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Title:Diel patterns of insect herbivory and plant secondary metabolites in understory shrubs on Barro Colorado Island
Author(s):Zehr, Luke N
Advisor(s):Berenbaum, May R.
Contributor(s):Suarez, Andrew V.; Dalling, James W.
Department / Program:Entomology
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Nocturnal herbivory
Barro Colorado Island
Plant secondary metabolites
Molecular networking
Plant defense
Insect herbivore
Abstract:Plants produce a broad diversity of secondary metabolites as defenses against herbivory. In response, herbivorous insects have evolved a diversity of behavioral and biochemical counter-adaptations to these defenses. Despite that a significant portion of leaf area removal by insects in the tropics occurs at night, virtually all studies of chemically mediated interactions between herbivorous insects and their host plants have been conducted primarily or entirely during the daytime. Accordingly, I set out to quantify if rates of herbivory differ between the day and night. I used leaf photographs and single plant herbivore exclosures on 126 individual plants of four species in the genera Piper and Psychotria on Barro Colorado Island, Panama, to quantify the timing of herbivory over the course of 56 diel cycles. I found that on young leaves, protecting plants from herbivores during the night causes significantly fewer leaves to be damaged than if plants are protected only during the day but are exposed at night. I then characterized qualitative differences in the secondary metabolite profiles of the leaves of these plant species during daytime and nighttime hours using ultra high-performance liquid chromatography (UHPLC), electrospray ionization and molecular fragmentation, and tandem mass spectrometry (MS/MS). Network analyses show that plant secondary metabolites varied greatly in their presence in leaves over the course of several hours, suggesting that ecologically significant differences exist in the overall chemical profile that herbivores would encounter in leaves during different times of a day. Whether these variations in putative defense compounds may affect the foraging times and behaviors of herbivorous insects remains unresolved. Most larval herbivores lack an effective means for dispersing to new hostplants. Therefore, feeding strategies that maximize assimilation and growth, and minimize time to pupation, are expected to be most advantageous. However, larval lepidopterans exhibit numerous behaviors that appear to interfere with maximizing the rate of foliage consumption. Caterpillars may limit their foraging times to specific periods of the diel due to uneven predation risk, to predictably variable abiotic (especially temperature and humidity) conditions, or to short-term variation in forage quality. While daily variation in forage nutrient levels and secondary metabolite concentrations has been well-studied in numerous systems, the influence of such variation on the daily rhythms of herbivore feeding activity has not yet been determined. I propose that the highly sensitive and specialized senses of olfaction and gustation in herbivores suit them well for limiting their feeding bouts to times of the day when plants are less well-defended (which I propose is the night). Such behavior is especially likely to occur in environments where there is relatively low variation between daytime and nighttime temperatures and humidity and where predation risk is always relatively high, or even higher during the nighttime than the day, as is likely the case in many tropical forests.
Issue Date:2017-07-20
Rights Information:Copyright 2017 Luke Zehr
Date Available in IDEALS:2017-09-29
Date Deposited:2017-08

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