The application of juvenile hormone (or chemical analogs, such as methoprene) to newly emerged adult male fruit flies (Diptera: Tephritidae) represents a promising method to improve the efficiency of the Sterile Insect Technique against economically important species. This procedure has been shown to accelerate male sexual maturity in species with a long pre-copulatory period, and could allow for release of sterile males at younger ages and a greater release rate of sterile males overall. Topical application of methoprene has been shown to enhance male mating competitiveness. The present study investigated the effect of methoprene on maturation speed in males of the oriental fruit fly, Bactrocera dorsalis (Hendel) (a ‘slow’ maturing species) and the Mediterranean fruit fly, Ceratitis capitata (Wiedemann) (a ‘fast’ maturing species). For both species, newly emerged males were treated with acetone containing methoprene (treated) or acetone alone (control). The mating propensity of males was then monitored in non-competitive environments with mature females. Contrary to other studies, we found no evidence that methoprene accelerated male sexual activity in either a wild-like or mass-reared strain of B. dorsalis or a mass-reared (genetic sexing strain) of C. capitata. Possible explanations for these results are discussed.
The Sterile Insect Technique (SIT) is widely used to suppress or eradicate fruit fly pests, in particular the Mediterranean fruit fly, Ceratitis capitata (Wiedemann) (Hendrichs et al. 2002; Klassen 2005; Enkerlin 2005). The process involves the mass production of the target species and the subsequent release of irradiated (sterile) individuals in the environment to achieve matings between sterile males and wild females, which yield infertile eggs and thus depress the reproductive potential of the pest population. Ultimately, the chief goal of the SIT is to produce maximally competitive sterile males at the lowest cost over the shortest time interval. Because the large-scale production and release of insects are inherently expensive, there is a persistent need to evaluate these processes and reduce associated costs.
In recent years, many studies have been conducted to assess the impact of adult diet (Aluja et al. 2001; Shelly et. al 2005; Barry et al. 2007; Yuval et al. 2007) and adult olfactory environment (Shelly et al. 2007) on the field performance of sterile fruit fly males. Another promising avenue of investigation involves the application of juvenile hormone (or synthetic analogs) to newly emerged males. This procedure may result in 2 significant benefits to SIT programs. First, Teal et al. (2000) demonstrated that topical application of juvenile hormone (or the synthetic mimics methoprene and fenoxycarb) to young adult males of the Caribbean fruit fly, Anastrepha suspensa (Loew), dramatically accelerated sexual maturation. For example, the mean age at which all males in a replicate mated was 7 d for control males compared to 4 d for hormone-treated males. Similar trends have also been reported for males of the West Indian fruit fly, A. obliqua (Macquart) and the Mexican fruit fly, A. ludens (Loew) (Teal et al. 2007). Particularly for species, like Anastrepha, with relatively long pre-maturation intervals of 1–3 weeks depending on strain and species (Aluja 1994), increased rate of sexual development allows for the earlier release of sterile males, which, in turn, promotes a greater release rate of sterile males into the environment. Methoprene treatment may be less effective on fruit fly species with shorter pre-maturation intervals, and Faria et al. (2008) found no methoprene-mediated effect on sexual development in mass-reared, male Mediterranean fruit flies, whose normal pre-copulatory period was only 2–4 d.
In addition, treating young males with methoprene may enhance their copulatory success. Working with A. suspensa, Pereira (2005) found that protein-fed males treated with methoprene achieved significantly more matings than sexually mature, protein-fed males that had not received methoprene. However, a comparable study on C. capitata yielded inconsistent results. Comparing mating success among 4 treatment groups (methoprene treated or untreated and protein-fed or deprived), Faria et al. (2008) found no effect of methoprene in field cage trials (regardless of diet regime) and in laboratory trials found a significant effect of methoprene among protein-deprived males but not protein-fed males.
The present study investigated the effect of methoprene application on sexual maturation in males of the oriental fruit fly, Bactrocera dorsalis (Hendel) and the Mediterranean fruit fly. In Bactrocera species, males of wild populations have lengthy pre-copulatory periods of 2–4 weeks (Yang et al. 1994; Wee & Tan 2000), while males of mass-reared strains mature in approximately 6–8 d (Vargas et al. 1984) Thus, the pre-maturation interval in Bactrocera species is comparable to that of many Anastrepha species, and we anticipated that methoprene would effectively accelerate sexual maturation in B. dorsalis, as reported for the melon fly, B. cucurbitae (Coquillett) (Haq et al. 2008). The work presented here on C. capitata was undertaken to confirm the results of Faria et al. (2008), i.e., the absence in this species of a methoprene effect on male sexual maturation.
MATERIALS AND METHODS
The majority of tests with B. dorsalis involved flies from a laboratory (“wild-like”) colony started with 300–500 adults reared from mangos (Mangifera indica L.) collected in Waimanalo, Oahu. The colony was maintained in a screen cage (1:w:h, 1.2 × 0.6 × 0.6 m) and provided a mixture (3:1 wt:wt) of sugar (sucrose) and enzymatic yeast hydrolysate. Water was supplied ad libitum, and papayas (Carica papaya L.) were introduced for oviposition. Infested papayas were held over vermiculite, and the pupae were sifted from vermiculite 16–18 d later. Adults used in the mating trials were separated by sex within 24 h of eclosion and held in screen-covered, plastic buckets (5-L volume; 100–125 individuals per bucket) with ample food (the sugar-yeast hydrolysate mixture) and water. Flies were held at 24–28°C and 60–90% RH and received natural and artificial light under a 12:12 (L:D) photoperiod. When used in the study, these wild-like flies were 4–5 generations removed from the wild.
As shown below, tests with the wild-like strain of B. dorsalis failed to show an effect of methoprene. Consequently, we performed a second set of tests using B. dorsalis from a mass-reared strain that had been maintained by USDA-ARS for over 20 years (D. McInnis, personal communication). Females of this strain oviposited in perforated tubes, and eggs were placed on an artificial diet (Tanaka et al. 1969) for larval development. Adults were handled in the same manner described above and held under the same environmental conditions.
For medfly, experiments were performed with mass-reared males and wild-like females. Mass-reared males were from a tsl (temperature sensitive lethal) genetic sexing system (Vienna-7/Tol-99) reared by the California Department of Food and Agriculture's (CDFA) Hawaii Fruit Fly Rearing Facility, Waimanalo, Oahu. In rearing this strain, eggs are exposed to high temperature, which selectively kills female embryos and allows production and release of males only (Franz et al. 1994). Undyed pupae were obtained 2 d before eclosion after irradiation in air at 150 Gy of gamma radiation with a 137Cs source.
Female medflies were from a