Moths coming to lights at night

[DR. JAMES ADAMS]

Dear Listers,

       Recently, there have been a few new queries to the list about 
moths coming to lights at night.  I think it is about time I reposted 
a rather lengthy composition of mine that I put together last year to 
clarify some points about moths coming to lights at night.  The names 
here represent people who had asked questions in last year's go 
around.

First things first.  GSlS dog asked about whether stars are really
bright enough to be used as navigational cues.  The answer is "yes."
In studies done on the sensitivity of moth optical neurons to light,
they have an extremely low threshold, meaning that even very low
levels of light will allow the moths to "see".  Not see sharply, but
undoubtedly enough to avoid large (dark) objects, as suggested by
Ian.  Liz asked some related questions, namely "1) Why would moths
need to navigate (using light)?"  Well, if you don't like the word
navigate, perhaps orient would be a better answer.  All organisms
need to be able to orient themselves in their environment in 
relation to other objects (foodplants, mates, finding shelter, etc.).  I grant
you that finding foodplants and mates for moths flying at night is
almost exclusively done using chemical cues.  However, when just
"cruising" moths need cues that will allow them to maintain
appropriate orientation to surrounding objects, the ground, etc., and
one of the cues that is consistent for moths is that light sources
(back before the advent of human-made light sources) were predictably
UP" (in the sky).  I grant you that the moon is not always at a
consistent angle upward in the sky, but it *is* always up, as are the
stars.  And, by the way, in response to your question "Do moths
migrate", the answer is also "yes".  Not necessarily in masses like
the Monarch Butterfly, and not necessarily in a particular direction
at a particular time of the year, but moths do fly long distances and
likely use the sky light cues in the sky *directionally* as well
(though little research has been done on this).  For instance, there
is some documentation for the movement of moths out of the west coast
lowlands of Costa Rica during the dry season up into the moister
mountainous areas to the east, and back down into the coastal dry
forest during the rainy season.  There are several other recorded 
instances of large scale moth migrations, big enough to be picked up 
on radar.  The observation that moths (appear
to [see below]) fly more strongly on humid, cloudy nights is also
well documented, but to say that it doesn't make much sense therefore
that they would use lights as a navigational cue doesn't necessarily
follow.  Most animals have *more than one* way to navigate through
their environment, and just because they don't necessarily use one
all the time doesn't mean they *can't*.  (Moths *do* have functional
eyes, after all!!).  Just as an off the wall example, homing pigeons
can use sun compass and magnetic cues to migrate -- a pigeon released
on a sunny day with a small magnet attached to its head (which screws
up their magnetic sense) can still navigate just fine.  Unencumbered
pigeons do just fine on a cloudy day as well.  However, release a
pigeon with a magnet on its head on a cloudy day and they fly around
aimlessly.  The point?  Moths undoubtedly can use lights as a
navigational cue, and along with gravitational cues, use the light
sources from above to maintain appropriate "up-down" orientation in
their environment.
 
A statement Martin Damus made seems completely appropriate to the     
point I was attempting to make.
 
> ". . . noctuid moths migrate using the moon as a primary reference
> point.  To calibrate the location in the sky with actual geographical
> direction, they periodically use an internal geomagnetic compass.  In
> fact, every hour, they alter their flight path by 16 degrees to
> correct for travel of the moon across the sky (for purists, rotation
> of the earth).  On moonless nights they navigate solely with the
> geomagnetic compass.  I guess using the moon is 'easier', and
> therefore they 'prefer' that when it is visible, hence the screwup
> when bright lights are visible."
 
Just as an aside, Jim Hanlon mentioned fish
coming to the surface of the ocean during a full moon -- this is well
documented for many fresh water fish as well.  Perhaps one of the
reasons is that insects flying through their environment on a
well moonlit night will be confused by *reflected* moonlight from
below and go cascading down into the water (more food for the fish).
I know this has come up in discussions with other entomologists
before, but I am unaware if this is actually scientifically
documented.

    Now, why do moths come to lights?  To address your other point
Liz, you mentioned something about moths perhaps being able to hear
the light and come to the light based on certain sound frequencies
being produced by the lights.  I agree that this may be a
possibility, but there are two things which suggest that this is
certainly not the only (main?) mechanism.  I doubt very seriously
that white light bulbs and white light bulbs painted yellow emit
significantly different sound impulses, and so this would not explain
why moths come in to different colored lights in significantly
different numbers.  Secondly, there are some families of moths which,
in essence, have *no* hearing capabilities whatsoever (for
instance, the Saturniidae [egs., Polyphemus, Cecropia, Io, Imperial,
etc.]), so without the ability to hear sound frequencies, saturniids
and some other moths would never end up at lights, and this is
certainly not the case.  The use of moon and stars as navigational
cues can at least partly explain why moths end up at lights.  Both
Martin Damus and David Britton bring up the point about moths
maintaining a certain angle between themselves and light sources,
which explains the "spiralling in" that is easily observable in many
species as they come to lights.  Interestingly, some of the species
that do this most strikingly, such as the saturniids, are also some
of the species that do not have hearing apparati.  The reason why
they stay at the lights (after "hit(ting) the moon, an accomplishment
they never evolved a decent response to." [nice quote, Martin, I like
it!]), is likely because, now close to a the bright light source, the
artificial "moon" has become the "sun", and the moths settle down.
The reason why some moths fall from the wall when tapped has to do with
thermoregulation.  Cooled moths (in many cases) require a certain
warm up period before flight (accomplished by "shivering" [muscular
thermogenesis]). Using nighttime celestial lights sources as navigational 
cues would also be a convenient explanation as to why it appears that fewer
moths come to lights on well moonlit nights.  Full moonlight is
harder to compete with using the superstimulating electric light
sources, as well as why it therefore appears that there is *more 
activity* at artificial light sources on cloudy nights.  However, 
this is certainly not the entire story.

    Many, many moths, if you watch them come to lights, fly
*directly* at the light source as they come in, with little
indication of any spiral.  This may get back to Liz's point about
potential use of sonic cues.  Many moths can hear, and it is *some*   
of these moths which appear to come more directly at the light.  
These moths are perhaps using *both* a light and sonic cue to
get to the light.  This does *not*, however, explain why the moths
"like" either the light or the sound.

     Hope clarifies some points, as well as muddies some others for
continued discussion!!!

     James Adams

P.S.  As to the decline of species because of artificial lighting,
the main reason for this (assuming it does happen) is the predation
that occurs on the placid moths sitting on walls, etc. in the morning
(not against some background that they might be camouflaged on).  I
have seen numerous birds "cleaning up" in the early morning, as well
as Bald-Face Hornets and wasps of the genus Tripoxylon.  There may
indeed be some selection going on for individual moths that do *not*
respond strongly to light cues (but, of course, if the navigational
argument is sound, it would be impossible to select this out of a
moth species!!).