2025 Rainbow Stag Beetle Breeding Trial

Meet the Parents

Male Type: Gold Full Metal
Emerged: July, 2023
Size: 36mm

Female Type: Normal
Emerged: October, 2024
Size: 33mm

Pairing Date: June, 2025

NOTE: How to set up the egg laying container and how to use kinshibins can be found on my main rainbow stag beetle page.

They were paired together at the start of June, 2025, for 2 weeks. The maximum temperatures were around the 23 to 25 degrees mark, perfect for breeding. They mated multiple times during this period.

The female was placed in an egg laying container. I used RTN's U-MAT. I waited until the female started eating the jelly before removing her, which was about one and a half weeks later. Eating jelly is usually a sign that she has finished laying eggs.

After removing the female from the egg laying container, I kept her separate from the male by using a divider for a few days. This was to give her time to rest, as laying eggs takes a lot of energy.

After a few days, I reintroduced her to the same male for another week to allow more mating.

The female was then placed in the same egg laying container as before. I waited until she started eating jelly again before removing her. This time it was only 1 week later.

After another 3 weeks, I saw first instar larvae moving in the egg laying container. I waited another week or so before looking through the humus.

I found 30 larvae in the egg laying container. I put 14 first instar larvae in 200cc G-cups, 8 bigger larvae in 550cc G-pots, and 8 larvae (a mixture of first instar and second instar) in RTN's original 550cc kinshibins. I tried to keep the larvae at 24 degrees by using the air conditioner. However, the summer temperatures in Japan regularly reached 34 degrees Celsius in July, and I simply couldn't keep the air conditioner on all the time. Regular temperatures above 28 degrees Celsius result in a higher death rate for larvae, so I needed to find an alternative cooling method.

At the end of July, I invested in a wine cellar. The temperature is not perfect, as the highest temperature I can set it to is 18 degrees Celsius, but the actual temperature seems to be between 17 and 19 degrees Celsius inside. It's below the recommended 24 degrees Celsius for rainbow stag beetles, but this is a better option than exposing the larvae to 30+ degrees Celsius temperatures every day, which causes greater stress. As long as the temperature in the wine cellar stays above 15 degrees Celsius, the larvae should continue to feed and grow, though at a slower pace.

The initial humidity inside the wine cellar was around 48%, which is too low. The kinshibins will dry out too quickly at this humidity, and rainbow stag beetles do better in moist conditions.

I added two wet towels on trays. This helped increase the humidity to over 60%, which is in a safer zone. As long as the humidity stays between 60 and 80%, it should be perfect. I change the wet towels every few days to prevent mold growing. As I open the wine cellar door to do this, it allows air transfer to occur, keeping the oxygen levels suitable for the larvae to survive. At the moment, the larvae seem to be doing well.

After spending a month in the wine cellar, the larvae in the 550cc G-pots were feeding actively on kinshi at 18°C. However, I noticed the larvae in the 200cc G-cups weren’t eating at all. This is typically a sign of death, so I decided to open them up for a check. Three were dead, but the remaining eleven were still alive, though inactive. They hadn’t eaten and were almost the same size as when I first put them in the G-cups.

I suspect the larvae in the 200cc G-cups were more affected by the 18°C temperature than those in the larger 550cc G-pots. The smaller G-cups clearly couldn’t shield the larvae from the cold in the wine cellar as effectively, and when larvae get too cold, they stop eating.
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I moved all the 200cc G-cups out of the wine cellar at the end of August and transferred the kinshi into small containers with U-MAT humus, placing them in a room that often had the air conditioner running. After this, the larvae became more active and started feeding again. I don’t think I’ll be using 200cc G-cups in the wine cellar again.

At the end of September, I decided to change out the kinshi in the containers that had U-MAT, just to check on how the larvae were doing. The 200cc G-cups are really only meant for one month’s use, so I kept the larvae in there longer than I should have, but they also had humus to eat. Since then, there’s been one more death, but nine larvae are still going strong. Another one, however, is covered in mites. All these larvae are a little on the small side. Below are all the larvae from the 200cc G-cup and humus combination.

