THE EFFECT OF LARVAL DIET AND SEX ON NECTAR NICOTINE FEEDING PREFERENCES IN MANDUCA SEXTA (LEPIDOPTERA: SPHINGIDAE)

Many Lepidoptera interact with the same plant species as both herbivorous larvae and nectar-feeding adults (Adler & Bronstein 2004), providing the potential for plant secondary compounds to influence both pollination and herbivory through expression in floral and foliar tissue. Secondary compounds in nectar may be costly to plants if they deter pollinators, but beneficial if they deter oviposition of herbivorous offspring. Several studies have examined how pollinators respond to secondary compounds in nectar (Detzel & Wink 1993; Adler 2000; Liu et al. 2004; Tadmor-Melamed et al. 2004; Adler & Irwin 2005; Singaravelan et al. 2005; Kessler & Baldwin 2007). However, foraging responses can vary between individuals due to several factors, including sex and previous experience. For example, female insects often require amino acids for egg maturation (Jervis et al. 2005), so sexes may differ in foraging criteria. Larval experience could also influence adult oviposition (e.g., Hopkin's host selection principle, Barron 2001). We asked how sex and larval exposure to nicotine affected adult feeding and oviposition in response to nicotine in nectar. Manduca sexta L. (Lepidoptera: Sphingidae) is a larval specialist herbivore on solanaceous plants including Nicotiana species (e.g., Madden & Chamberlin 1945; Lou & Baldwin 2003), and non-nicotine species such as tomato (Chen et al. 2005). Thus, some larvae consume nicotine while others do not. Several Nicotiana species have nicotine in nectar (Detzel & Wink 1993; Raguso et al. 2003; Adler et al. 2006; Kessler & Baldwin 2007). First instars of M. sexta (North Carolina State University Insectary) were placed in individual 170-g cups with plastic lids and randomly assigned to no-nicotine or nicotine treatments (n = 177 and 173) with synthetic diet (F9783B, Bioserv, Inc. Frenchtown, NJ, USA) with or without 2% incorporated nicotine by wet weight ((-) nicotine, Sigma-Aldrich, Inc. N3876), well within the range in tobacco plants (Sisson & Severson 1990). Larvae were provided fresh diet ad libitum and allowed to pupate. We recorded mortality, time to pupation, pupal weight, and sex for each individual. Pupae were then divided into 4 greenhouse (9.5:14.5 L:D, temperature 25-26°C) cages (55-61 cm3) by larval diet treatment and sex. After emergence, moths were marked on their forewings to indicate larval diet treatment and sex, and transferred to a holding cage for mating and feeding. We measured moth feeding preferences with artificial flowers (Kinko's gray fleck paper, Goyret & Raguso 2006) with 3 nectar compositions: high (0.0005% nicotine; 5 ppm), low (0.0001% nicotine; 1 ppm), and no nicotine. Flowers were placed into 1.5-mL microcentrifuge vials containing 0.5-mL of synthetic nectar without touching the nectar. Nectar was made from a 14% sugar solution (2:1:1 ratio of sucrose: glucose: fructose) to match the sucrose equivalents of AT. tabacum nectar (Adler, unpublished data) and the ratio of sugars in other Nicotiana species (Kaczorowski et al. 2005). Nicotine concentrations were based on levels in N.