The effects of fertilization and host plant on variability in herbivory and herbivore growth of the generalist Spodoptera exigua on the crop Brassica oleracea

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Alissa Cole, Mason C. Hoskins and Nahakuelua A Kekauoha, Environmental Science, Pacific University, Forest Grove, OR, Shaznae Marsh and Marley Shouldice, Biology, Florida State University, Tallahassee, FL, Jessie Mutz, Brian D. Inouye and Nora Underwood, Biological Science, Florida State University, Tallahassee, FL, Stacey L. Halpern, Biology Department, Pacific University, Forest Grove, OR

Background/Question/Methods
Fertilization and host variation are known to affect mean levels of herbivory. However, current knowledge is lacking on how these factors influence variability in herbivory. This information would have important applications in basic ecology and agriculture. In this study, we conducted two greenhouse experiments that addressed the following questions: (1) How does variability in herbivory and herbivore growth differ with fertilization? (2) How does variability in herbivory and herbivore growth differ with host variation? In both experiments, we used the generalist Spodoptera exigua as the herbivore, and measured percent leaf damage and larval relative growth rate. We placed one larvae on each plant, which was either alone or part of a cluster (allowing larval movement). To answer the first question, we compared variability in plant damage and larval growth among fertilized and unfertilized Brassica oleracea (Russian red kale). We predicted that variability in mean percent leaf damage would be lower and variability in larval relative growth rate would be higher among fertilized B. oleracea. To address the second question, we measured variability in herbivory among plants in a monoculture of B. oleracea and plants in a diculture of B. oleracea and Brassica juncea (giant red mustard). We predicted that variability in mean percent leaf damage would be lower and variability in larval relative growth rate would be higher among plants in the monoculture.
Results/Conclusions
Unfertilized B. oleracea had significantly more variable and 3x higher plant damage than fertilized B. oleracea. Spodoptera exigua on unfertilized B. oleracea had significantly more variable and 2x higher relative growth rates than those on fertilized B. oleracea. These results suggest that fertilization leads to low variability in herbivory; all plants are consumed relatively equally. If all or most plants in a field are of high nutritive quality, herbivores will likely spend less time moving between plants and more time eating from one or a few plants
In the host variation experiment, B. juncea had significantly more variable and 2x higher plant damage than B. oleracea. There was no significant difference in relative growth rates among S. exigua on B. oleracea and S. exigua on B. juncea.
In both experiments, plants in the movement treatment (where S. exigua were allowed to move among plants in the cluster) had significantly higher and more variable plant damage than plants in the no movement treatment (where S. exigua were restricted to one plant).