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The effect of Azolla on mosquito breeding
328
Parasitology Today, vol. 4, no. I I, 1988
for literature searching. Scientists should take advantage of the ease and costeffectiveness of online searching, but it is only one of the weapons in the armour/ for keeping up-to-date.
N e w Databases New databases are appearing at a rate that cannot sustain the commercial survival of them all, particularly as there is an increasing tendency for databases to cover narrower, more specialized subjects. For example, there are now databases devoted to subjects such as nudeotide sequencing (Nucleotide Sequence Search System), drug sideeffects (SEDBASE) and AIDS (AIDS). The more specialized the database, the more precarious will be its commercial future, and some producers of small databases have pooled their files to produce more attractive packages. The use of compact disk technology (CD-ROM) to mount bibliographic databases is now
a rapidly expanding field, and already most of the major databases such as MEDLINE, SCISEARCH and EMBASE are available in this medium 8. CD-ROM offers most of the advantages of online searching without the worry and expense of accessing remote hosts; searches can be conducted in a more leisurely fashion with more opportunities for 'browsing'. The medium may be particularly appropriate for use in developing countries where access via satellite telecommunications to remote databases may not be available. However, costs of individual CD-ROM packages are still high, making them more suitable for institutional or library purchase. New copyright legislation is being enacted in some areas of the world which will affect downloading of references for storage and reuse in in-house databases (text retrieval software makes this practice relatively simple). Individuals who store downloaded references in their own microcomputers should seek permission from database producers -
payment is usually required. In the UK, registration under the Data Protection Act is also advisable if references are to be stored in this way.
References I de Solla Price, D. (1981) in Coping with the BiomedicalLiterature: a Primerfor the Scientist and Clinician (Warren, K.S., ed.), pp 3-16, Praeger 2 Warren, I~S.( 1981) in Copingwith the Biomedical Literature: a Primer for the Scientist and Clinician(Warren,K.S.,ed.),pp 17-30,Praeger 3 Hall,J.L.(1987)OnlineBibliographicDatabases: a DirectoryandGuide(4thedn)Aslib 4 Farbey, R. (I 987) Medical Databases 1988 Aslib S Snow,B.(1985)Database8, 15-44 6 Turpie,G. (1987)GoingOnline 1988Aslib 7 Armstrong, C.J. and Large,J.A. (eds) (1988) Manualof OnlineSearchStrategiesGower 8 Churchill, K. (ed.) (1987) The CD-ROM DirectoryTFPLPublishing John Eyers is Readers' Services Librarian at the London School of Hygiene and Tropical Medicine, Keppel Street, London WC I E 7HT, UK. Angela Taylor is with the Bureau of Hygiene and Tropical Diseases,based at the same address.
The Effect of m Mosquito Breeding Lu Bao-lin Surface agents to smother mosquito breeding sites have been used extensively for mosquito larval control. Such agents range from oil films and chemical monolayers, to expanded polystyrene beads ~'2. Here, Lu Bao-lin discusses the use 0fAzolla - a floating fern - for a similar effect. The fern has the additional advantage of providing a useful source of green manure as a crop fertilizer. Azolla is a genus of free-floating ferns, belonging to the family Azollaceae. It consists of six species - A. caroliniana, A. filiculoides, A. rnexicana, A. microphylla, A. nilotica and A. pinnata. The fern lives in association with a blue-green algal symbiont, Anabaena azolla, which fixes enough atmospheric nitrogen for rapid growth of the plant. Thus the AzollaAnabaena complex offers a source of organic fertilizer. The mineralization rate of nitrogen fixed by the AzollaAnabaena complex differs among species and strains, but it can produce up to 1000 kg of green manure per hectare per day 3 and supply 70 kg nitrogen per hectare when grown 4-6 times per rice crop. This nitrogen promotes rice yields comparable to that obtained by 70- 100 kg/ha of chemical fertilizer 4. Azolla has been used in China and other countries as a means to improve fertility of rice paddy soil and as an animal feed. In 1983,
