Pairing (silk)moths

You’ve done it; after pouring months of work into raising the caterpillars and storing the cocoons, the adults begin to hatch. All you need to do to complete the life cycle is one thing: pairing! 

Pairing of Automeris io (Saturniidae) in captivity

Note: This page broadly generalises the pairing mechanics of moths to resemble that of Saturniidae, Lasiocampidae, Lymantriidae and other moth families with reduced mouthparts that heavily rely on pheromones. There are over 215.000 species of Lepidoptera in the world, and many of them have their own unique pairing habits. This page only describes the heavily generalised mechanics behind some Bombycoidea to help breeders reproduce them in captivity.

Pairing moths in captivity is either the easiest or the most frustrating part of completing the life cycle of a species. For some species it is notoriously difficult or even impossible. Others will pair with anything that moves, including other species and will breed like pests. What makes the difference?

Understanding pheromone

Pheromones are chemical compounds that insects use to communicate in several ways. When they are trying to find a sexual partner, the majority of moths heavily rely on one mechanic: pheromones. This page only focuses on sex pheromones which are odorous chemicals released by a moth, in order to attract a mate. In the majority of cases, the female releases these pheromones into the air in order to attract males. They are only odorous to other individuals of the same species; in most cases, releasing only a few nanograms of pheromone is enough to attract males. Males can be so sensitive to their species-specific pheromone blend that they can detect a single molecule of pheromone with their antennae. The smell is not noticable to humans or even other moths that are not of the same species. Note that in nature, exceptions to these rules can be found in many cases and that this text is just a heavy generalisation to help breeders pair their moths.

Females usually release pheromone against the wind from a gland in their abdomen; this act is called “calling”.  In some species it is very noticable when females are trying to call males, for they can be seen protruding the pheromone gland from their abdomen. To get a female to call it is important that they are stress free and have appropriate and natural lighting. They do this at specific times of the day. Most moths are active at night, but some moths (such as Callosamia promethea) do it during the day. This is why lighting is also important: the intensity and durating of the daylight regulates their biological clock and tells them what time of the day it is. Some moths have specific hours or times of the day during which males and females are active. These are best synchronised.

A female Dysphania species from Malaysia calling in captivity

The pheromone plume

The pheromone plume is an important concept. Pheromones don’t just randomly diffuse; that would make it hard for a male to pinpoint a location. Instead, pheromones are carried by the wind. The female does not constantly release pheromone usually: in most cases, she releases them in short intervals of time. This is what allows males to estimate the distance between him and her. As they are carried by the wind, the wafts of pheromone are dispersed and stretched out, increasing the distance between the molecules. The further  away the male is removed from the female, the more noticable these intervals become to his antennae. This grants the males the ability to judge if they are moving away from, or instead, are getting closer to the source. Usually they are seen flying in a zig-zag like pattern against the wind, trying to pick up the trail of pheromones, tracing it back to its origin.

This awesome paint illustration shows the pheromone plume:
A male tracks a female by remaining within the scent plume, tracing it back to its origin. Species: the very rare Bartus coppensii 

Copulation of Bartus coppensii (rarest moth on earth, endemic to Microsoft Paint)

Copulation

Once traced back to the female, the male gets close to her and starts to pair with her. On the end of their abdomen, males usually have two hooks or plates he can open and close in order to attach himself to the female (they are usually called claspers or valves). Because of this, males can dangle upside down below the female when pairing. They are suspended in the air and attached by their claspers only, although in some cases the male will prefer to grip on a surface.  In some cases he will simply grab the females abdomen. Then he will deposit a rubbery spermatophore inside the female, fertilising her.

It is not unusual for moths to remain together for 24 hours after pairing, even if the actual copulations only last 30 to 60 minutes. However, it is best not to disturb them as long as they are still attached.  Males possibly remain attached to the female for a longer time for this will lower her chances of pairing with another male, effectively blocking her for a day. However, soon they will part ways. In most cases males can pair with multiple females, and females can also pair with multiple males.  It is advised not to handle them during this process, in response to stress they can abort the pairing and decouple in order to escape.

The importance of ventilation

In captivity, males and females are often included together in small aerariums or cages made of netting. Ventilation is very beneficial for pairing adults. As you have read in the previous texts, males trace the source of the pheromone back to the origin by following her scent plume. However, in captivity, our houses can have stale air. Sometimes, they are unable to find the females and pair with them, because their whole room or container is filled with the females pheromones. Because the molecules are seemingly coming from everywhere, the males has no way of knowing where the female is. This is why they are best paired in front of an open window, outside, or in draughty places, and why they perform best in open and well-ventilated containers instead of airtight ones.

A male and female in a breeding cage, in an environment that lacks ventilation

A cage I use to pair moths outside in summer. A sheltered, cool spot with a breeze

Epiphora ploetzi pairing

Are your moths not pairing?

