Artificial mass-rearing and sterilization may affect field effectiveness of the Mediterranean fruit fly, Ceratitis capitata (Wiedemann) (Diptera: Tephritidae), drastically. Mass rearing conditions, selection genetic changes, irradiation and sterile insect technique (SIT) handling procedures may affect the vigor and behavior of sterile males and reduce strikingly their mating performance with the wild population in the field (Economopoulos 1996). Furthermore, recent evidence suggests development of behavioral resistance in the wild flies against sterile flies (McInnis et al. 1996). The limited success SIT had so far on several key insect pests was connected to a large degree with the quality of released insects. This became evident from the initial steps of the methodology and resulted in the development of tests which monitor the quality of sterile insects and the field effectiveness of the methodology (Calkins et al. 1996, Cayol et al. 1999). This effort culminated in the recent publication of a comprehensive manual of quality control for fruit flies (IAEA 1998).

The basic and closest to field conditions mating performance test available so far is that of field cage (Calkins & Webb 1983). Nevertheless, although the test is applied under natural conditions and involves a host tree, the fact that flies cannot freely fly away or “escape” from the host tree, or newcomers cannot mix with the caged tree flies reduces the value of the test. Recently, the interest on sterile insect competitiveness as deduced from egg hatch, first described in 1971 (Fried), has been renewed. Measurements of egg hatch from field oviposition in mock fruits are used for a more accurate evaluation of mating performance under completely natural conditions (Katsoyannos et al. 1999). Unfortunately, no practical method has been standardized so far on egg hatch measurement of field oviposited eggs in mock fruits.

In this study, the mating performance of a white female puparium strain has been evaluated under field cage conditions in citrus plantation.

Materials and Methods

Cylindrical field cages 2m h × 2.9m d were used (Synthetic Industries, Dalton, GA 30720, USA). Each cage was installed over a Navel orange tree in a mixed plantation with Navel and Valencia orange trees. The cage ceiling was covered by thick white fabric to provide shade. For the study of 1996 (Table 1), most wild flies were raised from larvae in sour oranges and loquats (early summer) and figs (late summer). The genetic sexing males were of the white female pupa strain T (Y;5) 1-61(95) (Franz et al. 1994) at generations 5-10. They were gamma-sterilized 1-2 days before adult emergence. Eight experiments were performed from June till October (see also caption of Table 1). Flies for the experiments in 1998 (Table 2, Table 3) were similar except that the genetic sexing males were of generations 33 and 36, respectively, and were not gamma-sterilized. In the June experiments, high mortality was observed on the second and third experiment days because of air-born toxicity due to near-by bait spraying. In both 1996 and 1998 matings were recorded from 09:00-18:00, every half hour.

In all experiments trees were pruned to fit the cages and make easy the census of fly activities and copulations. Males and females were separated soon after emergence and kept on standard adult diet (unless indicated differently in the table) prior to introduction into the field cage. Water was sprayed on the caged trees on the hot hours of the day to provide the flies with drinking water.

Results and Discussion

In 1996 mating performance experiments are presented in Table 1. All genetic sexing males included were gamma sterilized. In June-August (highest daily temperatures under shade between 30-36°C), the genetic sexing males produced only 0-33% of observed matings while expected values according to insect type ratios were 50-90%, i.e. 13 observed instead of 87 expected matings in total. In 3 out of the 5 experiments organized in this period, the genetic sexing males contributed zero to near zero of mating activity observed, while in the other 2 experiments their mating share was 1/2.7 and 1/2.5 of expected values, respectively. It is noted that the reduced performance of genetic sexing males in the June 18 experiment could had been intensified by the young age of laboratory flies mixed in this test (refer to Table 1 caption). On October 10 and 17 (when daily temperatures were between 16-29°C), genetic sexing males succeeded in getting 1/3.4 to 1/3.1 of their expected mating activity, i.e. 8 observed instead of 28 expected matings in total. That is, although the results in October were not as bad as in July, the genetic sexing males competed again poorly with wild males for matings with wild females. In all experiments the difference was highly significant (P < 0.001). Both type matings were recorded almost exclusively on the underside leaf surface, with sterile matings located relatively more at lower canopy than the wild type matings. The majority of both type matings was recorded at the same tree canopy sectors.

The results of the 1998 experiments are shown in Table 2 and Table 3. All genetic sexing males involved were not sterilized. The genetic sexing strain was already at generations 33 or 36 with extensive break down. In the June experiment (warm weather) the genetic sexing males, at 1:1:1 ratio (wild males: wild females: gen. sex. males) and fed complete diet, obtained more than expected matings, while at 1:1:3 ratio they obtained significantly fewer than expected matings. In September (cooler weather) at both insect ratios they obtained mating percentages lower than expected. It was observed that their performance was higher in the second and third experiment days as compared with the first one. At the ratio of 1:1:3, sugar fed laboratory flies obtained fewer than expected matings in both June and September. The difference was not significant in June, while in September it was highly significant. When insect ratios of 1:1:3 are compared with 1:1:1 ratios in June, the genetic sexing males of the high overall male: female ratio (4:1) obtained mating values lower than expected, while in the low overall male: female ratio (2:1) they obtained mating values higher than expected ones. In September the genetic sexing males of the low male: female ratio did not perform as well. It could be that the reduced male: female ratio improves the sterile male performance under warm weather. Also, if we compare the mating performance of genetic sexing males fed complete diet with sugar fed ones at 1:1:3 ratio, we observe that in the June experiments the sugar fed males had superior mating performance as compared with the complete diet fed ones. The opposite was true in September. In conclusion, the mating performance of non-irradiated genetic sexing males was considerably improved as compared with the performance of irradiated males in the 1996 experiments. This could be the result of no irradiation-damage and/or even of strain performance improvement because of sexing break down between 1996-98. The mating performance of genetic sexing males fed complete adult diet as compared with sugar only feeding, did not clearly prove superiority of any of the treatments.

The study of copulation site (1998, Table 3) concluded that the preferred site by both type matings is the underside of the leaf. There were some differences on mating site preference between June (higher temperatures) and September (lower temperatures). In June the preferred mating site by both type matings was the lower canopy while in September, matings moved higher in the canopy, the phenomenon been more striking with the genetic sexing type matings. As to tree sector and although differences were not significant, in June both type matings appeared to concentrate in the north and west of the tree canopy, the phenomenon been again more intense with the genetic sexing male matings. In September, wild type matings concentrated primarily in south, west and center while genetic sexing type matings concentrated in south, east and center of tree canopy. It is interesting to note that in June about half of total both-type matings concentrated in the cooler northern sector of the tree, while in September only 8% of matings preferred this part of the tree canopy.

In conclusion, genetic sexing sterilized males were found much inferior than their wild counterparts in mating performance under field cage conditions. Nevertheless they performed sexual activity mostly on the same canopy sites as the wild flies. Further research is needed, especially to elucidate the effect of protein feeding before releasing on the survival and mating effectiveness of sterile males.