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Expected effects of reducing Oestrus ovis L. mature larval weight on adult populations
Veterinary Parasitology 90 (2000) 239–246
Expected effects of reducing Oestrus ovis L. mature larval weight on adult populations R. Cepeda-Palacios a,∗ , S. Frugère b , Ph. Dorchies b a
Depto. de Zootecnia, Universidad Aut. de Baja California Sur, Km. 5.5 Carr. al Sur, P.O. Box 19-B, 23080 La Paz, Baja California Sur, Mexico b Lab. Associé de Physiopatologie Respiratoire des Ruminants, Ecole Nationale Vétérinaire, 23 Chemin des Capelles, 31076 Toulouse Cedex, France Received 7 October 1999; accepted 13 March 2000
Abstract In order to estimate the effects of eventual reductions in larval weight (LW) of Oestrus ovis L. as a measure of control, the correlation between mature LW and adult fly length (AL) in laboratory specimens (n=150) was calculated. The regression equation AL=5.62+10.65LW (r2 =0.76) was obtained. This equation was then applied to estimate the mature LW of wild larvipositing females (n=51) to predict the minimum mature LW at which fly viability would be compromised. The critical weight, 0.28 g (standard error limits 0.235, 0.323), was obtained from a small fly measuring 8.6 mm in length. Data from 383 mature third instars were used to estimate, by statistical analysis, the expected effects of decreasing the mature LW on subsequent fly population size. A considerable mean reduction (38%) in adult populations might be achieved by a 40% reduction of mature LW, but this eventual reduction may be temporary due to the high reproductive rate in this species. Sex differences in mature LW and fly size are also reported. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Oestrus ovis; Growth; Larva; Fly population; Larval weight
1. Introduction Larval weight (LW) has special importance in the Family Oestridae because these parasitic species of flies do not feed as adults, but must acquire during the larval stage all the energy reserves they will need during the pupal and adult stages (Wood, 1987). In the case of Oestrus ovis L., energy reserves in female flies are critical for mating, larval incubation, host-seeking and larviposition activities (Zumpt, 1965; Wood, 1987). Cepeda-Palacios et al. (1998) reported that, in O. ovis, weight and the degree of maturity of the third instars are associated with survival during the puparial period. LW of individuals that survived the puparial ∗ Corresponding author. Tel.: +52-112-8-08-02; fax: +52-112-8-08-80. E-mail address: [email protected] (R. Cepeda-Palacios)
0304-4017/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 0 1 7 ( 0 0 ) 0 0 2 3 7 - 5
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period was significantly higher than in those that did not survive; however, nonpigmented and slightly pigmented larvae were generally unable to survive outside the host. Likewise, low but significant correlations between LW and weight of adult female Hypoderma tarandi L. and Cephenemyia trompe Modeer have been reported (Nilssen, 1997). Populations of O. ovis are regulated by many factors, but establishment and survival of first instars in the host are of prime importance (Bart and Minár, 1992). Moreover, manipulating the larval growth of O. ovis inside the host in order to affect the viability of fly populations shows promise as a method of control. Reduction in the larval growth of Lucilia cuprina Weidemann using an immunological approach has been achieved (Johnston et al., 1992). However, no data about the relationship between mature LW and fly viability have been published for O. ovis, and therefore, no values of LW have been estimated that could be used as goals for manipulating larval growth. The objectives of this study were to analyze the relationship between larval and adult sizes of O. ovis, to estimate mature LWs at which fly viability may be compromised under natural conditions and to predict the possible impact of LW manipulation on populations of infecting flies. 2. Material and methods 2.1. Laboratory-reared adults (database 1) Mature O. ovis third instars were recovered from the sinus and horn cavities of slaughtered goats coming from the southern region of Baja California Sur, Mexico, to obtain 150 (81 male, 69 female) viable specimens. After collection, mature larvae were age-classified, weighed using an analytical balance and transferred to plastic jars partially filled with sterile sand to allow pupariation; the pupae were then transferred to vials. To classify the degree of maturation of the third instars, the scale described by Cepeda-Palacios et al. (1999) was used. Briefly, this scale is based on the advance of larval integumental pigmentation and includes six categories (namely, D1 . . . D6 ), where D1 (white larvae, posterior stigmas brilliant black, fully melanized) are the youngest, and D6 (fully dark-pigmented larvae) are the oldest. To measure the adults, removal of the puparial shell was performed 8 days post-pupariation (i.e. when head eversion had occurred) onwards up to emergence. During this period, the pharate (hidden) adult reaches the definitive adult size (Whitten, 1969; Hinton, 1971). Data of length (from the ptilinal suture to the posterior abdominal end) and width (mesothoracic width from a dorsal view, including the wing insertion) of pharate adults and newly emerged flies were obtained by direct measurement under the stereoscope, using a microrule. From collection to puparium removal, specimens were reared at a photoperiod of 12 h:12 h (L:D) and alternating temperatures of 32 and 16◦ C, respectively (Cepeda-Palacios and Scholl, 1999). 2.2. Wild gravid flies (database 2) During the springtime, females (n=51) were caught by hand while they were larvipositing on goats in two different flocks in the central region of Baja California Sur. Flies