The larva on the upper left is covered in pinkish mites, which are very difficult to remove without causing harm to the larva. This is also a sign that the larva is sickly, as healthy larvae are typically able to prevent mite infestations. Unfortunately, there’s a high likelihood that this larva will not survive.

Next to it is the larva that already died. Its body has dried out and shriveled, leaving only the orange head behind.

Now for the larvae that were originally put into the 550cc G-pots. Two months after being put in the 550cc G-pots above, I decided to put these larvae into new kinshibins. This is because most had eaten about 60-70% of the kinshi. Also, you should change the kinshibins every 2 to 3 months as well to keep the kinshi fresh. You can see that a few of the G-pots had started to turn a dirty yellow color. Many of the larvae from these G-pots were extremely big, significantly bigger than the larvae that were originally put in the  200cc G-cups. I upgraded the bigger larvae (males) to 800cc G-pots, while putting the smaller larvae (females) in new 550cc G-pots. However, there was one surprise larva- it was still extremely small, so I put that in a 200cc G-cup to monitor it more closely.

All 8 of the larvae that were originally put in the 550cc G-pots, are still alive. Seven of them are very healthy and should eventually become large adults. However, as you can see to the left, one larva didn't show any growth, so I'm interested to see if it will survive.

The picture to the right, shows one of the bigger larvae in his new 800cc G-pot. To be honest, it would be best to put the bigger larvae into 1500cc G-pots at this stage, but it becomes very costly to do so. I will consider doing that when I change the kinshibins in another 2 or 3 months.

The biggest disappointment came from the larvae I placed in RTN’s original 550cc kinshibins. These were more expensive than the 550cc G-pots, but resulted in noticeably smaller larvae. As you can see in this photo, unlike the G-pot kinshibins, there’s little to no visible evidence that the larvae were feeding at all.

I originally put eight larvae into these containers to compare their growth with those raised in the G-pots. When I opened them, I found that most were only slightly larger than the ones that had been in the 200cc G-cups with humus. Instead of burrowing and feeding throughout the kinshi, most had created small air pockets and stayed in one spot, barely moving—clearly not feeding much. That lack of activity likely explains their poor growth. On the upside, although smaller, all the larvae were very healthy, as you can see in the photos below.
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While RTN’s kinshibins might work well for raising large Dorcus beetles, they don’t seem suitable for rainbow stag beetles. In the future, I’ll be sticking with the G-pots. 

Winter Preparation

As November arrived, the weather finally turned cold, with temperatures dipping into the mid-teens. This is a bit too cool for rainbow stag beetle larvae if you’re aiming to raise larger adults, so I moved many of them into a Styrofoam cooler equipped with a reptile heater panel and a thermometer to monitor the temperature. You can find more details about this setup on the main rainbow stag beetle page. As shown in the image above, the room temperature is currently around 15°C. For healthy larval development, temperatures should stay above 18°C, with the optimal range being between 22°C and 24°C.

3rd Kinshibin Change

With more than two months passing since the last kinshibin change, I checked the kinshibins in my heater box system and also the G-cup/U-MAT humus containers at the end of November, 2025. Many of the larvae had eaten more than 70% of the kinshi, so I decided to change the kinshibin for these larvae. I also decided to weigh them to get a better understanding of whether they would be males or females, and also how big they could potentially become. I purchased three different sizes to cater for this; 550cc for the females, 800cc for the larger females or smaller males, and 1500cc for the larvae that should become larger males.

The 18.4g larva will hopefully become a male adult 60mm+. The other two will probably be males in the 55-59mm range.

The 12.5g and 11g larvae will probably become small males, less than 50mm. All the other larvae that are about 8g will probably become females in the 30-40mm range. The 6g larvae will probably be about 25mm.

The 8.7g, 8.6g, 8.5g, 8.0g, and 7.7g larvae were all from the G-cup/U-MAT combination.

I did not change seven other kinshibins and two G-cup/U-MAT containers, as the larvae had not eaten 60-70% of them, and the kinhsibins still looked okay.

