Ecology, 91(1), 2010, pp. 15–21 Ó 2010 by the Ecological Society of America Host-plant-induced larval decision-making in a habitat/host-plant generalist butterfly MAGNE FRIBERG 1 AND CHRISTER WIKLUND Department of Zoology, Stockholm University, SE-10691, Stockholm, Sweden Abstract. Phenotypic plasticity can be a passive response to fluctuating environmental conditions or an active and presumably adaptive (evolved) response selected for in different environments. Here we ask if the larval decision to enter diapause when reared on a host plant associated with a colder habitat is an active or a passive response to host-plant quality or suitability. We compare plasticity in larval propensity to enter diapause of the habitat generalist butterfly Leptidea sinapis and the meadow specialist Leptidea reali in a range of temperatures and long day length on a forest plant, Lathyrus linifolius, and a meadow- associated plant, Lathyrus pratensis. The warmer meadow habitat promotes direct development whereas the colder forest habitat is conducive to diapause. Larvae of L. sinapis had a higher propensity to enter diapause when reared on the forest plant L. linifolius across all temperatures. Conversely, the propensity of L. reali to enter diapause was consistently lower and did not differ between host plants. Larval growth rates were similar between and within butterfly species and between host plants. Hence, we conclude that larval pathway decision-making in L. sinapis is an active response mediated by information from their host plants. Key words: adaptation; development; generalist; growth rate; habitat; host plant; Lathyrus; Lepidoptera: Leptidea; life history; phenotypic plasticity; polyspecialist; specialist. INTRODUCTION The impact of environment on phenotypic trait expression has been acknowledged and discussed for centuries (Darwin 1859, Merrifield and Poulton 1899, Schmalhausen 1949, Bradshaw 1965, Danilevskii 1965, Shapiro 1976), but little research was devoted to the subject until the field virtually exploded between two and three decades ago (reviewed in, e.g., Scheiner 1993, Nylin and Gotthard 1998, West-Eberhard 2003, Whitman and Ananthakrishnan 2009). In a seminal article, West- Eberhard (1989) broadly defined phenotypic plasticity as ‘‘the ability of a single genotype to produce more than one alternative form of morphology, physiological state, and/or behavior in response to environmental condi- tions.’’ Several authors have since advocated a division of phenotypic plasticity into nonadaptive and adaptive plasticity (Gotthard and Nylin 1995, Spitze and Sadler 1996, Ghalambor et al. 2007, Whitman and Agrawal 2009). Nonadaptive plasticity can be defined as passive responses or susceptibilities in certain character traits to different levels of, e.g., temperature, food quality, or sun exposure (Whitman and Agrawal 2009). Gotthard and Nylin (1995) even distinguished between adaptive plas- ticity and plasticity as an adaptation denoting plasticity to be an adaptation only if the plastic trait has ‘‘... originated due to natural selection for performing that function.’’ Hence, a passive response to the environment can be adaptive in a general sense, but should not be seen as ‘‘adaptive plasticity’’ in the strict sense unless it can be shown that the plasticity has evolved in response to the heterogeneous environment. Gotthard and Nylin (1995) further highlighted that plasticity as an adaptation is often difficult to demonstrate, and that studies of phenotypic plasticity should benefit from including comparative tests involving sister groups. In such comparative analyses, an environmentally induced plas- tic response in only the sister group that is naturally exposed to that certain environment would support the argument that plasticity has evolved as an active, adaptive response. Highly predictable seasonal variation in temperate climates is likely to promote plasticity adaptations such as phenotypic plasticity in diapause (Gotthard and Nylin 1995, Meyers and Bull 2002, Whitman 2009). Insects can only be active and reproduce in sufficiently hospitable conditions, and temperate insects typically spend the cold part of the year in a species-specific diapause stage (egg, larva, pupa, or adult; Danilevskii 1965). Indeed, many widely distributed insects have only one generation per year at northern latitudes, but an increasing number of annual generations at increasingly southern latitudes (Nygren et al. 2008, Va¨lima¨ki et al. 2008). From the viewpoint of phenotypic plasticity and animal decision-making, the edge zone between the latitudes described above is of special interest, i.e., where individuals that reach the critical stage early in season Manuscript received 25 February 2009; revised 14 July 2009; accepted 28 July 2009. Corresponding Editor: S. J. Simpson. 1 E-mail: magne.friberg@zoologi.su.se 15 R eports