Selection of Oviposition Sites by a Lepidopteran Community of a Tropical Forest in Southeastern Brazil 1

PATIERNS OF HERHIVORE PREFERENCE IN R ~ m i m i i TO PLANT CONDITION have been discussed by several authors in recent years. Various hypotheses have been proposed such as the “plant stress hypothesis” (White 1969, 1984), in which plants become more susceptible to herbivore attack when physiologically stressed, and the “plant vigor hypothesis” (Price 1991a), in which groups of herbivores feed preferentially on more vigorously growing plants. Some insect herbivores are very selective in the individual plants they attack. Females might evaluate plant quality and lay eggs on rapidly growing plant tissues where larvae would probably remain and survive better (Price et al. 1990, Price 1991b); in this case, plant vigor would be the best predictor of oviposition (Price et al. 1990). Several other factors also may be related to choice of host plant, including leaf age, competition with other related species, the presence of previously laid eggs on the plant, and the presence of predators, like foraging ants (Shapiro 1981, Williams & Gilbert 1981, Pierce & Elgar 1985, Damman 1987, Freitas & Oliveira 1996). In fact, many of the plant discrimination mechanisms used by ovipositing lepidopteran females are still obscure (Chew & Robbins 1984). In the Neotropics little information is available concerning factors that affect host plant choice by herbivore assemblages. The great diversity of plant and insect species, along with the potential for yearround interactions in tropical communities, may contribute to different patterns from those found in temperate regions (Coley & Aide 1991, Fernandes & Price 1991). To examine the relationship between selection of oviposition sites by lepidopteran females and plant vigor, we addressed the following questions: (1) are immatures more commonly found on new or mature leaves? ; (2) does leaf size affect lepidopteran choice? ; and (3) are plants with longer internodes more likely to bear immatures than those with shorter internodes in the same patch? Both leaf size and internode length have been assumed as indicative of plant vigor (Price et al. 1987). Data were collected in the Serra do Japi, S5o Paulo, southeastern Brazil (23”11’S, 46“52’W), a mountain range (700-1 300 m elev.) covered by semideciduous mesophytic forest (Leitzo-Filho 1992). Fieldwork was carried out in early March at the end of the wet season, when most plants were at the end of their yearly vegetative growth (Morellato 1992). Six 100-m long transects were chosen along forest edges with small shrubs and herbaceous plants. These edges had been created by opening trails through the forest and are in an early successional growth stage. All plants on one side of the trail on each transect were searched carefully for lepidopteran immatures (eggs andlor larvae, but not pupae). We recorded the location of each immature on the plant (new or mature leaves) and whether the immatures were isolated or grouped. Grouped immatures were defined by their characteristic behavior and/or spatial distribution on the leaves (sensu Freitas 1991, 1993), and each group was considered as one record (since they all came from a single oviposition choice by a female). Leaf area and internode length were used as parameters to compare leaves within a plant and among plants along a path, respectively. We determined the size of the leaf in which the immatures were located, and the size of the adjacent leaf with no immatures on the same plant. Leaf area was estimated by multiplying the product of the longest length and longest width by a constant of two-thirds in dicotyledons and three-fourths in monocotyledons (.f: Norman & Campbell 1989). We compared leaf size (leaves with the same position on the plants) and length of the third internode from the apex of the nearest conspecific plant without immatures to those of the plant bearing immatures. In a total 182 records of immatures, we found 76 species of Lepidoptera (51 moth spp.) on 45 plant species. Almost all (90%) were eggs or first instar larvae. The latter were assumed to have low mobility within leaves of the host plant (in several cases, the egg shell was found on the same leaf). With five exceptions, each species of Lepidoptera was found on only one of the six transects. Conspecific plants (with and without immatures) were mostly (98%) < 2 m apart and of the same height.