In relation to the application of the sterile insect technique (SIT) for the South American fruit fly Anastrepha fraterculus (Wiedemann), we analyzed the effect on adult fertility of different doses of gamma irradiation and the age of pupae at the time of irradiation. In a first experiment, we applied doses of 50, 70, and 90 Gy to pupae at 24, 48, 72, and 96 h before adult emergence. In a second experiment we irradiated pupae 48 h before emergence with 20, 40, and 60 Gy and estimated male and female fertility and sperm transfer by irradiated males. The results indicated pupal age at irradiation does not significantly affect male fertility. If males irradiated with 60 Gy are crossed to non-irradiated females the fertility is about 1%. Females irradiated with 40 Gy did not lay eggs independently of the male to which they mated. No significant effects of radiation were observed with respect to the ability of males to transfer sperm. A dose of 70 Gy applied 48 h before adult emergence induces 100% sterility in both males and females.
The South American fruit fly Anastrepha fraterculus (Wiedemann) (Diptera: Tephritidae) is an important pest for fruit production in Argentina (Stone 1942). This species is native to the Americas, most probably South America, and is widely distributed throughout the tropical and subtropical regions (between the latitudes 27°N and 35°S). Its range includes southern USA (South Florida and Rio Grande Valley, Texas), Central America, Caribbean Islands, and South America, from Trinidad and Guyana to Central Argentina (Steck 1999; Aluja 1994; Hernández-Ortiz 1992).
There are at least 80 host species of A. fraterculus, including many economically important fruit species (Norrbom & Kim 1988). Tropical fruit flies not only cause great losses in fruit and vegetable production, but they also seriously impede international trade because of quarantine regulations (Klassen & Curtis 2005). In particular, the presence of A. fraterculus in the orchards reduces the possibility of exporting fruits and other horticultural products to the northern hemisphere (SENASA 1997). The export of fruits and vegetables to pest free areas or those that have implemented control programs against this pest requires the application of a quarantine treatment. Another common problem is that the intensive use of chemical insecticides is associated with environmental contamination. Furthermore, insects have been found to develop resistance to almost every chemical class of insecticide (Brown & Payne 1988). This includes some tephritids, such as Bactrocera oleae (Gmelin) (Vontas et al. 2002) and Bactrocera dorsalis Hendel (Hsu et al. 2004).
Recent studies indicate that populations of A. fraterculus from Argentina and Southern Brazil are not differentiated genetically (Alberti et al. 2002) and that 4 populations from different regions of Argentina do not show reproductive isolation (Petit-Marty et al. 2004). These findings suggest that the sterile insect technique (SIT) might be applied successfully against A. fraterculus at least at a regional scale.
In other tephritids, such as Ceratitis capitata (Wiedemann), the irradiation process may reduce the mating performance of the sterilized males (Calcagno et al. 2002; Lux et al. 2002). An essential requirement for a successful SIT is the application of a sterilization protocol to mass reared insects that ensures sterility with a minimal detriment of the mating competitiveness and viability of the released insect. Germ cells (oocytes and spermatids) are highly radiosensitive and when exposed to ionizing radiation, dominant lethal mutations are induced (Muller 1927). The dominant lethal mutations produced by radiation in insects depend mainly on the dose, insect type, size, and sex (Hooper 1989). Radiosensitivity also depends on other factors such as irradiation temperature, humidity, ploidy level, mitotic cycle phase, and metabolic condition (Enkerlin et al. 1997).
In species of the genus Anastrepha, studies on the effect of pupal age and radiation dose on the induced sterility are not completely consistent. Rhode et al. (1961) reported that Anastrepha ludens (Loew) pupae irradiated 96 h before emergence with 40 Gy showed 100% male sterility. By contrast, according to Velasco & Enkerlin (1982), the dose needed to induce sterility in the same species should be much higher. They reported that 40 Gy and 100 Gy induced 90% and 99% sterility, respectively, when pupae were irradiated 72 h before emergence. In the case of Anastrepha suspensa (Loew), Burditt et al. (1975) irradiated pupae with 40 Gy at 48 h before emergence and observed complete adult sterility whereas Calkins et al. (1988) reported that lower irradiation doses (30 Gy) applied 24-48 h before emergence induced high levels of sterility.
The efficiency of sterilized insect release programs depends to a great extent on the ability of laboratory reared sterile males to mate with, and transfer sperm to, wild females in the field (McInnis 1993). Usually, immediately after copulation 90% of sperm is found in the spermathecae of the female (Yuval et al. 1996). Mossinson & Yuval (2003) have shown that females with fewer sperm in their spermathecae show a higher tendency to remate. Remating may reduce the efficiency of the SIT if the second mating occurs with a wild male. However, this is very unlikely as sterile males far outnumber wild males in an SIT programme.
We analyzed under laboratory conditions the effect of different doses of gamma irradiation and the age of pupae at the time of irradiation on the induced sterility and the ability to transfer sperm in A. fraterculus.
Materials and Methods
The A. fraterculus individuals studied were from a strain reared since 1997 at Estación Experimental Provincial Obispo Colombres, Tucumán, Argentina. Pupae were sent to Buenos Aires (Centro Atómico Ezeiza, Grupo Agronómico, CNEA) by surface and there were kept under controlled conditions (25 ± 1°C, 75 ± 5% RH, and a photoperiod of 12:12 (L:D). Adult diet was composed of white sugar: yeast (Calsa, S:A:, Tucuman, Argentina) (3:1). Water was provided as 1% agar in 12-mL vials. Food and water were changed each once a week.
Pupae were irradiated at the Centro Atómico Ezeiza facility (Comisión Nacional de Energía Atómica, Argentina) in a Gammacell 220 (MDS Nordion, Canada) irradiator, with 60Co source (dose rate for the first and second experiment: 1.67 Gy min-1 and 1.60 Gy min-1).