Some things that can negatively impact pairings are:

  • Inbreeding. Moths are, just like many animals on this planet, simply less attracted to siblings. They can smell how strongly related they are to other males or females and are less attracted to pheromone that resembles their own. That being said, most moths will initially pair with brothers and sisters in captivity, but the resulting offspring will become less and less attracted to eachother if inbred for multiple generations, to the point of completely refusing to pair with eachother. In the wild, brother and sister couplings are generally avoided. It has higher chances of happening in captivity. Note: the level of inbreeding is not about the amount of generations produced in captivity, but about the genetic diversity – something that varies.
  • Stress. Moths are not bright creatures, and you don’t have to worry about their short term memory. Still, some species are very sensitive to stress. If handled very roughly or constantly for a long time, some individuals will lose their interest in pairing that day. Leave them alone and try again tomorrow.
  • Artificial light. The biological clock of moths, like many surface dwelling animals, is regulated by the day/night cycle. Artificial light can disturb their biological clock, causing desychronised periods of activity or inactivity. Think of it like this: You too would be less likely in the mood to have sex with your partner tomorrow if somebody forced you to sleep while all the lights are left on at night in your house.
  • Apathy. It’s true, some individuals will refuse to pair in captivity, no matter how well you treat them or give them what they need. Many breeders have complained to me about, excuse me for the language, what they describe as “retarded” individuals. It’s not a politically correct term but it illustrates the problem: for mysterious reasons, some individuals will simply not attempt to pair whatsoever. This is probably a complex problem that stems from many factors that are different in captivity. This is also more frequently seen in adults that hatch from cocoons that were shipped in the cocoon stage over a large distance; in this case the stress and shock from postage/transport has developmentally impaired the adults. While seemingly healthy looking, the damage or impaired development can be neurological or internal. Apathic adults can also be produced when the caterpillars are raised on a unusual diet: many moths biosynthesise pheromones from compounds in the specific host plants they eat. When fed artificial diet or plants they do not eat in the wild (substitutes), their diet could miss the specific compounds they need to produce pheromone; effectively making them infertile since they moths have no way to locate eachother, because they are unable to produce any pheromone. It can also happen due to reduced vitality and many other things!
  • No ventilation. In captivity, our houses can have stale air. Sometimes, they are unable to find the females and pair with them, because their whole room or container is filled with the females pheromones. Because the molecules are seemingly coming from everywhere, the males has no way of knowing where the female is. This is why they are best paired in front of an open window, outside, or in draughty places, and why they perform best in open and well-ventilated containers instead of airtight ones.
  • Insufficient temperature/lighting. Most moths are not picky and will pair in a dark room in front of a window on room temperature. But some have specific needs. Some need a high degree of humidity, UV light or specific temperatures to pair.
  • Intensity of the pheromone. Some moths have populations that live in very low densities – they have to call eachother from miles away, or in dense forests with not a lot of airflow where they need more concentrated  or stronger pheromones to be able to call males over a larger distance – in some have these very strong smelling pheromones to archieve this. These species are more prone to having a ‘sensory overload’ if not given a lot of space and ventilation.

Handpairing

If a natural pairing does not work, in some cases moths can be “forced” to pair. Forced is in quotation marks because you can never 100% force them to copulate, the moths need to co-operate to some degree. But you can make it easier for them by collecting the abdomen of the male to that of the female. I have been able to handpair moths from many families including Saturniidae, Lasiocampidae, Papilionidae, Nymphalidae.. et cetera. Still, it seems that some families are hard or impossible to handpair (I know nobody that has ever handpaired Sphingidae for example).

Here is a video that demonstrates handpairing:

What you basically need to do is get the male to open his claspers. This can be done by stimulating them with the abdomen of the female, and in some cases, with a stick or needle. You can also try to excite him by rubbing his antennae over the abdomen of the female. When opened, the claspers of the male can latch around the abdomen of the female in the correct angle (differs per species). Now, if you are lucky, he will grab her tight and soon start to copulate.

This method will not work if the males or females are  completely stressed out, it is best to gently try this, don’t treat them as toys you can squeeze together, but try to gently coax them. In some cases handpairing will fail even if you do everything right: some species will just not co-operate because they hate being grabbed and will struggle or do anything to escape. Some species also have internal claspers; this means you cannot help the male open his hooks around the female in order to attach her, making it seemingly impossible. However with some luck and skill, most silkmoth species can be handpaired, and many butterflies too. I use handpairing as a last resort, not as reliable method of reproducing them.

Handparing a H. cecropia male with a relaxed female

Pairing of Samia cynthia

Pairing of Hyalophora cecropia

A pairing of Citheronia laocoon

Handpairing Lasiocampidae? It is possible. Handpaired Gonometa nysa

Inter-species attraction? Pheromones consist of very specific chemical blends that allow Lepidoptera to differentiate between members of their own species and others. When a female releases pheromones, she releases them in such low amounts – usually only a few nanograms – that virtually no other animals are able to smell or detect them (virtually since some clever predators, like some wasps, can still smell them to hunt them down). In this case, the males are extremely sensitive to the composition of the females pheromone – some species can detect a single molecule of pheromone.

Usually moths are only attracted to members of their own species, but there are exceptions. Very closely related species can still get confused because they smell similar and pair with eachother. This results in hybrid moths. Hybridisation is an extremely rare event in the wild, but wild hybrids have been recorded. For example, some Saturnia species (such as pyri and pavoniella) are found to interbreed in very rare occasions – and species like Hyalophora too. Members of the genus Actias are moderately attracted to eachothers pheromones too.

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