Biological Development of the Fighting Beetle (Xylotrupes gideon) in a Confined Rearing System

Abstract

Background and Objectives : The fighting beetle (Xylotrupes gideon) plays an important role in the ecosystem, contributing significantly to nutrient cycling and soil enrichment through the consumption of decaying plant matter during their larval stages. Furthermore, the adult beetle is a viable food source for humans, presenting an opportunity for sustainable food production. Populations in their natural habitats are currently declining due to several factors, including the destruction of natural habitats, which limits the beetles' ability to reproduce, pollution from agricultural practices involving chemical use and overharvesting for consumption. Although some aspects of this insect's biology were explored, the complete biology of this species, particularly that of the pre-adult stages, which reside underground, remains little known due to the difficulty of observation. This study aimed to investigate the complete life cycle and developmental biology of the fighting beetle from the egg stage to the reproductive adult stage, under controlled, confined rearing conditions, utilizing an artificial diet to mimic their natural food sources.

Methodology : Five breeding pairs of healthy males and females were used for offspring production. Subsequently, mated females were allowed to oviposit in a 25×22 cm culture pot filled with a fermented sawdust artificial diet. After allowing for 5 days for oviposition, eggs were verified via careful examination. Ten eggs were carefully selected from a single female and were meticulously monitored under a stereomicroscope to analyze the developmental processes occurring before the larval stage. The majority of the eggs obtained from all five female beetles were incubated until they successfully hatched into first instar larva (L1). Subsequently, 100 larvae (L1) were individually reared in 32-ounce plastic cups until they developed into the reproductive adult stage. Throughout the developmental period, data on the number of eggs per female, larval morphology at each instar, body weight, developmental duration, survival rate, sex ratio, and adult lifespan were recorded.

Main Results : The study demonstrated that the fighting beetle can be successfully reared in a confined rearing system using a formulated artificial diet that mimics their natural food source, allowing them to develop into reproductive adults. The average number of eggs laid per female was 34.20 ± 12.19. The eggs were white, oval-shaped, and approximately 0.3 cm in length. Fertilized eggs exhibited progressive developmental changes as observed through stereomicroscopy. Observation of eggs collected 5 days after oviposition revealed a translucent appearance. By 10 days post-female oviposition, the eggs had increased in size, and an opaque spot was observed, indicating embryonic development. By 15 days post-female oviposition, larval development was observed with clear segmentation of the body. By 25 days post-female oviposition, significant development was apparent with pigmentation of the body and clearly distinguishable head and mandible structures. These eggs hatched into L1 larvae after approximately 25-30 days. Newly hatched L1 showed a white head and body with reddish-brown hair on the body. The head color transitioned to brown over time. Larval development progressed through molting, with larvae molting from L1 to L2 and L2 to L3, with each stage approximately 30 days. The L3 larval stage took approximately 180 days. At the late L3 stage, the larva made a pupation chamber and their transparent skin changed to yellow and wrinkled before developing into a pupa stage. The durations from the pupa-teneral adult, teneral adult-reproductive adult and reproductive adult-natural death stages were 21.38±2.26, 32.82±3.24 and 53.81±15.59 days, respectively. The survival rates (%) from L1-L2, L2-L3, L3-pupa, pupa-teneral adult, and teneral adult-reproductive adult stages were 100, 92, 82, 82, and 71%, respectively. Forming a pupation chamber by L3 larva is a prominent behavior, providing an open space for pupal and adult development. The pupation chamber of this species was oval-shaped with rounded ends and possessed thick and rigid walls. This study yielded 41 reproductive adult males and 30 reproductive adult females, resulting in a male-to-female sex ratio of 1.00:0.73. A chi-square test indicated that this ratio was not statistically significant. Males exhibited variations in horn length. The coloration of adult fighting beetles from this study was classified into two primary groups: reddish-brown and black. Reddish-brown was the predominant color observed in the reared beetles. Within the reddish-brown group, there were varying shades that didn’t form distinct subgroups.

Conclusions : This study successfully reared the fighting beetle, elucidating their biology from the egg to the reproductive adult stages, under confined condition and using an artificial diet. The diet was formulated with rubber sawdust, dried cow manure, soil, wheat flour and water, mixed in specific proportions. The 71% survival rate of the fighting beetle from the larval stage to the reproductive adult stage under solitary rearing conditions (limited space) indicates that the formulated diet is adequate for supporting their development to the reproductive adult stage. This study provides critical insights into the feasibility and optimization of large-scale rearing for various applications, including sustainable conservation, commercial sale and alternative protein production from edible insect.