- Email: [email protected]
Parasitization of Culex fatigans by Nosema stegomyiae
JOURNAL
OF
INVERTEBRATE
Parasitization
PATHOLOGY
18,
of Culex
420 (1971)
futigans
Infection of Culex fatigans mosquitoes was obtained by hatching mosquito eggsin water to which a known concentration of Nosema stegomyiae spores had been added (D. G. Reynolds, Bull. Entomol. Res. 60 339, 1970). The results were assessedby calculating net reproduct’ion rates (R) for the experimental populations. R as used here indicates the total number of offspring produced by one female during t#he course of the experiment (70 days). R in the control populat#ion (exposed to no spores as newly hatched larvae) was 323; that of the population exposed to 12,000 spores/ml of larval breeding water was 198, a reduction from the control figure of 38.7 %; that of the population exposed to 18,000 spores/ml was 159, a reduction of 50.8%; and that of the populat,ion exposed to 24,000 spores/ml wets256, a reduction of 20.7 %. These reductions in R in t’he infected popuIat,ions were caused by two mechanisms: (1) a reduct’ion in egg production in the parasitized populations as a whole, and (2) an increase in the number of nonhatching eggs produced by the parasit,ized populations. The average number of eggs produced by each female in each population was as follows : control-376.2; 12,000 population236.8, a reduction of 37.0%; 18,000 population-212.0, a reduct#ion of 43.6%; and 24,000 population-350.1, a reduct’ion of 6.9% from the control figure. The proportions of embryonated eggs which did not hat’ch-1.7 % in the control population, 1.8 % in the 12,000 population, 2.0% in the 18,000 populat,ion, and 1.9% in the 24,000 populat,ion-were similar (x2 = 10.6179, p lies between 0.02 and 0.01). Greater differences were found in the numbers of eggs oviposited which were not embryonated; 12.9 % in the control population,
by Nosema
stegomyiae
15.3% in the 12,000 population, 23.7% in the 18,000 population, and 25.6% in the 24,000 population. r\Tonembryonation of eggs has been found to be due to lack of fertilization (B. De Meillon, A. Sebastian, and Z. H. Khan, Bull. W.H.O. 36 7, 1967). It is possible that nonfertilization is the cause in this case, perhaps induced by Arosema infections in the testes of males. Such infections were found in 62 of a sample of 137 males examined. iYo Nosemainfections were found in the unembryonated eggs, and transovarial transmission was not achieved despite repeated attempts. Parasitization did not have a marked effect on the rate of mortality in the experimental populations. At the end of week 7, for example, 36.0% of the control females were still alive, as were 24.1% of the 12,000 females, 31.2% of the 18,000 females, and 24.5% of the 24,000 females. The males of all populations did not live solong; there were no marked differences in the rates of mortalit,y. Infection rates in the four populations were as follows: Control, 0 of 360 mosquitoes were infected; 12,000 population, 288 of 399 mosquitoes were infected, an infection rate of 85.0%; 18,000 population, 274 of 315 mosquitoes were infected, 87.0 %; and in the 24,000 population 196 of 288 mosquitoes were infected, 67.9 %. It is considered, on the above evidence, bhat Nosema stegomyiae,at the spore concentrat’ions used, is of little value as a potential biological control agent of Culex fatigans. D. G. REYNOLDS Medical and Health Department Victoria, Seychelles
429
ReceivedOctober 8, i.970
OF
INVERTEBRATE
Parasitization
PATHOLOGY
18,
of Culex
420 (1971)
futigans
Infection of Culex fatigans mosquitoes was obtained by hatching mosquito eggsin water to which a known concentration of Nosema stegomyiae spores had been added (D. G. Reynolds, Bull. Entomol. Res. 60 339, 1970). The results were assessedby calculating net reproduct’ion rates (R) for the experimental populations. R as used here indicates the total number of offspring produced by one female during t#he course of the experiment (70 days). R in the control populat#ion (exposed to no spores as newly hatched larvae) was 323; that of the population exposed to 12,000 spores/ml of larval breeding water was 198, a reduction from the control figure of 38.7 %; that of the population exposed to 18,000 spores/ml was 159, a reduction of 50.8%; and that of the populat,ion exposed to 24,000 spores/ml wets256, a reduction of 20.7 %. These reductions in R in t’he infected popuIat,ions were caused by two mechanisms: (1) a reduct’ion in egg production in the parasitized populations as a whole, and (2) an increase in the number of nonhatching eggs produced by the parasit,ized populations. The average number of eggs produced by each female in each population was as follows : control-376.2; 12,000 population236.8, a reduction of 37.0%; 18,000 population-212.0, a reduct#ion of 43.6%; and 24,000 population-350.1, a reduct’ion of 6.9% from the control figure. The proportions of embryonated eggs which did not hat’ch-1.7 % in the control population, 1.8 % in the 12,000 population, 2.0% in the 18,000 populat,ion, and 1.9% in the 24,000 populat,ion-were similar (x2 = 10.6179, p lies between 0.02 and 0.01). Greater differences were found in the numbers of eggs oviposited which were not embryonated; 12.9 % in the control population,
by Nosema
stegomyiae
15.3% in the 12,000 population, 23.7% in the 18,000 population, and 25.6% in the 24,000 population. r\Tonembryonation of eggs has been found to be due to lack of fertilization (B. De Meillon, A. Sebastian, and Z. H. Khan, Bull. W.H.O. 36 7, 1967). It is possible that nonfertilization is the cause in this case, perhaps induced by Arosema infections in the testes of males. Such infections were found in 62 of a sample of 137 males examined. iYo Nosemainfections were found in the unembryonated eggs, and transovarial transmission was not achieved despite repeated attempts. Parasitization did not have a marked effect on the rate of mortality in the experimental populations. At the end of week 7, for example, 36.0% of the control females were still alive, as were 24.1% of the 12,000 females, 31.2% of the 18,000 females, and 24.5% of the 24,000 females. The males of all populations did not live solong; there were no marked differences in the rates of mortalit,y. Infection rates in the four populations were as follows: Control, 0 of 360 mosquitoes were infected; 12,000 population, 288 of 399 mosquitoes were infected, an infection rate of 85.0%; 18,000 population, 274 of 315 mosquitoes were infected, 87.0 %; and in the 24,000 population 196 of 288 mosquitoes were infected, 67.9 %. It is considered, on the above evidence, bhat Nosema stegomyiae,at the spore concentrat’ions used, is of little value as a potential biological control agent of Culex fatigans. D. G. REYNOLDS Medical and Health Department Victoria, Seychelles
429
ReceivedOctober 8, i.970