Abstract
Nearly all herbivorous arthropods make foraging-decisions on individual leaves, yet systematic investigations of the adaptive significance and ecological factors structuring these decisions are rare with most attention given to chewing herbivores. This study investigated why an intracellular feeding herbivore, Western flower thrips (WFT) Frankliniella occidentalis Pergande, generally avoids feeding on the adaxial leaf surface of cotton cotyledons. WFT showed a significant aversion to adaxial-feeding even when excised-cotyledons were turned up-side (abaxial-side ‘up’), suggesting that negative-phototaxis was not a primary cause of thrips foraging patterns. No-choice bioassays in which individual WFT females were confined to either the abaxial or adaxial leaf surface showed that 35% fewer offspring were produced when only adaxial feeding was allowed, which coincided with 32% less plant feeding on that surface. To test the hypothesis that leaf biomechanical properties inhibited thrips feeding on the adaxial surface, we used a penetrometer to measure two variables related to the ‘toughness’ of each leaf surface. Neither variable negatively co-varied with feeding. Thus, while avoiding the upper leaf surface was an adaptive foraging strategy, the proximate cause remains to be elucidated, but is likely due, in part, to certain leaf properties that inhibit feeding.
Highlights
For most arthropod herbivores foraging on individual plants requires three hierarchical decisions: which branch to settle on, which leaf to settle on, and where to feed within an individual leaf
On normally-oriented cotyledons, an aversion to adaxial-feeding by Western Flower Thrips (WFT) was highly significant on both genotypes based on Monte Carlo simulations (P
No-choice feeding and relative fitness bioassay Under no-choice conditions Western flower thrips (WFT) fed 32% less on the adaxial leaf surface compared to the abaxial surface (χ2=34.494, d.f.=1, P< 0.001; Figure 2A; Table 1)
Summary
For most arthropod herbivores foraging on individual plants requires three hierarchical decisions: which branch to settle on, which leaf to settle on, and where to feed within an individual leaf. The final decisionswhere to feed on individual leaves- is one that most herbivores encounter, and yet surprisingly little is known about the factors affecting these foraging-decisions with most attention given to chewing (i.e., mandibulate) herbivores [1,2,3]. Leaves due to higher concentrations of allelochemicals such as glucosinolates[3]. Another chewer, Galerucella lineola (F.) prefers feeding in the rolled-leaf margins of their host plant, Salix viminalis L., due to increased protection from desiccation[1]. Comparatively less is known about the factors influencing the within-leaf foraging decisions of intracellular feeding herbivores that consume the cellular contents of host plants
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