PARENT SESSION
Oral Session #78: Insect Population and Community Ecology. Presiding: R. Denno.
Friday, August 10, 2001. 8:00 AM to 11:45 AM. Hall of Ideas I.
The effects of host-plant genotype and condition on a gall-forming midge and its parasitoids.
Briggs, Cheryl1, Latto, John1, 1
ABSTRACT- Using a field experiment we investigated plant effects in a gall-forming midge system. The midge, Rhopalomyia californica, forms galls only on the shrub Baccharis pilularis in California, and is attacked by a suite of parasitoids and predators. We have shown previously that parasitoids and predators can have major impacts on the midge population abundances. Here we investigate whether the genotype of the Baccharis plant and its condition (altered by water and nutrient availability) also influence the midge populations. In an experimental array we created 24 plots in which we planted 1 of each of 7 genotypes of Baccharis grown from cuttings from a nearby stand of Baccharis. Each plot received one of 3 experimental treatments: added water, added water + fertilizer, or no addition. We looked at colonization of the different plants and treatments by the midges and parasitoids from the surrounding natural populations, and at the success at gall formation and midge survival when the plants were exposed to experimentally added midges. Genotype and treatment both had highly significant effects on plant growth, which translated into differences in the number of midges colonizing per plant from the surrounding area, but not differences in the number of midges colonizing per unit plant area. Water and fertilizer treatments had no effects on the gall-formation success of experimentally added midges. There were, however, significant differences between the genotypes in gall-formation success.
KEY WORDS: Baccharis pilularis, Rhopalomyia californica, parasitoid, insect gall