In the spring of 2009, already I have been asked that question or something similar three times, so it is probably wise to document what I have tried, have read about, and will document this summer via some experimentation.
There are at least two proactive approaches that should be considered:
1) Female pupae are usually larger and heavier than males. Hence it usually takes them a bit longer to develop in pupal stage, and male and female cocoons/pupae stored under identical conditions, brought out of cold storage at same time, and again treated identically will usually (not always) result in males eclosing a day or a few days earlier than females.
Under completely natural conditions, hardly ever are any two cocoons overwintered under identical conditions. Amount of shading from direct sunlight, northern or southern exposures, snow cover, exposure to wind, all vary. A female pupa whose cocoon is lying on the forest floor on the south side of a tree, only ten feet from a male pupa whose cocoon is lying on the north side of the same tree, is not experiencing the same developmental triggers and conditions as the male, despite their proximity.
One thing you can do to increase the likelihood that you will have males and females out at the same time, is to take the heavier cocoons (anticipated as females) out of cold storage one to three or four days prior to taking out the lighter cocoons, anticipated to be males.
Occasionally I will get an order-request for four females and no males. I reply that we don't ship just females, but try to send roughly equal
numbers of males and females, determination by hefting, which is not always accurate. Most people would prefer to just have females, especially if they have wild populations
which could provide the males.
I try to explain that if we honoured all the early requests for just females, we would be left with a glut of just males, and that would not be fair
to those who order later in the season.
2) A second thing you should consider is the number of cocoons that you retain from your own rearings or obtain via a trade or purchase. Quite often I will get emails from people who are inexperienced with cocoons. They request one male and one female and indicate they would like to breed the moths when they emerge. I indicate they might get lucky and have a paring with only two cocoons in the cage, but they would have much better chances of obtaining pairings with a complement of six to eight cocoons, following the recommendation above.
This past summer I reared approximately 850 lunas to cocoon stage. About thirty larvae spun up late and did not reach a marketable size. I retain those smaller cocoons just to see what comes of them. Of the remaining 820 cocoons, I saved about twenty of the largest cocoons (anticipated as females) for my own rearing and for egg sales. The rest of the luna cocoons were sold, anticipating about 410 male and 390 females in the lot. If you order an odd number of cocoons, of any one species, you are most likely to get one more male than female.
I have a good local population (Montague, PEI) of lunas so usually have no trouble getting the females to call in and pair with wild males. I also make an annual trip to Malay Falls, Nova Scotia, where female lunas are usually taken at our lights.
However, to increase chances of getting pairings, and to spread out the rearing work load, I usually begin taking some luna cocoons out of cold storage around May 10, for late May eclosions and take a couple more out every two or three days so I can expect to have females available off and on throughout the month of June.
For those who cannot take advantage of local populations for pairing of moths, I recommend a purchase/retention of at least eight cocoons of any one species to increase the likelihood of having males and females out at the same time. I would still take all the heavier cocoons out of cold storage a day or so before taking the lighter ones out of cold storage.
If these approaches fail to work, there are a couple other "tricks" you might be able to pull out of your sleeve.
A number of years ago was rearing some species that were not locally indigenous. I could not rely on wild males to fly in a pair with my emergent females. I followed my proactive advice as indicated above, but still did not always get the results I wanted.
I invested in a small electric space heater. If I had males out and wanted females to eclose, or less frequently if I had females out and wanted males to eclose, I put the cocoons from which I wanted eclosions in a vacated upstairs bedroom, closed the windows and the door and turned on the space heater. Be careful. You don't want to cook or desiccate the pupae. I was surprised out how quickly an 8' x 12' bedroom will warm up. It is probalby also wise to have an open pan of water in the room to maintain an appropriate humidity.
I would use the heater to bring the room temperature up to about 80-85F and then turn off the heater. Then I would leave the room and check several hours later to see if I had eclosions or if the room had cooled. I stopped the procedure if I had the desired result. I turned on the heater if the temperature had fallen below 75F.
Perhaps you cannot justify the purchase of a space heater, but they are a nice little addition in fall or winter if you just want to take the chill off the air in a single room without lighting a fire or turning on the heat.
I have also read and tried with success the chilling of adult moths to prolong life spans. I do not know how long the life spans can be prolonged or what the optimum temperature would be, but I have placed freshly emerged and inflated male cecropia in plastic tubs and have placed those tubs in the fridge crisper for as many as four days waiting for a female to eclose. I was attempting this to obtain a late season pairing to have mature larvae I could take to school in September for a science class display. Usually the larvae from the normal flight time here have all cocooned before school starts.
I have read concerns about the viability of male sperm subjected to cold. I do not know how long the males can be chilled without losing some potency, but I have had at least one male who was put to work after four days in cold storage at about 38F. He revived himself (at indoor room temperature) within a couple of hours after coming out of cold storage and paired with a fresh female. All of the subsequent eggs proved fertile and yielded a good percentage of full sized healthy cocoons.
I have had people tell me they have kept cecropia alive for up to two weeks using a similar method. Perhaps the accounts are correct, perhaps they are a bit exaggerated. I do not know if the moth (male or female) later paired, or if any subsequent eggs were fertile.
I usually have some extra female polyphemus and lunas and I will probably do at least one experiment with each species this summer to see if a male or female chilled for at least four days at 38F will pair and produce viable offspring.
Males tend to visit lights much more frequently than females. I have had success capturing very large and attractive males at lights, putting them almost immediately into cold storage, and then using them for breeding when females became available a day or so later.
A few years ago one of the members indicated he prolonged the life of males by putting them in close proximity to a light. Instead of flapping around all night long in the cage, trying to escape, the male would become "stupified/mesmerized" by the light and would conserve his energy and prolong his life. That is also an approach worth trying, but I would be concerned that the extra heat from the lamp might contibute to desiccation. still its an approach worth trying if fridge crisper storage is not available.
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