Sitophilus zeamais adults have survival and nutrition affected by Schinus terebinthifolius leaf extract and its lectin (SteLL)

Alternative methods for controlling insect pests are required because of the hazards of synthetic chemicals to people and the environment. Lectins are proteins that have been reported as insecticidal agents; however, only one study on the effects of these proteins on Sitophiius zeamais Motsch. (maize weevil) has been performed. In the present study, we evaluated the effects of ingestion of artificial diets containing a saline extract from Schinus terebinthifolius Raddi leaves (LE) or its lectin (SteLL, S. terebinthifolius leaf lectin) on the survival and nutritional parameters of S. zeamais adults. The in vitro effects of LE and SteLL on the activity of insect digestive enzymes were also investigated. In addition to SteLL, the LE contained hydrolysable tannins (including gallic acid at 0.559 g%) and flavonoids. Ingestion of LE (100, 200, and 250 mg of extract per g of Triticum aestivum L. flour) impaired the survival of the S. zeamais adults, with mortality rates ranging between 94% and 97% after 12 days of incubation. A strong deterrent effect was detected, and the insects lost biomass during the assay. However, more than 60% of the insects in the SteLL (1-5 mg/g) treatments remained alive during the 34 days of the experiment. The lectin did not show a deterrent effect, but the biomass and efficiency in conversion of ingested food decreased in a dose-dependent manner. The LE was able to inhibit in vitro the protease activity of the insect gut, while SteLL inhibited protease activity and stimulated amylase activity. In conclusion, the leaf extract had insecticidal properties against S. zeamais, which may be due to starvation induction in consequence of the deterrent effect and interference with proteolytic enzymes. Although SteLL did not cause the mortality of the insects, it may be useful as an additive or synergistic agent that reduces pest fitness by affecting the food conversion into biomass.