in the Sichuan province of SW China alone, the total area in which Azolla was associated with rice cultivation exceeded 130 000 has. In the past, the question as to whether the coverage of standing water by freefloating plants, including Azolla, could be detrimental to mosquito breeding has been frequently raised 6 7' . However, no detailed study has been carried out to test this idea under experimental conditions. Recently, it was reported from
Guatemala that the aquatic fern Salvinia auriculata had a marked inhibitory effect on anopheline breeding 8. Cage tests indicated that floating vegetation could be a barrier to oviposition of gravid Anopheles albimanus. Studies of the effect of Azolla have since been reported from China, Japan and Sri Lanka9-11 . In China, laboratory studies of the effect of Azolla filiculoides on mosquito breeding have been carried out in cages under well-controlled conditions. Complete coverage of the water surface by this plant had an inhibitory effect on the oviposition of Culex tritaeniorhynchus, the principal vector of Japanese encephalitis9, Further studies under similar conditions not only confirmed the previous
Table I. The effect ofA. filiculoides on the oviposition of some culicine mosquitoes Number of egg rafts in different oviposition containers Number of Complete Mosquito species replicates coverase 2/3Coverage Control Culexmodestus I0 13 480 901 Cx paUens 7 0 64 223 Cx quinquefasciatus 8 0 105 429 Cx tritaeniorhynchus I0 I 136 201 ~) 1988, ElsevierScience Publishers Ltd, (UK) 0165-6147188/$02.00
329
Parasitology Today, voL 4, no. I I, 1988
results, but also indicated a similar inhibitory effect on the oviposition of other culicines II (Table I). During outdoor observations made on artificial containers (50 cm in diameter) with 100%, 50%, 25% and 0% coverage by A. caroliniana in Guangzhou the number of recovered egg rafts of Cx. quinquefasciatus was 3, 25, 6'9 and 105, respectively (Cui Kelen, pers. commun.). Oviposition of this species was totally suppressed in studies in Sri Lanka when Azolla completely covered the water surface, both in small laboratory jars and under natural conditions in an outdoor tank ~0. Similar studies showed no significant influence on the oviposition of Anopheles sinensis, the important vector of malaria and brugian filariasis in China, nor on Aedes albopictus, an important dengue vector (Refi II; Cai Kelen, pers. commun.). However, although the,. coverage by Azolla could not prevent the oviposition of An. sinensis, it did affect the emergence of pupae. Thus the emergence rate of pupae in the completecoverage container was 15.3%, as opposed to 95.8% in the control ones ~J. No effect of intact Azolla present in the water containers was observed on the larval mortality of either An. sinensis or Cx. tritaeniorhynchus 9. However, Mogi et al. ~2, in their studies on the survival of immature, rice-field Anopheles, reported that A. pinnata (=imbricata) increased the mortality among An. peditaeniatus and An. sinensis in the laboratory/. Since Azolla is widely used in paddy fields, as mentioned above, its effect on mosquito breeding is primarily aimed at the rice-field breeders, such as An. sinensis and Cx. tritaeniorhynchus in China t3. Field observations in China showed a significant reduction in larval density in rice fields covered by Azolla (Peng and Xi, unpublished; Zhang, Liu and Sun, unpublished). A recent study in Guidung County of Hunan Province, China I~, where nearly half the rice paddies (850 ha) had grown Azolla (mostly A. filicuIoides) showed that the larval density of Cx, tritaeniorhynchus was markedly lowered after 75% coverage of water surface by the plant, and that larval density was negatively correlated to the degree of surface coverage. The average reduction in larval density was 68.7% from June to August. The effect w~s not so marked for the larval density of An. sinensis, but there was a similar reduction in pupal density for both species. At present, it seems that complete coverage of water surfaces by Azolla can affect the breeding of some culicine mosquitoes by acting as a barrier to oviposition. Such effects are less marked
period' in China. We know that the growth rate of Azolla varies in different species or strains, the time taken to double its original mass ranging from 2.78 to 9.01 days4. Therefore, species or strains should be selected to give approi priate growth rates to coincide with rice-cultivation practice and seasonal variation in mosquito breeding cycles. In addition, management of other insect pests that can damage the Azolla may also require study.
