Bactrocera tryoni

They are part of a complex, or a group of morphologically similar but biologically distinct species.[2] These are called sibling species. B. tyroni has three sibling species: B. neohumeralis, B. aquilonis, and B. melas.[2] All of these flies are sympatric, meaning that they inhabit the same territory, except B. aquilonas, which inhabits a different geographical area in northwest Australia.[3] Genetic data has suggested that B. aquilonas is simply an allopatric population of B. tryoni.[4] Additionally, B. tryoni mate at night, while B. neohumeralis mate during the day.[5] More pertinently, B. neohumeralis are not pests; they do not destroy crops.[2] Despite this behavioral difference, B. neohumeralis and B. tyroni are nearly genetically identical: the two species are only differentiable based on newly-developed microsatellite technology..[6] The evolutionary relationship between the species within the B. tryoni complex is unknown.[2]

After passing through a two week pre-oviposition stage following emergence from the pupae, adult females deposit around seven eggs in a fruit puncture, and may deposit up to 100 eggs per day. Fruit punctures are holes in the skin of the fruit that allow the females to access the nutrient rich interior. Females often oviposit in punctures made by other fruit flies, such as the Mediterranean fruit fly (Ceratitis capitata), which results in many eggs occurring in a single cavity.[10] Additionally, B. tyroni females can create their own puncture to oviposit in the fruit, called a "sting."

Eggs hatch into white larvae in 2–4 days under favorable weather conditions. These larvae, or maggots, eat toward the center of the fruit with their cutting jaws, causing it to rot. The maggots may reach up to 9 mm in length; larval development is completed in 10-31 days. At this point, the fruit has likely fallen to the ground. Up to 40 larvae can be reared from a single piece of fruit.

The maggot chews its way out of the remaining fruit and enters the soil, where it enters the pupal stage of development. Pupal development requires various temperature ranges from one week in warmer weather to one month in cooler conditions. The flexible amount of time needed for pupal development has resulted in B. tyroni relative adaptiveness to different environments.

After the pupal stage is complete, adults emerge from the soil. This typically occurs near the end of the summer season. Unlike other fly pests, B. tryoni does not breed continuously, but spends the winter in the adult stage.[11] Adult females live many months, and up to four or five overlapping generations may occur annually. Adults may live for a year or longer.

B. tyroni have been found to infect almost all commercial fruit crops as hosts, including abiu, apple, avocado, babaco, capsicum, carambola, casimiroa, cherry, citrus, custard apple, granadilla, grape, guava, kiwifruit, mango, nectarine, papaya, passionfruit, peach, pear, persimmon, plum, pomegranate, prune, quince, loquat, santol, spodilla, tamarillo, tomato, and wax jambu, with the exception of pineapples.[12] B. tyroni strongly prefer to oviposit into rotting fruit, although some evidence suggests that they will oviposit into under-ripe fruit as well. B. tyroni prefer to select fruits that have an outer layer that is able to be punctured or has already been lesioned.[12] The majority of research on B. tyroni host selection has included just a few, economically important crops.[2]

Larvae feed only on the flesh of fruit until they mature into adulthood. Adult flies, however, rely on leaf surface bacteria as a major source of protein.[2] There is some evidence suggesting that the bacteria and flies co-evolved,[13] but other data suggest that this symbiosis does not occur as the presence of protein-providing bacteria is not consistent throughout B. tyroni populations.[14] Due to this dependence on protein originating from bacteria, it is possible to control the population by providing flies with artificial protein mixed with insecticide.[2]

B. tyroni flies mate at dusk. This is pertinent to control efforts because it is one of the few characteristics that distinguish it from sister species, B. neohumeralis, which are not a highly destructive species, even though the two are very closely related genetically and evolutionarily.[2]

B. tyroni males exhibit behavior termed cue-lure, meaning that they are strongly attracted to a specific scent.[2] While this scent is artificially made, it is closely related to compounds occurring in nature. Male B. tyroni respond most greatly to the lure in the morning, likely because this is their peak of foraging time; however, an evolutionary reason for the cue-lure is not fully known. Cue-lure is only exhibited in sexually mature males, indicating that mate finding is related to the cue-lure behavior.[2] However, other Bactrocera species have been identified as means of enhancing male competitiveness, or to afford protection from predation.[15]

B. tyroni lay their eggs in fruit. Females prefer to lay their eggs in fruit that is sweet, juicy, and not acidic.[11] The presence of other female flies in pre- or post- oviposition on a piece of fruit was found to have no bearing on another female's likeliness to land on the fruit; however, female flies were more likely to bore into a piece of fruit that other female flies were currently ovipositing into, therefore increasing the density of larvae within a single piece of fruit.[11] This is an example of reciprocal altruism as larvae are at an advantage at higher densities.