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were preserved in alcohol in vials until measurement, which was accomplished as with laboratory-reared specimens. 2.3. Mature larvae (database 3) To estimate the impact of LW reductions on adult population viability, weights of 383 mature larvae showing variable degrees of maturity, including those from D3 (white or creamy larvae with black dorsal bands) to D6 (black-pigmented larvae, even in the intersegmental dorsal areas) were used as the distribution database. These third instars were obtained from goats coming from the same locality during 1997 and 1998. 2.4. Statistical procedures Firstly, using the measurements of laboratory-reared specimens (database 1), Pearson’s correlation coefficients of third instar weight, fly length, width, and product length×width (as an additional variable for analysis) were calculated. Fly length was found to be closely correlated with LW. Thus, the regression equation AL = 5.62 + 10.65LW (r 2 = 0.76, standard error of estimate = 0.47, P <0.001), where LW is the weight of the third instar and AL is the length of the pharate or emerged adult, was obtained. Data from males and females were pooled for this equation because fly length was found to be similar (P>0.05) between males (11.1±0.73 mm) and females (11.2±0.73 mm). Since both LW and AL are random variables, the relationship described by the equation was then used to estimate LWs and standard errors corresponding to the gravid wild flies of database 2. For our purposes, the smaller wild fly from database 2 was considered as the minimum size which was able to survive successfully and infect hosts, and therefore, its estimated LW was considered to be critical to produce viable adults. Finally, 10–50% reductions in LW and their lower and upper limits (determined by the standard error of the estimation) were calculated for database 3 values. Data of each weight reduction category (i.e. 10, 20, 30, 40, 50%) generated normal curves. Then, these calculated values were standardized as Z-values to estimate the proportion of individuals under the normal curve having weights below the critical weight mentioned above. Descriptive statistics of the size variables for the laboratory-reared and wild specimens were also calculated. Comparisons between sex groups of larvae and flies were carried out in the database of laboratory-reared specimens by student’s t-tests. All data analyses were made using the statistica software (StatSoft, 1998). 3. Results 3.1. Size of laboratory specimens Descriptive statistics for larval and adult measurements are shown in Table 1. In this database, a wide variation in mature LW was observed, averaging about half a gram and
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Table 1 Larval weight and adult size in laboratory-reared and wild Oestrus ovis Fly origina
Variableb
n
Mean
s
Range
Laboratory
Larval weight Fly length Fly width
150 150 147
0.498 11.2 5.2
0.08 0.72 0.54
0.328–0.671 9.1–12.5 3.7–7.0
Wild
Larval weight Fly length Fly width
51 51 50
0.431 10.2 4.2
0.04 0.45 0.41
0.280–0.551 8.6–11.5 2.8–4.9
a Laboratory specimens were male and female pharate adults and newly emerged flies. Wild specimens were gravid females caught while larvipositing on goats and their larval weight was estimated by a regression equation. b Fly sizes are in millimeters and larval weights are in grams.
95% confidence limits (not shown in Table 1) of 0.485 and 0.511 g. This variation resulted in size differences in adults. Although female (0.511 g) larvae tended to be heavier than males (0.489 g, P=0.08), both sexes had similar (P>0.05) adult length (11.1±0.7 mm males and 11.2±0.7 mm females). Confidence limits (95% level) for the overall mean fly length (11.2 mm) were 11.0 and 11.3 mm. Also, adult males (5.4±0.5 mm) were significantly (P<0.05) wider than adult females (5.2±0.6 mm). 3.1.1. Size of wild gravid flies Mean values of sizes for wild flies and estimated mean LW are shown in Table 1. Mean LW was lower in wild flies compared to laboratory-reared flies. This was expected because laboratory flies were generally larger than wild flies, and wild LW was less variable because
Fig. 1. Expected effects of decreasing the mature larval weight of Oestrus ovis on the subsequent fly population size. The upper and lower dotted lines represent these effects when the limits of the standard error of estimation were calculated. The wild weight corresponds to the mean weight of highly viable mature third instars.
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estimated LWs were adjusted by the regression equation. The lowest estimated LW was for a wild fly measuring 8.6 mm in length, which corresponded to a mature LW of 0.280 g; however, due to the standard error of the estimation, this value may be found to be between 0.235 and 0.323 g. The highest estimate was about 0.550 g, corresponding to the largest wild fly (11.5 mm long) caught in the period. 3.2. Expected effects of reduction in mature LW The probable effects of decreasing LW on a fly population are shown in Fig. 1. In wild populations, less than 1% of the larvae are expected to produce nonviable adult females, while 20 and 30% decreases in mature LW would result in 2 and 15%, respectively, of nonviable flies and so on. Furthermore, a 40% decrease in mature LW would cause a strong reduction of 39% (standard error limits of 12 and 80%) in the fly population.
