Bactrocera carambolae

Bactrocera carambolae is a member of the Oriental fruit fly Bactrocera dorsalis species complex and are genetically similar except for slight differences in nuclear and mitochondrial DNA.[2] From DNA analyses it was determined that B. carambolae forms a reciprocally monophyletic sister group to a larger clade containing the B. dorsalis species complex.[7]

Bactrocera carambolae can be difficult to distinguish from other B. dorsalis complex species. This species is generally characterized by a predominantly black thorax, while featuring abdominal segments with brown lateral posterior markings and a medial longitudinal black band over all three tergum.[7] This species also features a yellow scutellum; triangle shaped portion of exoskeleton located between the base of the wings. In addition to other Diptera, this species has one pair of membranous wings, with hind wings reduced to knob-like structures. The head of this species is reddish brown with one pair of oval, black compound eyes, and sucking/piercing mouthparts. This species can reach a length of 6 to 8 mm (0.6 cm to 0.8 cm).

Female B. carambolae can be differentiated from male B. carambolae by observation of longer abdomens, and an ovipositor on the posterior end of the abdominal segments, where as males have shorter abdomens and a more rounded tip of their abdominal segments.[7] Compared to other members of the B. dorsalis complex, B. carambolae have shorter male reproductive organs, known as aedeagi, and broader costal bands on their wings.[7]

Different bacterial communities are associated with different developmental stages of B. carambolae, and includes four major phyla; Actinomycetota, Bacteroidota, Bacillota and Pseudomonadota.[10] Proteobacteria are dominantly present in every life stage, and Bacteroidetes are at largest in the larval and pupal stages.[10] The large abundance of Proteobacteria are essential in the act of courtship and reproduction of not only B. carambolae, but Bactrocera fruit flies in general.[10]

Carambola fruit without infestation

Fruit following infestation at larval stage

Bactrocera carambolae often feed prior to depositing eggs under the skin of a fruit, therefore you can characterize a fruit that has been fed on by the likelihood of dark spots, or imperfections.[1] Dark spots on the skin of fruits are induced by the female carambola fruit fly laying her eggs. Sometimes there may be no symptoms of infestation on the outside of the fruit following feeding, however dark spots are seen especially on carambola, cherry and guava fruits.[7] Carambola fruit flies prefer to feed on tropical fruits, such as mangos, papayas, and oranges, using their proboscis to pierce the skin and suck liquids from the underlying flesh.[5]

Only adult B. carambolae are capable of mating through sexual reproduction, although the larval stage is capable of producing pheromones to a certain degree.[3] Male B. carambolae are strongly attracted to methyl eugenol (ME), which is a secondary plant compound found worldwide.[3] As they feed on this compound, they convert it into a phenylpropanoid known as (E)-coniferyl alcohol (ECF).[3] This new compound is stored in the rectal gland of the male, ready to be released as a sex pheromone during courtship.[3] Endogenous production of major compound 6-oxo-1-nonanol, and minor compound N-3-methylbutyl acetamide, also contribute to courtship and reproduction. They are stored in the male rectal gland, and when released into the air they create upwind flight via zigzag movement, attracting a significant number of female B. carambolae for courtship.[11] The production of all three endogenous compounds increase with age, reaching its maximum at sexual maturity.[3][11] The mating period of this species correlates with the time of dusk.[11]

The endogenous volatile, 6-oxo-1-nonanol, produced by male B. carambolae is also used a deterrent for predators, such as several species of gecko.[12] The production of this endogenous compound is age-related. The gecko is an avid consumer of immature carambola larvae, however their consumption greatly decreases when it comes to sexually mature male B. carambolae.[12] This effect is also seen on female B. carambolae, who do not naturally create 6-oxo-1-nonanol, when extracted male volatiles are topically applied to the thorax.[12]