Drosophila suzukii (Matsumura) (Diptera: Drosophilidae) and Drosophila melanogaster Meigen were studied in a laboratory at 25 °C, 60% RH and 16:8 h L:D. Stage-specific developmental times, reproduction, stage-specific survival rates, and adult sex ratios were recorded and organized in separate life tables for each species. The intrinsic rate of increase (r), the finite rate of increase (λ), the net reproduction rate (R0) and the mean generation time (T) were 0.12 day-1, 1.13 day-1, 27.57 offspring, and 28.04 days, respectively, for D. suzukii and 0.17 day-1, 1.19 day-1, 38.17 offspring, and 21.27 days, respectively, for D. melanogaster. The use of the age-stage, two-sex life table method to study D. suzukii and D. melanogaster yielded considerably more accurate and useful data than would have been obtained by using the female-only age-specific life table. These life tables can be used for population growth projections, designing mass-rearing programs, and for pest management.
Drosophila suzukii (Matsumura) (Diptera: Drosophilidae) and Drosophila melanogaster Meigen are both key pests of fruits. Drosophila suzukii damages cherries (Van der Linde et al. 2006) and a variety of small fruits (Dreves et al. 2009). Oviposition in fruits produces little visible damage on the fruit surface. Upon hatching, larvae feed on the fruit, causing it to soften, turn brown and rot. Many fruits are damaged by D. suzukii, including blueberry, blackberry, cherry, strawberry, plums, peaches, grapes, figs, kiwi fruit, and pears (Dreves et al. 2009). In the United States, losses in strawberry, blueberry and raspberry caused by D. suzukii have reached 80, 40 and 70%, respectively (Bolda 2010).
Drosophila melanogaster has caused increasing damage to Chinese bayberries and cherries in China. In Tianshui, Gansu Province, some latematuring varieties of cherries proved especially susceptible to damage by D. melanogaster with loss rates generally above 35%, reaching 80% for some cultivars (Guo 2007). In the Aba region of Sichuan Province fruit damage to cherries due to D. melanogaster has been as great as 60% (Guo et al. 2007), and in Guiyang, Guizhou Province red bayberry losses have been 38%∼57% (Li et al. 2005). Commonly, red bayberry damage is about 60% if control measures are not promptly undertaken to suppress fruit flies (Yang et al. 1998).
Life tables are helpful tools for quantifying and analyzing mortality, development, reproduction, and intrinsic rates of increase of insect populations (Chi & Su 2006) and can be used to make population projections based on computer simulations (Chi 1990). Life tables have been used in diverse types of studies related to population ecology, such as the population biology of invasive species (Sakai et al. 2001), conservation strategies (Wilcox & Murphy 1985), demographic ecotoxicology (Stark & Banks 2003), harvesting theory (Chi & Getz 1988, Chi 1994), and timing of pest control measures (Chi 1990).
Materials and Methods
Drosophila suzukii and D. melanogaster populations used in this study were from colonies started with larvae from cherry orchards in Tai'an, Shandong Province in May 2012. Flies were raised in an insectary for about 9 generations at 25 ± 1 °C, 70 ± 5% RH, 16:8 h L:D and 10,000 lux before use in experiments reported here. Both flies were reared on table grapes grapes (‘Kyoho’ grape cultivar, Vitis vinifera L.; Vitales: Vitaceae) purchased from the local market. Grapes were rinsed three times in distilled water and dried before being provided to adult flies for oviposition and subsequent larvae development.
Life Table Construction
To create cohorts for life table analysis of development and survival, grapes were cut into halves and placed in a 2000-mL flat drum bottle with the cut surface upward to provide food for adult fruit flies. About 60 pairs each of mated D. suzukii and D. melanogaster females were selected and placed in the bottles to lay eggs for 4 h. They were kept in the same conditions as the insectary above. Then, 106 D. suzukii eggs and 103 D. melanogaster eggs were collected and placed in 209 Petri dishes (1.5 cm diam) with fresh grape halves. All experimental insects were observed daily with a dissecting microscope at 10:00 am until all eggs had either become adults or died at some earlier life stage. Adult flies were sexed, and pairs of one male and one female were placed in 2,000-L flat drum bottles, where they were provided with one grape cut into halves to provide food and an oviposition substrate. Grapes were replaced daily and eggs counted until the adults died.
