The litchi stem-end borer, Conopomorpha sinensis Bradley (Lepidoptera: Gracillariidae) is an important pest of litchi, Litchi chinensis Sonn. (Sapindales: Sapindaceae) and longan, Dimocarpus longan Lour. (Sapindales: Sapindaceae). Litchi stem-end borers not only damage the fruit, but also damage the shoots and young leaves due to larval boring and feeding. This results in a large number of dropped fruits (“dung fruits”), fallen flowers and dieback (FAO 2002). The adult moths become active soon after sunset (18:00–18:30 h), although a few moths may become active as early as 16:30 h on dark rainy d. Adult moths cease to be active near the dawn, and they remain quiescent during the photophase (Cai et al. 2011). As the litchi stem-end borer larvae are interval feeders, they are difficult to control with chemical pesticides. Also, in organic litchi production, most chemicals are strictly prohibited, which makes control by traditional methods very difficult (Tsang et al. 2007, 2011).

The sterile insect technique (SIT) is a method of pest control based on area-wide releases of sterile males in a ratio of sterile to wild males of the same species that is sufficiently great to drastically reduce the fertility of the targeted field population. The SIT depends greatly on the production of good quality sterile males that can locate and compete for wild females of the wild target population. Since the New World screwworm, Cochliomyia hominivorax (Coquerel) (Diptera: Calliphoridae) was eradicated from the Island of Curacao in 1954 using this technology, the SIT has received increased attention.

Many lepidopteran species are destructive pests of annual and perennial crops, forests, and stored products throughout the world. Because lepidopteran species are usually more radiation resistant than most other insect species (Dyck et al. 2005), a fully sterilizing radiation dose would be so great that it would reduce the quality of the irradiated males and reduce their ability to disperse in the field and compete for wild females. To increase the competitiveness of irradiated Lepidoptera, Proverbs (1962) investigated the potential of using sub-sterilizing doses of radiation on the codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae), and discovered that at less than fully sterilizing doses, the sons of irradiated males were fully sterile. This discovery of inherited (IS) or F₁ sterility promoted numerous investigations in many Lepidopteran pests.

Inherited sterility is a derivative of the SIT that is applicable to lepidopteran pests (Proverbs 1962; LaChance 1985; Carpenter et al. 2005; Kean et al. 2008), and—when compared with full sterility, provides several advantages for lepidopteran pest control. As explained by Carpenter et al. (2005), lepidopteran females are usually more radio-sensitive than males of the same species. Therefore in many cases it is possible to find a radiation dose that induces complete sterility in females but only partial sterility in males. Thus large numbers of irradiated males and females can be released in the field, but the released females cannot produce progeny that can damage the crop. However, when the partially sterile parental generation males mate with wild females the radiation induced deleterious genes are inherited by the F₁ generation. As a result the F₁ egg hatch is reduced and the F₁ offspring tend to be both sterile and predominantly male. Because the F₁ progeny are produced in the field they tend to be better adapted and distributed to compete for wild mates. The advantages of IS derive from the lowering of the radiation dose, which allows the release of more competitive and longer living males. In addition adult F₁ males tend to be stronger fliers that mate more frequently and are more competitive than moths irradiated with greater radiation doses (Carpenter 2000).

In a previous study, females irradiated with 200 Gy did not oviposit when mated with non-irradiated males (Fu et al. 2016), indicating that 200 Gy was the lowest fully sterilizing dose for females, and 200 Gy was confirmed to be the dose needed to induce complete sterility in the F₁ generation offspring from the parental cross untreated female (UF) × treated male (TM). Radiation-induced partial sterility of the litchi stemend borer was shown to be inherited in at least 3 consecutive generations beginning with the parental males that were irradiated with 150 Gy as pupae shortly before emergence. There were no significant differences in longevity between the parental generation and the F₁ generation when the male in the parental generation was irradiated with 150 Gy. Therefore, 150 Gy applied before emergence could be used to irradiate moths for large scale release programs, although a dose of 200 Gy may be more appropriate for use in SIT/IS programs. Very little work has been done on quality control aspects of C. sinensis, however, many workers have reported on various quality parameters used for other lepidopteran pests (Rogers & Winks 1993; Bloem et al. 1997; Vreysen & Hendrichs 2005). This study was carried out to assess the effect 150 and 200 Gy irradiation on the flight ability of male litchi stem-end borers in the laboratory and to assess their dispersal ability in the field.

