Fruit of litchi, Litchi chinensis, and rambutan, Nephelium lappaceum, were collected from the field in 2006 and 2007 and monitored for the emergence of West Indian fruit flies, Anastrepha obliqua. Fruit clusters of rambutan and litchi, with a piece of the peel removed to allow access to ovipositing females, were also placed in cages and exposed to 12-d-old post-eclosion male and female West Indian fruit flies for 48 h. These exposed fruit were then monitored for the emergence of A. obliqua. Mango fruit were simultaneously exposed to male and female A. obliqua in separate cages and monitored for the emergence of A. obliqua. Fruit fly traps baited with putrescine and ammonium acetate were placed in orchards of litchi and rambutan, as well as an adjacent orchard of carambola, Averrhoa carambola, to demonstrate the presence of fruit flies while litchi and rambutan were fruiting. Although we collected 3732 ripe litchi fruit (40.34 kg) and 5534 ripe rambutan fruit (166.60 kg), none of these yielded tephritid larvae. Litchi and rambutan fruit exposed to adult fruit flies in cages did not yield tephritid larvae, though similarly exposed mangoes did. We conclude that litchi and rambutan have an undetectably low probability of being infested by A. obliqua in Puerto Rico.
Litchi, Litchi chinensis Sonn., and rambutan, Nephelium lappaceum L., both in the Sapindaceae family, are valuable fruits native to southern China and Malaysia, respectively (Morton 1987). Although currently cultivated on a small scale on the island of Puerto Rico, growers have expressed an interest in expanding their market to include the North American mainland. Currently, Puerto Rico is home to 2 economically important tephritid fruit flies, Anastrepha obliqua Marquart and A. suspensa (Loew) (Diptera: Tephritidae) (Martorell 1976). Anastrepha obliqua is only occasionally reported from California and Texas, but otherwise is not found within the continental U.S. (Epsky et al. 2003). As a result, there are understandable fears that the importation of some fruit, such as litchi and rambutan, from Puerto Rico to mainland North America may facilitate the establishment of A. obliqua there. A search of a host plant database for Tephritidae (Norrbom 2004) found no reports of L. chinensis or N. lappaceum as hosts of A. obliqua as of Aug 2007. In light of the difficulties regulatory agencies face in determining the relative threat of importing exotic pests, particularly fruit flies in the family Tephritidae, guidelines have been published that outline methods thought to be sufficient in establishing that threat for a particular fruit species (Cowley et al. 1992).
Our objective was to observe the incidence of infestation in field-collected fruits, and in fruit exposed to A. obliqua females in no-choice laboratory tests. In addition, adult fly populations were monitored by trapping in orchards of litchi and rambutan, and in adjacent orchards of other species of fruit trees. We used the principles outlined in Cowley et al. (1992) as guidelines for our investigation. Identical methods have been used to demonstrate the non-host status of litchi and longan (Dimocarpus longan (Lour.): Sapindaceae) and mamey sapote (Pouteria sapota (Jacq.) H.E. Moore & Stearn: Sapotaceae) to A. suspensa and A. obliqua, respectively (Gould et al. 1999; Gould & Hallman 2001; Jenkins & Goenaga 2007).
Materials and Methods
Between May 2006 and Aug 2007 mature litchi fruits (Brewster, Bosworth-3, Groff, Mauritius, Kaimana, and Salathiel varieties) were harvested from an orchard in Adjuntas, PR, and mature rambutan fruits (Benjai, Gulu Batu, Jitlee, R-134, R-156, R-162, R-167, and Rongren varieties) were harvested from an orchard in Corozal, PR. Harvested fruit were counted, weighed, placed on a wire mesh over vermiculite in a screen-covered plastic bin, and were stored at 25-27°C in an environment of approximately 60% RH (never less than 50% RH). The vermiculite was monitored weekly for fruit fly larvae or pupae. These were collected and placed in a plastic Petri-dish with a small amount of moistened vermiculite and stored at 25°C in an environmental chamber (12:12 D:L) (White & Elson-Harris 1992). The Petri-dishes were monitored daily for the emergence of adults.
