More detailed comparison of moth trap bulbs

A comparison between two different bulbs using a Robinson trap.

1. 125W MV
2. 20W Blacklight “Eco” bulb supplied by Watkins and Doncaster

Throughout  2016 I attempted to find pairs of nights that were not more than a few days apart and with forecast weather conditions in terms of temperature, wind speed and rainfall as similar as possible.  On one of the two nights I would run my Robinson trap with a standard 125W MV bulb and on the other night I ran the same trap in the same place but with a 20W Blacklight (Eco) bulb.

In total 35 pairs of results were obtained.

A summary of the results is given below.

	125W MV	20W Eco	Percentage
Total number of moths caught	3078	1832	60
Mean number of moths per night	88	52
Mean number of species per night	25	17	68
Total number of different species caught	259	213	82
Number of species not caught with other bulb	83	39

 

 

 

 

 

The following table shows in which months each of the pairs of readings were obtained.

1,2	March
3,4,5,6	April
7,8	May
9,10,11,12,13,14	June
15,16,17,18,19,20,21	July
22,23	August
24,25,26,27	September
28,29,30,31	October
32,33	November
34,35	December

Although there were significantly more individual moths and species caught with the MV bulb the differences appeared to be less marked early and late in the season.  I wonder if some of the differences can be attributed to the brighter light from the MV bulb penetrating the surrounding vegetation (particularly deciduous trees and shrubs) in the summer months. It would be interesting to repeat the experiment in a more open situation than where I run my trap.

I have been asked whether there is any indication that the different bulbs attract different species. My feeling is that there was no evidence of this in my results. Of the species caught with only one type of bulb the majority were species caught in small numbers (often only one) and on very few occasions and which bulb attracted them was purely chance.

The few species where there might have been a difference are shown in the table below.  To produce this table I have only chosen species that were caught on 7 or more occasions and then picked the five species that seemed to favour each bulb. My own feeling is that these differences were probably due to chance and a lot more data would be needed to draw any conclusions concerning differences in species caught.

	125W MV	125W MV	20W Eco	20W Eco	Both traps	Percentage
	Total caught	No. of nights	Total caught	No. of nights	No. of nights	eco/MV
		caught		caught	caught
Marbled Beauty	5	3	14	4	7	280
Marbled Green	9	5	16	3	8	178
Common White Wave	4	4	7	5	9	175
Silver-ground Carpet	7	4	11	5	9	157
Hebrew Character	55	8	86	8	16	156
Coronet	33	10	3	2	12	9
Snout	31	12	2	2	14	6
Small Fan-footed Wave	24	5	1	1	6	4
Burnished Brass	13	7	0	0	7	0
Buff-tip	9	7	0	0	7	0

My conclusion from this experiment is that the 20W eco bulb is worth considering as an alternative to the 125W MV bulb especially as the latter become less readily available. It looks as if generally fewer moths and fewer species will be caught on each night the trap is run.

The eco bulb does however have certain advantages.

1. It uses less than one sixth of the electricity of the 125W MV bulb.
2. It does not require a choke and is therefore cheaper if starting from scratch.
3. The light produced is far less likely to disturb neighbours.
4. I understand that with an inverter it can be run from a battery.

I have no idea of how long the bulb is likely to last or whether it will deteriorate over time.

There are of course other alternatives to MV bulbs but I have no experience of these.

Andy Newbold.  Sibford Ferris, Oxon.