4. Discussion 4.1. Mature LW Very limited data exist about the importance of mature LW and adult size in oestrid fitness and biology. Larval burden (the number of larvae hosted by a goat or sheep) can be affected by factors such as the number of larvae deposited by female flies, the age, body and health condition of the host, the capacity of sinusal cavities of the host, geographic locality and climatic conditions (Meleney et al., 1962). Likewise, various factors attributed to the host (sinus capacity, weight, and immunological status) or larval (genetic capacity of growth, sex) origin or an interaction of both (i.e. larval density in the host) may affect the mature LW in O. ovis (Cepeda-Palacios et al., 1999). In general, our results on LW agree with those obtained by Breev and Sultanov (1975), who reported that, in O. ovis third instars, it ranged from 0.140 to 0.522 g. Unfortunately, no data on the degree of larval maturity was given in this study. Nilssen (1997) determined highly significant correlations (r=0.34–0.44) between mature LW and adult weight in newly emerged males and females in both H. tarandi and C. trompe. In these species, sexual size dimorphism was noted in adults as well. Lello et al. (1985) reported that Dermatobia hominis Linnaeus Jr. mature female larvae are heavier than males and that LW by itself is a reliable characteristic to distinguish both sexes at this stage. Similar weight differences between mature female and male larvae have been reported for Cuterebra emasculator Fitch (Bennett, 1972). The estimated critical or ‘lethal’ weights (<0.28 g) for mature larvae correspond to that reached by the larvae during the early L3 period (Cepeda-Palacios et al., 1999), which is the period when the highest LW gains occur. In our experience, the survivability of mature larva after they leave the host depends not only on the LW but also on the degree of maturity. Although D1 and D2 (white larvae with slight black dorsal bands) larvae are very often heavy enough to continue the life cycle, they seldom exit the host because they lack the abilities to burrow into the soil and to complete pupariation, which are acquired as integumental pigmentation advances.
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4.2. Adult size After emergence, the adult female must face tasks that require critical energy reserves such as mating, larval incubation, host seeking, and larviposition activities and a proportion of the fly population may be reduced during each of these stages. Thus, females that are successful in larvipositing should have attained at least the critical mature LW to ensure meeting these energy expenses. The lowest fly length found in this study was 8.6 mm which is below the lowest fly length (10 mm) reported in the literature reviewed (Porchinskii, 1913; Miller et al., 1961; Zumpt, 1965; Atencio-Leon and Ramirez, 1972). Such a low fly length was not found in a sample of 15 laboratory-reared females coming from the San Angelo Texas zone, which averaged 10.4±0.4 mm (range 9.9–11.4 mm), values very close to those obtained in our study. A noteworthy aspect found in this study is the apparent difference in average length (1 mm) between flies from southern and central zones of Baja California Sur, which may be either due to unknown factors or due to measuring bias. 4.3. Fly populations Results (Fig. 1) show that moderate reductions in mature LW (10–20%) would have a relatively low impact on fly populations, especially if these reductions included a large proportion of heavier larvae that normally would remain trapped in the head cavities and thus be unable to exit the host because of their size. It is clear that such a reduction in fly population would affect both sexes. Unfortunately, no field information is available on male performance. For example, it is not known if there is a critical fly size for successful reproductive males. Without doubt, when dealing with adult populations, other factors causing fly reduction such as diseases, predation, and climatic factors among others must be considered. However, in future population studies, the importance of these and other neglected sources of variation may be clarified. It is important to note that reductions in adult populations showed in Fig. 1 may be temporary due to the high reproductive rate in this species which would allow populations to regenerate rapidly. Meleney et al. (1963) reported a reduction from 49.8 to 8.7 in larval burdens of sheep when systemic control treatments were applied to animals on an isolated range for a 5-year period. A follow-up study demonstrated that this isolated parasite population was regenerated in a few years (Meleney and Apodaca, 1969). Price (1975) pointed out that insect control methods should focus primarily on reducing progeny survival to less than two per female on an average rather than killing percentages of the population without considering the fecundity of the females. In H. tarandi, the number of eggs produced by the females depended slightly on LW, but fecundity was found to be independent of adult weight or size (Nilssen, 1997). Therefore, reductions in mature LW probably would affect the reproductive potential, but the true relationships between these two characteristics remain to be explored in O. ovis. Furthermore, our results need confirmation with appropriate experimental work. Experiments to confirm our predictions require availability of techniques to reduce the mature LW in this species which are currently being developed (Cepeda-Palacios et al., 2000). It is also necessary to know the field behavior of O. ovis and the energy expenses of breeding adults.
