Proceedings of 6th International Fruit Fly Symposium 6 10 May 2002, Stellenbosch, South Africa pp. 91 95 Demographic parameters and biotic factors of two Dacini species, Bactrocera cucurbitae and Dacus ciliatus, on Réunion Island Jean-François Vayssières*, Yannick Carel & Mathieu Coubes Laboratoire d Entomologie de Kourou CIRAD-FLHOR, B.P. 701-97387, Kourou, Cedex, France Réunion Island is characterized by a particular orography and a variable distribution of host plants for Dacini species. A complex of 3 Dacini species infests 16 host plant species belonging to the family Cucurbitaceae. These three species of vegetable fruit flies represent the primary pests of this plant family and are the main insect problem for cucurbit crops. We included in this study the two species B. cucurbitae (melon fly) and D. ciliatus (Ethiopian fruit fly). Adults of B. cucurbitae are classified as R-strategists, spending more energy than D. ciliatus in effectively using environmental resources. Adults of D. ciliatus can be classified as K-strategists. B. cucurbitae is the species best adapted to exploit short-lived host plants. This adaptation explains its predominance over D. ciliatus under larval interspecific competition on Réunion Island. INTRODUCTION Réunion Island is characterized by a particularly diverse topography and variable distribution of host-plants for Dacini fruit fly species. A complex of three Dacini species infests 16 host plant species belonging to the Cucurbitaceae family. These represent the primary pests of this plant family and are the main insect problem for cucurbit crops. The present study focuses on two species, Bactrocera cucurbitae (melon fly) and Dacus ciliatus (Ethiopian fruit fly), both of which were introduced to Réunion Island and have different bio-ecological characteristics. The objectives of this study were: 1) to compare the main demographic parameters of two Dacini species living on Réunion island; 2) to compare the main biotic factors of these two Dacini species. MATERIALS AND METHODS Rearings of adults (wild strains). For both species, 30 male female pairs were randomly isolated in 30 individual cages 10 pairs were isolated on each of three cucurbits, squash (Cucurbita pepo), pumpkin (C. pepo), and cucumber (Cucumis sativus) at 25 ± 1 C and 75 ± 10% relative humidity. Adults were fed with protein hydrolysate and sugar. Adults could also feed on three slices of cucurbit laid on the wire mesh top of the cage. Females could also oviposit within these three slices. Rearing of larval instars. Eggs were collected, placed in Petri dishes in cohorts of 100 eggs, and then observed until eclosion. Four different incubator temperatures, 15 ± 0.2 C, 20 ± 0.5 C, 25 ± 1 C and 30 ± 1 C, were used for larval rearing in Petri dishes. Nine Petri dishes of each species and for each temperature were observed. *To whom correspondence should be addressed. E-mail: jean-francois.vayssieres@cirad.fr Statistical analysis was performed using the life test procedure (SAS 1997) for life span, with variance analysis, followed by the Newmann-Keuls test, P = 0.05, for the other parameters. [The life test procedure gives the point estimate (in days) of the survival distribution of each quantile (e.g. 50% of surviving flies) for each Dacini species with 95% confidence interval (lower, upper) for each host-species.] RESULTS AND DISCUSSION Pre-oviposition period Table 1 shows that the mean pre-ovipositional period was 10.9 days for B. cucurbitae and 11.2 for D. ciliatus (Table 2) at 25 C. No significant differences were observed for the pre-oviposition period (in days) between the two fruit fly species and the three host-plants. Oviposition period Table 1 shows that the mean oviposition period was 112.4 days for B. cucurbitae and 82.9 days for D. ciliatus (Table 2) at 25 C. Significant differences were observed between the two fruit fly species and between squash and cucumberpumpkin for melon fly. Life span For B. cucurbitae, 50% of the flies were still alive after five months on cucumber and 60% after six months on pumpkin at 25 C. For D. ciliatus, 50% of the flies were still alive after four months on pumpkin, and 50% after 105 days on squash at 25 C. Gross fecundity Table 3 shows that gross fecundity was significantly different for B. cucurbitae between the three
