Limitation of the Recruitment of the Shrub Haplopappus Squarrosus (Asteraceae) by Flower- and Seed-Feeding Insects

(1) The hypothesis, that flowerand seed-feeding insects limit the density of seedlings and the establishment of young plants, was tested experimentally under field conditions in California for the shrub Haplopappus squarrosus (Asteraceae). (2) Fortnightly application of insecticide during flower development and fruit maturation reduced the loss of fruits caused by tephritid flies, microlepidopteran moths, and chalcidoid wasps from 94% to 41%. (3) The experimental reduction in insect damage to flowers, developing ovules, and seeds had three major effects. The number of viable seeds (those which matured and remained undamaged) increased significantly; the density of seedlings increased significantly, and the number of young plants which were recruited after one year increased significantly. (4) The experimental results, combined with observations of density and mortality in subsequent stages of growth, suggest that flowerand seed-feeding insects are the proximate factor limiting the recruitment and abundance of Haplopappus squarrosus in this coastal environment.