Document(s) 5 of 41

More than 100 species of fish eat mosquito larva, and many authors have suggested species that are good at eating mosquito larva. In 1959, Chen Jiangxing and Gao Kai achieved good results using common carp fry (Cyprinus carpio), and in 1979, the Antiepidemic Station of Chengdu City, Sichuan Province, used silver crucian carp (Carassius auratus) and grass carp (Ctenopharyngodon idella) to eliminate mosquito larva. In 1976, the Antimalaria Group of Henan Province reported good results when fish were raised in ricefields to control mosquitoes. Although Dambusia affinis, Panchax panchax, Mocropodes apercularis, and Pseudorasbora parva are good for mosquito control, they are difficult to breed and there is a limited supply of fry. In addition, these fish species are of little economic value and are difficult to popularize.

There have been no reports on the ability of such fish as grass carp, silver common carp (first generation of crosses between red carp and silver crucian carp), and nile tilapia (Oreochromis niloticus) to eat mosquito larva. Earlier indoor experiments reported by the World Health Organization were mostly conducted with fish that were starved for 1–2 days before the mosquito larva were introduced.

A comparative experiment was carried out to determine the differences between the amount of food taken by starved fish and by those reared in natural conditions. Three common types of fish raised in ricefields were studied. The density of mosquito larva in one midseason ricefield and two late ricefields where grass carp were reared was also measured.

Materials and Methods

The research was conducted both indoors and in the field. The experiment on the feeding rate was done indoors under controlled conditions. Fish of different sizes were raised separately in a white round drum (40 cm in diameter, 20 cm of water). To test the food intake of hungry fish, the fish was starved for 24 h after they had adapted to their new environment. To test the food intake of fish under natural conditions, the feed most liked by fish was added after the fish had adapted to their new environment. Mosquito larva were added in batches. The amount of feed added or unconsumed, and the number of mosquito larva of different ages eaten by the fish, were recorded.

The survey of the density of mosquito larva in ricefields with and without fish was carried out by collecting water samples with a 500 ml aluminium ladle. If water depth in the ricefields is assumed to be 6 cm, every hectare of ricefield stores 600 m3 of water, which is equal to 1 200 000 of the ladles used to collect samples. A sample consisted of 150 ladles of water collected along the banks of a field. The mosquito larva were separated from the water using a glass pipette and fixed in a bakelite-capped tube that contained 30% alcohol. The larva were counted according to three categories: Anopheles sinensis, Culex spp., and others. Between July and August 1984, four samples were collected once every 2 weeks from the middle ricefield. In 1985, the survey on a large area of two seasons of late rice was conducted.

The survey of the mosquito larva density in the middle ricefield was conducted in the Lianhu Fish Farm, Mianyang County, Hubei Province. The farm had 57.7 ha of intensive fish-farming ponds and 31.7 ha of ricefields in continuous blocks. The ricefields were surrounded by 1.6 ha of ditches. The ricefields were divided into four blocks and managed by four different fish-farming teams. The area of the four blocks of ricefields was 1.8 ha and the three fields were about 7.8 ha each. The adjacent 5.3 ha of ricefields in which fish were not raised were used as the control field. The strains of rice in the fields were different, but all were middle rice (the distance between rows and plants was 13 cm x 20 cm). Tillage was not done in ricefields in which fish were raised. No fertilizer was applied, and pesticides were used only when necessary.

On 18 April 1984, sodium pentachlorophenate was used to eliminate weed, fish, and leeches from the ricefields, and fry were raised in the 0.8 ha of straight ditches around the four blocks of ricefields. On 19 May, 1.1 million grass carp fry were introduced. On 23 June, 2.6 million summer grass carp fingerlings were collected. In late June, 390 000 summer grass carp fingerling and 30 000 silver crucian carp fingerlings were released into the rice paddies (average 13 230/ha or 1–2/m²). In ricefields with fish, fish ditches were dug. No feed was put into the ricefields. The surveys of the density of mosquito larva in the two seasons of late rice where grass carp was raised and in the control fields were conducted on 10 ha in Chongyang County, Hubei Province.

Results

The food intake of hungry fish and fish under natural conditions were quite different. The number of mosquito larva eaten by grass carp under natural conditions was 73.4% of the intake when the fish were hungry (Table 82). Silver crucian carp consumed 36.3% under natural conditions compared with hungry conditions (Table 83). The number of mosquito larva consumed by nile tilapia under natural conditions was only 32.5% of that under hungry conditions (Table 84). Nile tilapia showed the greatest differences in intake under the two conditions, followed by silver crucian carp and grass carp. In the indoor experiment, grass carp almost always preferred mosquito larva to duckweed when they were fed together. This may explain why grass carp normally eat more mosquito larva than the other two species. Tables 83.and 84 show that when feed is given, the silver crucian carp and nile tilapia eat more fish powder than mosquito larva. The intake of mosquito larva is not correlated with increased body size (Tables 82–84). In both groups, grass carp eat the greatest number of mosquito larva, followed by silver crucian carp and nile tilapia.

