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2025 Rainbow Stag Beetle Breeding TrialMeet the ParentsMale 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.
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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.
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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.
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After a few days, I reintroduced her to the same male for another week to allow more mating.
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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.
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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.
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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.
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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.
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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. 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.
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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.
Next to it is the larva that already died. Its body has dried out and shriveled, leaving only the orange head behind.
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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.
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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. |
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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. 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. |
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Winter Preparation
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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
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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.
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The 18.4g larva will hopefully become a male adult 60mm+. The other two will probably be males in the 55-59mm range.
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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 kinshibins 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.
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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.
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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.
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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.
