How the sapling specialist shoot -borer, Cromarcha stroudagnesia (Lepidoptera, Pyralidae, Chrysauginae), alters the population dynamics of the Costa Rican dry forest tree Tabebuia ochracea (Bignoniaceae)

This dissertation details how herbivory by the shoot-borer Cromarcha stroudagnesia (Lepidoptera, Pyralidae, Chrysauginae), alters the growth and population dynamics of its host tree Tabebuia ochracea (Bignoniaceae) in the tropical dry forests of the Area de Conservacion Guanacaste, Costa Rica. Insect herbivory of tree saplings can affect tree population dynamics by increasing sapling prereproductive mortality and increasing the age of first reproduction (by decreasing the rate of sapling height increment). Herbivory of T. ochracea saplings by the larvae of C. stroudagnesia has large direct and indirect effects on both these variables. Just a single C. stroudagnesia larva attacking the top shoot of a canopy gap sapling reduces sapling annual height increment on average by 77.9%---equivalent to the difference in annual height increment between saplings in canopy gaps and understory shade. Each year, an average 40.1% of canopy gap saplings had their top shoot attacked by a C. stroudagnesia larva. Assuming a sapling remains at the top of canopy gap vegetation, this will result in a 45.1% increase in the duration of the pre-reproductive sapling lifestage (0.5 m to 10 m high) relative to undamaged plants. C. stroudagnesia herbivory increases the probability that canopy gap saplings are overtopped and shaded by surrounding vegetation, and so will further, indirectly increase the age of first reproduction. Sapling mortality was directly increased by C. stroudagnesia herbivory---10% of heavily damaged young saplings (initially 0.7--1.5 m high) died in four years while no protected saplings died. C. stroudagnesia herbivory will also indirectly increase sapling mortality by increasing the likelihood that a sapling is killed by falling debris before it reaches reproductive age. Cromarcha stroudagnesia shoot herbivory is greatest in areas of high sapling density, and so is predicted to cause a Janzen-Connell effect. Together, these results suggest that sapling herbivory, especially herbivory of the highest apical meristems of saplings, plays an important, but largely unexplored, role in the population dynamics of tropical tree species.