Table 82. Intake of mosquito larva by grass carp when hungry (H) and under normal feeding (NF) conditions.
	Fish		Feed in 24 h		Intake in 24 h		Intake of each fish (no.)	Ratio of normal to hungry feeding (%)
	No.	Size (cm)	Larva (no.)	Duckweed (g)	Larva (no.)	Duckweed (g)
NF	10	4.7	3650	50	2360	9.0	236
H	10	4.1–5.2	4000	—	3130	—	313	75.4
NF	10	4.7	3500	50	3363	8.5	336
H	10	4.1–5.2	4800	—	4670	—	467	72.0

Table 83. Intake of mosquito larva by silver crucian carp when hungry (H) and under normal feeding (NF) conditions.
	Fish		Feed in 24 h		Intake in 24 h		Intake of each fish (no.)	Ratio of normal to hungry feeding (%)
	No.	Size (cm)	Larva (no.)	Fish powder (g)	Larva (no.)	Fish powder (g)
NF	10	5.0	3000	14	742	11.5	72
H	10	5.0	4000	—	2016	2	202	36.8
NF	9	5.0	2350	10	1290	2	143
H	9	5.0	4000	—	3575	—	397	36.1

Table 84. Intake of mosquito larva by nile tilapia when hungry (H) and under normal feeding (NF) conditions.
	Fish		Feed in 24 h		Intake in 24 h		Intake of each fish (no.)	Ratio of normal to hungry feeding (%)
	No.	Size (cm)	Larva (no.)	Fish powder* (g)	Larva (no.)	Fish powder (g)
NF	10	4.4–6.0	2000	—	790	2.05	79
H	10	4.4–6.0	5000	—	2802	—	280	28.2
NF	10	4.4–6.0	1500	—	1002	1.85	100
H	10	4.4–6.0	3800	—	2709	—	271	37.0
* Figures missing in original Chinese publication.

The number of Culex spp. larvae consumed can be determined by counting the respiratory ducts that survive digestion intact. During the experiment, of the 300 Culex larva fed to the fish, only 261 respiratory ducts were recovered (253 from fish excrement and 8 from the sediment). This is only 87% of the number consumed. This suggests that when the number of mosquito larva consumed by fish is assessed, the number should be corrected by 13%.

The field surveys of the middle ricefields in Mianyang County (Table 85) show an average of 32 000 larvae when no fish were present, and one-third of that number when fish were present. The fields without fish had four times the number of Anopheles sinensis and five times the number of Culex spp., but the same number of other mosquito larvae. In a more extensive survey in Chongyang County (Table 86), the fields with grass carp had 900 larvae per hectare compared with 66 700 in the fields without fish (a difference of 99%). In this area, only one field with fish had any mosquito larva. This field monitoring clearly shows that grass carp can eliminate large numbers of mosquito larvae from ricefields.

Table 85. Mosquito larva density in middle ricefields with and without fish culture.
Date	Middle-rice	Anopheles spp. (no./ha x 1000)	Culex spp. (no./ha x 1000)	Others (no./ha x 1000)	Total (no./ha x 1000)
12 July 1984	With fish	8 II.	8 II.	8	24
	Without fish	24 II.III.VI	16 II.III.	0	40
25 July 1984	With fish	8 II.	0	0	8
	Without fish	16 II.III.	0	8	24
12 Aug 1984	With fish	0	0	8	8
	Without fish	8 II.	16 II.II.	16	40
24 Aug 1984	With fish	0	0	8	8
	Without fish	16 II.II.	8 III.	0	24
Average	With fish	4	2	6	12
	Without fish	16	10	6	32

Table 86. Density of mosquito larva in two seasons of late ricefields with and without fish culture, Chongyang County.
Location	Ricefields	Anopheles spp. (no./ha x 1000)	Culex spp. (no./ha x 1000)	Others (no./ha x 1000)	Total (no./ha x 1000)
Agro-Science Institute	Rice–fish culture				0
Xiexing village, Taishan township	Rice–fish culture				0
Xiaxing village, Shaping district	Rice–fish culture		8 IV		8
Wugang village, Shaping district	Rice–fish culture				0
Bailuo village group 6, Guikou township	Rice–fish culture				0
Bailuo village group 9, Guikou township	Rice–fish culture				0
Nanlin village, Qingshan town	Rice–fish culture				0
Taiping village, Huaqi township	Rice–fish culture				0
Lukou village group 9, Lukou town	Rice–fish culture				0
Bailuo village, Shaping district	Without fish	96 I.V.	24 II	8 I.	128
Nanlin village, Qingshan town	Without fish		16 II.IV	24 IV.II.II	40
Lukou village group 9, Lukou town	Without fish	16 II.IV		16 II.III	32
Average	Rice–fish culture	0	0.9	0	0.9
	Without fish	37.3	13.3	16	66.6

Discussion

Grass carp more effectively eliminate weeds and mosquito vectors in the ricefield than silver crucian carp and nile tilapia. Their body shape also seems to be better adapted to the shallow water in ricefields. Grass carp also have a higher economic return. When the excrement of grass carp that have eaten mosquito larva was examined, only the respiratory ducts were found entirely intact. Although the head of the mosquito larva is shell-like, it was ground to pieces and could not be counted. However, Anopheles spp. do not have respiratory ducts and this method of computation requires further discussion.

According to a report by Yi Mengjie and his colleagues in 1984, a 4.9-cm grass carp can catch 141 mosquito larva per night. In our experiments, each night grass carp the same size could catch 236 mosquito larva, which weighed 8.3 g (100 II-IV stage larva weighed 3.5 g).

Generally, the peak period for mosquito larva in ricefields is between late August and early September. Because of the field work needed for watering and harvesting, our survey continued only until late August when there was no peak and mosquito density was not high.

Summary

There are differences among the three species of fish in their food intake. The number of mosquito larva eaten under normal conditions is lower than when the fish are hungry. When the fish are the same size, grass carp eat the most larva, followed by silver crucian carp and nile tilapia. Fish reared in ricefields can eliminate up to 99% of the mosquito larva, Based on the indoor experiments, 100% of the mosquito larva in the ricefield could be eaten and this would still not fully satisfy the food requirements of the fish. Based on our experiments, grass carp would be the best species to control mosquitoes in ricefields.

Document(s) 5 of 41