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Manipulating larval growth to decrease mature LW by immunological (affecting larval nutrition) or pharmacological (i.e. use of growth regulators) means appears to be a promising research approach to control O. ovis. From the data presented here, it is feasible to diminish and predict the subsequent fly population size, after reducing mature LW, taking advantage of close relationships between larval and fly dimensions. Reducing a third of the current mature LW may be an initial realistic goal when trying to affect the viability of O. ovis populations.
Acknowledgements The authors thank Dr. Phillip J. Scholl from Fort Dodge Animal Health at Princeton for providing a sample of laboratory-reared flies and Dr. Donald R. Anderson for reviewing the manuscript. References Atencio-Leon, A., Ramirez, A.J., 1972. Miasis cavitaria de las ovejas. Rev. Vet. Venez. 32, 164–168. Bart, A.G., Minár, J., 1992. Probability description of regulation on the level of population and individual in the host–parasite system using Oestrus ovis (Diptera: Oestridae) as an example. Folia Parasitol. 39, 75–83. Bennett, G.F., 1972. Observations on the pupal and adult stages of Cuterebra emasculator Fitch (Diptera: Cuterebridae). Can. J. Zool. 50, 1367–1372. Breev, K.A., Sultanov, F.R., 1975. On some developmental peculiarities of nasal sheep botfly larvae, Oestrus ovis L. Parasitologia 9 (translated from Russian). Cepeda-Palacios, R., Scholl, P.J., 1999. Gonotrophic development in Oestrus ovis (Diptera: Oestridae). J. Med. Entomol. 36, 435–440. Cepeda-Palacios, R., Jiménez, M.L., Armenta, J.A., 1998. Viabilidad del ‘gusano del cuerno’ Oestrus ovis L. (D´ıptera: Oestridae) durante los per´ıodos de prepupa y pupa. Mem. del XXXIII Congr. Nac. de Entomol., Soc. Mex. de Entomol. Acapulco, Gro., pp. 516–520. Cepeda-Palacios, R., Ávila, A., Ram´ırez-Orduña, R., Dorchies, Ph., 1999. Estimation of the growth patterns of Oestrus ovis L. larvae hosted by goats in Baja California Sur, Mexico. Vet. Parasitol. 80, 119–126. Cepeda-Palacios, R., Frugère, S., Prevot, F., Tabouret, G., Duranton, C., Bergeaud, J.P., Dorchies, Ph., Jacquiet, Ph., 2000. Immunization of lambs with excretory secretory products of Oestrus ovis third instar larvae and subsequent experimental challenge. Vet. Res., in press. Hinton, H.E., 1971. Some neglected phases in metamorphosis. Proc. R. Ent. Soc. London 35, 55–64. Johnston, L.A.Y., Eisemann, C.H., Donaldson, R.A., Pearson, R.D., Voucolo, T., 1992. Retarded growth of Lucilia cuprina larvae on sheep and their sera following production of an immune response. Int. J. Parasitol. 22, 187–193. Lello, E.D., Gregório, E.A., Toledo, L.A., 1985. Desenvolvimento das gônadas de Dermatobia hominis (Diptera: Cuterebridae), Vol. 80. Mem. Inst. Oswaldo Cruz, Rio de Janeiro, pp. 159–170. Meleney, W.P., Apodaca, S.A., 1969. Regeneration of a population of Oestrus ovis in sheep on an isolated range. J. Am. Vet. Med. Assoc. 155, 136–138. Meleney, W.P., Cobbett, N.G., Peterson, H.O., 1962. The natural occurrence of Oestrus ovis in sheep from the Southwestern United States. Am. J. Vet. Res. 23, 1246–1251. Meleney, W.P., Cobbett, N.G., Peterson, H.O., 1963. Control of Oestrus ovis in sheep on an isolated range. J. Am. Vet. Med. Assoc. 143, 986–989. Miller, J.H., Johnson, H.E., Stout, A.L., 1961. Effects of organic phosphoramidate (Ruelene) in control of nasal botfly in sheep. J. Am. Vet. Med. Assoc. 138, 431–433. Nilssen, A.C., 1997. Factors affecting size, longevity and fecundity in the reindeer oestrid flies Hypoderma tarandi (L.) and Cephenemyia trompe (Modeer). Ecol. Ent. 22, 294–304.
