate that diverse Wolbachia infections are present in invasive populations of B. dorsalis.

Cold Response of the Mediterranean Fruit Fly (Ceratitis capitata) on a Lab Diet

Cold treatment at 0.0 °C with different exposure durations (0–12 days) was applied to the Mediterranean fruit fly Ceratitis capitata (Wiedemann) fed on a lab diet. The examined developmental stages were early eggs (<6 h), late eggs (>42 h), first instar, second instar and third instar larvae. Pupation, adult emergence and sex ratios of survived flies were investigated to study the C. capitata responses to this low temperature treatment. Our results showed that exposure time at low temperature has a clear effect on pupation and adult emergence. Based on pupation ratios, the first and third instar are the most cold tolerant stages, with LT99 = 7.3 for both of them. Cold tolerance at both stages are very close and no significant differences were detected. There were no significant differences on C. capitata sex ratios among different stages after treatment. This study improves our understanding of C. capitata responses to cold treatment, which may assist in the improvement of the current treatment strategies to control this destructive horticulture pest species.

Population structure of a global agricultural invasive pest, Bactrocera dorsalis (Diptera: Tephritidae)

Bactrocera dorsalis, the Oriental fruit fly, is one of the world's most destructive agricultural insect pests and a major impediment to international fresh commodity trade. The genetic structuring of the species across its entire geographic range has never been undertaken, because under a former taxonomy B. dorsalis was divided into four distinct taxonomic entities, each with their own, largely non-overlapping, distributions. Based on the extensive sampling of six a priori groups from 63 locations, genetic and geometric morphometric datasets were generated to detect macrogeographic population structure, and to determine prior and current invasion pathways of this species. Weak population structure and high genetic diversity were detected among Asian populations. Invasive populations in Africa and Hawaii are inferred to be the result of separate, single invasions from South Asia, while South Asia is also the likely source of other Asian populations. The current northward invasion of B. dorsalis into Central China is the result of multiple, repeated dispersal events, most likely related to fruit trade. Results are discussed in the context of global quarantine, trade, and management of this pest. The recent expansion of the fly into temperate China, with very few associated genetic changes, clearly demonstrates the threat posed by this pest to ecologically similar areas in Europe and North America.

Relative Tolerance of Six Bactrocera (Diptera: Tephritidae) Species to Phytosanitary Cold Treatment

To compare relative cold treatment tolerance across the economically important tephritid fruit flies (Diptera: Tephritidae), Bactrocera carambolae Drew & Hancock, Bactrocera correcta (Bezzi), Bactrocera cucurbitae (Coquillett), four populations of Bactrocera dorsalis (Hendel), Bactrocera zonata (Saunders), and Bactrocera tryoni (Froggatt), eggs (in vitro), and larvae (in infested fruit or on carrot diet) were cold treated at 2.0 ± 0.2 °C for selected durations. The study was performed to assess whether a single (i.e., generic) cold treatment could be developed that would control the entire group of fruit flies that were tested. Probit regression models showed that the hierarchy of cold resistance was third-instar larvae reared on carrot diet > third-instar larvae reared on orange > eggs test in vitro. Differences in mortality responses of third-instar larvae reared in oranges across populations of B. dorsalis were observed only at subefficacious levels of control. The majority of Bactrocera species responded the same at the high levels of control demanded of phytosanitary treatments, which indicated that cold treatments would be similarly effective across the species and populations tested. B. cucurbitae was found to be the most cold tolerant of all the species tested.