CÁC PHƯƠNG PHÁP THU MẪU TRONG NGHIÊN CỨU CÔN TRÙNG

Thu mẫu là khâu quan trọng trong nghiên cứu côn trùng, phương pháp thu mẫu đúng, cách thu mẫu thích hợp cho từng nghiên cứu, từng nhóm côn trùng sẽ đem lại kết quả thí nghiệm chính xác. Sau đây là một số phương pháp thu mẫu chính được IRRI đề xuất và giới thiệu.

Insect sweep net

The use of sweep net is a simple and inexpensive way to monitor the presence of a variety of arthropods in the ecosystem. If sampling effort is consistent (e.g. 20 sweeps or 30 seconds sweeping whilst walking slowly through vegetation) samples can also be used to infer relative abundance of arthropods within a vegetation type. The sweep net is a funnel- shaped net, which is made-up of a nylon or similar synthetic fabric. It is important that the net is mounted on a ridgit metal ring rather than wire. This allows the net to be swept through dense vegetation, dislodging arthropods. The net’s ring is attached to a long wood or metal handle. A standard sweep net has a diameter of 28 cm with a length of 71 cm

long. The stick handle is about 74 cm long.

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How to use a sweep net

1. Hold the sweep net near the end of the handle with the hoop end nearest to the ground in front of you.

2. Swing the net from side to side in a full 180o arc or forming a semicircle. Keep the circular frame of the open end of the net perpendicular to the ground and pointing to the direction of the swing.

3. Sweep one stroke per step as you casually w

alk through the field or down the row.

Do not swing the net up and down.

4. In short vegetation, swing the net as deeply as possible.

5. In taller vegetation, sweep only deeply enough to keep upper edge of the sweep net opening even with the top of the plants.

6. The net should not go more than 25 cm below the top of the plants during sampling.

Sampling arthropods by a sweep net

1. Sample arthropods once a week starting from the tillering stage (30 DAT) until the flagleaf stage (80 DAT) of the rice crop.

2. Sampling must be done at 10 am in the morning when all the morning dew has evaporated. Avoid sampling in wet weather.

3. Start to sweep from the end corner of the field towards the center of the field. Do twenty sweeps, which corresponds to one sample.The second twenty sweeps going to the center of the field and the third twenty sweeps going to the other end corner of the field. Or do the zig zag pattern of sweeping in random parts of the field.

4. A total of sixty sweeps (three samples) along a diagonal or zig zag pattern will be done in one-fourth hectare.

5. Swing the net as hard as possible after the last sweep. This will allow the insects to be deposited at the funnel end of the net.

6. Close the net by gripping the mid section by the palm.

7. Invert the net and put the collected insects in plastic bags and label with tags.
8. Transfer the collected insects in 3 labeled vials with 100 % alcohol.

The efficiency of a sweep net may vary depending on many factors. Different weather conditions, wind speed, air temperature, and intensity of solar radiation may affect the number of insects in the area while sweeping. Different habitats, especially the height of the plants, time of day, reflecting different cycles of behavior of the species, and different styles of sweeping are also factors to be considered.

Blower-vac machine

Blower-vac machine may be used for more quantitative studies of insects in rice. It is operated by a gasoline-powered motor. The machine sucks the insects from rice plants by vacuum pressure. This machine is similar to that described by Arida and

Heong (1992). However, instead of a plastic bucket, it will use a modified enclosure made of metal.

Sampling of arthropods by blower-vac machine

1. Sample arthropods biweekly or every two weeks starting from the tillering stage

(30 DAT) until the flagleaf stage (80 DAT) of the rice crop.

2. Biweekly sampling will consist of 10 Blower-Vac samples at every sampling date.

3. Sample arthropods from 10 randomly selected hills within the rice field.

4. To sample using the Blower-Vac, drop the enclosure over the rice plant to cover

1 hill.

5. Suck the arthropods from the nylon net sleeve, the air column, the plant

surfaces and finally the water

surface. The suction time will depend until all the insects are collected (suction time will later be prolonged as rice crop matures).

6. Place the collected insects in labeled vials with 100 % alcohol.

Yellow pan trap

Many small day-active insects are attracted to the color yellow. Yellow pan traps collect insects that are attracted to the color. They are inexpensive and simple means of passively sampling insects in an area. This trapping method uses small pans filled with a mixture of water and liquid detergent. The pans are then placed on the ground in conspicuous places in the morning. When flying insects land on the surface of the water they rapidly sink and drown. At the end of the day or after 1-2 days, the water is strained through a fine sieve and the specimens are collected.

Option 1

1. Use 500 ml bowl. Deeper bowls experience less evaporation in hot climates.

2. Cut holes near top of bowl and cover with mesh. In excessive rain this allows water to flow out of the bowl without losing any samples.

3. Paint with yellow UV paint (e.g. Sparvar Leuchtfarbe, Spray-Color GmbH, Merzenich, Germanyn or http://www.guerrapaint.com/tandc.html USA).

4. Place bowls at approximately the same height of vegetation (50-100 cm) using a wire frame.

5. Add a mixture of 400 ml water and 1.2 g of sodium be

nzoate preservative.

6. Place the first bowl in the vegetation bordering the crop/ on the bund and the others in a transect and at distances of 0 1 m, 2 m, 4m, 8m, 16m, etc into the centre of the paddy.

7. Cover each bowl with a coarse wire mesh to prevent scavanging of insects by birds.

8. Leave out for 48 hrs at a time.

9. Repeat a minimum of 6 times throughout the rice-growing season.

10. Use an aquarium net or fine sieve to collect the insects and place in 100 % ethanol.

Pitfall trap

Pitfall traps are often used to sample crawling or ground-dwelling insects. They are placed below the ground with the rim of the container below the surface of the soil. Detergent

may be added to reduce the surface tension allowing insects to sink into the liquid. Sampling by pitfall traps

Option

1. Install the pitfall traps two to three times a week before dibble and at seedling stage until harvest time.

2. Dig holes with the same size as the traps.

3. Dig holes at 0, 1 m, 2 m, 4 m, 8 m, 16 m, etc on the bunds and at the center of the field forming a straight line.

4. Make sure the rim of the trap is just below the surface of the soil.

5. Put some mixture of water and teepol on the trap. Ten percent picric acid can also be mixed with water to preserve the insects.

6. To keep rainwater out of these traps, a cover or a mylar roof supported by a metal wire can be placed above the opening of the trap.

7. Leave the traps for 1 day.

8. Retrieve the traps after 1 day.

9. To empty the traps, take the inside cup and turn it over in a labeled plastic bag.

10. Transfer the collected insects in labeled vials with 100 % alcohol.