Local Specialization And Landscape-Level Influence On Host Use In An Herbivorous Insect - Statistical Data Included

Ecology, August, 2000 by Mikko Kuussaari, Michael Singer, Ilkka Hanski

MIKKO KUUSSAARI [13]

MICHAEL SINGER [2]

ILKKA HANSKI [1]

Abstract. We studied host plant abundance, host use, and oviposition preference in metapopulations of the butterfly Melitaea cinxia within an area of 3500 [km.sup.2] in the Aland islands, southwestern Finland. In the study area, M. cinxia has [sim]400 small local populations on dry meadows with the larval host plants, Plantago lanceolata and Veronica spicata. Plantago lanceolata occurs in practically all meadows otherwise suitable for the butterfly, whereas the distribution of V. spicata is largely restricted to the northwestern part of the study area. Based on observations of 6500 prediapause larval groups during 1993-1996, we document spatial variation in host plant use in relation to their abundance (electivity). The fraction of larval groups found on V. spicata increased disproportionally with the relative cover of V. spicata in the habitat patches. Additionally, the probability of Veronica use in a population increased with increasing number of larval groups found on Veronica in the surrounding populati ons but decreased with increasing use of Plantago in the neighborhood. This regional effect on host use at the scale of migrating butterflies could be caused either by spatial variation in the insect (in either preference or performance) or by spatial variation in plants (in resistance to attack by the butterflies). To study the first possibility, we conducted oviposition preference experiments using butterflies from five metapopulations located 2-55 km from each other and characterized by differences in host plant availability and host use. We found clear genetic differences in oviposition preference between the five metapopulations consistent with the observed host use patterns in the field. We conclude that the spatial host use patterns of M. cinxia in the study area are driven both by direct effects of local host abundance and by indirect effects mediated through metapopulation-level adaptation to the regionally more abundant host plant.

Key words: host plant availability; host plant use; local adaptation; Melitaea cinxia; metapopulation; oviposition preference; Plantago lanceolata; plant-insect association; Veronica spicata

INTRODUCTION

The distribution of an insect species across a set of host plants at the landscape level can be described entomocentrically as "pattern of host use." Such patterns are influenced not only by spatial variation in host plant abundance (Wiklund 1974, Courtney and Forsberg 1988) but also by spatial variation in host plant quality and local adaptation in insect populations (reviews in Denno and McClure 1983, Jaenike 1990, Via 1994, Mopper and Strauss 1998; examples in Singer and Parmesan 1993, Bossart and Scriber 1995, Mayhew 1997). The host plant most often used locally by an oligophagous insect is not necessarily the most preferred one when the favored host is rare (Singer et al. 1989). Conversely, the most abundant host is not necessarily used most frequently, for instance when the insect prefers another host species over the most abundant one (e.g., Singer 1983, Singer et al. 1989).

The proportion of resources in the diet of a consumer as a function of their availability is known as electivity (Ivlev 1961). If insects encounter hosts in proportion to their abundances, and the probability of accepting each host type does not change with host abundance, then electivity will be constant across a landscape with varying relative abundances of host types. On the other hand, if electivity is spatially variable, insect diet varies in a manner that cannot be simply predicted from host availability. This could be caused by spatial variation in either plant traits or in insect traits, or both (Singer and Parmesan 1993, Mopper 1998, Strauss and Karban 1998). Plants could vary spatially in their suitability (ability to support insect growth and survival) or in their acceptability (attractiveness to ovipositing females or to feeding larvae when larvae are mobile). Insects could vary spatially in host preference and in performance (survival and growth) of the developmental stages. Spatial variation is known to occur in both insect traits (preference and performance) and in plant traits (acceptability and suitability; reviews in Denno and McClure 1983, Jaenike 1990, Thompson and Pellmyr 1991, Via 1994, Mopper and Strauss 1998).

Here, we document spatial variation in electivity in the Glanville fritillary butterfly, Melitaea cinxia (L.) (Nymphalidae), across a landscape measuring 50 X 70 [km.sup.2]. This insect inhabits a naturally highly fragmented environment in the Aland Islands in southwestern Finland, where its metapopulation structure and dynamics have been intensively studied (Hanski et al. 1994, 1995a, b, 1996, Kuussaari et al. 1998, Saccheri et al. 1998). In Aland, M. cinxia uses two host species, Plantago lanceolata (Plantaginaceae) and Veronica spicata (Scrophulariaceae). We examine here how these two host species are distributed in space and how the caterpillars are distributed among the host plants. We then ask whether spatial variation in diet can be simply explained by relative host abundance or whether electivity is spatially variable. Having documented spatial variation in electivity, we describe experiments investigating spatial variation in insect preference that could account for the observed patterns of host use and electivity in the field.