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Porchinskii, I.A., 1913. The sheep bot (Oestrus ovis L.). Its biology, characteristics, methods of control and relation to man. Trudy Biuro PO Ent. 10, 1–65 (translated from Russian). Price, P.W., 1975. Insect Ecology. Wiley, New York, pp. 137–143. StatSoft Inc., 1998. STATISTICA for Windows (Computer program manual). StatSoft Inc., Tulsa, OK. Whitten, J.M., 1969. Coordinated development in the fly foot: sequential cuticle secretion. J. Morph. 127, 73–104. Wood, D.M., 1987. Oestridae. In: Mc Alpine, J.F. (Ed.), Neartic Diptera, Monogr. No. 28, Vol. 2. Research Branch, Agriculture Canada, Ottawa, pp. 1147–1158. Zumpt, F., 1965. Myiasis in Man and Animals in the Old World. Butterworths, London, pp. 174–179.
Expected effects of reducing Oestrus ovis L. mature larval weight on adult populations R. Cepeda-Palacios a,∗ , S. Frugère b , Ph. Dorchies b a
Depto. de Zootecnia, Universidad Aut. de Baja California Sur, Km. 5.5 Carr. al Sur, P.O. Box 19-B, 23080 La Paz, Baja California Sur, Mexico b Lab. Associé de Physiopatologie Respiratoire des Ruminants, Ecole Nationale Vétérinaire, 23 Chemin des Capelles, 31076 Toulouse Cedex, France Received 7 October 1999; accepted 13 March 2000
Abstract In order to estimate the effects of eventual reductions in larval weight (LW) of Oestrus ovis L. as a measure of control, the correlation between mature LW and adult fly length (AL) in laboratory specimens (n=150) was calculated. The regression equation AL=5.62+10.65LW (r2 =0.76) was obtained. This equation was then applied to estimate the mature LW of wild larvipositing females (n=51) to predict the minimum mature LW at which fly viability would be compromised. The critical weight, 0.28 g (standard error limits 0.235, 0.323), was obtained from a small fly measuring 8.6 mm in length. Data from 383 mature third instars were used to estimate, by statistical analysis, the expected effects of decreasing the mature LW on subsequent fly population size. A considerable mean reduction (38%) in adult populations might be achieved by a 40% reduction of mature LW, but this eventual reduction may be temporary due to the high reproductive rate in this species. Sex differences in mature LW and fly size are also reported. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Oestrus ovis; Growth; Larva; Fly population; Larval weight
1. Introduction Larval weight (LW) has special importance in the Family Oestridae because these parasitic species of flies do not feed as adults, but must acquire during the larval stage all the energy reserves they will need during the pupal and adult stages (Wood, 1987). In the case of Oestrus ovis L., energy reserves in female flies are critical for mating, larval incubation, host-seeking and larviposition activities (Zumpt, 1965; Wood, 1987). Cepeda-Palacios et al. (1998) reported that, in O. ovis, weight and the degree of maturity of the third instars are associated with survival during the puparial period. LW of individuals that survived the puparial ∗ Corresponding author. Tel.: +52-112-8-08-02; fax: +52-112-8-08-80. E-mail address: [email protected] (R. Cepeda-Palacios)
0304-4017/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 0 1 7 ( 0 0 ) 0 0 2 3 7 - 5
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period was significantly higher than in those that did not survive; however, nonpigmented and slightly pigmented larvae were generally unable to survive outside the host. Likewise, low but significant correlations between LW and weight of adult female Hypoderma tarandi L. and Cephenemyia trompe Modeer have been reported (Nilssen, 1997). Populations of O. ovis are regulated by many factors, but establishment and survival of first instars in the host are of prime importance (Bart and Minár, 1992). Moreover, manipulating the larval growth of O. ovis inside the host in order to affect the viability of fly populations shows promise as a method of control. Reduction in the larval growth of Lucilia cuprina Weidemann using an immunological approach has been achieved (Johnston et al., 1992). However, no data about the relationship between mature LW and fly viability have been published for O. ovis, and therefore, no values of LW have been estimated that could be used as goals for manipulating larval growth. The objectives of this study were to analyze the relationship between larval and adult sizes of O. ovis, to estimate mature LWs at which fly viability may be compromised under natural conditions and to predict the possible impact of LW manipulation on populations of infecting flies. 2. Material and methods 2.1. Laboratory-reared adults (database 1) Mature O. ovis third instars were recovered from the sinus and horn cavities of slaughtered goats coming from the southern region of Baja California Sur, Mexico, to obtain 150 (81 male, 69 female) viable specimens. After collection, mature larvae were age-classified, weighed using an analytical balance and transferred to plastic jars partially filled with sterile sand to allow pupariation; the pupae were then transferred to vials. To classify the degree of maturation of the third instars, the scale described by Cepeda-Palacios et al. (1999) was used. Briefly, this scale is based on the advance of larval integumental pigmentation and includes six categories (namely, D1 . . . D6 ), where D1 (white larvae, posterior stigmas brilliant black, fully melanized) are the youngest, and D6 (fully dark-pigmented larvae) are the oldest. To measure the adults, removal of the puparial shell was performed 8 days post-pupariation (i.e. when head eversion had occurred) onwards up to emergence. During this period, the pharate (hidden) adult reaches the definitive adult size (Whitten, 1969; Hinton, 1971). Data of length (from the ptilinal suture to the posterior abdominal end) and width (mesothoracic width from a dorsal view, including the wing insertion) of pharate adults and newly emerged flies were obtained by direct measurement under the stereoscope, using a microrule. From collection to puparium removal, specimens were reared at a photoperiod of 12 h:12 h (L:D) and alternating temperatures of 32 and 16◦ C, respectively (Cepeda-Palacios and Scholl, 1999). 