Interactions among phytophagous insect species colonizing cones of white fir (Abies concolor).
The insect complex colonizing white fir (Abies concolor) cones is composed of 11 species that can be separated into 3 feeding guilds: the seed-mining guild (the torymids Megastigmus pinus and M. rafni and the lonchaeid Earomyia abietum); the cone- and seed-mining guild (the pyralid Dioryctria abietivorella, and the tortricids Eucosma probably E. siskiyouana, Barbara sp., and Cydia probably C. bracteatana); the scale- and bract-feeding guild (the cecidomyiids Asynapta hopkinsi, Dasineura probably D. abiesemia and Resseliella conicola and the anthomyiid Lasiomma abietis [Strobilomyia abietis]). In 3 of 4 study sites in California, the cone-crop was found to decrease from one year to the next. As cone crop size decreased there was a concomitant increase in the percentage of cones with more than one species. In addition, there was a shift towards an increased co-occurrence of members of different guilds within a single cone. Both positive and negative interactions were detected between selected species-pairs. Present-day competition was only inferred between species-pairs belonging to the cone and seed-mining guild. Decreasing resources over time, combined with increasing insect populations and the absence of acceptable alternate hosts appeared to be important factors for setting conditions conducive to interspecific competition. It is hypothesized that the aperiodicity of white fir cone crops is important in keeping insect populations below levels which would result in interspecific competition. The possible elimination of aperiodicity in cone crops of white fir, such as might occur in managed seed orchards, may lead to decreased species diversity via competitive exclusion and thereby simplify development of integrated pest management (IPM) programmes.