Oviposition Cues for a Specialist Butterfly–Plant Chemistry and Size

The oviposition choice of an insect herbivore is based on a complex set of stimuli and responses. In this study, we examined the effect of plant secondary chemistry (the iridoid glycosides aucubin and catalpol) and aspects of size of the plant Plantago lanceolata, on the oviposition behavior of the specialist butterfly Melitaea cinxia. Iridoid glycosides are known to deter feeding or decrease the growth rate of generalist insect herbivores, but can act as oviposition cues and feeding stimulants for specialized herbivores. In a previous observational study of M. cinxia in the field, oviposition was associated with high levels of aucubin. However, this association could have been the cause (butterfly choice) or consequence (plant induction) of oviposition. We conducted a set of dual- and multiple-choice experiments in cages and in the field. In the cages, we found a positive association between the pre-oviposition level of aucubin and the number of ovipositions. The association reflects the butterfly oviposition selection rather than plant induction that follows oviposition. Our results also suggest a threshold concentration below which females do not distinguish between levels of iridoid glycosides. In the field, the size of the plant appeared to be a more important stimulus than iridoid glycoside content, with bigger plants receiving more oviposition than smaller plants, regardless of their secondary chemistry. Our results illustrate that the rank of a cue used for oviposition may be dependent on environmental context

Trade-offs between chemical defence and regrowth capacity in Plantago lanceolata 2 3

Abstract -Resistance and tolerance are different strategies of plants to deal with herbivore 29 attack. Since resources are limited and resistance and tolerance serve similar functions for plants, 30 trade-offs between these two strategies have often been postulated. In this study we investigated 31 trade-offs between resistance and one aspect of tolerance, the ability to regrow after defoliation. 32 In order to minimize confounding effects of genetic background and selection history, we used 33 offspring derived from artificial selection lines of ribwort plantain (Plantago lanceolata) that 34 differed in their levels of leaf iridoid glycosides (IGs), allelochemicals that confer resistance to 35 generalist herbivores, to study genetic associations with regrowth ability. We tested whether 36 high-IG plants 1) suffer allocation costs of resistance in terms of reduced shoot and root growth, 37 2) have reduced regrowth ability (tolerance) after defoliation compared to low-IG plants, and 3) 38 whether such costs are more pronounced under nutrient stress. High-IG plants produced fewer 39 inflorescences and side rosettes than low-IG plants and showed a different biomass allocation 40 pattern, but since neither the vegetative, nor the reproductive biomass differed between the lines, 41 there was no evidence for a cost of IG production in terms of total biomass production under 42 either nutrient condition. High-IG plants also did not suffer a reduced capacity to regrow shoot 43 mass after defoliation. However, after regrowth, root mass of high-IG plants grown under 44 nutrient-poor conditions was significantly lower than that of low-IG plants. This suggests that 45 under these conditions shoot regrowth of high-IG plants comes at a larger expense of root growth 46 than in low-IG plants. We speculate therefore that if there is repeated defoliation, high-IG plants 47 may eventually fail to maintain shoot regrowth capacity and that trade-offs between resistance 48 and tolerance in this system will show up after repeated defoliation events under conditions of 49 low resource availability