Inflorescence Spiders: a Cost/Benefit Analysis for the Host Plant, Haplopappus Venetus Blake (Asteraceae)
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Oecologia (Berl) (1982) 55:185-191 Oecologia Springer-Verlag 1982 Inflorescence Spiders: A Cost/Benefit Analysis for the Host Plant, Haplopappus venetus Blake (Asteraceae) Svata M. Louda* Biology Department, San Diego State University, San Diego, CA 92182, USA Summary. Predators on flower visitors, such as spiders, hand, it is known that spiders show a numerical (Green- could influence plant reproduction by determining the bal- stone 1978) or reproductive (Wise 1975, 1979) response to ance between pollination and seed predation by insects. increased prey density in specific cases, and insects on plant This study examines the net effect of predation by the in- inflorescences do provide temporary increases in prey con- florescence spider, Peucet& viridans (Hentz), for seed pro- centration. Several families of spiders characteristically duction by a native plant species on which it hunts. Both hunt on flowers (Comstock 1940; Marden 1963; Gertsch pollination and seed set of Haplopappus venetus (Astera- 1979; Morse 1979, 1980), and these predators may be "part ceae) were reduced on branches with spiders; however, the of a plant's battery of defenses against herbivores" (Price release of viable, undamaged seed was higher on inflores- et al. 1980). However, such predator defense against inflo- cence branches with spiders than on those without. Occur- rescence herbivores carries an implicit cost, the potential rence of P. viridans was associated with the flat-topped in- reduction of pollination by insects; predation by spiders florescence branch structure characteristic of H. venetus may be analogous to interference by ants with pollinators rather than with the vertical structure of its congener, H. or with plant parasites in ant-plant mutualisms (Carroll squarrosus. Thus, the interaction should be a reinforcing and Janzen 1973; Bentley 1976; Messina 1981 ; Skinner and selective pressure on inflorescence branch morphology of Whittaker 1981). H. venetus over time. Two factors providing constraints If the balance between pollination and predation by on the degree and rate of coevolution of the plant-spider insects is positive for the plant when a spider is present, interaction are suggested by the results: (1) the critical role one would predict selective reinforcement of traits, such of phenological synchrony and (2) the opposing require- as morphological adaptations, which attract spiders and ments of interacting species and of subsequent life history which reinforce the interaction and facilitate the mutualistic stages within a species. relationship between the plant and its defenders (Janzen 1967). Few relevant studies exist to test this hypothesis. Furthermore, data on such higher order interactions may provide insight into constraints on the degree and the rate Introduction of coevolution among interacting species. Interaction with insects can be an important aspect of plant The purpose of my study was to examine the effect biology (Harris 1973). Not only are many plants dependent of predation by the inflorescence spider, Peucetia viridans on insect visitation for pollination (e.g. Faegri and van der (Hentz), on seed production by a plant, Haplopappus Pijl 1971 : Richards 1978) but, in addition, most plants are venetus Blake (Asteraceae), on which it hunts. The central subject to insect herbivory (Salisbury 1942; Whittaker question was: what is the net effect of predation by P. 1979), The visual and spatial concentration of floral re- viridans on inflorescence insects for the reproductive output sources that are necessary to attract pollinating insects of H. venetus? This involved two subsidiary questions: (1) (Baker and Hurd 1968) also make floral and developing is pollination decreased significantly in the presence of an seed tissues conspicuous to flower- and seed-feeding insects. inflorescence spider? and (2) is destruction of floral tissues Consumption of floral tissue and unreleased seed can have and seeds by insect seed predators reduced significantly by an important effect on plant fecundity (Salisbury 1942; spider foraging? In addition, I asked whether there was Janzen 1971; Bohart and Koerber 1972; Harper 1977; an association between spider occurrence and inflorescence Louda 1978; Lamb 1980; Zimmerman 1980b) and on plant branch structure. I hypothesized that: (1) localized foraging establishment (Louda 1978, 1982a, b, 1983). by P. viridans was consistent with optimal foraging theory Predators on flower visitors could influence the balance and contributed significantly to observed variation in seed between the opposing processes of insect pollination and set and seed release by H. venetus (Louda 1978, 1983); and insect-caused