Near the end of December, I checked all the kinshibi​ns again. I checked to make sure all the larvae I had recently changed were doing okay. Then I changed three kinshibins that I didn't change the previous month. The biggest larvae was 15.4g, which will probably become an adult male in the 55-59mm range.

At the start of January, 2026, I checked all the kinshibins again. I decided I should change the kinshibins for the larvae which hadn't been done yet. However, it appears I was too late. After taking out three of them, it seemed they had already started preparing their pupa chambers. They were a darker yellow color, which is also a sign that they are nearing pupation. Upon seeing this, I decided not to change the last kinshibin or the two G-cup/U-MAT containers. There is a chance that the three I changed may die if they don't have the energy to remake their pupa chambers, or if they do survive, they may just become pupae on top of the kinshibin.

I did not change the kinshibin on the far right of the picture above, as the larva appears to have started making its pupa chamber. I circled its position just to see if that is where it will stay.

This will be my final kinshibin change for all the larvae now, as it appears many of them are preparing to become pupae. The mite infested larva is still alive, as well as the very small larva. I didn't include their photos here, as I don't want to handle them too much. 

Spring has arrived.....

Spring has arrived, and with it, many of the larvae built their pupal chambers and transformed into pupae during April 2026. Unfortunately, the mite-infested larva died while it was constructing its pupal chamber, but all of the others appear to be developing well.

Now, fast forward to May, and the first adults are beginning to emerge. So far, some of the smaller males and many of the females have completed their transformation. I have already taken a few of them out for their first photo sessions, and the results have been very exciting.

As mentioned previously, the male parent was a Gold Full Metal, while the female parent was classified as a normal type. Because of this pairing, I was eager to see what traits would appear in the offspring.
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So far, the females have been a fascinating mix of colors and traits. Two females show normal-type characteristics, one female has developed a strong green coloration, and two females display a beautiful deep red color.
The males have been equally impressive. The 14 g larva developed into a beautiful green male measuring 52 mm in length, with excellent coloration. The other two males both show beautiful red tones. The 11 g larva became a smaller 40 mm male with a bronze-brown metallic mirror finish on its pronotum, while the 12.5 g larva developed into a larger 45 mm male with a striking gold metallic pronotum.
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It has been exciting to watch this bloodline develop, and I look forward to seeing how the remaining beetles color up over the coming weeks.

Three more females have emerged, and all were a similar size (35-38mm). Their larval weights were each around 8 g, and all of them developed beautiful mirror-like pronota.

​This photo, taken by my daughter, shows another beautiful male that emerged, perfectly demonstrating why they are called rainbow stag beetles. Its body truly looks like a rainbow, shining with brilliant reds, yellows, greens, blues, and purples.

Another female has emerged, measuring 35 mm. It seems that all the larvae that weighed around 8 g developed into female beetles in the 35–38 mm range, which matches what I expected based on previous data.

Pupa to Adult: Rainbow Stag Beetle Eclosion

I have also taken the risk of removing three of my larger male pupae from their natural pupal chambers in the hope of documenting their transformations more closely. To do this, I carefully placed them into makeshift artificial chambers that I created using the green floral foam blocks commonly used for flower arranging.

Although this will hopefully allow me to photograph the different stages of their emergence into adulthood, I am a little concerned that it may have been the wrong decision. Disturbing a beetle’s natural pupal chamber always carries a higher risk of deformities or even death, especially with larger males.

One of my biggest concerns at the moment is whether their wings will close properly after emergence. If the artificial chambers are not shaped correctly, or if the humidity and moisture levels are too high or too low, it can lead to problems during the transformation process. Even small mistakes can sometimes cause issues with wing formation, body shape, or hardening.
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For now, all I can do is monitor them carefully and hope the conditions are suitable for a successful emergence. If everything goes well, I should be able to capture some exciting photographs showing their transformation from pupae into fully developed adults.

