Oviposition preference
Gregarious species
Although P. brassicae females distributed their egg clutches unevenly between plants (χ2 = 19.65, df = 7, P = 0.04, GLM, Fig. 1a), a post hoc test did not reveal a significant preference (Supplementary Table 1). Arabidopsis thaliana, which was in the flowering stage (in contrast to all other plant species), did not receive any egg clutches from P. brassicae in this setup. Oviposition choices of P. brassicae did not correlate with plant fresh weight (S = 1720.7, ρ = 0.25, P = 0.24, Spearman correlation, Supplementary Figure S2).
Solitary species
Pieris rapae females significantly preferred to oviposit on B. nigra over all other seven simultaneously offered plant species (Supplementary Table 1). The plant species chosen least frequently for oviposition were B. oleracea and A. thaliana (χ2 = 292.67, df = 7, P ≤ 0.001, GLM, Fig. 1b). The oviposition preference of P. rapae did not correlate with plant fresh weight (S = 1897.2, ρ = 0.18, P = 0.41, Spearman correlation, Supplementary Figure S2).
Egg survival and effect of plant species and HR-like necrosis
Gregarious species
Plant species significantly affected P. brassicae egg survival (χ2 = 20.39, df = 6, P = 0.002, GLMM, Fig. 2a). Egg survival was highest on B. montana chosen most for oviposition, while it was lowest on H. incana and R. sativus compared to all other plants (apart from B. nigra) (Supplementary Table S2). Five out of the seven tested plant species expressed an HR-like necrosis in response to P. brassicae eggs (Fig. 2a, Supplementary Table S2). Overall, induction of HR-like necrosis did not significantly affect egg survival (χ2 = 0.41, df = 1, P = 0.52, GLMM), while the effect of HR-like necrosis on survival of P. brassicae eggs was plant species specific (χ2 = 30.83, df = 4, P < 0.001, GLMM). On B. montana, egg survival was much lower on the two HR-expressing plants than on the seven non-HR plants (Fig. 2a, Supplementary Table S3–4).
Solitary species
The plant species selected by P. rapae females did not significantly affect egg survival (χ2 = 11.19, df = 6, P = 0.08, GLMM, Fig. 2b). Six out of the seven tested plant species expressed HR-like necrosis induced by P. rapae eggs (Fig. 2b, Supplementary Table S2). A significantly higher proportion of P. rapae eggs survived on non-HR plants compared to plants expressing HR-like (χ2 = 13.58, df = 1, P < 0.001, GLMM). This effect of P. rapae egg-induced HR-like necrosis on egg survival was—in contrast to the P. brassicae egg-induced response—independent of the plant species (χ2 = 4.43, df = 4, P = 0.35, GLMM) (Fig. 2b, Supplementary Table S3–4).
Correlation between oviposition preference and egg survival
We did not detect a significant correlation between oviposition preference and survival of the eggs for either of the two butterfly species (proportion of eggs laid related to egg survival; Spearman correlation; for P. brassicae: S = 32,335, ρ = 0.10, P = 0.44, for P. rapae: S = 9086.3, ρ = 0.01, P = 0.97, Fig. 3a). However, the proportion of P. rapae eggs laid was positively correlated with the proportion of plants expressing HR-like necrosis against those eggs (S = 4.57, ρ = 0.96, P < 0.001, Spearman correlation, Fig. 3b). For P. brassicae, this correlation was not significant (S = 67.72, ρ = 0.19, P = 0.64, Spearman correlation, see Fig. 3b).