Fig. I. Drawing ofAzolla flliculoides var. rubra from the ventral surface.
for An. sinensis, but the fern does appear to inhibit adult emergence in this species. Thus, cultivation of Azolla may represent a useful adjunct to mosquito management in rice fields. Azolla is readily cultivated by the community for fertilizer and animal feed, so its effect on mosquito control is really an extra benefit. However, to optimize its effects for ricefield mosquito control there are several operational questions to be considered. For example, since mosquito breeding is mainly affected only after complete surface coverage by the plant, the timing of plant cultivation is of prime importance. Complete coverage should be reached before peak mosquito b r e e d i n g - ie. near the end of the 'turning-green
References I Curtis, C.F. and Minjas,J. (1985) Parasitology Today I, 36 2 Sharma, V.P. (1987) Parasitology Today 3, 222-226 3 FAO (1978)FAOBull. 4 I, 81 pp 4 Watanabe,I.et al. (I 981) IRRIRes.PaperSer. 69, II pp 5 ZhangChunlin(I 985) Poster Abstract (Workshop on AzollaUse), p. 2 6 Li FengsunandWuShucheng(1934)TheMethods of Mosquito Control ShanghaiCommercial Press(in Chinese) 7 Macgregor,M.E.( 1920)J.R. Army Med. Corps 34, 370-372 8 Hobbs, J.H. and Molina, P.A. (1983) Mosq. News43,456-459 9 Anonymous(1986)PEEMNewsletter 15, 3-4 10 Amerasinghe,F.P.and Kulasooriya,S.A.(1986) Mircen.J. 12,355-363 I I Lu, B. et al. (1987) ChineseJ. Biol. Contr. 3, 160-162(inChinese) 12 Mogi, M. et al. (1986)J. Med, EntomoL 23, 244-250 13 Buh, P. (1983) PEEM Doc. EPO/PEAIVP/93.SA, 7 pp 14 Hu, Y. et aL (1988) ChineseJ. BioL Contr. (in press)(in Chinese) Professor Lu Bao-lin is at the Academy of Military Medical Sciences, PO Box 130, Beijing 100850, People's Republic of China.
Parasitology Today, vol. 4, no. I I, 1988
for literature searching. Scientists should take advantage of the ease and costeffectiveness of online searching, but it is only one of the weapons in the armour/ for keeping up-to-date.
N e w Databases New databases are appearing at a rate that cannot sustain the commercial survival of them all, particularly as there is an increasing tendency for databases to cover narrower, more specialized subjects. For example, there are now databases devoted to subjects such as nudeotide sequencing (Nucleotide Sequence Search System), drug sideeffects (SEDBASE) and AIDS (AIDS). The more specialized the database, the more precarious will be its commercial future, and some producers of small databases have pooled their files to produce more attractive packages. The use of compact disk technology (CD-ROM) to mount bibliographic databases is now
a rapidly expanding field, and already most of the major databases such as MEDLINE, SCISEARCH and EMBASE are available in this medium 8. CD-ROM offers most of the advantages of online searching without the worry and expense of accessing remote hosts; searches can be conducted in a more leisurely fashion with more opportunities for 'browsing'. The medium may be particularly appropriate for use in developing countries where access via satellite telecommunications to remote databases may not be available. However, costs of individual CD-ROM packages are still high, making them more suitable for institutional or library purchase. New copyright legislation is being enacted in some areas of the world which will affect downloading of references for storage and reuse in in-house databases (text retrieval software makes this practice relatively simple). Individuals who store downloaded references in their own microcomputers should seek permission from database producers -
payment is usually required. In the UK, registration under the Data Protection Act is also advisable if references are to be stored in this way.
