Competition Versus Host Plant Phenotype in Species Composition: Willow Sawflies

Abstract

The influence of host plant phenotype and interspecific competition on the abundance of four willow sawfly species was tested on potted cuttings of the arroyo willow, Salix lasiolepis, in northern Arizona. Interspecific competition by a stem—galling sawfly, Euura lasiolepis, was demonstrated on a leaf galler, Pontania sp., a leaf folder, Phyllocolpa sp., and a petiole galler, Euura sp. The competitive influence of the stem galler on these species is likely to be absent at low stem gall densities. Density of the stem galler is spatially patchy, so competition should be variable among willow clones in the field. The three other sawfly species form galls that are distal to stem galls and/or do not affect the shoot in a way detrimental to the stem galler, therefore this competition appears to be asymmetrical, an "amensalism." All sawfly species exhibited significant differences in gall densities among potted, replicate willow clones under uniform environmental conditions. This suggests underlying genetic variation among clones in susceptibility to sawfly oviposition. Each sawfly species seemed to respond to variation among clone phenotypes idiosyncratically. Under experimental conditions, host plant phenotype accounted for 1.5 to 3.2 times as much variation in the density of subordinate species as did interspecific competition; under field conditions host plant phenotype is likely to play a more important role than competition in the community patterns of herbivorous insects.

PATTERNS OF SELECTION ON PHYTOPHAGE RESISTANCE IN IPOMOEA PURPUREA

“…One reason for this may be that it has seldom been recognized that there are many possible causes of nonindependent evolution of resistances. These include, but are not limited to (1) the existence of genetic correlations among traits conferring resistance to different parasites (e.g., Kinsman 1982;Rausher and Simms 1989;Maddox and Root 1990), which may under some circumstances constrain the evolution of resistance even if there is directional selection acting directly on each resistance trait (Rausher 1992); (2) effects of one parasite on the abundance of, or susceptibility ofa hostto, another (e.g., Carroll and Hoffman 1980;Kuc 1982Kuc , 1987Fritz et al 1986;Karban 1986;Karban et al 1987;Apriyanto and Potter 1990;Fritz 1992;Rausher et al 1993); and (3) nonadditivity of the impact of two parasites on host fitness (e.g., Dickinson 1979;Powell 1979;Strauss 1991). [A general model indicating how (2) and (3) lead to nonindependent evolution of resistances is provided by Hougen-Eitzman 1991.…”

Evolution of Gall Morphology and Host-Plant Relationships in Willow-Feeding Sawflies (Hymenoptera: Tenthredinidae)

“…In the oak leaf galler Cynips divisa, gall location in relation to other galls and the midrib affects gall size and growth (Sitch et al 1988;Hartley 1998) and galler mortality (Gilbert et al 1994). Fritz et al (1986) demonstrated the possibility of asymmetric competition between the stem galler E. lasiolepis and three other species representing different gall types on Salix lasiolepis.…”

Interactions of gall-forming species at different plant spatial scales

“…However, competition was demonstrated in that system. Fritz et al (1986) found competitive effects of stem-galling sawfly Euura lasiolepis on the densities of three gallforming sawflies species on six clones of arroyo willow Salix lasiolepis. Several studies have demonstrated the importance of bottom-up effects on population regulation of herbivore species (e.g., Fernandes 1990Fernandes , 1998Fernandes et al 2000), especially those related to plant resistance (Karban and Baldwin 1997).…”

“…A recent review detected evidence for competition or facilitation in 73% of all pair-wise interactions (see Kaplan and Denno 2007). Herbivore competition may vary among genotypes (Fritz 1990) and phenotypes of the host plant (Fritz et al 1986), and can be mediated by predators (Pallini et al 1998;Chase et al 2002), parasitoids (Gonzáles et al 2002), host plant (Denno et al 1995;Inbar et al 1999) and by shared natural enemies (apparent competition; e.g., Morris et al 2004).…”