The white-spotted chafer, Protaetia brevitarsis (Coleoptera: Scarabaeidae), is distributed throughout Japan, Taiwan, Korea, China and parts of Europe (Cho, 1969). The adults are observed from late June through July in Korea (Kim et al., 2005; Zhang, 1984). These beetles are holometabolous and undergo three larval instars prior to pupation. Larvae overwinter as third instars in the soil and then pupate. In Korea, P. brevitarsis are raised commercially and used as a traditional medicine for the treatment of liver cancer (Park et al., 1994; Kang et al., 2001; Yoo et al., 2007). They were recently listed as a temporal food ingredient by Korean Ministry of Food and Drug Safety. Therefore, rearing of this beetle has gained increasing attention in Korea, and P. brevitarsis has been mass-reared for commercial purposes since the late 1990s. Thus, it is important to optimize rearing conditions for beetles to improve their commercial quality.

Diet and growth conditions have been previously investigated (Kwon, 2009). In this study, we sought to characterize the effect of supplementing insect feed with adsorbents such as clay and charcoal because they have been shown to remove contaminants such as heavy metals and bio-waste (Babel and Kurniawan, 2003). To improve the quality of the larvae, these two additives were added to standard commercial diet used for P. brevitarsis larvae, consisting primarily of fermented sawdust. We measured body weight change and pupation characteristics of insects fed diet supplemented with seven combinations of clay and/or charcoal.

Second instar larvae of P. brevitarsis (Coleoptera: Scarabaeidae) were reared from a laboratory colony started from insects purchased from a commercial supplier, Teunteun Farm (Siheung-si, Kyeonggi-do, Republic of Korea). The purchased beetles were reared on fermented sawdust at 25°C with ca. 40% humidity for six months prior to use in experiments. From this laboratory colony, second instar larvae were collected based head capsule size for use in subsequent experiments.

Second instar larvae were fed fermented oak sawdust diet purchased from a commercial supplier in Hoengseong-gun, Gangwon-do, Republic of Korea. To test the effects of feed additives, charcoal and/or clay were added at the indicated concentrations (Table 1). These additives were suspended in an equal weight of tap water and then mixed with the basic feed as described in Table 1.

Second instars were reared in round petri dishes (98 mm diameter x 15 mm depth) with sufficient amount of the designated feed at 25 °C with ca. 40 % humidity and a 12:12 h (L:D) photoperiod. We measured the body weight of each larva, and provided feed once per wk. Ten larvae were randomly selected for each feed treatment and maintained for approximately 17 wk until most second instars had pupated. Seven different treatments of different additive combinations (10 larvae each) were replicated three times. Larvae were weighed once per wk for 17 wk and development stage of each individual was checked until emerging adults. Larval weight, weight gain per wk, cumulative gain, weight prior to pupation, and time of pupation were calculated. The significance of each treatment was determined and compared to the control treatment using a t-test.

Larval body weights generally increased 0.105 ± 0.004 g (mean ± S.D.) per wk over the 17-wk experiment (Table 2). However, larvae gained less weight during the latter part of the larval period (Fig. 1). Three treatments – clay 25, charcoal 25 + clay 25, and clay 50 – showed no significant differences compared to the control feed containing no adsorbent (Fig. 2). The maximum weight increase was observed at wk 3 for two treatments (control and charcoal 50 + clay 50) and at wk 4 for five treatments (charcoal 25, clay 25, charcoal 25 + clay 25, charcoal 50, and clay 50) (Table 2). After that time point, weight gain gradually decreased. Moreover, weight decreases were also observed after wk 10, which may be attributed to the secretion of special proteins to form the pupal cell as a defense mechanism against unfavorable environmental factors in preparation of pupation (Lee et al., 2002; Shapiro-Ilan and Russell, 2015). Weight loss prior to pupation is commonly observed in many insect species that under the soil (Ichikawa et al., 2012; Johns et al., 2014).

Larval body weight prior to pupation was also analyzed (Fig. 3), because it is an important correlate of pupation rate and ultimate size of the adult beetle and therefore, the weight and size of prepupal larvae can be used as an indicator of adult insect quality (Sehnal, 1985: Leather, 1988). Although not significant, four of the treatments resulted in lower pupal weights compared to the control treatment. The treatments with clay 25 and clay 50 were similar to the control treatment (Fig. 3). In addition, the time for pupation was also compared for seven treatments (Fig. 4), larvae reared on charcoal 50 required the longest time for pupation (17.7 wk) whereas those reared on clay 25 required the least time for pupation (16.0 wk).

Among six different feeding regimens, clay 25 was determined to be the best for rearing P. brevitarsis larvae because it did not adversely affect the larval growth and development relative to the control treatment. Clay is not an organic substituent and appeared to pass through the alimentary tract of the beetles without adverse effects. Given the assumption that clay would have no nutritional value, adding clay might have been a negative factor for rearing insect larvae. However, negligible effects were detected in the larvae reared on the feed with low concentrations of clay in terms of the growth parameters studied. Thus, addition of adsorbent to the diet during commercial production may improve the nutritional quality of this important beetle.