PLANT AGING AND THE DISTRIBUTION OF RHYACIONIA-NEOMEXICANA (LEPIDOPTERA, TORTRICIDAE)

A noted pattern of attack by larvae of pine tip moths, Rhyacionia spp. , has been that young trees are susceptible and older trees remain una ttacked. We undertook a quantitative test of this conventional wisdom and tested the following 4 hypotheses that may account for the pattern : (1) the plant age hypothesis, (2) the plant vigor hypothesis, (3) th e plant stress hypothesis, and (4) the induced defense hypothesis. Pon derosa pine, Pinus ponderosa Lawson, near Flagstaff, AZ, were examined for attack by southwestern pine tip moths, Rhyacionia neomexicana (Dy ar), over an age range of 300 yr. No current attacks were found on tre es older than 20 yr or taller than 4 m tall, providing quantitative su pport for conventional wisdom and the plant age hypothesis. As trees a ged, there was a significant increase in shoot diameter (r2 = 0.22, P < 0.0001) which could not account for the lack of attack after age 20. Within individual trees attack was concentrated on the largest shoots , supporting the plant vigor hypothesis instead of the plant stress hy pothesis. Attack in previous years did not influence attack in the cur rent year, indicating a lack of induced plant defense. The results ind icate the importance of genetically regulated ontogenetic aging as a s trong negative influence on attack. Yet within the young age group of attacked trees larger shoots were attacked more frequently than smalle r shoots. The results suggest that shoot borers and shoot gallers show similar relationships to their host plants and aid in the recognition of broad patterns in plant-herbivore interactions.