92 Proceedings of the 6th International Fruit Fly Symposium Table 1. Oviposition period and life span (in days) of the melon fly, Bactrocera cucurbitae, at 25 C in three host plants. Pre-oviposition Oviposition Life span Cucumber 10.0 a 121.8 a 143 a Pumpkin 12.0 a 133.1 a 211 b Squash 10.6 a 82.2 b 142 a Table 2. Oviposition period and life span (in days) of the Ethiopian fruit fly, Dacus ciliatus, at 25 C in three host plants. Pre-oviposition Oviposition Life span Cucumber 9.8 a 84.0 b 110 a Pumpkin 12.5 a 85.2 b 107 a Squash 11.2 a 79.4 b 116 a host-plants, with 1111 eggs for cucumber compared with 521 for pumpkin and 343 for squash at 25 C. The gross fecundity is the measure of the number of eggs a hypothetical cohort of females would lay if all lived to the last possible age. The net fecundity is the actual number of eggs laid by females observed in this study over their life. Table 4 shows that gross fecundity was also significantly different for D. ciliatus between the three host-plants, with 375 eggs for cucumber compared with 423 for pumpkin and 571 for squash at 25 C. Gross daily fecundity Table 3 shows that gross daily fecundity by host-plant is significantly different for B. cucurbitae: 8.6 eggs per day for cucumber versus 3.8 for pumpkin and 4.3 for squash at 25 C. Table 4 shows that gross daily fecundity by host-plant for D.ciliatus is also significantly different, Table 3. Fecundity, fertility and egg hatching success of Bactrocera cucurbitae at 25 C in three host plants. Number of eggs Gross Net Gross Net Gross Net Gross hatch fecundity fecundity fecundity/day fecundity/day fertility/day fertility/day (%) Cucumber 1111 a 1015 a 8.6 a 8.1 a 7.4 a 6.7 a 91 Pumpkin 521 b 435 b 3.8 b 3.6 b 3.0 b 2.5 b 83 Squash 343 c 313 c 4.3 b 4.2 b 3.8 b 3.5 b 91 Table 4. Fecundity, fertility and egg hatching success of Dacus ciliatus at 25 C in three hosts-plants. Number of eggs Gross Net Gross Net Gross Net Gross hatch fecundity fecundity fecundity/day fecundity/day fertility/day fertility/day (%) Cucumber 375 c 333 b 2.7 c 2.3 c 2.2 c 1.9 b 89 Pumpkin 423 b 357 b 3.5 b 3.1 b 2.8 b 2.3 b 84 Squash 571 a 533 a 6.6 a 6.1 a 5.8 a 5.4 a 93
Table 5. Egg incubation period and developmental rate of larval instars of Bactrocera cucurbitae and Dacus ciliatus at 30 C in three host plants. L1 = first larval instar; L2 = second larval instar; L3 = third larval instar. Cucumber B. cucurbitae 23.67 ± 0.58 a 8±1a 12.33 ± 0.58 a 65 ± 7.00 a 8.67 ± 0.58 a 317 ± 23.01 13.2 ± 0.96 a D. ciliatus 30.67 ± 1.15 b 15.67 ± 0.58 b 14.67 ± 8.14 a 92 ± 17.78 b 10 ± 0.58 b 385 ± 41.5 16 ± 1.73 b Pumpkin B. cucurbitae 24.33 ± 0.58 a 7.67 ± 1.15 a 14±1a 84±3.46 a 9±0a 346±6.20 14.4 ± 0.26 a D. ciliatus 31.67 ± 0.58 b 15 ± 1.73 b 14.33 ±1.53 a 86 ± 14 a 11±0b 407±17.8 17 ± 0.74 b Squash B. cucurbitae 23.67 ± 0.58 a 10±0a 12.33 ± 1.15 a 65 ± 7.21 a 9.00±0a 327±8.94 13.6 ± 0.37 a D. ciliatus 29.67 ± 0.58 b 15.67 ± 2.08 b 14.33 ± 8.50 a 81.33 ± 7.77 b 10±1b 381±42.9 15.9 ± 1.79 b Table 6. Egg incubation period and developmental rate of larval instars of Bactrocera cucurbitae and Dacus ciliatus at 25 C in three host plants. L1 = first larval instar; L2 = second larval instar; L3 = third larval instar. Cucumber B. cucurbitae 30±1a 23.33 ± 0.58 a 24±0a 64.67 ± 12.70 a 11.33 ± 0.58 a 414 ± 28.1 17.2 ± 1.17 a D. ciliatus 40.33 ± 0.58 b 23.33 ± 0.58 a 24±0b 71.33 ± 1.15 a 14 ± 0.58 b 503 ± 16.2 21 ± 0.67 b Pumpkin B. cucurbitae 30.67 ± 1.53 a 19 ± 3.61 a 24±0a 64.67 ± 12.70 a 12.67 ± 0.58 a 442 ± 31.7 18.4 ± 1.32 a D. ciliatus 41.67 ± 1.53 b 23.67 ± 2.08 a 24.33 ± 0.58 a 79 ± 9.64 a 15.67 ± 0.58 b 545 ± 27.7 22.7 ± 1.15 b Squash B. cucurbitae 31±0a 18.33 ± 2.31 a 22.67 ± 2.31 a 72±0a 11±0a 408±4.62 a 17 ± 0.19 a D. ciliatus 43±1b 22.67 ± 1.15 b 24.67 ± 1.15 b 71.33 ± 1.15 b 14.33 ± 0.58 b 506 ± 18.3 21 ± 0.76 b Table 7. Egg incubation period and developmental rate of larval instars of Bactrocera cucurbitae and Dacus ciliatus at 20 C in three host plants. L1 = first larval instar; L2 = second larval instar; L3 = third larval instar. Cucumber B. cucurbitae 49.67 ± 3.21 a 17.33 ± 2.31 a 