EFFECTS OF HERBIVORES ON GROWTH AND REPRODUCTION OF THEIR PERENNIAL HOST, ERIGERON-GLAUCUS

Experiments were conducted over several years to assess the effects of the three most abundant herbivores (a thrips, a spittlebug, and a plume moth) on the growth and reproduction of their long-lived, iteroparous host plant. In a greenhouse experiment, thrips reduced root biomass of seedlings, but not shoot biomass. No correlation was found between density of thrips and change in aboveground area of plants after monitoring for 3 yr in the field. Thrips fed on the phyllaries and corollas of flower heads, as well as on leaves, and were particularly damaging to ray petals. Placed in artificial arrays, flower heads with rays damaged by thrips were less likely to be visited by pollinators than were undamaged flower heads. Plants in a common garden were subjected to three seasons of feeding by either spittle-bugs, plume moth caterpillars, both, or neither. Survival, growth, and flower production were measured for those 3 yr and for an additional 3 yr following the herbivory treatments. Plant survival was unaffected by these insect herbivores, although gophers (Thomomys bottae) were a major source of mortality. Plants with caterpillars produced more rosettes than those without moths. By consuming apical buds the caterpillars released axillary buds, thereby causing plants to produce more side rosettes. This difference in architecture did not result in an increase in flower production. Plants that had experienced 3 yr of spittle-bug or caterpillar attack produced fewer flower heads than controls during their 3rd yr. One year later, and after the herbivory treatments were discontinued, plants that had been damaged by spittlebugs (but not plume moth caterpillars) still produced fewer flowers than those in other treatments. Thereafter, no long-term effects of herbivory could be detected. All three herbivores were found to adversely affect some components of plant growth and reproduction. Because they are consistently abundant at Bodega Bay, these herbivores may reduce the fitness of their host plant.