Effects of Prior Defoliation on the Timing of Life Cycle Events and Susceptibility to Natural Enemies of a Host Specific Gall-Former

Abstract

In short lived phytophagous insects specialized to exploit specific plant tissues, phenological synchronization between plant and insect life cycle events is crucial and likely under selection. Here I report on past (1995-2001) and recent (2006-2007) patterns of emergence phenology and survivorship of the asexual generation of the host specific gall-former, Belonocnema treatae (Hymenoptera: Cynipidae) that develops within and emerges from leaf galls on Quercus fusiformis, a live oak endemic to the Edwards Plateau region of central Texas. Comparisons of these patterns document an apparent shift in the timing of B. treatae emergence (Past: Oct.-Dec., Recent: Dec.-Mar.) and survivorship (Past: 0-50%, Recent: 0-13%) in the presence of natural enemies. These shifts in conjunction with past and recent defoliation of live oaks by oak leaf rollers (which alter the timing of tissue formation coinciding with B. treatae oviposition) motivated a defoliation experiment inducing variation in the timing of leaf flush, thus altering the timing of oviposition to test the following hypothesis: variation in the timing of leaf formation, induced by defoliation, creates temporal variation in the timing of oviposition, affecting subsequent gall maturation schedules, and B. treatae emergence phenology and survivorship. Galls induced by early and delayed oviposition mature at the same time and produce adult B. treatae that have similar emergence phenologies and levels of survivorship in the presence and absence of natural enemies. Thus B. treatae development is a plastic trait. However, survivorship, when exposed to natural enemies, differed significantly between early (mean± SE = 1.3 7% ± 1.29%) and delayed (mean± SE= 20.99% ± 1.43%) oviposition events. These results indicate that defoliation, as manipulated herein, created variation in oviposition timing but is not responsible for observed shifts between past and recent emergence phenologies. However, variation in oviposition timing, driven by defoliation can explain differences in survivorship in the presence of natural enemies. These results highlight the effects that common but patchy outbreaks of defoliators can have on the timing of life cycle events and survivorship of phytophagous insects.