Abstract Detail


Ivey, Christopher [1], Raether, Constantin [2], Wright, Jessica [3], Sork, Victoria [4].

Plant vigor and phenological synchrony explain variation in defense against galling cynipid wasps of valley oak (Quercus lobata) in a large common garden experiment.

As primary producers, plants are subjected to relentless attack from diverse natural enemies, including many insect herbivores. An enduring question is why variation in herbivory is commonplace. The “Plant Vigor Hypothesis” proposes that robust or vigorously growing individuals are more likely to be targeted for attack by herbivorous insects, because such plants are able to prioritize investment in growth over defense. The non-mutually exclusive “Phenological Synchrony Hypothesis” suggests that host selection depends on a match between plant and herbivore developmental stages. We tested these hypotheses in a large provenance trial of valley oak (Quercus lobata) that was fully replicated in two field sites. Valley oak is host to a diverse fauna of specialized galling herbivores in the wasp family Cynipidae, which induce tumor-like neoplasms on leaves and stems that house and nourish juvenile wasp life stages. On each of over 6,500 young valley oak trees, representing 644 family lineages collected throughout the species range, the abundance of leaf- and stem-galls was scored as a measure of vulnerability to herbivore attack. The change in tree height over a three-year period was used to calculate relative growth rate as a measure of plant vigor. In addition, leaf emergence was scored weekly throughout the spring growing season as a measure of plant phenological stage. To test predictions, we constructed mixed models that included terms to account for variation among plant families and populations. The provenance sites contrasted in gall abundance. In addition, we found significant variation among maternal lines and source populations in galling herbivores, relative growth rates, and leaf phenology when grown in the common garden, which may reflect genetic variation for these traits. At one of the provenance sites, where galls were more common, we found a positive relationship between relative growth rate and abundance of the most common gall species, as well as between relative growth rate and galling species richness. Both of these patterns were consistent with predictions of the Plant Vigor Hypothesis. In support of the Phenological Synchrony Hypothesis, we found a significant negative quadratic relationship between leaf phenology and gall abundance, which indicated that trees with intermediate rates of development had higher abundance of herbivores. At the second provenance site, located in a cooler area at higher elevation, gall abundance was lower, and we observed no significant relationships between galls and either relative growth rate or leaf phenology. Both plant vigor and leaf phenology explained variation in galling herbivores at the site where they were more common. Where herbivores impose a fitness cost, higher rates of herbivory may select against lineages with faster growth and intermediate rates of development.

1 - California State University, Chico, Biological Sciences, 400 W 1st St., Chico, CA, 95929, United States
2 - California State University, Chico, Biological Sciences, 400 W. First St., Chico, CA, 95929, USA
3 - USDA Forest Service, Pacific Southwest Research Station, 1731 Research Park Drive, Davis, CA, 95618, USA
4 - UCLA, ECOL & EVOL BIOL, Box 957239 , Los Angeles, CA, 90095, United States

valley oak
plant vigor
relative growth rate
bud burst
common garden
provenance trial

Presentation Type: Oral Paper
Number: EC12002
Abstract ID:534
Candidate for Awards:None

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