Bactrocera Dorsalis

''Bactrocera dorsalis'', previously known as ''Dacus dorsalis'' and commonly referred to as the oriental fruit fly, is a species of tephritid fruit fly that is endemic to Southeast Asia. It is one of the major pest species in the genus ''Bactrocera'' with a broad host range of cultivated and wild fruits. Male ''B. dorsalis'' respond strongly to methyl eugenol, which is used to monitor and estimate populations, as well as to annihilate males as a form of pest control. They are also important pollinators and visitors of wild orchids, ''Bulbophyllum cheiri'' and ''Bulbophyllum vinaceum'' in Southeast Asia, which lure the flies using methyl eugenol.

The fly is similar to the closely related species '' B. carambolae'' and '' B. occipitalis.'' The species name ''B. dorsalis'' is identical to other synonyms ''B. papayae, B. invadens'' and ''B. philippinensis''.

Description

''B. dorsalis'' is a species of tephritid fruit fly. Flies that belong to this family are usually small to medium-sized with colorful markings. In particular, ''B. dorsalis'' belongs to a complex of physically similar flies called the ''Bactrocera dorsalis'' complex, whose defining characteristics include a mostly black thorax and dark T-shaped marking on the fly's abdominal segment. The T-shape marking consists of a dark medial and transverse band along the fly's abdomen.

The ''B. dorsalis'' species has distinctive yellow and black markings on its thorax and abdomen, which may vary between flies. Two vertical yellow markings on the thorax and the dark T-shaped marking on the abdomen differentiate this species of fly from its close relatives. The wings are clear with a continuous costal band. The adult body is around 8.0 mm in length, with wings approximately 7.3 mm in length. The female adult has a tapered ovipositor for depositing eggs in host fruits, while in male adults this ovipositor is notably absent.

Distribution

Endemic to Southeast Asia, ''B. dorsalis'' is a highly invasive pest species that now has a presence in at least 65 countries. It has also been introduced to Hawaii, the Mariana Islands, and Tahiti. The fly is also found in most countries of sub-Saharan Africa. From 1910 to 1990, the fly species was only observed in 5 countries; however, in the last three decades, the rate of spread by ''B. dorsalis'' has sharply increased, with the species invading an additional 70 countries.

Elsewhere in the United States, ''B. dorsalis'' has been spotted in California and Florida. These appearances then trigger a cascade of eradication efforts. Four major oriental fruit fly eradication efforts occurred in response to infestations in California between 1960 and 1997. Two additional infestations were eradicated in 2006 and 2007, occurring within 3–4 years of reports of these infestations. In July 2010, flies were discovered in traps in the Sacramento and Placer counties of California. A quarantine was established, and eradication efforts followed. These sightings in the mainland United States are generally quarantined infestations that have been eradicated.

CLIMEX, a modeling software, has been used to map the future trajectory for the fruit fly in terms of opportunities for increasing its distribution. This was tested both under current and future predicted climate conditions, given the current research on climate change. Under current conditions, the fly's projected distribution includes much of the tropics and subtropics and extends into areas like Mediterranean Europe. The model predicts optimal climate conditions in the southeast United States. Under climate change conditions, the spread overall increases as the fly is less limited by cold weather. However, its distribution does possibly decrease in areas where precipitation decreases.

Habitat

These tephritid fruit flies are found in tropical areas. ''B. dorsalis'' also prefers to pupate in shaded rather than brightly lit areas, moist over dry soil, and in soil with larger particles (particle size greater than or equal to 2.5 mm) than in soil with smaller particles.

They can be found in the USA, China, the UAE, the rest of Southeast Asia, India, Brunei, Malaysia, the Philippines, The Mariana Islands, Timor Leste, Pakistan, Taiwan and the rest of South Asia. One male fly was recently found in New Zealand, but it is so far unclear as to whether a breeding population has been established.

Life history

Under summer conditions, development from egg to adult requires 16 days. Several stages can be delayed in cooler conditions. ''B. dorsalis'' eggs may take up to 20 days to hatch under cool conditions, extended from the usual single day. Pupariation occurs in the soil under the host plant and is normally completed within 10–12 days, but can be delayed up to 90 days under cool conditions. Flies typically live from 1–3 months, but this can be extended to up to 12 months under cool conditions.

Eggs

Mated female ''B. dorsalis'' puncture the skin of mature fruit and deposit eggs in a few batches of 3-30 eggs underneath a fruit's skin via ovipositor, depending on the quality and ripeness of the fruit. Eggs usually hatch within a day, although the hatching process can be delayed in cool conditions. Females are capable of laying over 3,000 eggs during their lifetime under optimal conditions, but in field conditions, reports a typical range from about 1,200 to 1,500 eggs laid per female.

Larvae

Eggs hatch to larvae and moult twice (there are three larval instars) while feeding on the flesh of the fruit for about 6–35 days. Larvae are creamy white in color with a maggot-like appearance and are about 10 mm in length.

Pupae

The third instar larvae exit the fruit and burrow into the soil under the host plant to pupate. Larvae generally pupate in the upper 4 cm of the soil. Pupation can take approximately 10–12 days, but depending on the season, this process can be delayed in cooler conditions.

Adults

In 1-2 weeks, the adult emerges from the pupae and matures. Most adults emerge from the soil between 7:00 AM and 10:00 AM. Once sexual maturity is reached (which takes approximately 9 days), adults engage in the mating process and the life cycle repeats. The adult lifespan for ''B. dorsalis'' is about 90 days, and the flies have been reported to travel up to 30 miles in search of new egg laying sites and food, such as decaying fruit and plant nectar. Adult females prefer to lay eggs in old egg deposit sites on fruits.

Food resources

''B. dorsalis'' has been seen in more than 200 kinds of fruit and nut plants, but the species lay eggs in mango, papaya, and avocado fruits most often. Adult flies feed on decaying fruit, plant nectar, and other substances during their lifetime and prefer to feed in the morning.

Social behavior

Mating

Polyandry has been observed in ''B. dorsalis''. For females, there is typically a re-mating refractory period. The length of this period does not vary based on whether the female is mating with a virgin or non-virgin male. However, when there was a refractory period, females lay more eggs. Females who were exposed to two males continuously without a refractory period in between lay fewer eggs, but still lay more eggs than females with only one male. Therefore, there appears to be a reproductive benefit for females with polyandry.

Reproductive senescence does appear to be present in this species, as male and female age correlates negatively with the rate of fertilization.

Flying

The flight capacity of ''B. dorsalis'' adult females at various life stages has been observed in order to better understand and prevent their spread. Past research has shown that the species can spread extensively following fruit harvests, with a dispersal radius up to 37 km observed in Hawaii. Transmission electron microscopy was employed to view changes in flight muscle ultrastructures. Researchers observed that flight speed and distance changed with the age of the fruit fly, reaching its maximum capacity at 15 days of age.

Long-distance flight

''B. dorsalis'' has been observed to be capable of long-distance flight. One study investigated the relationship between flight muscle structure and the flies' flight capacity: as the number of mitochondria increased, myofibril diameter increased, and sarcomere length decreased, the researchers found the fly's flight capacity to be maximized. This particular muscle structure was evident in 15-day-old female adult ''B. dorsalis'' flies.

Symbiotic relationships

''B. dorsalis'' has symbiotic relationships