Population Parameters of Spalangia endius Walker (Hymenoptera: Pteromalidae) on Pupae of Musca domestica L. (Diptera: Muscidae)

Parámetros Poblacionales de Spalangia endius Walker (Hymenoptera: Pteromalidae) Sobre Pupas de Musca domestica L. (Diptera: Muscidae)

BIOLOGICAL CONTROL

Population Parameters of Spalangia endius Walker (Hymenoptera: Pteromalidae) on Pupae of Musca domestica L. (Diptera: Muscidae)

FRANCISCO R. LA ROSSA¹, DIANA C. CRESPO^1,2 AND ROBERTO E. LECUONA¹

¹Inst. Microbiología y Zoología Agrícola (IMYZA), Centro Nacional de Investigaciones Agropecuarias (CNIA), Instituto Nacional de Tecnología Agropecuaria (INTA), C.C. 25, 1712, Castelar, Buenos Aires, Argentina

² CONICET, Buenos Aires, Argentina

Parámetros Poblacionales de Spalangia endius Walker (Hymenoptera: Pteromalidae) Sobre Pupas de Musca domestica L. (Diptera: Muscidae)

RESUMEN - Se determinó el incremento poblacional potencial del parasitoide Spalangia endius Walker (Hymenoptera: Pteromalidae) sobre pupas de Musca domestica L. (Diptera: Muscidae) en condiciones de laboratorio, en oscuridad a 27°C y 50 ± 10% HR. Se colocaron individualmente 30 hembras con menos de 12h de emergidas sobre 15 pupas de Musca domestica L de menos de 24h de edad, renovándolas diariamente. La supervivencia preimaginal fue del 90,3 %. La longevidad de los adultos fue estimada en 8,3 días en promedio con una tasa neta de reproducción de 24,549; tasa intrínseca de crecimiento natural de 0,119; tasa finita de crecimiento de 1,126; tiempo generacional medio de 28,162 días y tiempo de duplicación de 5,849 días. Estos valores de los parámetros indican el alto potencial de S. endius comparadas con otras especies congenéricas.

PALABRAS CLAVE: Tabla de vida, parasitoide de pupas, tasa intrínseca de crecimiento

ABSTRACT - The potential increase of the parasitoid Spalangia endius Walker was determined under laboratory conditions at 27°C, 50 ± 10% RH. and 24h darkness. Thirty <12h-old females of the parasitoid were individualized with 15 Musca domestica L. pupae <24h old. Survival of S. endius immature was 90.3 %. Adult females mean longevity was estimated in 8.3 days with a net rate of reproduction of 24.549; intrinsic rate of natural increase of 0.119; finite rate of increase of 1.126; mean generation time of 28.162 days; and duplication time of 5.849 days. These values indicate the high potential of S. endius as a parasitoid of M. domestica, as compared to other congeneric species.

KEY WORDS: Life table, pupal parasitoid, intrinsic rate of increase

The microhymenopteran Spalangia endius Walker is a well-known parasitoid of Musca domestica L. pupae (Morgan et al. 1976) and is used to control this dipterous in confinement animal production system.

Life tables are an important component in estimating the vital statistics of an insect population. This methodology has been used to study population dynamics of a particular species (Southwood 1995), to select natural enemies (Janssen & Sabelis 1992) and to evaluate the impact of a beneficial insect on an insect-pest (Bellows et al. 1992). In order to evaluate the possibility of success of a parasitoid release in rural facilities to control sinanthropic dipterans, Morgan et al. (1989) carried out laboratory studies towards determination of parameters such as: daily fecundity and adults survival and mortality rates, among others. Several authors (Morgan et al. 1989, Morgan et al. 1991, Costa et al. 1995) obtained population parameters of congeneric species such as Spalangia cameroni Perkins and S. gemina Boucek and studied their use as control agents.

The objective of this research work was to construct fertility life tables of S. endius in order to obtain population parameters.

Materials and Methods

The present study was carried out in the insectary of domestic fly of the IMYZA (INTA Castelar, Buenos Aires, Argentina. Thirty <12h S. endius females from a mass rearing were used. These females were individually placed into 15 cm x 1.5 cm cotton-plugged glass tubes, without any food, together with 15 pupae of M. domestica. Selection of pupae was performed when the presence of the larval cuticle of the third instar was evident in the pupary formation. These pupae, which were selected by their white color indicating the beginning of pupation, were maintained in incubators at 29 ± 1ºC temperature and 60% to 70% RH. Under these conditions, the fly pupae became purplish-brown in color within 24h after pupation started. The environmental conditions for the life table assay were: 27 ± 1ºC temperature and 50 ± 10% RH. Fly pupae were daily replaced until death of all parasitoid females. Before sexation, the progenies of parasitoides and flies emerged from non-parasitized pupae were computed. For each female the following parameters were obtained: longevity, pre-oviposition period and daily number of male and female descendents.

