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Fish as a component of integrated pest management in rice production

by Matthias Halwart


Integrated pest management

Integrated pest management (IPM) is a pest control concept that uses the best available mix of technologies for a particular pest problem. It promotes practices available to the farmer and permits pest control with the least use of chemicals to get high yields and maximum profits. Reducing pesticide application is an important factor because this control method is often uneconomical and unsafe to humans. Moreover, many pesticides kill both pests and their natural enemies at the same time, leaving the crop open to an uncontested invasion of pests.

IPM includes a variety of technologies, such as choosing pest and disease-resistant varieties, using crop rotation, fallows and simultaneous planting over wide areas. It also encourages the establishment of natural enemies of rice pests.

Economic threshold levels are used to help farmers decide when the application of pesticides is economically justified. The presence of parasites and predators, including fish, minimizes the need for the farmer to take action.

Fish as predators in ricefields

The most widely spread fish species in paddies are common carp (Cyprinus carpio), Nile tilapia (Oreochromis niloticus) and silver barb (Barbodes gonionotus). However, there is a large additional number of both stocked and wild fish species in ricefields.

Among those, there are larva-feeding and mollusc-feeding fish which are of considerable importance in the control of human vector-borne diseases, like malaria and schistosomiasis. Plant-eating fish species directly feed on weeds and are reported to be efficient in keeping irrigation canal free from vegetation. Turbidity and high water level add to the effects of fish controlling weeds.

Different fish species will affect different pests in different ways. Common carp, for instance, appear very effective at controlling golden apple snail. In Malaysia, tilapia was not able to control this pest but the giant walking catfish performed well.

The effectiveness of fish in fields where rice is broadcast is doubtful, since the rice will be too crowded to give fish good access to the field. There is no evidence for an optimal fish stocking density for pest control, but higher densities are presumed to be more effective. Fish eat older, outer rice leaves which are more likely to be infested with pathogens. This will make the rice plants healthier.

There are several reports documenting either reduced number of pests or less damage caused by pests and diseases with the presence of fish (Table 1). Most of this work has been done in concurrent rice-fish systems. In some cases, the underlying mechanisms have been described (Figures below). However, this list is far from complete. It is likely that more direct and indirect effects will be revealed with continuing research.

Table 1. Problematic organisms in rice production, predation of fish and susceptible life stage of prey.

Problematic organisms

Predation by fish

Life stage of prey

Insects

Whorl maggot

?

?

Gall midge

+

?

Caseworm

+

Larvae

Leaffolder

?

?

Stemborer

+

Larvae

Black bug

?

?

Brown planthopper

+

Nymphs/adults

Whitebacked planthopper

+

Nymphs/adults

Green leafhopper

?

?

Diseases

Rice blast

?

?

Sheath blight

+

Sclerotia

Bakanae

?

?

Brown spot

+

?

Bacterial blight

+

?

Bacterial leaf streak

?

?

Weeds

Grasses

+

Seedlings

Sedges

+

Seedlings

Broad-leaved weeds

+

Seedlings

Others

Golden apple snail

+

Hatchlings

Notes:
1. ? - no information available
2. + - based on documented observations

To date, feeding habits of fish in the ricefield habitat are not well-established. Generally, younger fish are more omnivorous and insects often form part of their diet. Abundance and behaviour of fish and rice pests will determine the relative importance of fish as a biocontrol agent in rice production.

Field observations are the basis of understanding the role of fish in ricefield ecosystems. If you are working in the field, please record and share your experiences with others on the activity pattern and feeding habits of fish. The important factors are species, size, prey and response to environmental changes.

Pest control mechanisms

Golden apple snails


Fish feeding on newly hatched snails

Stemborer


Fish feeding on dispersing stemborer larvae

Planthopper


Fish feeding on hoppers falling on water surface may actively shake rice hulls when nibbling on the stems

Caseworm


Fish feeding on floating case-worm larvae

Rice sheath blight


Fish feeding on floating sclerotia

Direct/Indirect
Weeds


Weeds are controlled by direct feeding, increased water turbidity and constant flooding

Indirect
Leaffolder


Disturbed moths fly up and are preyed upon by birds or other predators


Issues for further consideration

By feeding on rice pests, fish may reduce damage to the rice crop. At the same time, the stocking of fish helps in reinforcing IPM practices that have been the basis for increases in rice yields in many parts of Asia. Farming rice and fish and consciously avoiding the use of pesticides may be an important first step for farmers towards getting better prices particularly at markets near big cities where organically farmed products are of increasing demand.



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