Temperate fruit flies in the genus Rhagoletis (Diptera: Tephritidae) have narrow host ranges relative to those of tropical fruit flies, suggesting they will not attack or are incapable of developing in most novel fruit. We tested the hypothesis that apple maggot fly, Rhagoletis pomonella (Walsh), and western cherry fruit fly, Rhagoletis indifferens Curran, whose normal hosts belong to the Rosaceae, will not attack or develop in mango (Mangifera indica L.; Anacardiaceae) and other non-rosaceous tropical fruit. Of fruits hung in infested apple trees, at least 49% of apples (n = 77) produced R. pomonella puparia, whereas only 1% of mangoes (n = 291) and 0% of papayas (Carica papaya L.; Caricaceae) and 8 other tropical fruit produced puparia. In laboratory tests in 1.9 L containers, 33% of apples (n = 131), 7% of mangoes (n = 118), and 7% of papayas (n = 14) produced R. pomonella puparia; adult flies also eclosed from puparia from mango and papaya. Females of R. pomonella landed approximately 4 to 9 times more often on apple than mango. When exposed to R. indifferens in laboratory tests in 1.9 L containers, 6% of mangoes (n = 32) and 0% of papayas (n = 23) versus 33 to 73% of sweet cherry, plum, and nectarine, and 0% of peach (all Prunus species; Rosaceae) produced puparia; no eggs were detected in mango and papaya. Contrary to our hypothesis, larvae of R. pomonella and R. indifferens were capable of developing in some tropical fruit under laboratory conditions. How findings here relate to fly quarantines versus basic fly biology is unknown and needs further study.
Temperate fruit flies in the genus Rhagoletis (Diptera: Tephritidae) include some of the major quarantine pests of tree fruit in North America, but the threat these flies pose to orchard crops as a whole is unclear because their natural host ranges tend to be narrow relative to those of tropical or subtropical fruit flies. Typically, the host plants of any one Rhagoletis species are confined to a specific family, within which only plants in 1 genus or in related genera are utilized (Bush 1969). This contrasts with, for example, the melon fly, Bactrocera cucurbitae (Coquillett), and Mediterranean fruit fly, Ceratitis capitata (Wiedemann), which attack approximately 136 plants from 62 genera in 30 families (McQuate et al. 2017) and approximately 321 plants from 157 genera in 62 families (Liquido et al. 2014), respectively.
The utilization by temperate Rhagoletis flies of plants mostly within specific families suggests the flies will not attack or are incapable of developing in novel fruit evolutionarily distant from them. Such fruit include those of economically important tropical plants. This hypoth esis could have implications for fly quarantines. The non-use of these fruit by flies would reduce the threat of the flies establishing in tropical countries even if they were accidently introduced and some could tolerate the climates there.
Two Rhagoletis species in the western USA that have adapted to cultivated host plants but still have relatively narrow host ranges within the Rosaceae are apple maggot fly, Rhagoletis pomonella (Walsh), and western cherry fruit fly, Rhagoletis indifferens Curran, which are quarantine pests of apple (Malus pumila Miller) and cherries (Prunus species), respectively. Rhagoletis pomonella is native to eastern North America and the highlands of Mexico (Bush 1969; Rull et al. 2006) and was introduced into the western USA sometime in the mid-1900s. It is not found outside North America. Its ancestral hosts are hawthorns (Crataegus species; Rosaceae), but it adapted to cultivated apple in the eastern USA about 150 yr ago (Bush 1969). Rhagoletis pomonella infests about 54 plant species in nature, but all in the Rosaceae; furthermore, 28 are Crataegus species and another 25 species are rarely infested (Yee et al. 2014). Rhagoletis indifferens is native to the western USA and British Columbia, Canada (Foote et al. 1993); it is found nowhere else. Its ancestral host is bitter cherry (Prunus emarginata [Douglas ex Hook.] Walp.), but the fly has adapted to cultivated sweet cherry (Prunus avium [L.] L.) and tart cherry (Prunus cerasus L.) throughout its range. Rhagoletis indifferens infests 15 plant species in nature (Yee et al. 2014), but only 4 of them are commonly used.
The main objective here was to test the hypothesis that R. pomonella and R. indifferens will not attack or develop in tropical fruit. We focused on mango (Mangifera indica L.; Anacardiaceae) because it is economically important (Shah et al. 2010), is taxonomically distant from Rosaceae (Angiosperm Phylogeny Group 2016), and has qualities (e.g., smooth and soft skin) that make it a candidate for attack by these flies, thus presenting a challenge for our hypothesis. We also determined the use of various temperate fruit by these flies as a basis for comparison with tropical fruit.
Materials and Methods
Tropical fruit were chosen based on a list of major fruit produced in Indonesia (World Bank 2007). For R. pomonella, the only fruit tested known to be a host for the fly was apple; for R. indifferens, it was sweet cherry. Fruit were purchased in local markets in western and central Washington. Tropical fruit originated mostly from Mexico, Chile, and Peru. Temperate fruit used in field tests originated from the USA, Chile, and Peru. In all experiments, smooth-skinned fruit were rubbed under water by hand and air dried before testing.
INFESTATION OF FRUIT HUNG IN APPLE TREES BY R. POMONELLA
Tests to measure whether flies attack and larvae can develop in apple and tropical fruit in the field were conducted by hanging fruit in fly-infested apple trees at sites in Woodland (Cowlitz County), Vancouver (Clark County), and in Skamania County in western Washington State in 2013 and in Woodland in 2014. A 0.64 cm wide beige strip of hook-and-loop fastener was wrapped around the center of each fruit. Green floral wire was attached to the strip on opposite sides. The wires were attached to 1 office binder clip, which was clipped onto a branch approximately 1.5 m above ground.
At the Woodland site in 2013, fruit were exposed in 13 trees during 10 to 31 Jul: 30 apples (10 ‘Gala'; 20 ‘Golden Delicious'), 49 mangoes (20 ‘Ataulfo’; 29 ‘Kent’), 11 Mexican papayas (Carica papaya L.; Caricaceae), 9 plantains (Musa acuminata Colla; Musaceae), 18 bananas (M. acuminata), 9 kumquats (Citrus japonica Thunberg; Rutaceae), 10 oranges (Citrus × sinensis (L.) Osbeck; Rutaceae), 10 mandarin oranges (Citrus reticulata Blanco; Rutaceae), 10 lemons (Citrus × limon [L.] Burm. f.; Rutaceae), 5 pineapples (