Facing the difficulty in obtaining information on parasitoid development inside the host pupa, the overall percentage of survival of immature stages was estimated by subtracting the number of flies emerged, the number of parasitoids emerged and the number of naturally dead pupae from the total pupae offered. Data on this parameter were obtained by performing observations on 50 pupae daily taken from the same mass rearing used for the assays previously described.

Based on the data obtained, a fertility life table was built for each cohort (Southwood 1995) where x represents the average point of the age interval, lx is the life expectancy of the age x expressed on a base of 1,000 females, mx is the specific fertility or number of females per female born in the age x, and finally lxmx represents the total number of females produced in the age x.

The population parameters computed were: net rate of reproduction (R_o), which represents the total number of females produced in one generation; intrinsic rate of population increase (r_m), which expresses the innate-capacity of increasing in number; mean generation time (T), which is the mean extent of one generation; finite rate of increase (l), which is the number of times that the population increases per unit of time; and time of duplication (D), which represents the time that a given population needs to duplicate itself in number.

The mean longevity of adult females was estimated based on the Weibull distribution, determining a function in which the values estimate a survival curve as a function of time (Sgrillo 1982).

The confidence intervals of each population parameter estimated with a=0.05, which are indispensable for comparing among cohorts, were computed by the "jackknife" method (Hulting et al. 1990, Thomazini & Berti Filho 2000).

Results and Discussion

The percent survival of immature stages of S. endius on M. domestica was 90.3%. The value for this parameter was lower for S. endius than for S. cameroni (97.5%) and S. gemina (96.5%) (Morgan et al. 1989). The pre-oviposition time varied from >24h to six days, being 3 the most frequent value. The fertility curve (Fig. 1 and Table 1) reaches its maximum value at the 7^th day and at that age more than 50% of descendents had already been produced, followed by a decreasing trend.

The survival curve followed the Weibull distribution (Sgrillo 1982) with c² = 4.81(n.s.) estimating the mean longevity in 8.3 days. All females were dead at the 18^th day (Fig 2). Morgan et al. (1976) found that 50% of females died at 4.5 days and the maximum longevity was nine days. This difference is probably due to either the different rearing conditions or racial characteristics. In the case of S. gemina that percentage was reached at 17.3 days and all females were dead after 20 days (Morgan et al. 1991).

The number of times that a population of S. endius multiplies per generation (R_o) was 24.55, which indicates that in average a female can produce other 24 to 25 new females in each generation (Table 2). In other congeneric species, Morgan et al. (1991) found a R_o of 40 for S. gemina, while Costa (1995) found a figure of 64.14 for this parameter in the same species. On the other hand, the R_o calculated for S. cameroni was 13 females/female/generation (Morgan et al. 1989). The population of S. endius herein studied would be at an intermediate point between the two species cited, as far as R_o is concerned, even considering that the discrepancy found in S. gemina could indicate a high variability of this parameter within the same species.

Southwood (1995) points out that the parameter r_m is the rate of increase per individual or the intrinsic rate of natural increase in an environment where the fecundity and the survival are maximal, in the absence of external mortality factors. On the other hand, the bigger is the r_m value, the bigger will be the potential of a species in reproducing and increasing in number within a given environment. This is also the most important parameter obtained from a life table since it allows the comparison of the potential increase among species. In that particular case it facilitates the evaluation of a parasitoid concerning its use in strategies where biological control is included (Jansen & Sabelis 1992). The r_m value was 0.119 (Table 2), which was higher than the figure of 0.09 found for S. gemina at 25ºC temperature and 70% RH (Costa 1995). The finite rate of increase (l), which is the number of times that the population is multiplied per unit of time is another important parameter obtained from a life table and is also used for comparisons among species or races of parasitoids. This ratio was 1.126, being higher than that of 1.1 found with S. gemina (Costa 1995). The mean time among generations (T) was 28.16 days, sensibly lower than that of 43.57 days found for S. gemina (Costa 1995). The higher r_m and lower T indicate that S. endius possesses a high potential of population increase as compared to S. gemina.

Acknowledgments

To Mr. Luís Díaz and to the Agronomist Juana Luna for their collaboration during the development of the present research work and to the Librarian Mrs. Estela Favret for her help in the bibliographic searching.

Literature Cited

Received 22/02/02. Accepted 15/09/02.

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Publication Dates

• Publication in this collection
  20 Feb 2003
• Date of issue
  Oct 2002

History

• Accepted
  15 Sept 2002
• Received
  22 Feb 2002