References I de Solla Price, D. (1981) in Coping with the BiomedicalLiterature: a Primerfor the Scientist and Clinician (Warren, K.S., ed.), pp 3-16, Praeger 2 Warren, I~S.( 1981) in Copingwith the Biomedical Literature: a Primer for the Scientist and Clinician(Warren,K.S.,ed.),pp 17-30,Praeger 3 Hall,J.L.(1987)OnlineBibliographicDatabases: a DirectoryandGuide(4thedn)Aslib 4 Farbey, R. (I 987) Medical Databases 1988 Aslib S Snow,B.(1985)Database8, 15-44 6 Turpie,G. (1987)GoingOnline 1988Aslib 7 Armstrong, C.J. and Large,J.A. (eds) (1988) Manualof OnlineSearchStrategiesGower 8 Churchill, K. (ed.) (1987) The CD-ROM DirectoryTFPLPublishing John Eyers is Readers' Services Librarian at the London School of Hygiene and Tropical Medicine, Keppel Street, London WC I E 7HT, UK. Angela Taylor is with the Bureau of Hygiene and Tropical Diseases,based at the same address.
The Effect of m Mosquito Breeding Lu Bao-lin Surface agents to smother mosquito breeding sites have been used extensively for mosquito larval control. Such agents range from oil films and chemical monolayers, to expanded polystyrene beads ~'2. Here, Lu Bao-lin discusses the use 0fAzolla - a floating fern - for a similar effect. The fern has the additional advantage of providing a useful source of green manure as a crop fertilizer. Azolla is a genus of free-floating ferns, belonging to the family Azollaceae. It consists of six species - A. caroliniana, A. filiculoides, A. rnexicana, A. microphylla, A. nilotica and A. pinnata. The fern lives in association with a blue-green algal symbiont, Anabaena azolla, which fixes enough atmospheric nitrogen for rapid growth of the plant. Thus the AzollaAnabaena complex offers a source of organic fertilizer. The mineralization rate of nitrogen fixed by the AzollaAnabaena complex differs among species and strains, but it can produce up to 1000 kg of green manure per hectare per day 3 and supply 70 kg nitrogen per hectare when grown 4-6 times per rice crop. This nitrogen promotes rice yields comparable to that obtained by 70- 100 kg/ha of chemical fertilizer 4. Azolla has been used in China and other countries as a means to improve fertility of rice paddy soil and as an animal feed. In 1983,
in the Sichuan province of SW China alone, the total area in which Azolla was associated with rice cultivation exceeded 130 000 has. In the past, the question as to whether the coverage of standing water by freefloating plants, including Azolla, could be detrimental to mosquito breeding has been frequently raised 6 7' . However, no detailed study has been carried out to test this idea under experimental conditions. Recently, it was reported from
Guatemala that the aquatic fern Salvinia auriculata had a marked inhibitory effect on anopheline breeding 8. Cage tests indicated that floating vegetation could be a barrier to oviposition of gravid Anopheles albimanus. Studies of the effect of Azolla have since been reported from China, Japan and Sri Lanka9-11 . In China, laboratory studies of the effect of Azolla filiculoides on mosquito breeding have been carried out in cages under well-controlled conditions. Complete coverage of the water surface by this plant had an inhibitory effect on the oviposition of Culex tritaeniorhynchus, the principal vector of Japanese encephalitis9, Further studies under similar conditions not only confirmed the previous
Table I. The effect ofA. filiculoides on the oviposition of some culicine mosquitoes Number of egg rafts in different oviposition containers Number of Complete Mosquito species replicates coverase 2/3Coverage Control Culexmodestus I0 13 480 901 Cx paUens 7 0 64 223 Cx quinquefasciatus 8 0 105 429 Cx tritaeniorhynchus I0 I 136 201 ~) 1988, ElsevierScience Publishers Ltd, (UK) 0165-6147188/$02.00
329
Parasitology Today, voL 4, no. I I, 1988