Before emergence, rainbow stag beetle pupae should ideally rest on their side, as this position allows them to move more easily and helps their wings close properly during transformation.
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At the moment, the two outer pupae have settled into the correct position. However, the middle pupa is currently lying on its back. If it does not reposition itself naturally, or if it is not gently rolled onto its side, there is a risk that it could develop deformities during emergence.

On May 12th, 2026, the middle larva became darker. Its underside turned pinker, and parts of its body developed a rainbow-like shimmer.

By May 14th, the middle larva had transformed from a pupa into an adult. Its pronotum was a reddish-brown color.

You can see that the outer wings (elytra) are orange as they begin to cover the hindwings. The hindwings must dry properly before being covered by the elytra, which normally happens when the beetle is standing on its legs. Its mandibles are still curved and extremely soft at this stage.

By May 15th, the elytra had completely covered the hindwings and had turned a reddish-brown color. The mandibles were also starting to straighten.

By May 16th, the mandibles had straightened into their normal position. The beetle was still mostly reddish-brown, but parts of it were beginning to turn green.

By May 18th, it had developed a much stronger green metallic color, while its legs had changed from reddish-brown to black. It had also moved inside the artificial pupal chamber and was now facing the opposite direction.

By May 21st, the middle beetle had developed its rainbow colors and a shiny, mirror-like pronotum. Now it just needs to be left alone for a couple more weeks. It should be happy to stay in its artificial pupal chamber during that time.
The pupa on its right also looks very close to emerging now.

By May 22nd, the middle beetle had started moving around its section of the container, so I moved it into a separate container. Its pronotum had darkened into a beautiful mirror-like finish, and its wings had developed a lovely rainbow sheen. It is amazing to see how much variation there is among the beetles emerging from the same two parents.
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As a larva, this beetle weighed 15 g. As an adult, it measures 56 mm long, which is within the expected range.

The Main Event.....

On May 24th, my largest larva, which weighed 18.4 g, finally emerged. This was the beetle I was most worried about, but thankfully, everything seems to be going smoothly.
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I sat and watched for three hours as it emerged from its pupa. I took photos at different stages, which was the main reason I placed it in the artificial pupa chamber I had made for it. I also recorded video. I have put it at 30x speed.

In the morning, it was on its back, which was a little concerning.

However, when it was ready to emerge, it rolled over onto its front.

It was interesting to watch the beetle emerge without any complications. In the last picture, you can see that its wings are still slightly wet, so it needs time to dry them before they are fully covered.

Over the next few days, the wings should gradually cover its body, turn reddish-brown, and then begin developing their rainbow colors as they harden. I am very intrigued to see which coloration will prevail, whether it will develop a mirror-like pronotum, and how long it will be. My fingers are crossed for 60+ mm.

By May 25th, its elytra had turned reddish-brown. Its hindwings were still visible beneath.

By May 26th, the hindwings were completely covered by the elytra.

Occassionally, the beetle flips over onto its back. Its mandibles have straightened, but they are still soft.

By May 27th, its rainbow colors started to show. It was still diffiult to know its true color traits.

I finally decided to measure it to see if it hit the 60 mm mark. Sadly, it registered 59 mm. It wasn't quite what I was hoping for, but he is a beautiful emerald color which I will use in future breeds.

Larva Weight to Beetle Length

The 18.4 g larva became a 59 mm male. I decided to make a video to compare three more male beetles that recently emerged. One of the males in the video has similar color traits to my 59 mm male. Another one has a mirror-glazed dark pronotum, and the other has a stronger red variation. it really is interesting to see the varying traits from the same two parent beetles.

The final male from the artificial pupal chamber I made emerged with his right mandible longer than his left. I could already see that this was going to happen when he was still a pupa. He had a full-metal green coloration.

After checking the final kinshi bottles, I found that the extremely small larva had emerged as a healthy but tiny female with a strong red coloration.
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Unfortunately, another kinshi bottle contained a female that died while emerging. Looking at the photo, you can see that her wings did not close around her body properly, possibly because the pupal chamber had collapsed. This was a shame, as she would have been a large female.​

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