The data were analyzed using age-stage, twosex life tables as described by Chi & Liu (1985) and Chi (1988). The mean development period for each developmental stage, the longevities of adult males and females, the adult pre-oviposition periods (APOP), the total pre-oviposition periods (TPOP), and the fecundities of D. suzukii and D. melanogaster were calculated. The average APOP was calculated based on meaurements of the pre-ovipositional periods of the adult females, whereas the average TPOP included the developmental times of the pre-adult stages plus the preovipositional period of the adult females. The agestage specific survival rate (sxj) (where x is the age and j is the stage), the age-stage specific fecundity (fxj), the age-specific survival rate (lx), and the agespecific fecundity (mx) were calculated from the daily records of the survival and fecundity of all individuals in the cohort. The population parameters (r, intrinsic rate of increase; λ, finite rate of increase; R0, net reproduction rate; and T, the mean generation time) were also calculated. The intrinsic rate of increase was estimated by using the iterative bisection method from the equation:Goodman 1982). In the age-stage, two-sex life table, the lx and mx are calculated as: Chi & Liu, 1985). The life expectancy (exj) was calculated according to Chi & Su (2006). The mean generation time is defined as the period of time needed by a population to increase to R0-fold of its size (i.e., erT =R0 or λT =R0) at the stable age-stage distribution and is calculated as T = (ln R0)/r, where R0 = Σlxmx, λ = er. The gross reproductive rate (GRR) = Σmx. For the tedious and complicated calculations involved in the analyses of raw data and of the life table, a computer program (TWOSEX-MSChart) was used in this analysis (Chi 2012). This program is written in Visual BASIC for the Windows operating system and is available at http://126.96.36.199/Ecology.
There were significant differences between D. suzukii and D. melanogaster in the duration of the egg, L1 and pupal stages (P = 0.003, P = 0.001, P = 0.000, respectively). The pupation rates of D. suzukii and D. melanogaster were 70.5 and 75.7%, respectively, and every pupa reached adulthood. The emerged D. suzukii males and females had a sex ratio of 1:1.4, while the sex ratio of D. melanogaster was 1:2.1. The adult pre-ovipositional periods (APOP) of D. suzukii and D. melanogaster were 3.02 ± 0.16 days and 2.87 ± 0.25 days (P > 0.05), and the total pre-ovipositional period (TPOP) of D. suzukii (19.95 ± 0.65 days) and was significantly longer than that of D. melanogaster (15.62 ± 0.39 days) (P = 0.000), largely because D. suzukii had a longer pupal stage (Table 1).
The age-stage specific survival rate (sxj) (Fig. 1) shows the probability that a newly laid egg will survive to age x and develop to stage j. The curves also shows stage differentiation due to variable developmental rates among individuals. Considering the variable developmental rates among individuals, the stage survival curves can describe generational overlapping. In the D. suzukii cohort, pupae were present up to day 39, but all D. melanogaster pupae molted to adults by day 19. The horizontal survival curve of D. suzukii male adults showed almost no mortality during the age interval 22–39 days, and by the 53rd day all adults of both D. suzukii and D. melanogaster had died.
Developmental times, longevities and fecundities of Drosophila suzukii and Drosophila melanogaster.
Ignoring stage differentiation, a single agespecific survival rate (lx) (Fig. 2) was calculated that expresses the probability that an egg will survive to age x. The curve of age-specific fecundity (mx) showed that reproduction began at day 13 for D. suzukii and lasted for 37 days, while D. melanogaster reproduction began at day 9 and lasted for 36 days. The age-specific reproduction (lxmx) is the population rate of reproduction at age x, incorporating both fecundity for that age of female and the probability of a fly living to that age.
Based on the age-stage, two-sex life table, the age-stage-specific life expectancy (exj) gives the expected life span of an individual of age x and stage j (Fig. 3). For example, the D. suzukii life expectancy of an egg was 33 days and a 34-dayold female will likely live another month. Because this study was conducted in the laboratory without the adverse effects of field conditions, life expectancy decreased gradually with age.
