Flower vs. Leaf Feeding by Pieris brassicae: Glucosinolate-Rich Flower Tissues are Preferred and Sustain Higher Growth Rate

Abstract

Interactions between butterflies and caterpillars in the genus Pieris and plants in the family Brassicaceae are among the best explored in the field of insect-plant biology. However, we report here for the first time that Pieris brassicae, commonly assumed to be a typical folivore, actually prefers to feed on flowers of three Brassica nigra genotypes rather than on their leaves. First-and second-instar caterpillars were observed to feed primarily on leaves, whereas late second and early third instars migrated via the small leaves of the flower branches to the flower buds and flowers. Once flower feeding began, no further leaf feeding was observed. We investigated growth rates of caterpillars having access exclusively to either leaves of flowering plants or flowers. In addition, we analyzed glucosinolate concentrations in leaves and flowers. Late-second-and early-third-instar P. brassicae caterpillars moved upward into the inflorescences of B. nigra and fed on buds and flowers until the end of the final (fifth) instar, after which they entered into the wandering stage, leaving the plant in search of a pupation site. Flower feeding sustained a significantly higher growth rate than leaf feeding. Flowers contained levels of glucosinolates up to five times higher than those of leaves. Five glucosinolates were identified: the aliphatic sinigrin, the aromatic phenyethylglucosinolate, and three indole glucosinolates: glucobrassicin, 4-methoxyglucobrassicin, and 4-hydroxyglucobrassicin. Tissue type and genotype were the most important factors affecting levels of identified glucosinolates. Sinigrin was by far the most abundant compound in all three genotypes. Sinigrin, 4-hydroxyglucobrassicin, and phenylethylglucosinolate were present at significantly higher levels in flowers than in leaves. In response to caterpillar feeding, sinigrin levels in both leaves and flowers were significantly higher than in undamaged plants, whereas 4-hydroxyglucobrassicin leaf levels were lower. Our results show that feeding on flower tissues, containing higher concentrations of glucosinolates, provides P. brassicae with a nutritional benefit in terms of higher growth rate. This preference appears to be in contrast to published negative effects of volatile glucosinolate breakdown products on the closely related Pieris rapae.

Formation of Simple Nitriles upon Glucosinolate Hydrolysis Affects Direct and Indirect Defense Against the Specialist Herbivore, Pieris rapae

“…Several specialist insects have developed counteradaptations to circumvent the toxic effects of glucosinolates and their hydrolysis products (Müller et al 2001;Alibadi et al 2002;Ratzka et al 2002;Wittstock et al 2004;Agerbirk et al 2006;Vergara et al 2006;Wheat et al 2007). Moreover, glucosinolates and isothiocyanates often serve as attractants or oviposition and feeding stimulants for these insects (Van Loon et al 1992;Wittstock et al 2003;Miles et al 2005;Schoonhoven et al 2005;Renwick et al 2006;Barth and Jander 2006;Smallegange et al 2007). Still, several studies have shown negative correlations between the glucosinolate content of the diet and the larval performance of herbivorous insects specialized on Brassicaceae (Mewis et al 2005(Mewis et al , 2006Gols et al 2007Gols et al , 2008Kim and Jander 2007).…”

Central Metabolic Responses to Ozone and Herbivory Affect Photosynthesis and Stomatal Closure

“…P. brassicae oviposition on the host plant is influenced by glucosinolates (sulfur-and nitrogen-containing glucosides) that function as chemical cues for the butterfly (Fahey et al, 2001;Petersen et al, 2002;Halkier and Gershenzon, 2006;Textor and Gershenzon, 2008). Young caterpillars primarily feed on mature leaves of B. nigra, but after the third instar, they move to fresh tissues with higher glucosinolate content, such as flowers and buds (Smallegange et al, 2007). Therefore, descriptions of plantherbivore interactions frequently emphasize specialized (secondary) metabolism (Simmonds, 2003;van Dam et al, 2004;Poelman et al, 2010;Boeckler et al, 2011;Lof et al, 2013;Onkokesung et al, 2014).…”

Florivory as an Opportunity Benefit of Aposematism

“…If the animal is cryptic when small, thenfrom the arguments above-it should feed on interior leaves during initial instars (Smallegange et al 2007;Agerbirk et al 2010;Bandeili and Müller 2010). Thus, larval florivores should be later instars, should be chemically defended, and should advertise those defenses.…”

“…We use dynamic programming of insect life history to find the fitness advantages of a combination of florivory and aposematism relative to only crypsis and/or folivory. We characterize a herbivore, in line with Smallegange et al (2007), not as sequestering toxins but rather as storing them for responsive defenses (Higginson and Ruxton 2009), such as the regurgitation exhibited by many lepidopteran larvae (Grant 2006). For example, Pieris brassicae stores highly concentrated glucosinolate hydrolysis products in the foregut (Smallegange et al 2007) that are costly to use (Higginson et al 2011;Daly et al 2012).…”

“…We characterize a herbivore, in line with Smallegange et al (2007), not as sequestering toxins but rather as storing them for responsive defenses (Higginson and Ruxton 2009), such as the regurgitation exhibited by many lepidopteran larvae (Grant 2006). For example, Pieris brassicae stores highly concentrated glucosinolate hydrolysis products in the foregut (Smallegange et al 2007) that are costly to use (Higginson et al 2011;Daly et al 2012). The relative nutrition and toxicity of flowers, compared to those of leaves, varies greatly (McCall and Irwin 2006), and so we explored the impact of this variability for the evolution of florivory.…”