From 1982 through 1987, I investigated whether sticky exudates released by Cirsium discolor and Penstemon digitalis in their inflorescences provide defense against seed predators. I tested two hypotheses: 1: Exudates directly deter seed predators, and 2: Insects struggling in exudates attract predatory arthropods that remain and defend the inflorescences against seed predators. These hypotheses predict that neutralizing the stickiness will increase seed predation (by allowing access to more seed predators, or by decreasing the number of predatory arthropods attracted), and therefore decrease successful seed production. Results did not support either hypothesis, with no increase in seed predators (nor decrease in predatory arthropods), and no decrease in seed production, when traps were neutralized.
Thomas, Patricia A.
"Sticky Exudates on the Inflorescences of Cirsium Discolor (Asteraceae) and Penstemon Digitalis (Scrophulariaceae) as Possible Defense Against Seed Predators,"
The Great Lakes Entomologist, vol 36
Available at: https://scholar.valpo.edu/tgle/vol36/iss2/3