Aphids Limit Fecundity of a Weedy Annual (raphanus Sativus)1

Few previous studies document effects of herbivores on the reproduction of wild plants in situ. We examined the impact of aphids' on seed production in wild radish (Raphanus sativus L.), an annual herb. Aphid infestation increased during the three-month flowering period. Flower and fruit production declined during the season, in part due to aphids. Inflorescences of lateblooming plants more than doubled their fruit production when aphids were removed. Thus, aphids curtailed the blooming period of wild radish, perhaps conferring a selective advantage on early-flowering plants. A few individuals were not susceptible to aphid colonization. HERBIVORES cause dramatic reductions in the yield of agricultural species, yet few previous studies concern their effects on the fecundity of wild plants. Plant reproductive success is obviously affected when severe damage results in mortality (e.g., Fedde, 1973; Schmitt and Antonovics, 1986). More often, herbivores merely reduce plant growth, usually leading to a decrease in fecundity (Rausher and Feeny, 1980; Louda, 1984; Marquis, 1984; Whitham and Mopper, 1985; but see Hendrix, 1979; Paige and Whitham, 1987). The detrimental effects of sucking insects, such as aphids, are generally less visible than the damage caused by leaf-chewing ·herbivores. However, these sap-feeders can severely reduce plant growth and reproduction (in agricultural systems: Kennedy and Stroyan, 1959; Harper, 1963; Banks and Macaulay, 1967; Dixon, 1971a, b; Vereijken, 1979). Further ecological studies are needed to understand how herbivory affects lifetime reproduction in wild plants, thereby promoting evolutionary change. Relatively little is known about the impact ofsap-feeding insects on wild species. Here we describe the effects of aphid feeding and variation in flowering time on some fitness components of Raphanus sativus. 1 Received for publication 30 March 1987; revision accepted 16 October 1987. We thank Dr. W. H. Lange ofUC/Davis for identifying aphids, H. Hasbrouck and C. McGee for technical assistance, and R. Nakamura, M. Rausher, and T. Whitham for reviewing a draft ofthis paper. This work was supported by NSF grants BSR 84-11077 to AAS and DEB 82-14508 to MLS. 2 Current address: Smithsonian Environmental Research Center, PO Box 28, Edgewater, MD 21037. MATERIALS AND METHODSWild radish is a cosmopolitan annual that became naturalized in California in the late 1800's (Panetsos and Baker, 1967). This economically important weed is largely self-incompatible, relying on insect vectors for pollen transfer and seed set. Flowers open sequentially on indeterminately growing branches. Local pollinators include honeybees, lepidoptera, syrphid flies, and other insects (Stanton, 1987a). In this and several previously studied California populations, seed production was not limited by pollinator service (Stanton, 1987b). We studied patterns of flowering and fruit set during the 1983 growing season (MarchMay). Field work was conducted at a population of several thousand wild radish plants growing in an abandoned field at the University ofCalifornia at Davis, CA. (See Stanton, 1987a, for description of "Arboretum" site.) The 11.5 m tall plants each had several hundred inflorescences (also referred to as flowering branches) that began producing fruits in midMarch. Aphids did not appear until later in the season, so their effects on plant reproduction were determined using plants that reached peak flowering in late April. These plants will be referred to as late-flowering plants. Prior to the arrival of aphids, we measured fruit set on 18 haphazardly chosen plants that flowered early in the season. Starting on 19 March and 20 April we labelled all open flowers on each of 3 randomly selected flowering branches per plant. This procedure was repeated 3 times over a period of 10 days. flowers were marked by attaching a small adhesive label to the stem section below each pedicel. Fruit set from these flowers was recorded ap-