José Roberto TRIGO
Laboratório de Ecologia Química, Departamento de Zoologia, Instituto de Biologia,
UNICAMP, Caixa Postal 6109, Campinas, SP, 13083-970, Brazil.
Plants that contain pyrrolizidine alkaloids (PAs) are known to attract specialist PA-insects. It is suggested that this interaction is mutualistic since pollination may occur-1,2, and specialist insects sequester PAs from nectar and other tissues pharmacophagy) for chemical defense and communication-3. Nectar from PA-plants may contain PAs-4, but the toxicity of these to generalist visitors is questionable-5,6.
This work quantified visit rates of insects to inflorescences of two PA-plants (Eupatonum laevigatum and E. inulaefolium, Asteraceae), one non-PA-plant (Bidens brasiliensis, Asteraceae), and withered leaves of Heliotropium indicum (Boraginaceae), known to attract only males of PA-insects. It was observed, that the ratio of PA specialists to non-specialists vvas almost the same for the three flowers (13:87 for E. Iaevigatum, 5:95 for E. inulaefolium and 13:87 for B. brasiliensis) with, 100% specialists on the Heliotropium baits. Most of the specialist visitors were males of Ithomiinae butterflies (e.g. Hypothyris ninonia daeta, Pseudoscada erruca), followed by Danainae butterflies and Arctiidae moths. The non-specialists Vera mostly Actinote butterflies (Acracinae, Nymphalidae), very common at this time of the year in some forests in southeast Brazil, follovved by Papilionidae. Nymphalidae. Heliconiini, Pieridae and Hesperidae butterflies, bees, wasps and flies. Similar results were already published by Pliske-2. The behavior of PA-insects when feeding on inflorescences of PA-plants is typical: after landing the lepidopterans stay a long time on the inflorescence heads, sucking nectar from open flowers, and sucking and scratching the tissues around the open and closed flowers, in the same way that they act on Heliotropium-baits to obtain PAs. Sometimes they were seen visiting and sucking buds or vvithered inflorescences and leaves of E. laevigatum. The behavior of non-PA-insects is variable, depending up on the species, but the time they spent on the inflorescence head was short and they sucked only open flowers. Chemical analysis by GC-MS of dichloromethane extracts of inflorescences from E. laevigatum and withered leaves of H. indicum showed in the former only mono- and sesquiterpene hydrocarbons, and in the latter several structures whose fragmentation patterns resembled dihydropyrrolizines (PA derivatives that attract PA-insects, and are also present in the androconial organs of PA-butterfly males).
It is suggested that PA-plants would not invest in chemicals only to attract PA-specialist visitors, and the high concentration of PAs in inflorescences-4,7 may be related to the protection of these costly organs-8, against attack by herbivores (FAPESP, CNPq).