DIY Fermented Substrate Guide | How to Make Your Own Substrate for Rhinoceros Beetle and Stag Beetle Larvae

What is Fermented Substrate and Why Make Your Own?

"Fermented substrate" is bedding material made by fermenting and decomposing sawdust and hardwood chips using microorganisms — an essential component for raising rhinoceros beetle and stag beetle larvae. Since larvae feed on this substrate to grow, its quality significantly affects growth speed and final size. Substrate that is not sufficiently fermented cannot provide adequate nutrition to the larvae, while over-fermented substrate generates harmful gases and increases mortality risk.

Commercial fermented substrate is convenient with stable quality, but costs 600–2,000 yen per bag (5–10L). For serious breeders managing multiple enclosures, substrate costs represent a significant fixed expense. Making your own substrate can drastically reduce material costs and offers the advantage of adjusting fermentation level and nutritional composition to your preference.

However, making substrate requires time, effort, and space. Additionally, insufficient fermentation risks generating harmful gases (ammonia and carbon dioxide) that can injure larvae. We recommend starting by combining homemade substrate with commercial products as you gain experience.

Required Materials and Tools

Gather the materials needed to make fermented substrate.

• Base Material (Wood Base): Sawdust from hardwoods (Kunugi, Konara, Nara) is optimal. You can purchase it at home centers or agricultural supply stores. Never use softwood (Cedar, Cypress) as they contain compounds harmful to insects. Wood pellets (wood fuel pellets) rehydrated with water can also serve as an alternative material. They are valued for large-scale production due to low cost and easy availability.
• Fermentation Accelerants: Common agricultural byproducts include rice bran, wheat bran, and cornmeal. Nutrients in these materials promote microbial activity and aid wood decomposition. Add approximately 5–15% of sawdust by weight. Excessive amounts cause over-fermentation and gas generation. By varying the ratio of additives, you can adjust the substrate to suit different larval development stages (first, second, third instar).
• Moisture: Adjust moisture needed for fermentation. The ideal level is when the substrate forms a clump in your hand but crumbles lightly when released (approximately 45–55% moisture). Insufficient moisture slows fermentation; excess moisture causes decay.
• Fermentation Container: Use a large plastic container (60–120L) or vinyl bag. The ideal setup provides airflow while allowing a sealed environment.

How to Make Fermented Substrate: Procedure

Step 1: Mixing Materials Place sawdust and fermentation accelerants (rice bran, wheat bran) in a large container and mix uniformly. At this stage, add moisture and adjust to the consistency where it forms a clump but crumbles when squeezed. Wear gloves during work and be mindful of skin irritation and sanitation.

Step 2: Initial Fermentation (Primary Fermentation) Loosely cover with a lid to allow air circulation and place in an outdoor or well-ventilated location. An environment of 25–35°C is ideal for fermentation. Within 1–3 days, the temperature at the center will rise (potentially reaching 50–70°C). This heat generation is a sign of normal fermentation. If temperature does not rise, consider whether moisture, ambient temperature, or accelerant is insufficient.

Step 3: Turning Over Every 2–3 days, mix the entire batch thoroughly (turning over) and provide air circulation to ensure uniform fermentation. With each turning, check for ammonia or sour odors. Strong odors indicate uneven fermentation, so mix thoroughly. Turn the pile at least 3–4 times, and keeping a record of temperature changes each time will serve as reference for the next batch.

Step 4: Maturation and Finishing (Secondary Fermentation) After 4–6 weeks, when the entire batch has turned uniformly tan to dark brown and develops a distinctive earthy aroma (similar to commercial rhinoceros beetle bedding), completion is near. Finally, allow heat generation to subside and temperature to stabilize for an additional 1–2 weeks. If color is not uniform or white mold appears in patches, turn over again. While white mold may indicate beneficial fermentation fungi, green and black mold warrant caution.

Pre-Use Confirmation and Precautions

Before placing the finished substrate in an enclosure, verify the following points.

• Gas Venting: Substrate stored in a sealed state may accumulate harmful gases. Leave the enclosure uncovered for 1–3 days to allow gases to dissipate before adding larvae.
• pH Check: The proper pH for fermented substrate is approximately 5.5–7.0. Confirm with pH test paper; if overly acidic, add a small amount of lime to adjust. Excessive acidity negatively affects larvae's gut bacteria and can cause digestive upset and stunted growth.
• Small-Scale Trial: When using homemade substrate for the first time, place a few larvae in a small enclosure and observe for 1–2 weeks before full-scale use. If larvae fail to burrow or emerge on the surface unexpectedly, discontinue use. Larvae are sensitive and react quickly to substrate quality issues.
• Storage: Keep finished substrate in a resealable plastic bag or sealed container in a cool, dark place. Removing air and sealing tightly is important to prevent re-fermentation.

Homemade substrate is appealing not only for cost savings but also for the flexibility to adjust nutritional composition according to larval development stage. The safest and most reliable approach is to start by mixing it with commercial substrate, then gradually increase the proportion of homemade material.