Synergistic effects of direct and indirect defences on herbivore egg survival in a wild crucifer

Abstract

Evolutionary theory of plant defences against herbivores predicts a trade-off between direct (anti-herbivore traits) and indirect defences (attraction of carnivores) when carnivore fitness is reduced. Such a trade-off is expected in plant species that kill herbivore eggs by exhibiting a hypersensitive response (HR)-like necrosis, which should then negatively affect carnivores. We used the black mustard (Brassica nigra) to investigate how this potentially lethal direct trait affects preferences and/or performances of specialist cabbage white butterflies (Pieris spp.), and their natural enemies, tiny egg parasitoid wasps (Trichogramma spp.). Both within and between black mustard populations, we observed variation in the expression of Pieris egg-induced HR. Butterfly eggs on plants with HR-like necrosis suffered lower hatching rates and higher parasitism than eggs that did not induce the trait. In addition, Trichogramma wasps were attracted to volatiles of egg-induced plants that also expressed HR, and this attraction depended on the Trichogramma strain used. Consequently, HR did not have a negative effect on egg parasitoid survival. We conclude that even within a system where plants deploy lethal direct defences, such defences may still act with indirect defences in a synergistic manner to reduce herbivore pressure.

Keywords: PR1; Pieris rapae; Trichogramma; defence trade-offs; hypersensitive response; oviposition-induced plant volatiles.

Figure 1.

Frequency of B. nigra plants expressing HR-like necrosis induced by Pieris eggs and effects of HR-like necrosis on Pieris egg survival and PR1 gene expression. (a) Percentage of HR-like necrosis on plants of a Dutch field population (Steenfarbiek) and in the greenhouse. (b) Percentage (estimated mean ± s.e.) of caterpillars emerging from Pieris eggs on plants of a Dutch field population (Steenfarbiek) (2010 and 2011, generation 1 and 2); ***p < 0.001; n.s., not significant (GLM, p > 0.05). (c) Variation in HR severity and effect on P. rapae egg survival (estimated mean ± s.e.) in the greenhouse (different severities were visually characterized as in the pictures); different letters indicate significant differences (GLM and LSD, p < 0.05). (d) Effect of HR-like necrosis induced by P. rapae eggs on expression changes (mean ± s.e.) of HR-marker gene PR1 at 24 and 72 h after oviposition; different letters indicate significant differences (ANOVA, Tukey test p < 0.05, n = 3–5 biological replicates). Numbers inside the bars represent number of plants for data of 2011 and number of eggs for data of 2010. Hypersensitive response (HR), in light grey HR−: no necrotic zone observed, in dark grey HR+: necrotic zone. (Online version in colour.)

Figure 2.

Effect of Pieris egg-induced HR on Trichogramma parasitism and performance in a natural B. nigra population collected in three different seasons in 2010 and 2011 (generations 1 and 2). (a) Percentage (estimated mean ± s.e.) of eggs that were parasitized by Trichogramma wasps. Picture shows a parasitized egg with HR-like necrosis. (b) Percentage of parasitized eggs (estimated mean ± s.e.) from which Trichogramma wasps emerged. Numbers inside the bars represent number of plants for data of 2011 and number of eggs for data of 2010. Different plant phenotypes: hypersensitive response (HR), HR−: no necrotic zone observed, HR+: with necrotic zone. ***p < 0.001, *p < 0.05; n.s., not significant (GLM, p > 0.05). (Online version in colour.)

Figure 3.

Proportion (mean ± s.e.) of female Trichogramma wasps choosing OIPVs of B. nigra plants. Plants were infested with single eggs of P. rapae. Columns represent the mean proportion of choice for OIPVs of isofemale lines from Trichogramma brassicae (lab strain Y175) (light grey) and T. evanescens (native strain RU124, medium grey and native strain RU 263, dark grey) tested in a Y-tube olfactometer. Numbers in the columns represent number of responding wasps. All experiments were conducted in a two-choice situation between plants infested with eggs of different ages (24 and 72 h) and uninfested control plants. *p < 0.05; n.s., not significant (p > 0.05); one-sample t-test. The dashed line indicates 0.5 = no preference. Each treatment combination was replicated with five to seven plant pairs and 10–20 wasps of each strain per plant pair (n = 70–100 wasps per treatment/strain). Different plant phenotypes: hypersensitive response (HR), HR−: no necrotic zone observed, HR+: necrotic zone. (Online version in colour.)