Sublethal Effects of Solanum nigrum Fruit Extract and Its Pure Glycoalkaloids on the Physiology of Tenebrio molitor (Mealworm)

Abstract

Background: Solanaceae plants produce glycoalkaloids (GAs) that affect various physiological processes of herbivorous insects and they are being tested as potential alternatives for synthetic pesticides. They cause lethal and sublethal effects. Nevertheless, their mode of action remains unclear. Therefore, we examined the effects of Solanum nigrum fruit extracts and pure glycoalkaloids on a model beetle, Tenebrio molitor. Methods: Plant extracts or pure alkaloids were added to the food of the larvae for three days. The lipid, glycogen, and protein content in the fat body and the midgut were determined, and the contractility of the heart, hindgut, and oviduct muscles was tested using the video-microscopy technique. Finally, the ultrastructure of the fat body and the midgut was observed using electron microscopy. Results: No lethal effects were noted. Sublethal changes were observed in the content of biomolecules, malformations of organelles, chromatin condensation, and heart and oviduct contractility. The observed effects differed between the tested glycoalkaloids and the extract. Conclusions: Both the extract and pure GAs have a wide range of effects that may result in impaired development, food intake, and reproduction. Some early effects may be used as bioindicators of stress. The effects of the extract and pure alkaloids suggest that the substances produced by the plant may act additively or synergistically.

Solanum nigrum Fruit Extract Increases Toxicity of Fenitrothion—A Synthetic Insecticide, in the Mealworm Beetle Tenebrio molitor Larvae

“…In the second variant of the experiment, the comparison of the amount of lipids after application of FN (300 μg/mL) and FN (300 μg/mL) with pre-treatment of EXT gave significant differences. The additive effect can be confirmed by the previous observations of lipid content in the fat body after the EXT application [ 12 ]. In this case, the acute toxic effect was postponed, and some detoxification mechanisms could appear.…”

“…Sublethal effects on the fat body of T. molitor larvae caused by S. nigrum extract were previously described [ 12 ]. The EXT caused a significant decrease in the glycogen level at a concentration of 0.1% and decrease of lipids content at a concentration 1% in the fat body cells.…”

“…In our previous study, the EXT in the concentration of 0.01% given for 3 days to larvae, did not show any visible changes in the midgut cells ultrastructure. The concentration of 1% was responsible for the disruption of nuclear membranes and 10% EXT additionally for the appearance of glycogen vacuoles, a sign of changes in its metabolism in the midgut cells [ 12 ]. The above-mentioned observations were neither visible after the application of FN and FN–EXT blends nor in the midgut cells from larvae pre-treated with the EXT for 24 h and then FN for 48 h. Possibly, the exposure on the EXT was to short and as a result of an acute toxic action of FN, EXT did not penetrate the cells.…”

“…In previous reports, we checked the sensitivity of yellow mealworm beetle to Solanum nigrum fruit extract (EXT) and its main glycoalkaloids, given in the diet and in in vitro tests. We described the altered amount of biomolecules, ultrastructural malformations, increased chromatin condensation, and altered heart and oviduct contractility effects [ 12 , 13 ], which showed that the extract and also its main glycoalkaloids solasonine and solamargine might have affected the insect metabolism, development, and reproduction. Hence, even if natural substances may not have a lethal effect, they may significantly decrease the vitality of the insects.…”

“…Therefore, we decided to focus on these two tissues due to their importance for entrance, metabolism and storage of toxic substances in the organism. The effects of the S. nigrum extract in used concentrations on the midgut and fat body cells were previously described [ 12 ].…”