2.2. Wild gravid flies (database 2) During the springtime, females (n=51) were caught by hand while they were larvipositing on goats in two different flocks in the central region of Baja California Sur. Flies
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were preserved in alcohol in vials until measurement, which was accomplished as with laboratory-reared specimens. 2.3. Mature larvae (database 3) To estimate the impact of LW reductions on adult population viability, weights of 383 mature larvae showing variable degrees of maturity, including those from D3 (white or creamy larvae with black dorsal bands) to D6 (black-pigmented larvae, even in the intersegmental dorsal areas) were used as the distribution database. These third instars were obtained from goats coming from the same locality during 1997 and 1998. 2.4. Statistical procedures Firstly, using the measurements of laboratory-reared specimens (database 1), Pearson’s correlation coefficients of third instar weight, fly length, width, and product length×width (as an additional variable for analysis) were calculated. Fly length was found to be closely correlated with LW. Thus, the regression equation AL = 5.62 + 10.65LW (r 2 = 0.76, standard error of estimate = 0.47, P <0.001), where LW is the weight of the third instar and AL is the length of the pharate or emerged adult, was obtained. Data from males and females were pooled for this equation because fly length was found to be similar (P>0.05) between males (11.1±0.73 mm) and females (11.2±0.73 mm). Since both LW and AL are random variables, the relationship described by the equation was then used to estimate LWs and standard errors corresponding to the gravid wild flies of database 2. For our purposes, the smaller wild fly from database 2 was considered as the minimum size which was able to survive successfully and infect hosts, and therefore, its estimated LW was considered to be critical to produce viable adults. Finally, 10–50% reductions in LW and their lower and upper limits (determined by the standard error of the estimation) were calculated for database 3 values. Data of each weight reduction category (i.e. 10, 20, 30, 40, 50%) generated normal curves. Then, these calculated values were standardized as Z-values to estimate the proportion of individuals under the normal curve having weights below the critical weight mentioned above. Descriptive statistics of the size variables for the laboratory-reared and wild specimens were also calculated. Comparisons between sex groups of larvae and flies were carried out in the database of laboratory-reared specimens by student’s t-tests. All data analyses were made using the statistica software (StatSoft, 1998). 3. Results 3.1. Size of laboratory specimens Descriptive statistics for larval and adult measurements are shown in Table 1. In this database, a wide variation in mature LW was observed, averaging about half a gram and
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Table 1 Larval weight and adult size in laboratory-reared and wild Oestrus ovis Fly origina
Variableb
n
Mean
s
Range
Laboratory
Larval weight Fly length Fly width
150 150 147
0.498 11.2 5.2
0.08 0.72 0.54
0.328–0.671 9.1–12.5 3.7–7.0
Wild
Larval weight Fly length Fly width
51 51 50
0.431 10.2 4.2
0.04 0.45 0.41
0.280–0.551 8.6–11.5 2.8–4.9
a Laboratory specimens were male and female pharate adults and newly emerged flies. Wild specimens were gravid females caught while larvipositing on goats and their larval weight was estimated by a regression equation. b Fly sizes are in millimeters and larval weights are in grams.
95% confidence limits (not shown in Table 1) of 0.485 and 0.511 g. This variation resulted in size differences in adults. Although female (0.511 g) larvae tended to be heavier than males (0.489 g, P=0.08), both sexes had similar (P>0.05) adult length (11.1±0.7 mm males and 11.2±0.7 mm females). Confidence limits (95% level) for the overall mean fly length (11.2 mm) were 11.0 and 11.3 mm. Also, adult males (5.4±0.5 mm) were significantly (P<0.05) wider than adult females (5.2±0.6 mm). 3.1.1. Size of wild gravid flies Mean values of sizes for wild flies and estimated mean LW are shown in Table 1. Mean LW was lower in wild flies compared to laboratory-reared flies. This was expected because laboratory flies were generally larger than wild flies, and wild LW was less variable because
Fig. 1. Expected effects of decreasing the mature larval weight of Oestrus ovis on the subsequent fly population size. The upper and lower dotted lines represent these effects when the limits of the standard error of estimation were calculated. The wild weight corresponds to the mean weight of highly viable mature third instars.
