The relative importance of food quality vs. enemy‐free space remains an unresolved but central issue in the evolutionary ecology of host use by phytophagous insects. In this study, we investigate their relative importance in determining host‐plant use by a generalist caterpillar, Estigmene acrea (Lepidoptera: Arctiidae). In nature, E. acrea late‐instar caterpillars preferred Senecio longilobus (Asteraceae), which contains pyrrolizidine alkaloids that the caterpillars sequester, over Viguiera dentata (Asteraceae), a natal host, and typically suffered a 28% mortality risk from parasitoids. We hypothesized that the natural, mixed diet of caterpillars provides high‐quality food via hosts like Viguiera as well as antiparasitoid defense via sequestered toxins from Senecio. We found that a pure Viguiera diet provides superior growth performance over a pure Senecio or mixed diet in the absence of parasitism. However, when parasitism risk is at least moderate, the mixed diet provides a survival advantage over the pure diets of Viguiera or Senecio. We therefore conclude that the balance between benefits of growth (food quality) and defense (enemy‐free space) maintains the use of a mixed diet in nature. Furthermore, the value of enemy‐free space supercedes the value of food quality in determining the host‐plant preference of late‐instar caterpillars.

Abstract.  1. Seventy‐seven individual last‐instar caterpillars foraging in the field were examined for 6 h each. They represented four species of Arctiidae of similar size and habitat use. Two, Hypocrisias minima and Pygarctia roseicapitis, are specialists restricted to particular plant genera. The other two, Grammia geneura and Estigmene acrea, are extreme generalists that use many host plant species from multiple plant families.2. Parameters of behavioural efficiency were monitored. Generalists spent more time walking, rejected more potential host plants, took longer to decide to feed after inspecting a plant, and took relatively more small feeding bouts compared with specialists.3. This is the first test of differential foraging efficiency in the field in relation to diet breadth of insects and the data indicate that generalists are less efficient in their foraging activities and may suffer from divided attention. The need for attentiveness to enhance efficiency and thereby reduce ecological risk is discussed.

The underlying mechanisms mediating the use of multiple host plants were investigated in Parrhasius polibetes (Lycaenidae), a florivorous and facultative myrmecophilous butterfly. Plant traits such as presence of ant-treehopper associations as a source of enemy-free space, flower bud dimensions, toughness, thickness, trichomes, and the corresponding performance and wear of P. polibetes mandibles were examined for three natural hosts: Schefflera vinosa (Araliaceae), Pyrostegia venusta (Bignoniaceae) and Luehea grandiflora (Malvaceae). Parasitism levels of larvae found on the three hosts were also determined. Almost all Luehea had ant-treehopper associations, and all larvae found on this host were non-parasitized. Parasitism was low in larvae found on Schefflera, half of which hosted ant-treehopper associations. No ant-treehopper association was found on Pyrostegia, where parasitism was significantly higher compared to other hosts. In the laboratory, P. polibetes performed well on Schefflera, followed by Pyrostegia. No larvae survived when fed with Luehea. Flower buds of Luehea were thicker and tougher than those of Schefflera and Pyrostegia. Indeed, mandibles of larvae reared on Luehea showed substantial wear, whereas those reared either on Schefflera or Pyrostegia presented no significant damage. Additionally, we suggest that co-occurrence with ant-treehopper associations on a plant provides parasitoid-free space for P. polibetes larvae. Our results support the hypothesis that ecological trade-offs among host plants (i.e., food quality and enemy-free space) promote polyphagy in natural populations of P. polibetes. Host morphological traits seem to play a relevant role in P. polibetes performance. To our knowledge, this is the first report showing the costs of polyphagy in a myrmecophilous butterfly.

