Plant pH level regulates the preference of hormone-treated Ceratapion basicorne (Coleoptera: Apionidae) for yellow starthistle

Mass-rearing a univoltine biocontrol agent faces 2 main challenges: terminating reproductive diapause and enhancing the egg substrate. While cold treatment and topical application of insect hormones can terminate reproductive diapause, methods to increase the oviposition rate into the egg substrate remain unclear. Plant pH is an important functional trait that can attract or deter biocontrol agents. To assess the preference of a biocontrol agent for different plant pH levels, I investigated how the rosette-feeding weevil, Ceratapion basicorne, distinguishes between a pH-matched leaf (i.e., an excised leaf with a pH level equivalent to the pH level of a whole rosette plant by dissolving sodium bisulfate in tap water) and a control leaf in tap water. The pH-matched leaf was a pH of 5.8, but the control leaf was a pH of 6.3; both leaves were collected from the same plant. Female weevils preferred to feed on the pH-matched leaves 3.6 times more than on control leaves. They also oviposited an average of 2.1 ± 0.3 eggs on pH-matched leaves and 0.8 ± 0.2 eggs on control leaves. These findings indicate that leaf pH is a variable to consider when developing enhanced egg substrates for mass-rearing programs of C. basicorne in insectaries and laboratories in the United States.