46.33 ± 0.58 a 96±1a 16.33 ± 1.15 a 601 25.1 ± 1.45 a D. ciliatus 72.33 ± 1.53 b 21±2a 49.67 ± 1.15 b 179 ± 11.79 b 21.67 ± 0.58 b 842 35.1 ± 1.26 b Pumpkin B. cucurbitae 49.67 ± 3.21 a 17 ± 1.73 a 46.67 ± 1.53 a 110.33 ± 12.42 a 16.33 ± 1.15 a 616 25.6 ± 1.94 a D. ciliatus 75.33 ± 1.15 b 24 ± 5.29 b 46±4a 185±8.19 b 21±0b 834 34.8 ± 0.77 b Squash B. cucurbitae 49.33 ± 2.08 a 19.33 ± 1.53 a 48±0a 112.33 ± 14.36 a 16.33 ± 1.15 a 621 25.8 ± 1.90 a D. ciliatus 71.66 ± 1.15 b 23±1b 46.33 ± 2.31 b 168.33 ± 3.06 b 20.66 ± 0.58 b 826 33.5 ± 0.89 b Vayssières et al.: Demographic parameters and biotic factors of two Dacini species on Réunion Island 93
94 Proceedings of the 6th International Fruit Fly Symposium with 2.7 eggs per day for cucumber versus 3.5 for pumpkin and 6.6 for squash at 25 C. Egg hatching success Tables 3 & 4 show that there was no significant difference in egg hatch between the two fruit fly species; more than 88% of eggs hatched for both species at 25 C. Egg incubation period at four temperatures At all temperatures tested, 30 C (Table 5), 25 C (Table 6), 20 C (Table 7) and 15 C (Table 8), melon fly had a significantly shorter incubation period than the Ethiopian fruit fly. Rate of development of larval instars at four temperatures At 30 C (Table 5), 25 C (Table 6), 20 C (Table 7) and 15 C (Table 8), the larval instars of melon fly developed significantly faster than those of the Ethiopian fruit fly. The terms R-selection and K-selection can be used to describe extreme strategies of exploitation of host plants (McArthur & Wilson 1967). This classification considers demographic parameters in relation to their main biotic factors. R-selection corresponds to unpredictable environmental conditions (short-lived host plants) and K-selection to stable ones (long-lived host plants). Adults of B.cucurbitae are classified as R-strategists, are large and have great mobility, high fecundity (more than 1000 eggs per female on the most favourable host plant), high daily fecundity (an average of eight eggs per day per female), a high fertility (more than 90%), a long life span (more than five months), and a short egg eclosion period (24 h at 30 C). Other plants belonging to the families Solanaceae and Passifloraceae are occasionally infested by larvae of the melon fly. B. cucurbitae spend more energy than D. ciliatus in effectively using environmental resources. Adults of D. ciliatus are smaller and have a lower mobility, lower fecundity (~500 eggs per female on the most favourable host plant), lower daily fecundity (an average of two eggs per day per female), lower fertility (83%), a shorter life span (four months), and a longer egg eclosion period (31 h at 30 C). Owing to these characteristics, the Ethiopian fruit fly can be classified as a K-strategist. Significant differences in demographic parameters and main biotic factors (egg incubation and instar development) between the two fruit flies species explain the predominance of B. cucurbitae Table 8. Egg incubation period and speed of development of larval instars of Bactrocera cucurbitae and Dacus ciliatus at 15 C on pumpkin. L1 = first larval instar; L2 = second larval instar; L3 = third larval instar. Pumpkin B. cucurbitae 96±0a 48±0a 72±24a 205±41.57 a 26.67 ± 0.58 a 1061 44.2 ± 2.52 a D. ciliatus 120±0b 96±24b 120±10b 224±13.86 a 63±1b 2072 86 ± 12 b
Vayssières et al.: Demographic parameters and biotic factors of two Dacini species on Réunion Island 95 on D. ciliatus on Réunion Island. Furthermore, the classification of the melon fly as an R-strategist can explain its worldwide distribution and its ability to colonize new areas such as in West Africa in 1999 and 2000 (Vayssières 2000), which were then already occupied by the Ethiopian fruit fly (Mali, northern Ivory Coast, Guinea, Senegal). REFERENCES McARTHUR, R.H. & WILSON, E.O. 1967. The Theory of Island Biogeography. Princeton University Press, Princeton, N.J. SAS INSTITUTE 1997. The SAS system for Windows, ver. 6, 4th edn, vol. 2, Lifetest procedure. 1027 1070. SAS Institute, Cary, NC. VAYSSIÈRES, J.F. 2000. Rapport de mission au Mali sur le complexe des mouches des fruits (Diptera Tephritidae) inféodées au manguier. Projet CAE/SEG/USAID.