Materials and Methods

Results

RELEASE AND RECAPTURE EXPERIMENTS

Recaptures of Conopomorpha sinensis Males

Weather conditions during the first release experiment were favorable with almost no precipitation, and steady light winds (1.6–1.9 m/s) mostly from the SE (Table 3) under a mostly clear sky. In the first release experiment, a total of 1,500 adult male moths were released, and a total of 87 males (5.8%) were recaptured. The recapture rates of the non-irradiated control males, and males irradiated with either 150 or 200 Gy in the first release experiment were 6.2%, 5.4%, and 5.8%, respectively. The irradiation treatments (150 and 200 Gy) had no significant effect on the recapture rates as compared with the nonirradiated control (χ² = 0.29; P = 0.86) (Table 4).

Weather conditions during the second release experiment were somewhat turbulent with precipitation on all 5 d of the experiment, and light but variable winds (0.8–2.0 m/s) of quite variable directions fluctuating mostly between SE and NE under a mostly cloudy sky (Table 3). In the second release experiment, a total of 1,500 males were released, and a total of 123 males (8.2%) were recaptured. The results were similar to those of the first release in that no significant differences were found in recapture rates of the males irradiated either with either 150 or 200 Gy and the non-irradiated control males (χ² = 1.49; P = 0.48) (Table 4).

Table 1.

Flight parameters of Conopomorpha sinensis males tethered on a flight mill system. The males were non-irradiated or irradiated either with 150 or 200 Gy and allowed to fly for 24 h. If a flight stopped for ≥⃒ 1 min, it was considered the end of that flight.

Table 2.

The influence of fluorescent dyes on longevity in the laboratory of adult Conopomorpha sinensis males.

Dispersal Direction and Dispersal Distance

The maximum distances that the adult males dispersed were 30 m and 80 m in the first and second release experiments, respectively. Most of the recaptured males (67.8% in the first experiment and 62.8% in the second experiment) were caught in traps located only 5 m from the release point. The number of recaptured males decreased exponentially with distance from the release point (Fig. 2).

Fig. 1.

Survival (days) of non-irradiated Conopomorpha sinensis males either dyed with 1 of 3 different fluorescent colors or undyed (control) in the laboratory. Each treatment involved 30 males.

A Kruskal-Wallis test indicated that there were no significant differences in the dispersal distances of males of the 2 treatment groups as compared with the non-irradiated control males in the first (W = 0.00, P = 1) and second experiment (W = 0.2, P = 0.905). In view that no effect of irradiation could be detected with respect to recapture rate and dispersal distance, all data in each experiment was analyzed by regression analysis. The analysis indicated a median dispersal distance of 11.3 m and an arithmetic mean of dispersal distance of 8.7 m in the first release experiment. In the second experiment the median dispersal distance was estimated at 10.6 m and the arithmetic mean of the dispersal distance was estimated at 15.6 m (Table 4).

Table 3.

The weather conditions during the field release experiments with Conopomorpha sinensis adult males in a litchi orchard of the South China Agricultural University, Guangzhou, China. Meteorological data were obtained from a station of the Guangdong Meteorological Service located 5 km from the release orchard. The data presented are the mean values for each 24 h period.

Litchi stem-end borer males dispersed in all directions, but in both experiments most males were recaptured in 3 directions (S, W and SW) (Fig. 3). These dispersal directions were similar to the wind directions in each experimental period (Table 3). The dispersal direction was not significantly different for the males of the 2 treatment groups as compared with the non-irradiated control males; for the first release (P = 0.50) and for the second release (P = 0.80). Analyzed percentages of recaptured males in each treatment and experiment were significantly different among the 8 directions (Table 5).

Discussion

Our study showed an average flight speed of non-irradiated male litchi stem-end borers of 0.42 m/s, as measured in a flight mill system in the laboratory. This recorded speed was lower than the one observed for 1 d old Asian corn borers, Ostrinia furnacalis (Guenée) (Lepidoptera: Crambidae), i.e., 1.01 m/s at 24 °C and 70–75% RH (Lu et al. 2005), but similar to the one observed for 1 d old pink bollworms, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), i.e., 0.54 m/s at 28 °C and 80% RH (Wu 2006). In addition, the greatest total flight distance flown by a C. sinensis male using the flight mill system was 13.93 km.

Table 4.

Analysis of parameters of field release experiments with adult Conopomorpha sinensis males in a litchi orchard of the South China Agricultural University, Guangzhou, China.

