Japanese Stag Beetle Larva Rearing Guide | Fungal Substrate Bottles, Substrate Mat, Temperature Management and Tips for Raising Large Individuals
Detailed explanation of how to raise Japanese stag beetle larvae. Covers the difference between fungal substrate bottles and fermented substrate mats, replacement timing, temperature management, tips for raising large individuals, and management methods from pupation to emergence.
Key Takeaways
Detailed explanation of how to raise Japanese stag beetle larvae. Covers the difference between fungal substrate bottles and fermented substrate mats, replacement timing, temperature management, tips for raising large individuals, and management methods from pupation to emergence.
The Japanese stag beetle (scientific name: Dorcus hopei binodulosus) is a large stag beetle representative of Japan, and many breeders aim to raise individuals exceeding 80mm in size. Proper larval rearing management is the key to achieving larger sizes, and the precision of fungal substrate bottle selection, temperature management, and replacement timing greatly affects the final body size. This article provides a detailed guide to larval rearing management for raising large Japanese stag beetles.
Larval Instars and Growth Stages
Japanese stag beetle larvae grow through repeated molting from 1st instar → 2nd instar → 3rd instar after hatching from eggs. The characteristics of each stage are as follows.
Instar
Characteristics
1st instar larva
Very small stage of only a few millimeters in body length, with low food consumption
2nd instar larva
Body length reaches approximately 1–3 cm, and appetite increases
3rd instar larva
The most important stage; management during this stage greatly affects the final body size
Late 3rd instar (stage approaching pupation)
Appetite drops sharply, and the body takes on a yellowish tint. Avoid replacing the fungal substrate bottle and observe quietly
The sex of larvae can be identified at the 2nd–3rd instar stage by the presence or absence of a "V-shaped marking" (yellow ovarian marking) on the abdomen (5th abdominal segment). If a V-shaped marking is present, the larva is female; if absent, it is male. Since males are more likely to grow larger, when aiming for larger individuals, special attention should be given to managing male specimens.
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Comparison of Fungal Substrate Bottles and Fermented Substrate Mats
There are two main approaches to raising Japanese stag beetle larvae: "fungal substrate bottles" and "fermented substrate mats." Fungal substrate bottles are made by cultivating mushroom fungi (oyster mushroom, king oyster mushroom, turkey tail, etc.) on wood chips, allowing larvae to efficiently obtain nutrients by decomposing and consuming the mycelium. For Japanese stag beetles in particular, oyster mushroom-type and turkey tail-type mycelium substrates are commonly used. Fermented substrate mats are made by fermenting wood chips from hardwoods and are less expensive than fungal substrate bottles. The characteristics of each are as follows.
Item
Fungal Substrate Bottle
Fermented Substrate Mat
Material
Wood chips with mushroom fungi (oyster mushroom, king oyster mushroom, turkey tail, etc.) cultivated on them
Fermented wood chips from hardwoods
Advantages
Fast growth rate, easier to produce large individuals
Inexpensive, and suitable in some cases for restless individuals or smooth transition through pupation and emergence
Disadvantages
Expensive and time-consuming to manage (temperature and monitoring for deterioration)
Many breeders believe the potential for large size growth is inferior to fungal substrate bottles
As a general approach, many breeders adopt a hybrid method of "replacing 2–3 fungal substrate bottles → transitioning to mat in late 3rd instar."
Selecting Fungal Substrate Bottles and Capacity
It is common practice to use different volumes of fungal substrate bottles depending on the larval instar for Japanese stag beetles.
1st bottle (for 1st–2nd instar) should be 800–1000 ml
2nd bottle onward (for 3rd instar) should be 1400–2300 ml or larger
For 3rd instar male larvae aiming for large size, use large-capacity bottles of 2000–3000 ml (to ensure adequate food and space for movement)
Regarding brands, oyster mushroom-type mycelium from various manufacturers with proven results is widely adopted among domestic breeders. In terms of mycelium type, oyster mushroom-type has high versatility and is easy to handle for beginners. Turkey tail-type is more nutrient-rich but has the drawback that uneaten portions deteriorate more easily.
Timing for Replacing Fungal Substrate Bottles
The timing for replacing fungal substrate bottles should be determined while observing the larval instar and condition. Guidelines are as follows.
Initial placement of 1st–2nd instar larvae in a bottle should occur 1–2 months after hatching
Replacement with the 2nd bottle should occur 2–3 months after initial placement (when feeding marks reach 60–70% of the total)
For the 3rd bottle (final bottle), 2–3 months is the guideline while monitoring temperature management and mycelium condition
The following signs indicate the need for replacement.
Feeding marks occupy most of the entire bottle
Mycelium has deteriorated and turned brown (excess moisture)
Larvae frequently move around the bottle walls ("thrashing")
When thrashing occurs, it is often a sign of mycelium deterioration or larval stress, so prompt replacement with a fresh bottle is necessary.
Temperature Management and Tips for Raising Large Individuals
Temperature management is one of the most important factors in raising large Japanese stag beetles. The effects of different temperature ranges are as follows.
Temperature Range
Effect
Approximately 18–23℃ (lower)
The larval period becomes longer (1–2 years), and as a result, the body tends to grow larger
28℃ or higher (high)
Growth accelerates, but there is a tendency for the adult size at emergence to be smaller
Many breeders manage air conditioning to maintain 25℃ or below in summer (ideally 20–22℃) and a minimum of 18℃ or higher in winter. Simple cooling boxes with cooling packs and low-temperature management using converted wine coolers are also widely practiced. Thorough low-temperature management is said to be the most important point for raising large individuals.
Managing Pupation and Emergence
In late 3rd instar, larvae begin preparing for pupation and create a hollow called a "pupal chamber" within the fungal substrate bottle. Replacing the fungal substrate bottle at this timing will damage the pupal chamber and lead to pupation failure; therefore, it is a principle not to move the bottle after pupal chamber formation. If the pupal chamber is damaged, move the larva to an artificial pupal chamber (commercial product or homemade with oasis foam) to address the situation.
The pupal period lasts approximately 3–5 weeks, and after emergence, the adult remains in the pupal chamber until the body hardens. It is safe not to remove the adult for 1–3 months after emergence confirmation, and to remove it only after the exoskeleton has sufficiently hardened (approximately 30–50 days after emergence). The adult should be judged as mature only after confirming post-emergence feeding (when the adult begins eating). Male maturity (reproductive readiness) is estimated at approximately 6 months to 1 year after emergence. Recording the final body length and keeping breeding records is useful for planning the next generation of breeding.