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estimated LWs were adjusted by the regression equation. The lowest estimated LW was for a wild fly measuring 8.6 mm in length, which corresponded to a mature LW of 0.280 g; however, due to the standard error of the estimation, this value may be found to be between 0.235 and 0.323 g. The highest estimate was about 0.550 g, corresponding to the largest wild fly (11.5 mm long) caught in the period. 3.2. Expected effects of reduction in mature LW The probable effects of decreasing LW on a fly population are shown in Fig. 1. In wild populations, less than 1% of the larvae are expected to produce nonviable adult females, while 20 and 30% decreases in mature LW would result in 2 and 15%, respectively, of nonviable flies and so on. Furthermore, a 40% decrease in mature LW would cause a strong reduction of 39% (standard error limits of 12 and 80%) in the fly population.
4. Discussion 4.1. Mature LW Very limited data exist about the importance of mature LW and adult size in oestrid fitness and biology. Larval burden (the number of larvae hosted by a goat or sheep) can be affected by factors such as the number of larvae deposited by female flies, the age, body and health condition of the host, the capacity of sinusal cavities of the host, geographic locality and climatic conditions (Meleney et al., 1962). Likewise, various factors attributed to the host (sinus capacity, weight, and immunological status) or larval (genetic capacity of growth, sex) origin or an interaction of both (i.e. larval density in the host) may affect the mature LW in O. ovis (Cepeda-Palacios et al., 1999). In general, our results on LW agree with those obtained by Breev and Sultanov (1975), who reported that, in O. ovis third instars, it ranged from 0.140 to 0.522 g. Unfortunately, no data on the degree of larval maturity was given in this study. Nilssen (1997) determined highly significant correlations (r=0.34–0.44) between mature LW and adult weight in newly emerged males and females in both H. tarandi and C. trompe. In these species, sexual size dimorphism was noted in adults as well. Lello et al. (1985) reported that Dermatobia hominis Linnaeus Jr. mature female larvae are heavier than males and that LW by itself is a reliable characteristic to distinguish both sexes at this stage. Similar weight differences between mature female and male larvae have been reported for Cuterebra emasculator Fitch (Bennett, 1972). The estimated critical or ‘lethal’ weights (<0.28 g) for mature larvae correspond to that reached by the larvae during the early L3 period (Cepeda-Palacios et al., 1999), which is the period when the highest LW gains occur. In our experience, the survivability of mature larva after they leave the host depends not only on the LW but also on the degree of maturity. Although D1 and D2 (white larvae with slight black dorsal bands) larvae are very often heavy enough to continue the life cycle, they seldom exit the host because they lack the abilities to burrow into the soil and to complete pupariation, which are acquired as integumental pigmentation advances.
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4.2. Adult size After emergence, the adult female must face tasks that require critical energy reserves such as mating, larval incubation, host seeking, and larviposition activities and a proportion of the fly population may be reduced during each of these stages. Thus, females that are successful in larvipositing should have attained at least the critical mature LW to ensure meeting these energy expenses. The lowest fly length found in this study was 8.6 mm which is below the lowest fly length (10 mm) reported in the literature reviewed (Porchinskii, 1913; Miller et al., 1961; Zumpt, 1965; Atencio-Leon and Ramirez, 1972). Such a low fly length was not found in a sample of 15 laboratory-reared females coming from the San Angelo Texas zone, which averaged 10.4±0.4 mm (range 9.9–11.4 mm), values very close to those obtained in our study. A noteworthy aspect found in this study is the apparent difference in average length (1 mm) between flies from southern and central zones of Baja California Sur, which may be either due to unknown factors or due to measuring bias. 4.3. Fly populations Results (Fig. 1) show that moderate reductions in mature LW (10–20%) would have a relatively low impact on fly populations, especially if these reductions included a large proportion of heavier larvae that normally would remain trapped in the head cavities and thus be unable to exit the host because of their size. It is clear that such a reduction in fly population would affect both sexes. Unfortunately, no field information is available on male performance. For example, it is not known if there is a critical fly size for successful reproductive males. Without doubt, when dealing with adult populations, other factors causing fly reduction such as diseases, predation, and climatic factors among others must be considered. However, in future population studies, the importance of these and other neglected sources of variation may be clarified. It is important to note that reductions in adult populations showed in Fig. 1 may be temporary due to the high reproductive rate in this species which would allow populations to regenerate rapidly. Meleney et al. (1963) reported a reduction from 49.8 to 8.7 in larval burdens of sheep when systemic control treatments were applied to animals on an isolated range for a 5-year period. A follow-up study demonstrated that this isolated parasite population was regenerated in a few years (Meleney and Apodaca, 1969). Price (1975) pointed out that insect control methods should focus primarily on reducing progeny survival to less than two per female on an average rather than killing percentages of the population without considering the fecundity of the females. In H. tarandi, the number of eggs produced by the females depended slightly on LW, but fecundity was found to be independent of adult weight or size (Nilssen, 1997). Therefore, reductions in mature LW probably would affect the reproductive potential, but the true relationships between these two characteristics remain to be explored in O. ovis. Furthermore, our results need confirmation with appropriate experimental work. Experiments to confirm our predictions require availability of techniques to reduce the mature LW in this species which are currently being developed (Cepeda-Palacios et al., 2000). It is also necessary to know the field behavior of O. ovis and the energy expenses of breeding adults.