predispersal seed predation. The role of high- (2) the morphological form of the inflorescence branch of er order interactions in plant reproduction is not well H. venetus was attractive to spiders, enhancing continued known (Cates et al. 1977; Price et al. 1980). On the other interaction and net positive outcome for the plant. Conse- quently, data were collected on spider occurrence on H. * Present address: Duke University, Pivers Island, Beaufort, venetus, with a flat-topped inflorescence branch (Fig. 1 A), NC 28516, USA and on a closely related co-occurring species, H. squarrosus 0029-8549/82/0055/0185)$01.40 186 1979). Consequently, female spider site tenacity is excep- tionally high, from bud initiation through seed release. The newly hatched first instars remain in the egg sac 12-14 days before they molt and leave the sac (Whitcomb et al. 1966). The spiderlings thus emerge at the end of September or beginning of October (Lowrie 1963; Louda, personal obser- vation). The newly emerged spiderlings feed and eventually disperse, overwintering as second or third instar spiderlings. Seven to 9 instars (286-301 days) are required to reach ma- turity (Whitcomb et al. 1966). 4. Flower Head Insects Three groups are attracted to the flower heads: phytopha- gous species, parasitoid/hyperparasitoid species, and pollen or nectar foraging species. At least eleven phytophagous species forage on the developing flowers and seeds (Louda 1978, 1983). The most conspicuous are three tephritid flies: Urophora formosa Coquillet, Trupanea femoralis Thomp- son, and Paroxyna murina Doane. The most destructive phytophagous species are microlepidopterans in three fami- Fig. 1. Inflorescence branch structure of Haplopappus. A=H. lies: Pterophoridae, Tortricidae, and Gelechiidae (Soph- venetus Blake, B = H. squarrosus H.&A. ronia sp.). In addition, the developing ovules are fed on by larvae of two pteromalid wasps and of a curculionid H.&A., with a vertical inflorescence branch (Fig. 1 B) and weevil, Anthonomus ochreopilosus Dietz. Additionally, the with higher pollination and predation rates in the same flowers attract phytophagous thrips: Frankliniella occiden- climatic area (Louda 1978, 1982a, b). talis (Pergando), F. minuta (Moulton), and Thrips tabaci Lindeman. The phytophagous insects attract parasitoids System Studied and hyperparasitoids. A eurytomid (Eurytoma sp.), a eulopid (Tetrasticus sp.), and a parasitoid species of ptero- I. Host Plant malid attack the tephritid flies. An ichneumonid parasitoid Haplopappus venetus Blake is a small shrub, 50-150 cm tall, attacks the moth larvae. Finally, the pollen and nectar for- that is characteristic of the coastal sage scrub vegetation agers attracted to the flower heads include honey bees (Apis from central California to central Baja California, Mexico mellifera), a chrysidid wasp, and a halictid bee. (Munz and Keck 1959; Mooney 1977). H. venetus occurs primarily in disturbed microhabitats, such as alluvial fans, arroyos, and overgrazed horse pastures. Vegetative growth Methods occurs in winter and spring. In July, flower heads are ini- tiated and flowers occur in August-September; seeds are Plant Phenology released in October-November (Fig. 2). Development of flower head buds, flower presentation and seed maturation were recorded for all heads on three inflo- 2. Study Area rescence branches on each of three plants (N= 9 branches). The main study site was a coastal, disturbed plot at the The censuses included growth and number of flower heads junction of Carmel Valley Road and Interstate Highway in five developmental stages and were done biweekly from 5 in Del Mar, California, 22 km north of the City of San 22 July to 27 December 1976. Developmental stages of Diego. This site was adjacent to the back of Penasquitos flowers and seeds were defined as follows: (1) small buds Lagoon, an area with some of the largest stands of H. were heads less than 4.0 mm total length; (2) large buds venetus observed in San Diego County. Site characteristics were unopened heads from 4.0 mm up to presentation of and vegetation description are presented elsewhere (Louda floral buds and less than three opened flowers; (3)flowering 1978). heads were those with at least three florets with open, bright yellow floral tubes; (4) maturing heads were those with 3. Inflorescence Spider fewer than three fresh flowers but with no more than two Peucetia (Oxyopes) viridans (Hentz), the Green Lynx seeds missing and released; and (5) releasing heads were Spider, is the most conspicuous and common member of those with more than two seeds dispersed but at least two the Oxyopidae, occurring in the southern United States, seeds remaining in the head. Following the latter stage, Mexico, and Central America (Gertsch 1979). The adults the heads were considered empty. (female= 14-16 mm body length, male= 11-13