Breaking diapause - character errors.

[Paul (ELG)]

A number of errors occurred in my previous mail regarding Gardiners advice
on diapause. It appears that the little "degrees" character was converted
to an extra "zero" in the temperatures, making all the values x10. My
apologies for any confusion this may have caused.  Corrected text below:

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Dear Kristina,

Tony (Mothman)'s reply to you earlier more or less agreed with my comments
regarding the possibility of cold treatment, indeed, we have suffered the
same mistake by receiving shipment after shipment (from the same source)
and keeping them too warm. 

For your information (and others) and as Brian Gardiner has been a valued
collegue of mine for many years, I am sure he will not mind that I quote a
small section from "The Silkmoth Rearer's Handbook" which will be relevant
to your problems. This is published by our society - The Amateur
Entomologists' Society and copies of the book can be purchased by visiting
their website at http://www.theaes.org

Verbatim: From the revised 2nd edn (my 3rd edn is not at hand):

BREAKING DIAPAUSE AND SYNCHRONISING EMERGENCE

Once the conditions necessary to induce the diapause State have occurred,
then this stage will continue until development is reactivated. This
commences when the right hormonal balance is achieved and this in turn is
triggered by the conditions experienced by the diapausing stage. Unless
these meet the normal requirement of the species the diapause State is
liable to continue. Now the diapause state is a period where the moth (in
whatever stage) is lying dormant in order to ride out the storm of that
adverse weather condition, winter. Therefore until it has had that winter
it will continue to wait for it. This is the mistake so often made by
rearers and one of the reasons why some pupae have been recorded as 'lying
over' for two or more years. They have been kept too warm. Diapausing
stages require a winter. That is to say a period of cold. Not until this
has been experienced will the appropriate glands react to warmer conditions
and secrete the hormones that break the diapause and allow development to
continue. Just how long the winter cold should be, and of what intensity,
is often a matter for conjecture and depends on a number of factors, not
least the conditions which have caused diapause in the first place. Usually
with obligate univoltines there is no trouble. But when we are dealing with
a facultative light-controlled diapause requirement the situation becomes
complex and species may then be in 'weak' diapause or they may be in
'strong' diapause. Weak diapause is produced when the light threshold is
marginally insufficient to prevent diapause, or, if more than normally
sufficient, has been partially overridden by rearing at an excessively high
temperature. Weakly diapausing specimens may well react simply to warmth
and emerge after a period that is often only a few weeks (or a month or
two) longer than they would normally take were they not in diapause.
Specimens in normal diapause will require a period of cold which is given
by the formula 'Time x Degrees of temperature below a certain threshold =
K' where K is a constant for the species.

This temperature threshold is very variable as between species and is
completely unknown for any but a few. It is however pretty safe to assume
that it lies between 5-12'C. As an example suppose that K = 700 and the
temperature threshold is 10'C. Then at 8'C the time taken to break the
diapause would be Tx(10-8)=700  or  T=700/2=350  days,  but  at  3'C  then
Tx (10-3)=700 orT=700/7= 100 days.

In the majority of cases with which I am acquainted it is my experience
that the diapause of temperate zone species is broken by giving them 2-4'C
for a minimum period of 12 weeks.

The above is rather a simplified version of the true state of affairs but
gives the general principle. Once diapause has been entered then it can
only be terminated by a period of cold followed by warmth. In the past,
until this physiological fact was investigated and proved, it was the
custom of many rearers to continue keeping pupae that were in diapause warm
and this explains the many cases both of mortality (usually by drying out)
and of greatly extended emergence times.

Now the breeder of Saturniids, especially when he only has a few pupae to
act as the mother stock, requires the moths to emerge as simultaneously as
possible. Frequently they trickle out one after another, first a male, then
another, then a female a week later just after the second male has died. So
often have I heard of this happening (and had happen) and it is most
disappointing. In nature the animals manage these things rather better and
if all the Emperor moths we rear were to be kept under natural conditions
then no trouble would be experienced. But this is not so. The majority of
breeders keep them warmer, and have put forward (or conversely delayed) the
normal emergence time or have ensured an extra generation. Therefore
additional steps must also be taken to try and ensure a synchronous
emergence from the pupa for species which have been reared under what is to
them unnatural (but by no means unsuitable) conditions.

I have stated above the minimum conditions for breaking the diapause state.
This is unlikely to ensure synchronous emergence but a scatter over several
weeks. Consider the normal course of events. A caterpillar feeds up in
early summer (lengthening daylight) produces non-diapausing pupa and hence
a second generation. The larvae resulting from these are now feeding up in
autumn (shortening daylight) and produce diapause pupae. These remain
comparatively warm through late autumn, and early winter. They then have
the cold of winter proper followed by the warm-up of spring and will then
tend to emerge when the late spring or early summer temperature rises above
a certain mean. In captivity, breeding as we do species from many regions
and differing climatic conditions, it is not possible to give natural
conditions to any but local species and a different ploy must therefore be
used. For species which diapause under cold conditions I have always found
that the most synchronous emergence results if the pupae are first kept
warm, 15-25'C for six to eight weeks, then at 4'C for 16 weeks, followed by
one to two weeks at 10'C and then brought into 25-30'C. An alternative to
the 4'C is to leave them, in England, in an unheated shed or garage for six
months (October to end of March). when waiting for leaf to appear as food
for the larvae this period can be extended, as indeed can the period at
4'C.
Indeed for the hardier Palaearctic species a lower temperature could be
tolerated. Fig. 6 summarises the requirements of three species.

The above remarks apply primarily to temperate zone species that normally
experience frost.

Tropical or subtropical species should be treated rather differently and
should never be subjected to even a touch of frost as this could well kill
them. Indeed I would suggest that tropical species be always kept above 5'C
and perhaps 10-12'C is a better proposition for most of them. However, it
is worth pointing out that not all tropical species are in fact 'tropical'.
If they come from high mountain regions, even on the equator, then they may
well be true temperate zone species, and can and should be treated as such.

Having given an account of the treatment required to ensure synchronous
adult emergence from diapause pupae, it is worth pointing out that the same
principle can be followed with non-diapause and with tropical species, both
as a means of delaying emergence and, more particularly, ensuring the
adults emerge over a short period. Naturally neither the low temperature
nor the rather long period of time is required. Both non-diapause or
tropical pupae can be held at 10-12'C for several weeks. This both slows
and synchronises development. They should then be brought up to around 25'C
and emergence will happen about a week later. Even when kept warm, however,
it will be found that some tropical species have a long pupal period of two
to four months.

Many of the obligate univoltine species tend either to emerge early in the
year or else they diapause as eggs and the adults emerge in the autumn. In
some cases the larvae first develop inside the egg and so these should be
maintained for a few weeks at moderate temperatures (12-18'C) before being
subjected to the real cold of winter. Although it is not possible to
prevent the diapause of obligate univoltines it is possible by suitable
management and the use of a refrigerator to obtain two (or even three)
generations a year. This is done, be it noted, by terminating the diapause
by suitable minimum cold treatment, not by preventing it in the first
place. It is wise to proceed with caution and it is worth pointing out that
the larvae issuing from eggs hatching early in the year always seem to
require very fresh growing foliage and if you are going to try for another
generation the same year it will be in autumn and it will be necessary to
hard prune the foliage several weeks in advance to encourage some new
shoots to be put forth and indeed it may well be essential to have a heated
greenhouse to grow it in.  [Brian. O.C. Gardiner].

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Paul W. Batty
The Entomological Livestock Group has over 550 members Worldwide.
Website:  http://www.pwbelg.clara.net/index.html