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Manipulating larval growth to decrease mature LW by immunological (affecting larval nutrition) or pharmacological (i.e. use of growth regulators) means appears to be a promising research approach to control O. ovis. From the data presented here, it is feasible to diminish and predict the subsequent fly population size, after reducing mature LW, taking advantage of close relationships between larval and fly dimensions. Reducing a third of the current mature LW may be an initial realistic goal when trying to affect the viability of O. ovis populations.
Acknowledgements The authors thank Dr. Phillip J. Scholl from Fort Dodge Animal Health at Princeton for providing a sample of laboratory-reared flies and Dr. Donald R. Anderson for reviewing the manuscript. References Atencio-Leon, A., Ramirez, A.J., 1972. Miasis cavitaria de las ovejas. Rev. Vet. Venez. 32, 164–168. Bart, A.G., Minár, J., 1992. Probability description of regulation on the level of population and individual in the host–parasite system using Oestrus ovis (Diptera: Oestridae) as an example. Folia Parasitol. 39, 75–83. Bennett, G.F., 1972. Observations on the pupal and adult stages of Cuterebra emasculator Fitch (Diptera: Cuterebridae). Can. J. Zool. 50, 1367–1372. Breev, K.A., Sultanov, F.R., 1975. On some developmental peculiarities of nasal sheep botfly larvae, Oestrus ovis L. Parasitologia 9 (translated from Russian). Cepeda-Palacios, R., Scholl, P.J., 1999. Gonotrophic development in Oestrus ovis (Diptera: Oestridae). J. Med. Entomol. 36, 435–440. Cepeda-Palacios, R., Jiménez, M.L., Armenta, J.A., 1998. Viabilidad del ‘gusano del cuerno’ Oestrus ovis L. (D´ıptera: Oestridae) durante los per´ıodos de prepupa y pupa. Mem. del XXXIII Congr. Nac. de Entomol., Soc. Mex. de Entomol. Acapulco, Gro., pp. 516–520. Cepeda-Palacios, R., Ávila, A., Ram´ırez-Orduña, R., Dorchies, Ph., 1999. Estimation of the growth patterns of Oestrus ovis L. larvae hosted by goats in Baja California Sur, Mexico. Vet. Parasitol. 80, 119–126. Cepeda-Palacios, R., Frugère, S., Prevot, F., Tabouret, G., Duranton, C., Bergeaud, J.P., Dorchies, Ph., Jacquiet, Ph., 2000. Immunization of lambs with excretory secretory products of Oestrus ovis third instar larvae and subsequent experimental challenge. Vet. Res., in press. Hinton, H.E., 1971. Some neglected phases in metamorphosis. Proc. R. Ent. Soc. London 35, 55–64. Johnston, L.A.Y., Eisemann, C.H., Donaldson, R.A., Pearson, R.D., Voucolo, T., 1992. Retarded growth of Lucilia cuprina larvae on sheep and their sera following production of an immune response. Int. J. Parasitol. 22, 187–193. Lello, E.D., Gregório, E.A., Toledo, L.A., 1985. Desenvolvimento das gônadas de Dermatobia hominis (Diptera: Cuterebridae), Vol. 80. Mem. Inst. Oswaldo Cruz, Rio de Janeiro, pp. 159–170. Meleney, W.P., Apodaca, S.A., 1969. Regeneration of a population of Oestrus ovis in sheep on an isolated range. J. Am. Vet. Med. Assoc. 155, 136–138. Meleney, W.P., Cobbett, N.G., Peterson, H.O., 1962. The natural occurrence of Oestrus ovis in sheep from the Southwestern United States. Am. J. Vet. Res. 23, 1246–1251. Meleney, W.P., Cobbett, N.G., Peterson, H.O., 1963. Control of Oestrus ovis in sheep on an isolated range. J. Am. Vet. Med. Assoc. 143, 986–989. Miller, J.H., Johnson, H.E., Stout, A.L., 1961. Effects of organic phosphoramidate (Ruelene) in control of nasal botfly in sheep. J. Am. Vet. Med. Assoc. 138, 431–433. Nilssen, A.C., 1997. Factors affecting size, longevity and fecundity in the reindeer oestrid flies Hypoderma tarandi (L.) and Cephenemyia trompe (Modeer). Ecol. Ent. 22, 294–304.
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