Kristen S. Walker, Jessica L. Bray, Mary E. Lehman, Amanda J. Lentz-Ronning
BIOS 85 (2), 95-101, (1 May 2014) https://doi.org/10.1893/0005-3155-85.2.95
Females of the cabbage white butterfly, Pieris rapae, lay eggs (oviposit) on plants in the Brassicaceae family (crucifers) that will serve as foodplants for their larvae. The presence of plant secondary metabolites (PSM), specifically glucosinolates in crucifers, serves as the primary cue for feeding and oviposition. However, the influence of other PSM, such as phenolic acids, has not been well characterized. Seedlings of Brassica rapa were treated with a phenolic acid (p-coumaric acid, ferulic acid, salicylic acid or protocatechuic acid) to see if they would deter or stimulate oviposition. Female butterflies were provided a choice of whether to lay eggs on control plants sprayed with deionized water or plants sprayed with a phenolic acid. Ferulic and p-coumaric acid, which are very similar in chemical structure, both had significant stimulatory effects on oviposition. The other two phenolic acids did not affect ovipositional choices. The effects of PSM may interact with other factors such as the nutrient content of host plants. Plant nutritional status was manipulated by supplying Hoagland's nutrient solution at concentrations of 1/2 X (control) or 1/8 X (reduced nutrient treatment). Plants from both nutrient treatments were sprayed with deionized water (control) or 1.0 mM p-coumaric acid (allelochemical treatment). A significantly higher number of eggs were deposited on the 1/2 X control plants than on the 1/8 X reduced nutrient treatment, but the more subtle effects of p-coumaric acid treatments were not detected in this factorial experiment. However, caterpillars that fed on cabbage leaf discs soaked in p-coumaric acid weighed significantly more than those that fed on leaf discs soaked in deionized water. Overall, the results of this study suggest that some phenolic acids may act as secondary cues that help P. rapae identify the most suitable host plants among a selection of plants that all possess the primary cue of glucosinolates. However, results also suggest that other factors, such as nutrient status of host plants, may be even stronger determinates of ovipositional preference.