Piper fuligineum (Piperaceae) Essential Oil on Euschistus heros: Lethal and Sublethal Toxicities and Selectivity to an Egg Parasitoid Used in Applied Biological Control

Euschistus heros is a major soybean pest in Neotropics, requiring sustainable control alternatives. This study characterized the chemical composition of Piper fuligineum essential oil (EOPF) and evaluated its bioactivity against different developmental stages of E. heros, as well as its selectivity toward the egg parasitoid Telenomus podisi. Chemical analysis identified 19 compounds accounting for 85.37% of EOPF, with a profile dominated by sesquiterpenes, particularly β-caryophyllene (10.60%), β-eremophyllene (10.46%), and eremophyllene (9.09). The major monoterpenes were 1-naphthalenol (9.88%), β-ocimene (5.48%), and β-pinene (5.16%). EOPF exhibited pronounced stage-dependent ovicidal activity, causing up to 91% egg inviability during early embryogenesis at the highest concentration tested (40,000 mg L−1). Exposure also markedly reduced the subsequent survival of nymphs emerging from treated eggs and caused concentration-dependent mortality in postembryonic stages, reaching, at 40,000 mg L−1, 53% in third-instar nymphs and 46% in adults. Selectivity toward T. podisi strongly depended on the timing of exposure. In pre-parasitism assays, parasitism and emergence were reduced to near 0% at ≥10,000 mg L−1, while in post-parasitism conditions, parasitism remained high (91–95%) but emergence declined to 0% at the highest concentration. Free-choice tests showed a pronounced repellent effect of EOPF, with females preferentially parasitizing untreated eggs. According to the IOBC classification, the EOPF ranged from slight to harmful, particularly affecting parasitoid emergence. These findings highlight the need to optimize concentration, exposure timing, and application strategy before integrating EOPF with parasitoid-based biological control.