The third instar larvae of P. brevitarsis (Coleoptera: Scarabaeidae) were collected from the laboratory colony that was established by purchasing from a commercial seller, Smurf Bugs Farm (Namyangju-gun, Kyeonggi-do, Republic of Korea), and a private seller (Hoengseong-gun, Gangwondo, Republic of Korea). The purchased beetles were kept together to hybrid and to increase genetic diversity in the laboratory condition on the fermented sawdust at 25℃ with ca. 40% humidity. From this laboratory colony, the third instar larvae were collected based on its head capsule size for this experiment.

The basic feed was fermented oak sawdust that was mixed with 60% of oak sawdust, 30% of wheat bran, and 10% of previous fermented oak sawdust to provide useful microbes that help fermentation. To test the effects of feed additives, we added rice chaff and/or soybean cake based on the Table 1. The rice chaff and soybean cake were mixed with fermented oak sawdust. After mixing it, the feed were fermented for one more month.

The third instar larva was reared in a round petri dish (98 mm diameter X 15 mm depth) with enough designated feed. The rearing room was set as 25, 27.5 and 30℃ with ca. 40% humidity and 12:12(L:D) light condition. We measured body weight and state of each larva, and changed feed once per a week. For each treatment, 10 individuals for the third instar larvae were maintained for 14 wk. It was enough time for the most of second instar larvae to pupate. Three replications for each treatment were conducted. Each of three biological replications, independent batches of different feeds and temperatures, consisted of ten technical replications. A technical replication was a set of 12 different treatments ( four feeds X three temperatures). Averages and standard deviations of weight increase for each week, accumulated weight increase, and weight before pupation were calculated and compared each feed treatment to control feed with t-test and ANOVA analysis.

Larval weight gain was significantly different by rearing temperatures (df = 2; F = 5.01; p = 0.0071) and feed with four different combinations with soybean cake and rice chaff (df = 3; F = 3.30; p = 0.0205) when analyzed by ANOVA (Table 2). However, the interaction between three rearing temperature and two feed ingredients did not show significant effect on the larval weight gain (df = 6; F = 2.10; p = 0.0530) when analyzed by ANOVA (Table 2). When compared three temperature conditions, larvae grown at 25 ℃ (1.18 ± 0.48 g) gained significantly less weight than those at other temperatures: 27.5℃ (1.34 ± 0.45 g) and 30 ℃ (1.33 ± 0.48 g) (Fig. 1). As the temperature increased, the larval weight gain is also increased.

As analyzing the effects of different feed additives, a feed with soybean cake and rice chaff showing the best effect on the larval weight gain (Fig. 2). However, the difference between the feed with soybean cake and rice chaff and the feed with soybean cake was not significantly different. However, the feed with rice chaff showed the least larval weight gain during 14-wk experiment. Even though the difference was not significant, adding rice chaff did not show meaningful effects on larval weight gain. The effects of different composition of feed on the larval weight gain of P. brevitarsis were reported (Kwon, 2009). In this experiment, rice chaff showed negative effect when it was added to the control feed. However, it showed positive effects with 5 % of soybean cake. It might be compensate the nutritional balance between protein and carbohydrate.

When the interaction between rearing temperatures and different feed additives analyzed, the larval weight with control feed and feed with soybean cake was increased as the temperature increased from 25 to 30℃ (Fig. 3). In addition, the larval weight with soybean cake and rice chaff at 27.5℃ was lower than other two temperature condition, 25 and 30℃. Interestingly, the larval weight gain with rice chaff at 30℃ compared to 27.5℃. These results showed that the larval weight gain is different by temperature as well as the feed additives.

With four different feeds and three temperature conditions, we concluded that larvae reared with soybean cake at 30℃ showed the best result for the larval weight gain. However, the difference between the larval weight gain with soybean cake at 27.5 and at 30℃ was not significant, but it requires lots of resources to keep 30℃. Therefore, we concluded that it is the best to rear P. brevitarsis larvae with conventional feed mixed with soybean cake at 27.5℃.