Effects of herbivory on the growth and reproduction of Picris hieracioides in the Mediterranean region

Abstract

Abstract. The impact of herbivorous insects on the growth and reproduction of Picris hieracioides (Compositae) was followed in three consecutive years by comparing plants treated with an insecticide and untreated plants in an old‐field population abandoned for 15 yr (P15). In the third year the interactions between herbivory and competition were analysed as well. Pre‐dispersal seed predation of seed heads was investigated in three adjacent old‐fields abandoned for one (P1), 15 (P15) and 40 yr (P40) in the region of Montpellier, France. Fields differed in vegetation cover and floristic composition and also in density and above‐ground biomass of Picris hieracioides plants (from 246 g per plant in P1, 13.3 g in P15, down to 3.3 g in P40, for the first year of study). The cumulative rainfall between January and July, the period of Picris growth, decreased from 1992 to 1994. Moreover, in 1992, three times as much rainfall occurred in May and June – the period of stem elongation – as in 1993 and 1994. As a consequence, environmental stress increased during the study period.Herbivorous invertebrates had a negative effect on the growth and reproduction of individual plants of Picris hieracioides in P15. The effect was not constant with time, being apparent in only two years out of three. These two years (1992 and 1994) had contrasting climatic characteristics, whereas the year with no apparent effect (1993) was climatically intermediate. When herbivory had effects on growth, the insecticide‐treated plants had a significantly higher above‐ground biomass and produced twice as many seed heads as the control plants. There was no interaction between herbivory and competition.The percentage of damaged seed heads was significantly lower in the P1‐field than in the P40‐field. The P15‐field had intermediate levels of predation. Predation on seed heads increased whereas biomass and reproductive output decreased between 1992 and 1994 in the three old‐fields. Therefore, the results on seed predation (but not for above‐ground biomass and reproduction) tend to confirm‘the environmental stress hypothesis’ of Mattson & Haack (1987) that states that plants under stress are more liable to be attacked by grazers.