B. tyroni have evolved to disperse widely, which was greatly influenced their ability to cause damage to farms.[16] When fruit is available, the flies often do not disperse far distances (only a few hundred meters to a kilometer), but they have been found to travel large distances in the absence of fruit. In addition to lack of resources, adult flies may also move to locate overwintering sites or avoid dry or cold weather.[16]

Farmers in effected regions are encouraged to use a lure and kill tactic to combat the presence of B. tyroni.[17] Lure and kill tactics include the use of some sort of bait that attracts the pest, or a lure.[18] This can include semiochemical lures such as pheromones, food attractants, host mimics, or color attractants.[2] The killing mechanism often involves pesticides, liquid traps in which the pest drowns, or sticky traps that the pest cannot escape from.[2] At low densities of B. tyroni, lure and kill tactics are most effective as a mechanism to monitor the frequency of B. tyroni; at high densities, they effectively combat the pest via population reduction.[17] Two of the most common lure and kill approaches for B. tyroni are the male annihilation technique (MAT) and the protein-bait spray (PBS).[2]

Both male and female B. tyroni require proteins produced by bacteria found on the leaves of plants in order to reach sexual maturity.[2] The protein-bait spray takes advantage of this behavior by combining necessary proteins normally acquired from leaf bacteria with deadly insecticides.[19] The combination of protein and insecticide attracts B. tyroni of both sexes, resulting in elimination of adult flies.[20] Neither the effectiveness of this technique, nor the scientific underpinnings of what protein exactly attracts B. tyroni to the spray, are well investigated.[2]

B. tyroni sexually mature males are strongly responsive to specific scents that may be associated with mating, or a cue-lure.[21] A specific cue-lure, Willson's lure, was found to be incredibly effective at attracting sexually mature B. tyroni males.[22] When combined with insecticides, artificially developed cue lures may be an effective elimination method of sexually mature males. While extensive research has suggested this is an effective strategy in other fly species, very few controlled experiments have been conducted to determine the effectiveness of the male annihilation technique in B. tyroni.[2]

Control efforts include submerging post-harvest fruit and treating fields of fruit trees with the chemicals dimethoate and fenthion. As of October 2011, the use of these chemicals was under review by the Australian Pesticides and Veterinary Medicines Authority. As a result, dimethoate was suspended from use.[23] As of 2014, fenthion was no longer commercially available in Australia.

Containment efforts have included irradiating pupae in order to induce sterility. A study testing the viability of this technique found that sterility was dose independent, meaning that a single ionizing event was enough to render the male sperm sterile.[24] Additionally, it was found that emergence and flight ability remained unaffected by the ionizing event. This indicates that males sterilized via a low dosage of radiation were equally as competitive as males that were not irradiated.[24]

A popular method for controlling populations of invasive or destructive species of flies involves producing a strain of fly that is incapable of reproducing.[25] If this can be accomplished, this strain can be mass produced and released into the wild without necessitating repeated exposure to irradiation as required in methods such as the Sterile Insect Technique (SIT).[24]If they have the same sexual competitiveness as wild type males, then the species’ overall population will presumably decrease. In order to accomplish developing such a strain in B. tyroni, molecular tools capable of genetically transforming B. tyroni must be implemented.[25] One such strain of a genetically compatible fly has been developed in Drosophila melanogaster.[25] The phenotypic marker for the presence of an efficient vector for gene transfer is white eye color.[25] Development of a genetically engineered B. tyroni strain that is compatible with gene transfer was successful; however, scientists have yet to develop a sterile strain that can be released into the wild.[25]

Rising CO₂ levels may influence the distribution of B. tyroni[27]. Based upon recent studies which utilize computer programs to simulate B. tyroni distribution in the event of rising temperatures, it was predicted that there will be an overall increase in Queensland fruit fly damage, but the fruit flies will re-localize to more southerly locations as northern and central Queensland will begin to exceed the maximum habitable temperature of B. tyroni.[27] However, these simulations may not accurately predict the future distributions of B. tyroni as they have exhibited an immense capability to adapt to various conditions.[19] The prediction is also complicated by the uncertainty of how the relative humidity will change in regions across Australia as temperature increases, and B. tyroni survival is heavily dependent upon a humid climate.[19]