Effect of plant species, egg infestation and HR on larval performance
Gregarious species
The weight of 7-day-old P. brassicae caterpillars did not vary significantly depending on the plant species they were feeding on (χ2 = 12.44, df = 6, P = 0.05, LMM, Fig. 4a). However, the plants’ response to prior egg deposition significantly affected performance of P. brassicae larvae. Seven-day-old larvae developing on plants that previously had received eggs (EF) significantly gained less weight than those on plants that had not received eggs (F) (χ2 = 5.27, df = 1, P = 0.02, LMM, Fig. 4b). Caterpillars feeding on plants without prior egg deposition gained about 5% more weight. This egg-mediated effect on performance was independent of the plant species (no interactive effect between plant species and egg infestation on larval weight; χ2 = 2.51, df = 6, P = 0.87, LMM). HR-like necrosis induced by previously laid eggs did not affect the weight of caterpillars (χ2 = 0.72, df = 1, P = 0.40, LMM, see Fig. 4c), and neither did plant species nor did the interaction between plant species and HR-like necrosis (χ2 = 5.76, df = 6, P = 0.45 and χ2 = 5.46, df = 4, P = 0.24, LMM). The weight of 3-day-old caterpillars is presented in the supplementary material (Supplementary Figure S3).
Solitary species
When considering 7-day-old P. rapae caterpillars on both egg-free and previously egg-deposited plants, their weight was not affected by the plant species they were feeding on (χ2 = 5.04, df = 6, P = 0.54; LMM, Fig. 4a). When excluding the occurrence of HR-like necrosis induced by egg deposition, egg infestation preceding larval feeding did not affect larval weight (χ2 = 0.001, df = 1, P = 0.97; LMM, Fig. 4b). Neither did the interaction between egg infestation and plant species affect larval weight (χ2 = 1.09, df = 6, P = 0.98, LMM, Fig. 4c). Yet, larvae feeding on EF plants expressing an HR-like necrosis were significantly heavier than those feeding on EF plants that did not show HR-like necrosis (χ2 = 4.14, df = 1, P = 0.04, LMM, Fig. 4c). Neither plant species nor the interaction between plant species and HR-like necrosis affected caterpillar weight on previously egg-infested plants (χ2 = 3.73, df = 6, P = 0.71 and χ2 = 3.93, df = 3, P = 0.27, LMM). The weight of 3-day-old caterpillars is presented in the supplementary material (Supplementary Figure S3).
Correlation between oviposition preference and larval performance
To assess whether there was a correlation between adult oviposition preference and larval performance, we first analyzed the relationship between the proportion of eggs laid and the weight of three (Supplementary information and Supplementary Figure S4) or 7-day-old caterpillars feeding on previously oviposited EF plants and egg-free F plants for each plant species.
Gregarious species
Weight of 7-day-old P. brassicae larvae significantly and positively correlated with the number of eggs laid. Seven-day-old P. brassicae larvae were the heaviest on those plant species that received most egg clusters (S = 15,964,000, ρ = 0.17, P < 0.001, Spearman correlation, Fig. 5a). The proportion of egg clusters laid did not correlate with the weight of caterpillars feeding on egg-free plants (S = 34, ρ = 0.39, P = 0.40, Spearman correlation, Fig. 5b).
When considering the weight of 7-day-old P. brassicae caterpillars with respect to the plant’s capability to express HR in response to the eggs, weight of caterpillars feeding on previously egg-deposited HR-expressing plants did not correlate with the proportion of egg clutches per plant (S = 22, ρ = − 0.1, P = 0.95, Spearman correlation). Neither was a correlation found between the proportion of plants expressing HR-like necrosis in response to oviposition and the caterpillar weight (S = 36.65, ρ = 0.35, P = 0.45, Spearman correlation).
Solitary species
In contrast to P. brassicae, the weight of 7-day-old P. rapae larvae feeding on previously egg-infested plants neither correlated with the proportion of eggs laid (S = 340,680, ρ = 0.13, P = 0.17, Spearman correlation, Fig. 5c) nor with egg load when larvae were feeding on egg-free plants (S = 50, ρ = 0.11, P = 0.84, Spearman correlation, Fig. 5d). Weight of 7-day-old caterpillars feeding on HR + plants did not correlate with the proportion of eggs laid (S = 32, ρ = − 0.6, P = 0.35, Spearman correlation). Furthermore, the weight of 7-day-old larvae was not correlated with the proportion of plants expressing HR-like necrosis (S = 29.52, ρ = 0.47, P = 0.28, Spearman correlation).