This is a complex question. The short answer is that any bulb will work. Bulbs with more ultraviolet typically work better. This includes mercury vapor bulbs and fluorescent "black light" bulbs. But there are problems:

Some insects could not care less about the light.

Some insects are attracted to the light up to a point, but no closer.

So the real issue is what is your goal? Are you monitoring for a specific insect, or doing a survey? Are these moths, beetles, flies, wasps, or everything?

If the goal is survey, you might consider a light trap and some other techniques.

It would also be a good idea to figure out what you are going to do with the data. So you have counts of a species for each night of trapping .... now what? You could look for seasonal patterns with several years of data. You could look for changes in biodiversity between seasons or between different trapping locations. All of this requires you to think about what constitutes a "replicate" which will subsequently influence your sampling design.

Some other things to consider:

1) Trapping area. Given a light trap, how far away can an insect be before it is unlikely to be captured? This may be influenced by background light (moon phase, city lights, etc...).

2) How effectively does the trap catch the target insects? If the trap is placed in the middle of a population of insects at 300 insects per km, how many insects should I expect in the trap?

3) Am I catching rare residents or transients?

4) Are my differences caused by treatments or seasonal changes in abundance?

5) The trap may attract predators. Are these influencing my data by eating specimens before they can be captured and preserved?

6) If this is a long term intensive study, how will you handle changes in the environment caused by your trapping efforts. This can be anything from the path you make to get to your trap, to the effect of removing rare individuals from the trapping area thereby reducing their local abundance.

I am sure I am forgetting some points that should be considered, but this will make a good starting point.

Ultraviolet light trap cathes more insect than conventional traps such as visible light trap

We use mercury bulb to set up the light trap during our field trip.

This is a complex question. The short answer is that any bulb will work. Bulbs with more ultraviolet typically work better. This includes mercury vapor bulbs and fluorescent "black light" bulbs. But there are problems:

Some insects could not care less about the light.

Some insects are attracted to the light up to a point, but no closer.

So the real issue is what is your goal? Are you monitoring for a specific insect, or doing a survey? Are these moths, beetles, flies, wasps, or everything?

If the goal is survey, you might consider a light trap and some other techniques.

It would also be a good idea to figure out what you are going to do with the data. So you have counts of a species for each night of trapping .... now what? You could look for seasonal patterns with several years of data. You could look for changes in biodiversity between seasons or between different trapping locations. All of this requires you to think about what constitutes a "replicate" which will subsequently influence your sampling design.

Some other things to consider:

1) Trapping area. Given a light trap, how far away can an insect be before it is unlikely to be captured? This may be influenced by background light (moon phase, city lights, etc...).

2) How effectively does the trap catch the target insects? If the trap is placed in the middle of a population of insects at 300 insects per km, how many insects should I expect in the trap?

3) Am I catching rare residents or transients?

4) Are my differences caused by treatments or seasonal changes in abundance?

5) The trap may attract predators. Are these influencing my data by eating specimens before they can be captured and preserved?

6) If this is a long term intensive study, how will you handle changes in the environment caused by your trapping efforts. This can be anything from the path you make to get to your trap, to the effect of removing rare individuals from the trapping area thereby reducing their local abundance.

I am sure I am forgetting some points that should be considered, but this will make a good starting point.

Good answer Timothy.

Actually, I'm going to conduct light trapping during 4 am until 6 am in the morning to trap bagworms (Psychidae) Lepidopteran family. In my case, I want to trap them as many as possible by using light to attract them and then they fall into the attached bucket. It can fly about about 150 m far away.

So, is it possible and reliable to use light trap?

I use mercury bulb in light trap. That work whole night. Ther is only 2 to 10 Psychidae per year, per trap! Only two or three species was cought in period 20 year! I think beter methods is collect larvae in caracteristic sack. I never use other light source, so you can try. Best regards

Possible: yes, all you need is the equipment.

Reliable: yes, the results will be repeatable again and again.

Accurate: maybe. If there are 200 adults within 150 m of the trap do you always catch 3 of them each season, or all 200. This I do not know, and probably the only way to really find out would be to run an experiment. Tag a bunch of adults. Release them from several distances from the trap and see how many you recover. There are several very good papers on the effectiveness of light traps and their radius of attraction. One approach is to use traps to catch moths, and then mark and release them the following night. If you don't get many moths per trap, you might have to put out a large number of traps, or use moths from a colony (with the inherent problems that entails).

If you know the host plants, Dragan's approach may be effective -- unless they are all 30 m up in a tree. Knowing where the larvae live may provide a clue as to where the adults will fly and this would be important for your light trap, as trap height off the ground does influence your catch. Are you sure that the adults are most active from 4am to 6am? I don't doubt that this is possible, but read carefully the methods of the source for this information.

In our use of blacklight traps in New Jersey for monitoring crop pests, we occasionally catch the male bagworm moths.  Considering the size of the population of bagworms that we have at times, the number of males caught doesn't seem to relate well to the population.  Depending upon your goal, there may be better means of trapping the males than using blacklights.

There is nothing left to tell you more because Timothy expressed it briefly. Go and see the literature and start your work

I am beginning the 48th year of continual light trapping in the state of Louisiana (every night of every year, since 1969). I have developed, designed, fabricated and tested over 400 various insect traps including numerous hundreds of lamps, bulbs, devices, and accoutrements of various types, for the attraction and automatic capture of different insects.  Attracting and capturing various insects is not difficult, as I often capture numerous millions of individual insects of all types in a single day.  The issue then becomes how to collect them in such a manner as collecting huge amounts of specimens and dispatching them within seconds in a collection chamber, and then allow quick sorting. And to do this automatically, so there is no need to be physically present, other than to pick up the resulting captures. Then, if these specimens are part of a scientific survey for eventual placement in a museum, research material, or reference collection, there are many other subsequent issues to consider.  As, I previously stated, capturing material is not the problem, but rather solving the many problems one is presented with after one has the insects.  Specifically, the best lamps to consider should be based upon the wavelengths (spectral distributions) emitted by the hundreds of potential lamp models.  Ideally, the greatest amounts of insects are attracted to lamps which emit high percentages of ultraviolet radiation in the range of 365nm.  See link I provided here.  

https://www.academia.edu/9665262/Images_of_some_of_the_insect_traps_designed_fabricated_and_operated_by