The flight ability of released sterile insects is an important factor for the success of the SIT/IS. Although we cannot expect that the flight performance of insects with a flight mill will accurately reflect their performance under field conditions, the flight mill does provide standardized and quantitative estimates of the insect's intrinsic flight capacity (Taylor et al. 1992). Within the known limitations of the system (Armes & Cooter 1991; Beerwinkle et al. 1995; Riley et al. 1997), flight mills have been used to examine insect flight activity as a function of age, sex, size, mating status, and physiological state (Colvin & Gatehouse 1993; Rankin et al. 1994; Lu et al. 2007; Sarvary et al. 2008; Tsang et al. 2011), as well as lipid utilization in flight (Williams & Robertson 2008).

Fig. 2.

Numbers of adult Conopomorpha sinensis males recaptured in traps deployed at various distances (m) from the release point in 2 release/recapture experiments in a litchi orchard of the South China Agricultural University, Guangzhou, China. The males were either non-irradiated or irradiated either with 150 or 200 Gy. A: First release, B: Second release.

Irradiation of adult litchi stem-end borer males with either 150 or 200 Gy did not impair their flight ability as measured with a flight mill. Bloem et al. (2006) reported that the mobility of adult C. pomonella males was significantly influenced by an interaction of radiation and ambient temperature, but that the relatively low dose of 150 Gy had little effect on the flight activity of the codling moths. In contrast wind tunnel studies with the light brown apple moth, Epiphyas postvittana Walker (Lepidoptera: Tortricidae), indicated that either 100 or 250 Gy reduced upwind flight by 14% and 46%, respectively (Suckling et al. 2011). Previous studies had shown that relatively low doses of gamma irradiation did not significantly affect the flight ability of C. sinensis, and the results of this study are consistent with the previous studies.

Li et al. (2009) reported that 80.5% of litchi stem-end borer adults in 1 experiment flew 3–7 m, with an average flight distance of 4.97 m and a maximum distance of 12 m. These data are similar to the results of our release recapture experiment, where 59.1% of the released adult males were recaptured in traps that were located at 5 m from the release point, and the median dispersal distance as obtained with the regression analysis was about 11 m. However, we need to recognize that the released males were recaptured in traps baited with the female's sex pheromone, and it is very likely that the released males would have dispersed much longer distances if pheromone sources had been absent.

In the 2 release experiments, the main dispersal direction of the adult male litchi stem-end borers corresponded with the main wind direction, and it was skewed toward the west. We hypothesize that there are 2 other possible factors that may have affected the results. First, our study site was located on an east-west slope, and the experimental area was surrounded by trees except on the north-eastern side. This topography may have enhanced the effect of the wind. Second, the odor of vegetation including oleander, Nerium indicum Mill. (Gentianales: Apocynaceae) and wax apple, Syzygium samarangense Merr. & Perry (Myrtales: Myrtaceae) outside of the study area may have affected the dispersal direction. Generally, these factors—as well as the effect of some artificial lights to which insects may display a negative phototaxis (Li et al. 2008)—may combine to affect the dispersal direction (Kumano et al. 2009).

Fig. 3.

Frequency distributions of the dispersal directions of adult Conopomorpha sinensis males in 2 release/recapture experiments in a litchi orchard of the South China Agricultural University, Guangzhou, China. The males were either non-irradiated or irradiated either with 150 or 200 Gy. A: First release, B: Second release.

Table 5.

Tests of dispersal direction of adult Conopomorpha sinensis males in field release experiments in a litchi orchard of the South China Agricultural University, Guangzhou, China.

In the present study, we demonstrated that irradiating adult litchi stem-end borer males with either 150 or 200 Gy did not affect their flight ability as measured with a flight mill in the laboratory, nor did these levels of gamma irradiation affect their dispersal ability in the field as measured in 2 release recapture studies. It is clear that operational and commercial insect pest management programs with a SIT component can only be successful when there is consistent production and release of sterile insects of high biological quality and performance (Calkins & Parker 2005). Effective quality assurance tests for monitoring and providing feedback on the performance of insects during each step of production, handling and release are crucial to insure quality control and success of these programs (Huettel 1976; Singh & Ashby 1985; Calkins & Parker 2005). Additional studies are needed to evaluate the quality and performance of litchi stem-end borers released in the field, and to develop mass rearing and mass handling procedures.