Ceratitis capitata

C. capitata eggs are characterized by their curved shape, shiny white color, and smooth features.[1] Each egg is approximately 1 millimetre (5⁄128 in) in length.[1] As seen in other fruit flies, the egg possess a micropylar region with a clear tubular shape.[1]

Larvae of C. capitata have been described as having a common fruit fly larval shape that is cylindrical with a narrow anterior end and flattened caudal tail.[1] By the end of the third and final instar of the medfly, the larvae measure between 7 and 9 millimetres (35⁄128 and 45⁄128 in) and about 8 fusiform areas.[1]

Larva of the medfly

The adult flies typically measure 3 to 5 millimetres (15⁄128 to 25⁄128 in) in length. There are numerous visually defining characteristics of the C. capitata’s bodily features. The thorax is a creamy white to yellow with a characteristic pattern of black blotches, and the abdomen is tinted brown with fine black bristles located on the dorsal surface and two light bands on the basal half. The medfly's wings contain a band across the middle of the wing with dark streaks and spots in the middle of the wing cells.

In a study done by Siomava et al., researchers utilized geometric morphometrics to analyze wing shape in three different fly species including C. capitata. Through their findings, the researchers showed that the medfly exhibits extensive sexual shape dimorphism (SShD) between the proximal and distal part of the wing. This difference can be used to distinguish between the two sexes since male wings tend to be wider and shorter in comparison to females. This anatomical difference is important because this allows males to displace more air and create a more audible “buzzing” effect during mate attraction.[1][2]

The males also can be distinguished by the fact that they have two setae between their eyes; the setae are conspicuously flattened and black at their ends.[3]

Medflies can complete their life cycles in 21 days in optimum conditions. In cooler temperatures, the life cycle of the medfly can take up to 100 days to complete. In temperatures that are below 50 °F (10 °C), development of the fly ceases. Oviposition in females ceases to occur in temperatures below 60 °F (16 °C).[1]

The lifespan of the C. capitata is quite short as half of most populations die in under 60 days. However, cool conditions and proper sustenance can enable some flies to live 6 months or up to a year. In lab conditions, under controlled diets of sugar and protein, the life expectancy of females is usually longer than that of males by 1.5 days. On average, the lifespans of flies in captivity are 10 days longer than those of wild flies.[5][6]

The lifespans of certain species are also affected by periods of food deprivation, which is a key driver of invasion success, adaptation, and biodiversity. Starvation resistance is a plastic trait that varies due to the relation between environmental and genetic factors. Recent studies into the starvation resistance (SR) of C. capitata has found that SR decreases with increasing age and that age-specific patterns are shaped in relation to adult and larval diet. Furthermore, females exhibited higher SR than males, and the greatest influence on SR in C. capitata was due to age and adult diet followed by gender and larval diet.[7]

Because nutrition is a crucial determinant of adult size and development, larva prefer to eat fleshy host fruit. Higher concentrations of glucose and sucrose boost development and the percentage of emerging larva in comparison to high starch and maltose diets.[5][8][9]

By manipulating larval diets with relation to brewer's yeast and sucrose, researchers were able to show that varying the levels of yeast and sucrose in the diet changes the proportion of proteins to carbohydrates which affects the ability of pupating larvae to accumulate lipid reserves. Diets with high protein to carbohydrate ratios produced larvae with high protein and lipid contents. Conversely, diets with a low protein to carbohydrate ratio led to pupating larvae having relatively reduced loads of lipids.[10] Parental condition may affect larval responses to the immediate dietary environment through a process known as maternal effects.[11]

Research into the correlation between citrus variety, fruit part and stage of C. capitata has found strong effects on larval performance, smaller effects on pupae, and no effects on eggs. The highest survival rate was shown to be on bitter oranges; however, the shortest developmental time and heaviest pupae were obtained from orange cultivars. In short, pulp chemical properties such as acidity and soluble solid contents had little effect on larval and pupal survival but larger effects on pupal weight.[12]

Adults tend to gain their carbohydrate intake from ripe fruit and protein from decomposing fruit or leftover bird feces. While larva prefer the middle of the fruit, adults prefer the fruit portion that contains more nutritional value in comparison to the flesh. Their diet preferences have been proven by studies in which medflies placed at the top of oranges and papayas consistently moved lower to the nutrient dense parts whereas flies placed near the bottom remained in their starting location. Adult flies typically feed in the mid-morning/late afternoon.[5][8]

With respect to reproductive success of male C. capitata, males that are fed a diet consisting of no protein copulated at a significantly lower rate than males who were fed protein. In short, male diets are a significant factor in the mating success of male C. capitata as dictated by the receptivity of females to further copulations.[10]

It was shown, that adults of C. capitata host diazotrophic bacteria from the Enterobacteriaceae family in their gut. These symbionts actively fix nitrogen by the enzyme nitrogenase which can alleviate nitrogen limitation and thus can be beneficial for the host.[13]

Field observations conducted in various localities within the Hawaiian Islands, specifically in Kula, Maui and in Kona, Hawaii, showed researchers a clear distinction in the mating behavior of C. capitata. The mating ritual in this species of fly can be separated into two basic phases: (1) lek behavior and (2) courtship.

