Distribution. Mediterranean fruit fly, commonly known as "medfly," probably originated in tropical Africa, but as its common name suggests, it first attracted attention in the Mediterranean region. It now is widespread in South America and Africa, and also occurs in southern Europe, Central America, and Australia. Southeast Asia is not infested, and most of North America is without permanent populations. In California, Texas and Florida, populations are occasionally detected and apparently eradicated. This pest is well-established in Hawaii, where it was first observed in 1910. Its potential range in North America likely includes all of the southern United States from Georgia to northern California, and all areas further south.
Host Plants. This insect has been known to develop successfully in the fruit of over 400 plants from numerous plant families, including several wild and ornamental plants that produce small berries. The most common North American host plants, however, are such stone, pome, and citrus fruits as almond, apple, apricot, nectarine, peach, pear, plum, grapefruit, orange, and tangerine. Other fruits at risk are fig, guava, grape, kumquat, loquat, lychee, passion fruit, quince, and persimmon. In general, thin-skinned, ripe fruits are preferred by ovipositing females. If the skin is cracked, other fruits such as avocado, banana, and papaya are attacked. Mediterranean fruit fly is not usually considered as a serious vegetable pest, but occasionally vegetables are attacked, and among those known to be hosts are eggplant, pepper, and tomato. Strawberry also is reported as a host.
Natural Enemies. Several general predators such as ants (Hymenoptera: Formicidae) and rove beetles (Coleoptera: Staphylinidae) have been reported to attack Mediterranean fruit flies, but only parasitoids are considered to be significant natural enemies. Many wasps were imported to Hawaii from western Africa, but most of them did not survive the trip or failed to establish when released. Additional research on para-sitoids followed the introduction of oriental fruit fly, Bactrocera dorsalis (Hendel), into Hawaii in 1945, and many parasitoids introduced for oriental fruit fly control also attack Mediterranean fruit fly. Biosteres arisanus (Sonan), B. longicaudatus Ashmead, and Diaschasmimorpha tryoni (Cameron) (all Hymenoptera: Braconidae) are the dominant parasitoid species in Hawaii, with the relative importance of individual species changing through the year (Wong and Ramadan, 1987). Together, they cause about 30-50% mortality in fruit fly larvae, but its effect varies considerably depending on the host fruit. In most countries where parasitoids have been introduced there has not been an appreciable reduction in damage by this fly. Host-specific parasitoids have not yet been identified (Headrick and Goeden, 1996). The taxonomy and biology of the parasitoids was discussed by Wharton and Gilstrap (1983).
Competition with other fruit flies seems to limit abundance of Mediterranean fruit fly. Its abundance and damage in Hawaii, especially at lower elevations, were diminished by the introduction of oriental fruit fly. This phenomenon has been observed elsewhere in the world with other fruit fly competitors.
Life Cycle and Description. The life cycle varies considerably in duration. Under ideal conditions a complete generation may be completed in about 18 days, but 30-40 days is common. The longevity of the life cycle under inclement conditions may be 3-4 months. Under favorable conditions in Hawaii 15-16 generations are estimated annually. Because the females are long-lived and continue oviposition over a considerable length of time, overlapping generations are common and virtually all stages are found throughout the year. The developmental threshold for egg, larval and pupal stages is about 9.7°C.
pointed head and a legless, cylindrical body that expands in diameter at the posterior region. The body color is usually creamy-white, but it may be tinted with other colors depending on the food source. There are three instars. The first instar measures only about 1 mm long, and bears spiracles only at the posterior end. The mouth hooks are very small, and bear two distinct teeth. The second instar is usually 3-5 mm long, though it may overlap the third instar in size. The second instar bears anterior spiracles on the second segment, in addition to the posterior spiracles. The mouth hooks in the second instar also have two teeth. The third instar measures 7-8 mm long, bears both anterior and posterior spiracles, and the mouth hooks consist principally of one large, sharply pointed tooth. The anterior spiracles possess 9-10 lobes. Larval development time is normally 6-10 days, but as for eggs, it may be extended to about a month in cool weather. The first two instars usually complete their development in 1-2 days, with the third instar requiring 2-8 days.
Mediterranean fruit fly larva.
Mediterranean fruit fly larva.
Complete treatment of Mediterranean fruit fly biology and management was given by Back and Pemberton (1918b), but many aspects were updated in Morse et al. (1995). Hendrichs and Hendrichs (1990) provided many important behavioral observations. Rearing procedures were given by Tanaka et al. (1969).
Females puncture the skin of fruit with their ovipositor and deposit eggs just beneath the surface at a depth of about 1 mm. The oviposition process often results in access to the fruit by plant pathogenic microorganisms. The larvae burrow deeply into the fruit, enhancing the movement of microorganisms. Infested
fruit usually drop from the plant and degrade into a rotten mass.
In addition to direct damage caused by flies and plant disease, Mediterranean fruit fly inflicts indirect economic injury by denying growers the ability to ship produce to unifested areas. Quarantines against movement of potentially infested crops are widespread throughout the world.
Cultural Practices. Few cultural practices are effective for Mediterranean fruit fly suppression (Aluja,
1996), and the most important tactic for North America is exclusion. In Hawaii, fruit is sometimes covered with bags to protect it from ovipositing females. Destruction of fallen fruit reduces reinfestation potential, but fruit must be collected before larvae complete their development and move to the soil. Mass trapping has been attempted, but most traps are not sufficiently attractive to be effective at reasonable cost. Rectangular yellow-sticky traps capture numerous flies, but these traps must be used at high densities and as they are not selective so they rapidly become covered with all types of insects. Placement of traps around the perimeter of orchards could eliminate flies that are invading from wild hosts.
Biological Control. The biological control organisms are generally considered to be incompatible with eradication programs unless they can be mass produced and applied repeatedly. Preliminary research has demonstrated that mass release of the parasitoid, Diachasmimorpha tryoni is compatible with sterile fly release, and may result in greater suppression of Mediterranean fruit fly than release of sterile flies alone (Wong et al., 1992). Also, application of the entomo-pathogenic nematode Steinernema carpocapsae to the soil surface at a rate of 5000 nematodes per square centimeter greatly reduced the emergence of Mediterranean fruit flies (Lindegren et al., 1990).
Sterile Insect Release. Mass release of laboratory-cultured, sterilized insects is used under the premise that wild females will mate with sterile males, and fail to produce offspring. This can be accomplished if sterile males are competitive with wild males, and the ratio of sterile to wild males is sufficiently high that the probability of wild females encountering a wild male is low. However, it is imperative that sterile flies originate from a vigorous colony, and that large number be available on a continuing basis. A ratio of 100-sterile flies for each wild fly is considered to be minimal for success. It has proven impossible, thus far, to eliminate well-established wild fly populations through release of sterile flies without the initial use of insecticide to reduce pest abundance. The release of sterile flies has few negative impacts, though the sterile females do sting the fruit in an attempt to deposit their eggs.
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