Parry, D. 2000. Induced responses of poplars to defoliation and their effects on leaf-feeding Lepidoptera. Ph.D. Dissertation, Michigan State University, East Lansing, Michigan, USA.

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An enduring hypothesis for explaining cyclical populations of forest Lepidoptera has been phytochemical resistance to herbivory elicited in trees subjected to defoliation. Severe defoliation of trees elicits both rapid and delayed changes in host quality for herbivores, known collectively as ‘induced-resistance’. While rapid-induced resistance (RIR), which affects the generation of insects causing the damage, is thought to contribute to population stability, delayed-induced resistance (DIR), lasting a year or more, may lead to unstable dynamics and generate population cycles. DIR has typically been investigated in trees following a single defoliation event. However, many outbreak folivores defoliate trees for consecutive years and single year studies may not capture the full effects of DIR. Thus, cumulative effects of successive years of defoliation on tree physiology and insect herbivores are poorly understood.

I conducted two long-term studies designed to emulate the defoliation intensity and temporal scale of natural outbreaks. Chapter 2 describes an outbreak population of gypsy moth ( Lymantria dispar L.) established experimentally in large-scale stands of hybrid poplar ( Populus sp.), 1996-1999. In Chapter 3, I experimentally established forest tent caterpillar ( Malacosoma disstria Hübner) populations and defoliated two stands of trembling aspen ( Populus tremuloides ) over a four-year period (1997-2000).

Gypsy moth defoliation increased total phenolics and condensed tannins in leaves while having only marginal effects on phenolic glycosides and other minor secondary compounds (Chapter 2). There were strong within-year, rapid-induced effects on secondary metabolites, and condensed tannins and total phenolics remained high one year after the cessation of defoliation. Nitrogen was reduced within the year that defoliation occurred, especially in late season leaf samples. Effects on nitrogen did not extend to the following season and levels had recovered to predefoliation levels within a year of cessation of defoliation. Gypsy moth pupal mass, development time and fecundity were not affected by the treatments in 1996, whereas female pupal mass and fecundity were marginally reduced in 1997 and significantly reduced in 1998. Gypsy moth defoliation reduced the performance of competing species of Lepidoptera in 1998. Forest tent caterpillar, poplar tent maker ( Clostera inclusa ), and big poplar sphinx ( Pachysphinx modesta ) all suffering reduced pupal mass, fecundity, and/or increased development time. Fall webworm ( Hyphantria cunea ), was unaffected by defoliation or fertilization. Defoliation in previous years did not affect gypsy moth and three other species in 1999, suggesting that herbivore performance was independent of phenolic and tannin levels.

Chronic defoliation of aspen (Chapter 3) had larger effects on tent caterpillars than defoliation of poplars had on gypsy moth (Chapter 2). Relative growth rates of final instar females were significantly reduced although growth of male fifth instars was unaffected. Mean fecundity was reduced by 8-21% in the first year, 13-16% following three years, and 20-21% on trees with four consecutive years of defoliation. Development time and survival from hatch to adult was unaffected by the treatments. Defoliation affected parasitism rates by two tachinid flies. Parasitism appeared to be mediated by caterpillar density rather than induced host tree responses to defoliation, and also varied significantly among aspen clones. Intraspecific differences in phytochemistry among clones may alter host detection abilities in tachinids using olfactory cues from herbivore damaged foliage.

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