PARENT SESSION
Poster Session #2: Plant-Animal Interactions.
Monday, August 6, 2001. Presentation from 10:30 AM to 12:00 PM. Exhibition Hall

Greenhouse bioassay for evaluating performance of the western spruce budworm on grafted Douglas-firs.

Clancy, Karen¹, Chen, Zhong², Kolb, Thomas², ^{1 2}

ABSTRACT- We reared western spruce budworm (Choristoneura occidentalis) larvae from our nondiapausing and diapausing cultures on grafted Douglas-firs (Psuedotsuga menziesii var. glauca) for one complete generation so that we could evaluate the importance of host tree phenotype (resistant versus susceptible to budworm defoliation) on the number of F₁ offspring produced, or the realized fitness of C. occidentalis. Two different bioassays were used to assess the importance of bud burst phenology as a factor determining host plant resistance. The bioassay techniques are described, and strenghts and weaknesses of this approach are pointed out. We concluded that this type of comprehensive bioassay is critical to advancing understanding of how mechanisms of host plant resistance to insect herbivores actually function to affect the population dynamics of the insects. Individual measurements of budworm performance (e.g., potential fecundity estimated from female pupal weights) did not differ between resistant and susceptible trees. However, when we combined the empirical data on survival through various life stages (including the egg stage) with data on realized fecundity to predict fitness (i.e., population growth) over multiple generations, clear distinctions emerged between the resistant versus susceptible trees. Furthermore, we demonstrated that phenological asynchrony between the emergence of budworm larvae and budburst of host trees is a critical factor determining host plant resistance to budworm defoliation.

KEY WORDS: Choristoneura occidentalis, greenhouse bioassay, realized fitness, mechanisms of resistance