Agave salmiana is a plant of great economic and cultural value in Mexico, whose interaction with the insect chinicuil (Comadia redtenbacheri), a phytophagous species that develops in its root system, represents a key example of the plant–insect relationship. The objective of this study was to identify and relate the bioactive compounds present in both A. salmiana and the chinicuil to better understand this interaction. Samples were collected from A. salmiana free of chinicuil (RSC) and infected by chinicuil (RIC), as well as chinicuils from the infected agave. Both materials were dried, pulverized, and subjected to hydroethanolic extraction (70:30) at a 1:10 w/v ratio. Subsequently, phytochemical screening was performed on the roots and entomochemical screening on the chinicuil. Total polyphenols were quantified using the Folin–Ciocalteu method (GAE mg/g), flavonoids with the AlCl3 method (QE mg/g), and tannins with the Vanillin–HCl method (CE mg/g), followed by statistical analysis with ANOVA and Tukey’s test (p < 0.05).

The results showed that RIC presented a greater diversity and concentration of compounds compared to RSC, including flavonoids, tannins, terpenes, glycosides, quinones, and coumarins, with up to a fourfold increase in polyphenol, flavonoid, and tannin content. This behavior could be attributed to the plant’s activation of metabolite production as an induced defense response to the biotic stress caused by the chinicuil. In contrast, the chinicuil extract showed significant concentrations of polyphenols (14.801 ± 0.310 mg/g), tannins (53.574 ± 0.131 mg/g), and flavonoids (21.600 ± 0.086 mg/g), though lower than in RIC. The screening revealed that the insect performs a selective uptake of metabolites, with low compound accumulation, indicating a detoxification mechanism that allows it to exploit the plant’s metabolites without suffering adverse effects. These findings confirm a close biotic relationship, providing valuable insights for the development of functional foods and sustainable agricultural strategies based on ecological interactions.