Predation on Scheelea Palm Seeds by Bruchid Beetles: Seed Density and Distance from the Parent Palm

In a test of the following hypotheses, the first was rejected and the second accepted. Rejected: (1) "The farther a freshly cleaned [pile of] palm nuts] (endocarp) is placed from a fruiting parent, the lower will be the probability of the nut[s'] being found by a bruchid. " Accepted: (2) "The larger the pile of freshly cleaned nuts at a site where there is no parent, the more likely that it will be found by bruchids. " The study was carried out in a palm forest in Guanacaste Province, Costa Rica, from March until July 1971. In addition to testing the above hypotheses, the study generated natural history data on bruchid oviposition behavior, oviposition intensity, survivorship of the larvae, 8and developmental rate. Beetles of the family Bruchidae are important predators on the seeds of a wide variety of plants (Janzen 1969). These small "pea weevils" lay their eggs on seeds or seed pods and the larvae develop inside the seed while feeding on the endosperm and embryo of the seed. The intensity and pattern of this seed predation may have a variety of effects on the population biology of the host plants (Janzen 1970). This study is a direct test of the following two hypotheses posed in Janzen's (1971) Costa Rican study of predation by bruchids on Scheelea rostrata seeds beneath the parent palm: (1) "The farther a freshly cleaned [pile of] palm nuts] (endocarp) is placed from a fruiting parent, the lower will be the probability of the nutss] being found by a bruchid. " (2) "The larger the pile of freshly cleaned nuts at a site where there is no parent, the more likely that it will be found by bruchids. " (This experiment mimics the effect of a rodent leaving a pile of nuts at a feeding site in the forest.) MATERIALS AND METHODS The seeds were set out in the relatively undisturbed hillside forest at the "Barranca site" along the PanAmerican highway in Guanacaste Province, Costa Rica (Janzen 1971). This small palm forest has a density of adult Scheelea palms of about 8-20/ha mixed in with at least 20 other genera of large dicot trees (e.g., Luhea, Licania, Enterolobium, Chlorophora, Gyrocarpus, Brosimnum, Terminalia, Lonchocarpus, Albizzia, Ficus, Guazuma, Apeiba, Tabebuia, Lysiloma, Schizolobiurn, Bursera, Spondias, Pseudo' Received October 14, 1971; accepted May 5, 1972. 2 Present address: Bird and Mammal Laboratories, Bureau of Sport Fisheries and Wildlife, National Museum of Natural History, Washington, D.C. 20560. 2 Present address: Department of Zoology, University of Michigan, Ann Arbor, Michigan 48104. boinbax, A nacardiurn, Sterculia, Triplaris, Pithecellobium). The understory vegetation contains a large amount of a forest grass (Pharus) mixed with Marantaceae, a variety of herbaceous dicots, 1to 3-m tall shrubs (e.g., Bactris, Bixa, Ardisia, Capparis, Cassia, Stemmadenia), and saplings of the canopymember trees. Although this sole remnant of Guanacaste lowland forest (Tropical moist forest, following the Holdridge terminology) along the Pan-American highway from Puntarenas to Libera is not larger than 100 ha, it still contains a substantial rodent population, including agoutis (Dasyprocta punctata), pacas (Cuniculus paca), and squirrels (Scittruis variegatoides). The site experiences a severe dry season from late December through most of April (cf. Fig. 1 in Janzen 1967). but has a nearly permanent stream running through it. Nuts (properly, the seed contained in a bony endocarp) were put out in the forest during the last half of the dry season (March 12, 1971) and about I month before normal fruit drop occurs. The first rain was on April 3 and rainy weather started between April 14 and 18 (Fig. 1). The nuts used in the experiment were obtained by cutting a large mature infrutescence from a precocious tree at the Barranca site and allowing the fruits to rot and desiccate for several weeks. The husk and pulp (exocarp) were then scraped by hand from the nut to a level approximating the degree of cleanliness achieved when a rodent eats off the pulp. (In a pilot experiment, where more than 1,500 fruits were put out without first removing the pulp, all were removed by rodents in 3 days, and presumably discarded at other places once the pulp was eaten.) The cleaned nuts were then placed under 40by 20by 15-cm screen cages with 1-cm mesh at five kinds of locations: This content downloaded from 157.55. 39.177 on Wed, 16 Nov 2016 04:24:31 UTC All use subject to http://about.jstor. org/terms Late Summer 1972 SCHEELEA PALM SEEDS AND BRUCHID BEETLES 955