Adult body size, a key life history component, varies strongly within and between Heliconius erato phyllis (Lepidoptera: Nymphalidae) populations. In the present study, we determined whether seasonal variation in adult body size is temperature related and/or determined by seasonal changes of host plants (Passifloraceae) used by the larval stage. A population of H. erato phyllis located in a Eucalyptus plantation (Barba Negra Forest, Barra do Ribeiro County, Rio Grande do Sul State, Brazil) was sampled every 45 days from March 1997 to October 1998 to quantify seasonal variation in adult body size and use of larval host plants. In the laboratory, the effects of the following factors on adult body size were quantified: (i) host plant species ( Passiflora misera or Passiflora suberosa ); (ii) food quantity consumed by larvae (experimentally manipulated for each passion vine species); (iii) winter and summer temperatures (15 and 25 Њ C, respectively); and (iv) the interaction between host plant species and temperature. Adults emerging during summer were larger than those emerging in other seasons. Female butterflies oviposited selectively on P. misera even when the dominant passion vine was P. suberosa . They only switched from using P. misera to P. suberosa during later autumn and winter, when P. misera vines were completely defoliated. The laboratory feeding trials with both passion vines showed a strong positive association between food quantity consumed by larvae and adult size. They also confirmed that adults are larger when their larvae are reared on P. misera than on P. suberosa . Temperature during larval development had no effect on H. erato phyllis adult size. Thus, seasonal variation of H. erato phyllis adult size in a given place is primarily determined by the availability and quality of host plant species used by the larval stage.

Adult body size, one of the most important life-history components, varies strongly within and between Heliconius erato phyllis (Lepidoptera: Nymphalidae) populations. This study determines if this variation is caused by geographical changes in host-plant used by the larval stage, whose reproductive parameters are influenced by female body size, with estimates of the corresponding heritability. The variation in adult body size was determined together with a survey of passion vine species (Passifloraceae) used by the larvae in seven localities in Rio Grande do Sul State: three located in the urban area of Porto Alegre and Triunfo Counties, two within Eucalyptus plantations (Barba Negra Forest, Barra do Ribeiro County, and Águas Belas Experimental Station -Viamão County), one in a Myrtaceae Forest (Itapuã State Park -Itapuã County) and one in the Atlantic Rain Forest (Maquiné Experimental Station -Maquiné County). Effects of female body size on fecundity, egg size and egg viability were determined in an outdoor insectary. Size heritability was estimated by rearing in the laboratory offspring of individuals maintained in an insectary. The data showed that adults from populations where larvae feed only upon Passiflora suberosa are smaller than those that feed on Passiflora misera. The larvae prefer P. misera even when the dominant passion vine in a given place is P. suberosa. Fecundity increases linearly with the increase in size of females, but there is no size effect on egg size or viability. Size heritability is null for the adult size range occurring in the field. Thus, the geographical variation of H. erato phyllis adult size is primarily determined by the type, corresponding availability and quality of host-plants used by the larval stage. Within the natural size range of H. erato phyllis, the variation related to this caracter is not genetically based, thus being part of H. erato phyllis phenotypic plasticity.Key words: body size, passion vines, heliconian butterflies, reproductive success, heritability. RESUMO

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Learning ability allows insects to respond to a variable environment, and to adjust their behaviors in response to positive or negative experiences. Pollinating insects readily learn to associate floral characteristics, such as color, shape, or pattern, with appetitive stimuli, such as the presence of a nectar reward. However, in nature pollinators may also encounter flowers that contain distasteful or toxic nectar, or offer highly variable nectar volumes, providing opportunities for aversive learning or risk-averse foraging behavior. Whereas some bees learn to avoid flowers with unpalatable or unreliable nectar rewards, little is known about how Lepidoptera respond to such stimuli. We used a reversal learning paradigm to establish that monarch butterflies learn to discriminate against colored artificial flowers that contain salt solution, decreasing both number of probes and probing time on flowers of a preferred color and altogether avoiding artificial flowers of a non-preferred color. In addition, when we offered butterflies artificial flowers of two different colors, both of which contained the same mean nectar volume but which differed in variance, the monarchs exhibited risk-averse foraging: they probed the constant flowers significantly more than the variable ones, regardless of flower color or butterfly sex. Our results add to our understanding of butterfly foraging behavior, as they demonstrate that monarchs can respond to aversive as well as appetitive stimuli, and can also adjust their foraging behavior to avoid floral resources with high variance rewards. Ethology 270Ethology 116 (2010) 270-280 ª

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