results, but also indicated a similar inhibitory effect on the oviposition of other culicines II (Table I). During outdoor observations made on artificial containers (50 cm in diameter) with 100%, 50%, 25% and 0% coverage by A. caroliniana in Guangzhou the number of recovered egg rafts of Cx. quinquefasciatus was 3, 25, 6'9 and 105, respectively (Cui Kelen, pers. commun.). Oviposition of this species was totally suppressed in studies in Sri Lanka when Azolla completely covered the water surface, both in small laboratory jars and under natural conditions in an outdoor tank ~0. Similar studies showed no significant influence on the oviposition of Anopheles sinensis, the important vector of malaria and brugian filariasis in China, nor on Aedes albopictus, an important dengue vector (Refi II; Cai Kelen, pers. commun.). However, although the,. coverage by Azolla could not prevent the oviposition of An. sinensis, it did affect the emergence of pupae. Thus the emergence rate of pupae in the completecoverage container was 15.3%, as opposed to 95.8% in the control ones ~J. No effect of intact Azolla present in the water containers was observed on the larval mortality of either An. sinensis or Cx. tritaeniorhynchus 9. However, Mogi et al. ~2, in their studies on the survival of immature, rice-field Anopheles, reported that A. pinnata (=imbricata) increased the mortality among An. peditaeniatus and An. sinensis in the laboratory/. Since Azolla is widely used in paddy fields, as mentioned above, its effect on mosquito breeding is primarily aimed at the rice-field breeders, such as An. sinensis and Cx. tritaeniorhynchus in China t3. Field observations in China showed a significant reduction in larval density in rice fields covered by Azolla (Peng and Xi, unpublished; Zhang, Liu and Sun, unpublished). A recent study in Guidung County of Hunan Province, China I~, where nearly half the rice paddies (850 ha) had grown Azolla (mostly A. filicuIoides) showed that the larval density of Cx, tritaeniorhynchus was markedly lowered after 75% coverage of water surface by the plant, and that larval density was negatively correlated to the degree of surface coverage. The average reduction in larval density was 68.7% from June to August. The effect w~s not so marked for the larval density of An. sinensis, but there was a similar reduction in pupal density for both species. At present, it seems that complete coverage of water surfaces by Azolla can affect the breeding of some culicine mosquitoes by acting as a barrier to oviposition. Such effects are less marked
period' in China. We know that the growth rate of Azolla varies in different species or strains, the time taken to double its original mass ranging from 2.78 to 9.01 days4. Therefore, species or strains should be selected to give approi priate growth rates to coincide with rice-cultivation practice and seasonal variation in mosquito breeding cycles. In addition, management of other insect pests that can damage the Azolla may also require study.
Fig. I. Drawing ofAzolla flliculoides var. rubra from the ventral surface.
for An. sinensis, but the fern does appear to inhibit adult emergence in this species. Thus, cultivation of Azolla may represent a useful adjunct to mosquito management in rice fields. Azolla is readily cultivated by the community for fertilizer and animal feed, so its effect on mosquito control is really an extra benefit. However, to optimize its effects for ricefield mosquito control there are several operational questions to be considered. For example, since mosquito breeding is mainly affected only after complete surface coverage by the plant, the timing of plant cultivation is of prime importance. Complete coverage should be reached before peak mosquito b r e e d i n g - ie. near the end of the 'turning-green
References I Curtis, C.F. and Minjas,J. (1985) Parasitology Today I, 36 2 Sharma, V.P. (1987) Parasitology Today 3, 222-226 3 FAO (1978)FAOBull. 4 I, 81 pp 4 Watanabe,I.et al. (I 981) IRRIRes.PaperSer. 69, II pp 5 ZhangChunlin(I 985) Poster Abstract (Workshop on AzollaUse), p. 2 6 Li FengsunandWuShucheng(1934)TheMethods of Mosquito Control ShanghaiCommercial Press(in Chinese) 7 Macgregor,M.E.( 1920)J.R. Army Med. Corps 34, 370-372 8 Hobbs, J.H. and Molina, P.A. (1983) Mosq. News43,456-459 9 Anonymous(1986)PEEMNewsletter 15, 3-4 10 Amerasinghe,F.P.and Kulasooriya,S.A.(1986) Mircen.J. 12,355-363 I I Lu, B. et al. (1987) ChineseJ. Biol. Contr. 3, 160-162(inChinese) 12 Mogi, M. et al. (1986)J. Med, EntomoL 23, 244-250 13 Buh, P. (1983) PEEM Doc. EPO/PEAIVP/93.SA, 7 pp 14 Hu, Y. et aL (1988) ChineseJ. BioL Contr. (in press)(in Chinese) Professor Lu Bao-lin is at the Academy of Military Medical Sciences, PO Box 130, Beijing 100850, People's Republic of China.