The parsnip webworm (Depressaria pastinacella) and the wild parsnip (Pastinaca sativa) together represent a potentially "coevolved" system in that throughout their ranges the plant has relatively few other herbivores and the insect has virtually no other hosts. Individual wild parsnip plants within a central Illinois population vary in their content and composition of furanocoumarins, secondary compounds with insecticidal properties. Half-sib and parent-offspring regression estimates of the heritability of furanocoumarins demonstrate that this variation is genetically based. Wild parsnip plants also vary in their resistance to damage by the parsnip webworm, which feeds on flowers and developing seeds. In an experimental garden, seed production in the primary umbel ranged from 0 to 1,664 seeds among individuals, and mean seed production of half-sib families ranged from 3.7 seeds to 446.0 seeds. Approximately 75% of the variation in resistance among half-sib families to D. pastinacella was attributable to four furanocoumarin characteristics-resistance is positively related to the proportion of bergapten and the amount of sphondin in seeds, and negatively related to the amount of bergapten and the proportion of sphondin in leaves. Each of the four resistance factors had significant heritability. Thus, resistance in wild parsnip to the parsnip webworm is to a large extent chemically based and genetically controlled. Genetic correlations among fitness and resistance characters, however, tend to limit coevolutionary responses between herbivore and plant. In greenhouse plants protected from herbivory, several of the resistance factors have negative genetic correlations with potential seed production. Ostensibly, highly resistant plants in the absence of herbivory would be at a competitive disadvantage in the field. The selective impact of the herbivore is also limited in this population by a negative genetic correlation among resistance factors. Selection to increase one resistance factor (e.g., the proportion of bergapten in the seed) would at the same time decrease the amount of a second resistance factor (e.g., the amount of sphondin in the seed). The wild parsnip and the parsnip webworm, then, appear to have reached an evolutionary "stalemate" in the coevolutionary arms race.

The parsnip webworm (Depressaria pastinacella) and the wild parsnip (Pastinaca sativa) together represent a potentially "coevolved" system in that throughout their ranges the plant has relatively few other herbivores and the insect has virtually no other hosts. Individual wild parsnip plants within a central Illinois population vary in their content and composition of furanocoumarins, secondary compounds with insecticidal properties. Half-sib and parent-offspring regression estimates of the heritability of furanocoumarins demonstrate that this variation is genetically based. Wild parsnip plants also vary in their resistance to damage by the parsnip webworm, which feeds on flowers and developing seeds. In an experimental garden, seed production in the primary umbel ranged from 0 to 1,664 seeds among individuals, and mean seed production of half-sib families ranged from 3.7 seeds to 446.0 seeds. Approximately 75% of the variation in resistance among half-sib families to D. pastinacella was attributable to four furanocoumarin characteristics-resistance is positively related to the proportion of bergapten and the amount of sphondin in seeds, and negatively related to the amount of bergapten and the proportion of sphondin in leaves. Each of the four resistance factors had significant heritability. Thus, resistance in wild parsnip to the parsnip webworm is to a large extent chemically based and genetically controlled. Genetic correlations among fitness and resistance characters, however, tend to limit coevolutionary responses between herbivore and plant. In greenhouse plants protected from herbivory, several of the resistance factors have negative genetic correlations with potential seed production. Ostensibly, highly resistant plants in the absence of herbivory would be at a competitive disadvantage in the field. The selective impact of the herbivore is also limited in this population by a negative genetic correlation among resistance factors. Selection to increase one resistance factor (e.g., the proportion of bergapten in the seed) would at the same time decrease the amount of a second resistance factor (e.g., the amount of sphondin in the seed). The wild parsnip and the parsnip webworm, then, appear to have reached an evolutionary "stalemate" in the coevolutionary arms race.

Fruits ofPastinaca sativa (Apiaceae), the edible parsnip, contain six different furanocoumarins that are differentially capable of ultraviolet-mediated cross-linkage of DNA and inhibition of DNA transcription. Individually, none of the other furanocoumarins present in parsnip seeds is as toxic as the photosensitizer xanthotoxin. Nevertheless, the natural mixture of compounds is toxicologically more effective againstHeliothis zea (Lepidoptera: Noctuidae), both in the presence and absence of UV light, than is an equimolar amount of xanthotoxin. The difference in toxicity diminishes with increasing light levels. Thus, a series of structurally related natural products can display toxicity lacking in individual compounds and may represent an adaptive compromise to varying environmental conditions.

An isolate of the fungus Chaetomium globosum produced culture broths that inhibited in vitro egg hatch and juvenile mobility of root-knot nematode (Meloidogyne incognita) and hatch of soybean cyst nematode (Heterodera glycines). Extraction and bioassay-directed fractionation of the culture broth ltrate determined that avipin, a low molecular weight compound, was the fungus metabolite responsible for most of the nematode-antagonistic activity. Synthesis of avipin permitted evaluation of the compound as a suppressor of nematode populations on plants in glasshouse studies. Muskmelon (Cucumis melo) plants in steamed and unsteamed soil were inoculated with root-knot nematodes and various concentrations of avipin were applied to the soil. Contrary to expectations from the in vitro studies, the number of galls per g of roots increased with avipin treatment at the 14-day harvest. No effect of avipin on nematode populations was found at the 55-day harvest. In general, plant growth and nematode populations were greater in plants grown in steamed soil.

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Geographic differences in host plant availability are thought to result in regional adaptation of herbivores to their sympatric host species. Larvae of Papilio troilus, the spicebush swallowtail butterfly, feed only on plants of the Lauraceae. Although this insect occurs throughout the eastern United States, each host species has a distribution smaller than the range of P. troilus. In peninsular Florida, the major host is Persea borbonia; Sassafras albidum and Lindera benzoin are the predominant hosts elsewhere. The hypothesis that P. troilus are better adapted to their sympatric host plants was tested using a split—brood design comparing the larval performance of Florida, Midwest, and hybrid Florida ° Midwest full—sib families on Persea, and Lindera. Florida larvae had higher first—instar growth rates and survival on Persea compared to Midwest larvae. In contrast, Midwest larvae had higher total larval growth rates on Sassafras and Lindera relative to Florida larvae. A significant host ° geographic strain interaction was also found for pupal mass. Performance of hybrid larvae was intermediate to the parental strains, or equalled the best performing parental strain, depending on the host and performance measure. Regional differentiation in genetically based traits for host use indicates the existence of geographic races in P. troilus.

SummaryParthenocarpy, the production of fruits without viable seeds, is a widespread phenomenon in plants. While failure to effect pollination or fertilization is often cited as the cause of parthenocarpy, this explanation alone is inadequate to explain why plants produce, maintain and further develop fruits. Wild parsnips (Pastinaca sativa) frequently produce parthenocarpic fruit. When parsnip webworms (Depressaria pastinacella), specialist feeders on wild parsnip, were given choices between normal fruit and parthenocarpic fruit, they exhibited a strong preference for parthenocarpic fruit. However, on parthenoearpic fruit, insects fed less efficiently and grew more slowly than insects fed normal fruit. Parthenocarpic fruits, then, may act as decoys that divert herbivores away from fruits that contain plant offspring.

No abstract