Effect of Different Sowing Seasons, Growth Stages, Leaf Positions, and Soybean Varieties on the Growth of Clanis bilineata tsingtauica Mell Larvae

Clanis bilineata tsingtauica Mell (Lepidoptera: Sphingidae, CBT), as a traditional edible insect, is becoming popular in China due to its high nutritional value, but production needs to be improved to meet the expanding market. In the present study, CBT eggs were artificially inoculated on soybean leaves to evaluate the effects of leaf position, growth stage, sowing season, and soybean variety on CBT larval growth, respectively. The results showed that (1) the larval weight and survival rate were poorly correlated, so they could represent two different larval growth and development indicators for CBT. The 21-day-old larval weight was significantly different between the sowing seasons and between soybean growth stages, which was suitable as a key indicator for evaluating CBT larval rearing factors. (2) Compared with autumn-sown soybeans, the weight of 21-day-old larvae feeding on V6 stage (sixth trifoliolate) leaves of summer-sown soybeans was significantly higher, with an average increase of 44.7%. (3) Under autumn sowing conditions, the weight of 21-day-old larvae feeding on soybeans in the V6 stage was significantly higher than those fed on soybeans in the R3 stage (beginning pod), increasing by 33.9%. (4) Under summer sowing conditions, the weight of 21-day-old larvae feeding on the third-top leaf (the third leaf from the top of the soybeans' main stem) was significantly higher than those feeding on the third-bottom leaf (the third leaf from the bottom of the soybeans' main stem) at V6 stage by 35.7%. Similar results also appeared in autumn sowing; the average weights of 21-day-old larvae feeding on the third-top leaf increased significantly by 29.9% compared to those feeding on the third-bottom leaf. Moreover, the survival rate of larvae fed with the third-top leaf was significantly higher than that of those fed with the third-bottom leaf at the V6 stage in autumn sowing. Leaf position is the main factor affecting the survival rate of larvae. (5) Under summer sowing conditions, the weights of larvae fed with the third-top leaf of the susceptible-soybean varieties NN89-29 and NN1138-2 were significantly higher than that of those fed with the third-bottom leaf of these varieties. This difference was significantly reduced with autumn sowing. In conclusion, the CBT eggs inoculated on the third-top leaf of NN89-29 and NN1138-2 at the V6 stage in summer sowing could achieve maximum larval yield.