Mushroom Bottle (Kinshi-Bin) Selection & Replacement Guide — Secrets to Raising Large Beetles

Fungal substrate bottles are one of the most effective methods for raising stag beetle larvae. As mushroom mycelium breaks down sawdust, the nutritional value increases and promotes larval growth. Selecting the right fungal substrate bottle and replacing it at the correct time is key to producing large specimens.

How Fungal Substrate Bottles Work and the Differences Between Fungal Species

A fungal substrate bottle is a rearing container where mushroom mycelium is cultivated in hardwood sawdust. The mycelium breaks down lignin and cellulose in the sawdust, making it easier for larvae to digest. There are three main types of fungi used. King oyster mycelium is the most common, with good larval feeding responses and suitability for many species, including native giant stag beetles. Kawara mushroom mycelium is essential for certain species like Tarandus and Regius beetles, and is used for species that cannot develop on king oyster mycelium. Oyster mycelium has similar properties to king oyster mycelium but is slightly more cold-hardy. For beginners, fungal substrate bottles using king oyster mycelium are easy to handle and recommended. Different manufacturers use different sawdust species, particle sizes, and additives, which affect larval growth.

Choosing the Right Capacity and Size

Fungal substrate bottles are selected based on capacity. For 1st to 2nd instar larvae, use small 500-800ml bottles; for 3rd instar males, use medium 1400-1500ml bottles; for large species or when aiming for large specimens, use large 2000-3000ml bottles. Females often require only two 800ml bottles from 1st instar to adult emergence. For male giant stag beetles, the typical relay system involves 800ml (1st-2nd instar), then 1400ml (early 3rd instar), and finally 1400ml or 2000ml (late 3rd instar to pupation) for a 3-bottle relay. Wide-mouth bottle shapes make it easier to introduce larvae and observe them, and also reduce stress during substrate changes. Purchasing blocks (refill fungal substrate blocks) and packing them into bottles yourself can reduce costs, but there is a risk of contamination by unwanted microorganisms, so work must be done in a clean environment.

Determining the Right Time to Change Substrate

Correctly determining when to change the fungal substrate is a crucial factor directly linked to producing large specimens. As a general guideline, when feeding marks (shokokon: the brown portions eaten by larvae) visible from the outside of the bottle reach 70-80% of the total, it is time to change the substrate. However, the feeding speed varies depending on the species and temperature. A change is also necessary if the mycelium has deteriorated and turned brown, or if moisture has accumulated at the bottom. If mushrooms are growing abundantly, it is a sign that the nutrients in the mycelium are being consumed. Conversely, if there are signs that the pupa chamber is beginning to form, the substrate should not be changed. Destroying the pupa chamber can cause emergence failure. If the pupa chamber cannot be confirmed from the outside of the bottle, check that the progression of feeding marks has not stopped, and gently lift the bottle to confirm there are no cavities. When changing the substrate, carefully remove the larvae from the old bottle without damaging them, and measure and record their body weight to help track growth.

Temperature Management and Storage of Fungal Substrate Bottles

To maximize the performance of fungal substrate bottles, temperature management is important. For giant stag beetles, the ideal rearing temperature for larvae is 20-23°C. At high temperatures (25°C or above), mycelium deterioration accelerates and larvae tend to pupate early and become smaller. Conversely, at low temperatures (18°C or below), growth slows, but with time, they may develop into larger specimens. Common temperature management methods include wine coolers, incubators, and air-conditioned rooms. The ideal storage for purchased fungal substrate bottles is a cool, dark place (18-20°C); if left in a hot place for a long time, they will deteriorate before use. If you place the fungal substrate bottle in the rearing temperature location for 1-2 days before use to acclimate it, and then introduce the larvae, you can reduce stress from temperature differences.

Consult with Br-choku Breeders About Choosing Fungal Substrate Bottles

There are many manufacturers and brands of fungal substrate bottles, and evaluations differ depending on the species being raised and the breeder's preferences. At Br-choku, you can learn from breeders with a track record in stag beetle breeding about which fungal substrate bottle brands have actually produced results. Since the final size can change depending on which fungal substrate bottle is used, even with the same bloodline, a breeder's experience is extremely valuable. Some breeders also sell fungal substrate bottles as a package when you purchase larvae, so it is recommended to consult with them about everything together.