Aquatic insects

Among various orders of class Insecta (Phylum Arthropoda), the following orders are commonly found in aquatic environment.

1.                  Order Odonata (Dragonfly and Damselflies)

2.                  Order Hemiptera (Bugs)  

3.                  Order Coleotera (Bettles)

Aquatic insects commonly have destructive and beneficial role in aquaculture ponds.

Destructive role:

Ø    Among odonatans, dragonfly nymphs (Anisoptera) are predatory and feed upon carp spawn.

Ø    Among aquatic bugs (Hemiptera) a good number are adverse to fish seed. Ex. Notonecta, Anisops, Plea sp. (Black swimmers) attack and eat carp spawn as soon as it is released into the ponds. They not only attack carp spawn but small fishes also.

Corixa (the water boatman) also attacks the larval fishes. Water scorpions (Ranatra, Nepa) usually attack young fishes.

Ø    Predaceous diving beetles (Coleopterans) belonging to genera like Cybister, Hydaticus, Sandrocottus, Eretes, Hyphoporus, etc. feed on all kinds of aquatic metazoans, the species commonly attacking small fish, tadpoles and even dragonfly nymphs.

The larvae of Cybister sp. are highly destructive to carp fry and even fingerlings.

Similarly, the larvae of whirligig beetle and water scavenger beetle kill fish spawn and small fishes.

Beneficial role

Ø    Insects as food for many fishes

Ø    Insects act as bio-indicators

Ø    Pollution indicators

Ø    Insects as scavengers

Control of aquatic insects

Proper preparation of nursery ponds for stocking with spawn thus also aims to total eradication of predatory insects. The common methods adopted for control of aquatic insects are: manual/ mechanical, chemical and biological.

a. Manual/ mechanical: In Bangladesh, where labour is cheap, manual methods are often employed to remove aquatic insects with the help of locally available some mechanical devices.

Aquatic insects controlled by-

Ø    Clear all the vegetation from the pond. Cut the grass short on the wet dike slopes.

Ø    Three to four days before stocking, slowly drag a fine-mesh net through the water to capture as many insects as possible and destroy them.

Ø    Refilling nursery pond with water not more than two weeks before stocking.

b. Chemical methods: The basic method is to apply a thin oily film over the pond surface which chokes the respiratory tubes of aquatic insects. The spawn and fish food organisms remain unaffected. Some of the common treatment methods are as follows:

Ø    Soap-oil emulsion method is widely used in aquaculture farm. An emulsion is prepared after mixing any cheap washing soap with mustard oil, coconut oil, ground nut oil or linseed oil and then sprayed on the water surface.

A ratio of 56:18 kg/ha of soap to oil, respectively is suggested to prepare emulsion.

Ø    0.6 to 1 ppm gammexane (lindane, C₆H₆Cl₆) is found sufficient to kill aquatic insects within 60 minutes to 11 hours.