In the summer of 2006, collapsible nylon cages (60 × 60 × 60 cm) (Bioquip, Rancho Dominguez, CA) were filled with clusters of ripe litchi with one half of the peel removed from each fruit to expose the fleshy pulp. Twenty female and 20 male Anastrepha obliqua individuals, reared from mango, Mangifera indica L. and/or fruit of Spondias mombin L. (Anacardiaceae), were placed into each cage 12-d post-eclosion. As simultaneous positive controls, nylon cages were filled with mango fruit that had been covered with brown paper bags (Lawson pollination bags, No. 400) 2 weeks prior to prevent field infestation by A. obliqua. Twenty female and 20 male A. obliqua adults, 12 d old, were placed into each cage. All cages were held in a greenhouse (mean temperature 24.8°C, RH 74%) and contained a single seedling of Manilkara zapota van Royen (Sapotaceae) to provide a suitable microclimate for the flies. Prior to placement in cages, flies were given slivers of carambola, Averrhoa carambola L. (Oxalidaceae), to provide carbohydrates and water. Mangoes that had been bagged but not subsequently exposed to ovipositing A. obliqua were monitored for the emergence of fruit flies to ensure that the paper bags prevented infestation. After 48 h of exposure, all fruit in all cages were harvested and monitored as described above for the emergence of adult A. obliqua. All fruit were monitored for 3 weeks post-harvest and then discarded. Each laboratory exposure was replicated 3 times for each fruit variety and for the control exposures with mangoes. This experiment was repeated with fruits of rambutan in the summer of 2007.
In addition, 5 plastic Multilure traps® (A Better World, Inc., Fresno, CA) baited with ammonia acetate and putrescine (Suterra, Bend, OR) were placed in each litchi and rambutan orchard and monitored weekly for fruit flies. Five traps also were placed in a carambola orchard near the rambutan orchard and monitored weekly.
A total of 3732 litchi fruit weighing 40.34 kg were collected from Adjuntas, PR, none of which yielded tephritid pupae (Table 1). Similarly, 5534 rambutan fruits were collected, weighing a total of 166.60 kg, none of which yielded tephritid pupae (Table 2). Litchi and rambutan fruit exposed to A. obliqua adults in laboratory studies yielded no tephritid larvae, while mango fruit similarly exposed yielded pupae (Table 3 and Table 4, respectively). Mangoes that had been bagged prior to use but that were not exposed to A. obliqua females did not yield any tephritid pupae.
Multilure traps baited with putrescine and ammonium acetate yielded A. obliqua and A. suspensa females simultaneous to fruit collection, indicating that these flies were active in the area when fruit were harvested. The number of A. obliqua trapped in the rambutan orchard was always 0, whereas a nearby carambola orchard the number fluctuated between 0 and 25 A. obliqua adults per trap per d. The number of A. obliqua adults captured in the litchi orchard ranged from 0 to 4.3 flies per trap per d while these fruits were being harvested.
It is impossible to prove that a fruit is never a host to a species of insect, only that a fruit is used by a species of insect. However, our data show that the likelihood of infestation of the varieties of litchi or rambutan that we assayed by A. obliqua is very small. We conclude that these fruit varieties are extremely unlikely to contain A. obliqua and therefore represent minimal threat of transporting this pest when exported.
Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. We thank Elkin Vargas for his excellent field and labwork.
Collections of litchi by date and variety.
Collections of rambutan by date and variety.
Table 3 Fly pupae recovered from litchi fruit exposed to 20 male and 20 female Anastrepha obliqua (12-d post-eclosion) for 48 h.
Table 4. Fly pupae recovered from rambutan fruit exposed to 20 male and 20 female Anastrepha obliqua (12-d post emergence) for 48 h.