Small-scale resource tracking in a population of a long-lived insect

Olof Widenfalk,Christer Solbreck,Hanna L Bergeå

doi:10.1002/ece3.378

Olof Widenfalk, Christer Solbreck + Show 1 more

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https://doi.org/10.1002/ece3.378

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Abstract

How plant-feeding insects distribute themselves and utilize their host plant resources is still poorly understood. Several processes may be involved, and their relative roles may vary with the spatial scale considered. Herein, we investigate small-scale patterns, namely how population density of a gall midge is affected by individual growth form, phenology, and microsite characteristics of its herb host. The long-lived plant individuals vary much with regard to number of shoots, flower abundance, and flowering phenology. This variation is connected to site characteristics, primarily the degree of sun exposure. The monophagous insect galls the flowers of the host plant – an easily defined food resource. It is a poor disperser, but very long-lived; diapausing larvae can stay in the soil for many years. Galls were censused on individual plants during 5 years; from a peak to a low in gall population density. Only a very small fraction of the flowers produced (<0.5%) were galled even in the peak year. Nevertheless, most plant individuals had galls at least 1 year. In a stepwise multiple regression, plant size (number of shoots) was found to be the most important predictor of gall density (galls/flower). However, gall density decreased more than one order of magnitude over the plant size range observed. There was also a weak effect of plant phenology. Early flowering plants had lower gall densities than those starting later. Sun exposure had no direct effect on gall density, but a path analysis revealed indirect effects via the timing of flowering. Gall population change was highly synchronous in different parts of the study area with no significant decrease in synchrony with distance.

Highlights

The distribution of plant-feeding insects in the landscape is often affected by multiple processes, each of which may have a dominating effect on a different spatial scale (e.g., Menendez and Thomas 2000; Krawchuk and Taylor 2003; Rabasa et al 2005; Singer and Wee 2005; Roslin et al 2006; Gripenberg and Roslin 2007)
We explore, in a 5-year study, how structural characteristics of the individual plant and of the plant site affect the distribution of a gall midge, Contarinia vincetoxici Kieffer galling the flowers of a long-lived perennial herb, White Swallow-wort, Vincetoxicum hirundinaria Medicus (L.)
The total gall population varied more than one order of magnitude during the period 1997–2003 (Fig. 4)

Summary

Introduction

The distribution of plant-feeding insects in the landscape is often affected by multiple processes, each of which may have a dominating effect on a different spatial scale (e.g., Menendez and Thomas 2000; Krawchuk and Taylor 2003; Rabasa et al 2005; Singer and Wee 2005; Roslin et al 2006; Gripenberg and Roslin 2007). Many studies have been concerned with the factors determining insect occurrence or abundance in host plant patches on the landscape scale (Hanski and Gilpin 1991; Harrison 1991; Hanski 1994, 1997; Forare and Solbreck 1997; Thomas and Kunin 1999; Kery et al 2001). How do insects exploit the resources and microhabitats provided by individual herbaceous plants within the landscape-scale patches?.

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