While the effects of plant allelochemicals on herbivores and, in several cases, their natural enemies have been widely documented, few studies have compared interpopulational differences in plant chemistry and their effects on higher trophic levels. Here, we examine the development of the generalist herbivore, Trichoplusia ni, and its specialist polyembryonic egg-larval parasitoid, Copidosoma floridanum, when reared on four populations of cabbage, Brassica oleracea, in a common greenhouse experiment comprising 50 seeds from each of four populations of cabbage (3 wild cabbage populations along the south coast of Dorset, England: ‘Kimmeridge’, ‘Old Harry’, ‘Winspit’; one commercial cultivar: ‘Copenhagen’). Six weeks after germination, 5 neonate T. ni (either parasitized or unparasitized) were placed on the leaves of each plant. Developmental mortality of T. ni was recorded. Pupae (unparasitized T. ni) were weighed and adult longevity and lifetime fecundity was measured. Clutch sizes of C. floridanum broods were calculated. The leaf content of 9 glucosinolates for each plant was analyzed. The effect of plant population and glucosinolate concentrations on T. ni pupal weight and C. floridanum clutch size was measured with an Analysis of Covariance (ANCOVA; 9 glucosinolates as covariates). Survivorship data was analyzed with logistic regressions.
Results/Conclusions:
The four cabbage populations differed in their glucosinolate profiles. ‘Copenhagen’ had lower levels of all nine glucosinolates (glucoiberin: F3,24=17.23, P<0.001; glucoraphanin: F3,24=12.25, P<0.001; gluconapin: F3,24=30.74, P<0.001; progoitrin: F3,24=9.24, P=0.009; glucobrassicin: F3,24=4.77, P<0.01; neoglucobrassicin: F3,24=8.71, P=0.007; 4-methoxy glucobrassicin: F3,24=6.63, P=0.003) with the exception of sinigrin, which was higher than the three wild populations (F3,24=7.33, P=0.002) and 4-OH glucobrassicin, which did not differ among the four cabbage varieties (F3,24=1.51, P=0.24). Survival of T. ni was strongly dependent on the host plant population on which larvae developed (‘Copenhagen’>‘Kimmeridge’>‘Old Harry’>‘Winspit’; Wald χ2 = 167.31). T. ni pupae were smaller when they developed on ‘Winspit’ plants (F3,134=5.32, P=0.0017). After accounting for T. ni pupal weight, host plant identity had no effect on either adult lifespan (F3,43=0.58, P=0.6344) or lifetime fecundity (F3,30=2.31, P=0.0967). C. floridanum clutch sizes were affected by host plant population (‘Copenhagen’>‘Old Harry’=‘Kimmeridge’>‘Winspit’; males: F3,70 = 2.45, P = 0.0708; females: F3,50=2.95, P=0.0418; mixed: F3,95=2.89, P=0.0393). Conclusions: Both herbivore and parasitoid fitness correlates differ among the cabbage populations; these differences strongly correspond to differences in glucosinolate profiles. Interestingly, the parasitoid appears to be more adversely affected by glucosinolate concentration than the herbivore. Furthermore, herbivore and parasitoid respond to different glucosinolates.
