**Background:** Modern agriculture seeks sustainable inputs that are non-toxic and can enhance crop stress tolerance and nutritional quality. Biostimulants and elicitors—natural products from plants, animals, or microorganisms—have gained attention for improving plant growth (biostimulant effect) or strengthening immune responses (elicitor effect) without heavy reliance on synthetic agrochemicals. Insect frass (excreta) is a byproduct of the growing insect-rearing industry and may serve as a source of chitin, beneficial microorganisms, and nutrients. This study evaluated the biostimulant and elicitor potential of cricket frass (Acheta domesticus) in tomato (Solanum lycopersicum) under greenhouse conditions.
**Methods:** The experiment was conducted in a 30 m² greenhouse in Amazcala, Querétaro, Mexico, from 15 April to 15 June 2022. Tomato seedlings (Saladette type, 6–8 true leaves) were transplanted into 500-mL containers with coconut fiber substrate. Treatments consisted of inert sand mixed with cricket frass at 0% (control), 0.1%, 0.5%, and 1.0% (w/w). Twenty-four plants were used per treatment in triplicate, arranged in a randomized block design with 8 plants per block and a planting density of 2 plants/m². Plants were fertigated every 2 days with 300 mL of a standard nutrient solution (pH 6 ± 0.2, electrical conductivity 1.6–1.8 dS/mL). Greenhouse conditions were 25–18 °C day/night and 80–85% relative humidity. Morphological measurements (plant height, basal stem diameter, leaf number) were taken weekly. Antioxidant enzyme activities—superoxide dismutase (SOD), catalase (CAT), and phenylalanine ammonia lyase (PAL)—were measured from leaf samples at the end of the experiment using spectrophotometric methods. Statistical analysis used ANOVA and Tukey's HSD post hoc test (p < 0.05).
**Key Results:** Morphological variables showed a clear dose-dependent response. The 0.1% frass treatment produced significantly higher plant height, basal stem diameter, and leaf number compared to control, consistent with a biostimulant effect. In contrast, the 0.5% and 1.0% treatments resulted in significantly lower values for all three morphological variables compared to control, suggesting an elicitor (stress-inducing) effect, though no visual toxicity symptoms were observed. For antioxidant enzymes: PAL activity was significantly decreased in the 0.1% treatment and showed the greatest decrease in the 1.0% treatment compared to control. CAT activity was significantly increased in both 0.1% and 1.0% treatments, with the highest activity in the 1.0% group. SOD activity showed no significant difference from control in any treatment. The authors interpret these patterns as reflecting a trade-off between growth and defense: the 0.1% treatment allocated resources to growth (biostimulation), while the 0.5% and 1.0% treatments allocated resources to secondary metabolism and ROS detoxification (elicitation). The overall response pattern is described as hormetic—a biphasic dose-response curve where low doses are stimulatory (eustress) and higher doses are inhibitory (distress) but still within a positive adaptive range.
**Clinical Implications:** This study provides proof-of-concept that cricket frass, a waste product from insect farming, can be repurposed as a sustainable agricultural input. At very low concentrations (0.1% w/w in substrate), it acts as a biostimulant, enhancing vegetative growth in tomato. At slightly higher concentrations (0.5–1.0%), it acts as an elicitor, activating plant defense pathways without causing visible toxicity. This dual functionality could allow growers to tailor frass application to specific goals—promoting early growth or priming plants for stress tolerance. The findings support circular economy principles by valorizing insect-rearing byproducts. However, the study was limited to 60 days (not a full tomato cultivation cycle) and did not measure fruit yield, quality, or pest/disease resistance. Future research should evaluate complete crop cycles, effects on fruit production, and the role of frass-associated microorganisms in mediating plant responses.