The reproductive value (vxj) is the contribution of individuals of age x and stage j to the future population (Fig. 4). The reproductive value for a new egg (v01) is the finite rate of increase (λ); the reproductive value gradually increased with age (x) and stage (j), then drops to zero. The peak reproductive value occurred for females 23 and 18 days old for D. suzukii and D. melanogaster, respectively, implying that in comparison to other ages, these females make the highest contribution to the population.
If all individuals are included, the intrinsic rate of increase (r), the finite rate of increase (λ), the gross reproduction rate (GRR), the net reproduction rate (R0) and the mean generation time (T) of D. suzukii were 0.12 ± 0.0053 day-1, 1.13 ± 0.006 day-1, 47.01 ± 5.14 offspring, 27.57 ± 3.48 offspring, 28.05 ± 0.55 days, and r, λ, GRR, R0, and T of D. melanogaster were 0.17 ± 0.0074 day-1, 1.19 ± 0.0088 day-1, 70.67 ± 8.36 offspring, 38.17 ± 4.16 offspring, 21.29 ± 0.63 days (Table 2). With the exception of the R0 values, significant differences were found between the r, λ, GRR, and T values for these two fruit fly species.
This study found that D. suzukii had a longer mean generation time than D. melanogaster, while D. suzukii adult males and females lived longer than those of D. melanogaster. The finite rate of increase (λ) (days-1) indicated that rate of increase of D. suzukii from one generation to next was significantly less than that of D. melanogaster. In this study, there was no significant difference in fecundity between D. suzukii and D. melanogaster, so did their net reproduction rates (R0) (offspring), were less than that when they are reared groups. Thus when the flies live together in groups, the females may lay more eggs than when reared in pairs. This apparent effect of isolated rearing seems more obvious in the case of D. melanogaster than that of D. suzukii. However this hypothesis requires additional investigation.
Zhang et al. (2010) reported that the developmental periods of D. melanogaster stages reared on grape at 25 °C were 1.08 days for the egg stage, 4.39 days for the larval stage, and 3.9 days for the pupal stage; these values are slightly shorter than in our study. A variety of factors, including host plants, temperature, and rearing methods have been reported to cause differences in the developmental times. Guo (2007) reported that the durations of the egg. larval and pupal stages of D. melanogaster reared on cherries at 25 °C were 0.95 days, 4.5 days and 4.1 days, respectively; and that the number of days required by D. melanogaster reared on cherries to reach the adult stage at various temperatures were 9.5 days at 25 °C, 15.7 days at 20 °C and about 30.9 days at 15 °C. Considering that cherries are greatly influenced by the season, and since grapes are one of the hosts of these two fruit fly species, grapes were used to rear D. suzukii and D. melanogaster. Also in this study the eggs were reared singly to the adult stage, and this may have influenced the developmental times of the various life stages.
Population parameters for Drosophila suzukii and Drosophila melanogaster.
The developmental times analyzed by agestage, two-sex life tables of D. suzukii and D. melanogaster in our study are generally consistent with those found by previous studies calculated in traditional way. Kanzawa (1939) reported that the developmental durations of D. suzukii were 2-72 h, 3–13 days and 3–15 days for egg, larvae and pupal stages, respectively, and the lifecycle was completed in 21–25 days at a constant temperature of 15°C, and in 9–11 days at 25 °C.
Because the age-stage, two-sex life table incorporates variations among individuals in developmental rates, stage overlaps in the survival rate can be observed in this study. Both D. suzukii and D. melanogaster can produce over 10 generations in a single year (Kanzawa 1939; Guo 2007; Steck et al. 2009), both species have overlapping stages and generations in nature.
Although insects are rarely subject to constant temperatures in nature, controlled laboratory studies can provide basic and valuable insights into the population dynamics of a particular species (Summers et al. 1984). The results obtained in this study provide information useful for predicting the population potentials of D. suzukii and D. melanogaster as key targets in biological control programs. The use of the age-stage, two-sex life table method to study D. suzukii and D. melanogaster yielded considerably more accurate and useful data than would have been obtained using the female-only age-specific life table. These life tables can be used for population growth projections, designing of mass-rearing programs, and for timing interventions in pest management.
This work was supported by the Shandong Provincial Modern Agricultural Industry Technology System Innovation Team Foundation, China (SDAIT-03-022-08) and Ministry of Agriculture Agricultural Research Exceptional Talents and Innovation Team Foundation, China.