Plant Chemistry, Insect Adaptations to Plant Chemistry, and Host Plant Utilization Patterns

Abstract

The role of plant chemistry in determining the suitability of herbivore—free individuals within a plant population has rarely been investigated. In this study, we examined the importance of furanocoumarin chemistry in determining distribution patterns of the parsnip webworm (Depressaria pastinacella) in a population of its host plant, wild parsnip (Pastinaca sativa). Parsnip webworms feed almost exclusively on wild parsnip, a species that produces furanocoumarins, compounds that are toxic or deterrent to many insects. Samples of plants hosting actively feeding parsnip webworms (attacked) and plants exhibiting no damage (unattacked) were collected in the field and brought to the laboratory, where they were examined for content of three furanocoumarins, xanthotoxin, bergapten, and sphondin. In addition, they were bioassayed with ultimate—instar parsnip webworms; growth, consumption rate, and rate of in vitro metabolism of furanocoumarins were compared between larvae fed undamaged tissue from attacked plants and larvae fed tissue from unattacked plants. Attacked plants contained less than half the amount of xanthotoxin and sphondin, two of the three furanocoumarins assayed, than did unattacked plants. Growth and consumption rates of larvae fed tissue of unattacked plants also were reduced significantly (40 and 24%, respectively) compared to larvae fed attacked plants. Moreover, in vitro metabolism of xanthotoxin and bergapten was reduced by 50% in larvae fed tissue from unattacked plants compared to larvae fed tissues of attacked plants. Cluster analysis placed plants into three groups based on furanocoumarin chemistry. One group, characterized by high concentrations of all three furanocoumarins, contained only unattacked individuals. A second group consisting of plants intermediate in furanocoumarin concentration, contained predominantly unattacked individuals, and last group, characterized by low concentrations of all three furanocoumarins, was dominated by attacked individuals. When large larvae are fed plant tissues with below—average total furanocoumarin concentrations, consumption rate is the most important larval characteristic determining growth rate. However, when larvae are fed plant tissues with above—average concentrations of furanocoumarins, the capacity to metabolize furanocoumarins is also important. These results indicate that plant chemistry, relative to larval ability to tolerate plant chemicals, can to a large extent explain the distribution of this insect within a population of its host plant.