In lek behavior, males begin by acquiring territory and jockeying with each other for optimal position. Leks are always located in positions that optimize the amount of sunlight penetrating the leaves.[14] Mating in the C. capitata fly typically begins with males stationed at the bottom of the surface of leaves during the late morning or early afternoon. Once males are stationed at these locations, they begin the mating process by forming leks and releasing sex pheromones to attract virgin females. If successful, mating will occur during this time period. Another important location for copulation is on the fruit itself during the late morning or early afternoon. Males position themselves here in an attempt to copulate with already-mated females through seduction or force.[15] A study conducted by Churchill-Stanland et al., showed that a male's size can dictate their mating success rate. Researchers found that flies weighing approximately 8–9 mg had optimum mating success while smaller flies (i.e. <6 mg) had significantly less mating success.[1] Furthermore, when males were equal or larger in size, mating frequency was equal and events such as eclosion, flying, and mating speed were positively correlated with pupal size.[16]

During the courtship phase, a series of signals are exchanged between the male and the female. As the female approaches, the male tucks his abdomen under his body with his abdominal pouches still inflated and wings still vibrating. Once the female is within 3–5 millimetres (15⁄128–25⁄128 in) of the male, the male will begin a series of head movements. Within 1–2 seconds of head movement initiation, the male begins rhythmically wing its fannings and moves closer to the female. Once close enough, the male then leaps onto the females back and begins copulation.[14]   

Male C. capitata

It has been shown that during mating, females experience a switch in olfactory-mediated behaviors. Specifically, virgin females prefer the pheromones of sexually developed males over the host fruit odor. Females exhibit this preference until mating occurs, following which they prefer the host fruit odor.[17] This finding has been evidenced by a specific protein, CcapObp22, that shows approximately 37% identity with the pheromone binding protein of Drosophila melanogaster. In a recent study, this protein was shown to bind male pheromone components, specifically farnesene, a highly strong hydrophobic terpene.[18]

Medflies were first detected in the region in Costa Rica in 1955. From then on, the medfly spread northward, reaching Guatemala in 1976 and Mexico in 1977. In order to begin eradication efforts, the Mass-rearing and Sterilization Laboratory was producing 500 million sterile flies weekly by the end of 1979. By releasing these sterile flies into the wild, scientists were able to not only prevent the northward spread of the fly, but officially declare it as eradicated from all of Mexico and large areas in Northern Guatemala in September 1982.[26]

Utilizing the Sterile Insect Technique, the medfly was eradicated in December 1984 from Carnarvon, Western Australia. In the 1980s, the Western Australia Department of Agriculture conducted a feasibility study into using the Sterile Insect Technique to eradicate the medfly population. Phase 1 of this study utilized 70 traps to establish the seasonal abundance of wild fly prior to releases. In Phase 2 of the study, the Department of Agriculture released 7.5 million sterile flies per week; however, this was insufficient in limiting the wild fly population. During phases 3 and 4, the number of released sterile flies increased to 12 million a week and was combined with chemical controls. After wild flies were no longer detected, phase 5 was initiated, withdrawing chemical controls from further distribution. Eradication was declared when neither wild flies nor larvae were found during the period of October 1984 to January 1985. This period corresponded to 3 fly generations; a threshold of eradication utilized by Hendrichs et al. (1982) in the eradication of the medfly in Mexico.[27]

Much research has been dedicated to means of controlling the medfly.[28] In particular, use of the sterile insect technique has allowed the species to be eradicated from several areas.

A typical trap used to capture C. capitata.

In 1981, California Governor Jerry Brown, who had established a reputation as a strong environmentalist, was confronted with a serious medfly infestation in the San Francisco Bay Area. He was advised by the state's agricultural industry and the US Department of Agriculture's Animal and Plant Health Inspection service (APHIS) to authorize airborne spraying of the region. Initially, in accordance with his environmental protection stance, he chose to authorize ground-level spraying only. Unfortunately, the infestation spread as the medfly reproductive cycle outpaced the spraying. After more than a month, millions of dollars of crops had been destroyed and billions of dollars more were threatened. Governor Brown then authorized a massive response to the infestation. Fleets of helicopters sprayed malathion at night, and the California National Guard set up highway checkpoints and collected many tons of local fruit. In the final stage of the campaign, entomologists released millions of sterile male medflies in an attempt to disrupt the insects' reproductive cycle.

Ultimately, the infestation was eradicated, but both the governor's delay and the scale of the action has remained controversial ever since. Some people claimed that malathion was toxic to humans, animals, as well as insects. In response to such concerns, Brown's chief of staff, B. T. Collins, staged a news conference during which he publicly drank a small glass of malathion. Many people complained that, while the malathion may not have been very toxic to humans, the aerosol spray containing it was corrosive to car paint.[29]

During the week of September 9, 2007, adult flies and their larvae were found in Dixon, California. The California Department of Food and Agriculture and cooperating county and federal agricultural officials started eradication and quarantine efforts in the area. Eradication was declared on August 8, 2008, when no "wild" (i.e. non-sterile) medflies were detected for three generations.

On November 14, 2008, four adult flies were found in El Cajon, California. The San Diego County Agricultural Commission implemented a treatment plan, including distributing millions of sterile male flies, local produce quarantines, and ground spraying with organic pesticides.[30]