It’s been a couple of weeks since I last ran the trap thanks to a combination of cold weather and busy weekends so I was delighted to find two December Moths in there on the night of 2nd December.
I love these moths. The very fact that they come out when the weather is at its coldest, their ‘woolly’ appearance, the smart patterning and the fabulous antennae of the males makes for a great catch every time.
Other than that it’s been very quiet with insects – the odd Silver Y being the only other moths of interest (see photo at top) – and so I’ve been taking the opportunity to do some reading. One thing I came across via Twitter was an article which went some way to answering a question that I’ve been thinking about for some time now, namely, what fraction of the moths near my trap am I actually catching? Do moths come from miles away to the MV light, or do they need to be passing close by? And does it depend on the moth? This seems like an important question to me, not just from the point of view of wanting to catch as many species as I can, but in terms of keeping records and contributing them to the National Moth Recording Scheme.
I know already, of course, that not all moths come to light. Some species/families need pheromones to attract them, for example the elusive clearwings. I’m also aware that keeping records that are informative about populations and commonality requires other factors in place, such as constant effort, as well as expertise in identifying the more difficult ones. However, if these things are controlled it still assumes that moths come in equal proportions to any single trap. In a 2014 volume of Insect Conservation and Diversity Merckx and Slade (2014) report on a field study in which various species of moths from three families – Geometrids, Noctuids and Erebids – were captured, then marked (with a pen) and re-released to see if they could be recaptured in a nearby portable, actinic trap; a so-called Mark, Release, Recapture (MRR) test. I have quoted the full abstract below, but in essence the experiment suggests that moths from different families are attracted at quite different ranges and that even for those flying within a metre of the light the proportion becoming trapped was significantly different in each family.
In terms of the first of these, erebids (the family including Snouts, Herald, Straw Dot etc.) had a range of 27m, measured in terms of a minimum 5% recapture limit(i.e. beyond 27m the fraction of moths recaptured dipped below 5% of those released). In contrast, Noctuids had only a 10m radius for the same 5% limit for recapture.
In terms of the second, of moths flying within 1 metre of the trap 55% of erebids were recaught, but only 10% of noctuids and 15% of geometrids. It’s also worth mentioning that the experiment was conducted in two locations – woodland and open space – and that moths were released from the same radii at different angles round the trap (N, S, E, W etc.). The direction round the trap had some small wind-speed effects, but the traps in (dark) wooded areas had much better recovery rates pointing to the negative effect on recapture of extraneous light in the atmosphere (and see also here for another paper which has demonstrated the quite severe effect on breeding success of light for geometrids).
So what does all this tell me? Well, first that it’s still fun to trap moths! … but also that I may need to be more cautious about drawing conclusions about the relative density of particular species in the area. It is about 10m from the space at the back of my house to the end of the garden and hence if noctuids’ range is about this distance that will be telling me the numbers that are in, or flying over/close to my garden itself. On the other hand, if erebids and geometrids are more likely to be pulled in from distances of 25m+, I’m likely to be gathering these moths from the local surroundings, which for me include a number of mature trees and shrubs. Moreover, of the moths coming close-in to the light the noctuids may be under-represented in terms of numbers compared to other families. Nonetheless, it’s worth also being aware that the experiment was conducted with a low-wattage actinic lamp and I run a high wattage MV bulb, so it may well be different. Moreover, as Merckx and Slade note:
[C]are must be taken in relating the abundance of the sample to absolute local abundance. Frequent sampling can provide adequate data on relative temporal change in the local macro-moth fauna, however.
As we enter the end of the year I’m looking forward to reviewing my catches from 2017 and comparing them to 2016 … but with appropriate care in terms of my conclusions.
Merckx, T., & Slade, E. M. (2014). Macro-moth families differ in their attraction to light: implications for light-trap monitoring programmes. Insect Conservation and Diversity, 7(5), 453-461. doi:10.1111/icad.12068
Abstract. 1. Light traps are used to make inferences about local macro-moth
communities, but very little is known about the efficiency with which they
attract moths from varying distances, and how this may differ among families.
2. We released 731 marked individuals, from three of the most common and
species-rich macro-moth families, at several distances from low-wattage actinic
light traps in open and woodland habitat.
3. Logistic regression showed family-specific sampling areas: erebids were
attracted from up to 27 m, geometrids from up to 23 m, and noctuids from up
to 10 m from the light source, with these distances corresponding to a 5%
recapture rate. Sampling size was also family-specific: a maximum of 55% of
erebids, 15% of geometrids, and 10% of noctuids were predicted to be trapped
when flying near (0–1 m) light traps.
4. Our study demonstrates that weak light traps: (i) have remarkably local
sampling ranges, resulting in samples that are highly representative of the local
habitat, and (ii) attract small, and family-specific proportions of individuals
within these ranges.
5. We suggest that the local sampling ranges of weak light traps make them
excellent tools to monitor nocturnal macro-moth communities. As trap efficiency
differs among macro-moth families, care must be taken in relating the
abundance of the sample to absolute local abundance. Frequent sampling can
provide adequate data on relative temporal change in the local macro-moth