The Addition of Sterols and Cryoprotectants to Optimize a Diet Developed for Eldana saccharina Walker (Lepidoptera: Pyralidae) Using the Carcass Milling Technique

Simple Summary Despite being essential for growth, insects cannot synthesize sterols. Naturally, sterols are obtained from insects' host plants, but in artificial diets are sometimes missing or not optimal, leading to reduced growth. By adding 1.0 g of stigmasterol, and a combination of 0.2 g each of cholesterol and stigmasterol to two standard minimum specification (MS) diets for Eldana saccharina, pupation was faster (72% and 70%, respectively) than the control diet (15%), 20 days after neonate inoculation. Nevertheless, a reproductive mass reared insect could become a poor field performer, as reproductive output and field performance might be traded off. To increase E. saccharina's cold performance, cryoprotectants were added to a standard MS diet. Males from MS diets containing both concentrations of a proline/trehalose mix, and the highest concentration of trehalose, recovered 30-33% faster from chill coma than males from the remaining diets. Fertility of females that entered chill coma was reduced (<44%) when fed as larvae on cryoprotectant supplemented diets. Females not exposed to chilling treatment had 84% fertility when mated with males from the same source. The MS diet plus 0.2 g each of cholesterol and stigmasterol should become the standard diet, as larval development time was decreased by 60% without obvious trade-offs. Various combinations and concentrations of cholesterol (C) and stigmasterol (S) were added into a base diet developed for Eldana saccharina. Survival of inoculated neonate was high on all diets (>92% at day 20 and >95% at day 27). Fastest larval development occurred on the minimum specification (MS) (+1.0 gS) and MS (+0.2 gC: 0.2 gS) diets (72 and 70% pupation respectively at day 20). Significantly slower development (15% pupation) occurred on the control diet at day 20. Female pupal weight increased when larvae fed on the MS (+0.1 gC), (+0.1 gS) and (+0.2 gC:0.2 gS) diets (0.2143 +/- 0.00 g, 0.2271 +/- 0.01 g and 0.2252 +/- 0.01 g, respectively) as compared with the control diet (0.1886 +/- 0.00 g). Adult emergence was significantly higher (100%) from the MS (+0.1 gS) and MS (+0.2 gC:0.2 gS) diets, as compared with the remaining sterol (95%) and control diets (97%). To potentially increase E. saccharina's cold tolerance, inclusion of cryoprotectants L-proline (P) and trehalose (T) into the MS diet was investigated. Males from the MS (0.2 gP:0.2 gT), MS (0.5 gP:0.5 gT) and MS (1.0 gT) diets recovered fastest from chill coma treatment (204 +/- 44 s, 215 +/- 7 s and 215 +/- 9 s, respectively) than those from the remaining cryoprotectant diets (305 +/- 22 s). The addition of cryoprotectants severely reduced female fertility (<44%) when mated with non-chill coma exposed males. In contrast, eggs from females not exposed to chilling treatment were 84% fertile when mated with males from the same source. The MS (0.2 gC:0.2 gS) diet is the preferred choice to replace the currently used diet, reducing the larval growth period by 60% without negative effects on key life cycle parameters of E. saccharina.

Automated summary

The study reveals that adding sterols can accelerate the pupation process of Eldana saccharina, while cryoprotectants can enhance its cold tolerance. However, the addition of cryoprotectants has negative effects on the fertility of female insects. The preferred diet for E. saccharina is the one containing 0.2g cholesterol and stigmasterol, as it shortens the larval growth period without trade-offs in key life cycle parameters.