Insect mortality in Spathodea campanulata Beauv. (Bignoniaceae) flowers

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INSECT MORTALITY IN Spathodea campanulata BEAUV. (BIGNONIACEAE) FLOWERS

TRIGO, J. R.^1,2 and SANTOS, W.F. dos ²

¹Laboratório de Ecologia Química, Departamento de Zoologia, Instituto de Biologia, Unicamp, C.P. 6109, CEP 13083-970, Campinas, São Paulo, Brazil

²Departamento de Biologia, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, USP, Av. dos Bandeirantes, 3900, CEP 14040-901, Ribeirão Preto, SP, Brazil

Correspondence to: José Roberto Trigo, Laboratório de Ecologia Química, Departamento de Zoologia, Instituto de Biologia, Unicamp, C.P. 6109, CEP 13083-970, Campinas, São Paulo, Brazil,

e-mail: [e-mail]

Received November 18, 1998 ¾ Accepted July 26, 1999 ¾ Distributed May 31, 2000

The African tulip tree, Spathodea campanulata, native of West African tropical forests, has been widely introduced as an ornamental plant in several regions of tropical America. In Brazil it is frequently used in urban forestry. PortugalAraujo (1963) found more than 200 dead insects (meliponine bees, flies and ants) in flowers of one inflorescence of S. campanulata in West Africa. He suggested that the mucilage into the young flowers and buds would be dissolved in the nectar, and is responsible for the insects' death. The use of this plant in Brazil is controversial, since it could cause serious losses to beekeepers of native bees (Nogueira-Neto, 1970).

Flowers of S. campanulata, in their original region, are pollinated by non-hovering birds (Gentry, 1974) and perhaps by lemurs (Sussman & Raven, 1978); in Panama, bats pollinated them (Ayensu, 1974). No record about the mortality of pollinators was veri-fied. The aim of this research was to seek the mortality causes of insects in Spathodea flowers, and suggest what evolutionary forces lead to this phenomena.

To study insect mortality in Brazil, flowers of S. campanulata were observed on five trees in the vicinity of the Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, during March and April 1982. Large buds were marked, one day before the anthesis. Every 24 hours dead insects were counted and identified. Recording was kept until the flowers fell off (about 5 days after anthesis). Mucilage was collected directly from the buds and diluted 1:1 directly in 50% sucrose solution, giving a 25% sucrose solution or used pure, without dilutions. These sugar solution diets were given to freshly emerged Scaptotrigona postica Latr. (Apidae: Meliponinae) to bioassay whether the mortality of insects was related to chemicals of the mucilage. For each test,

20 individuals of S. postica were kept in a box (10 x 10 cm), with two tubes of 1 ml, one with the diet and the other with distilled water. The number of dead individuals in each diet was recorded every 24 hours. Controls were 25% sucrose and distilled water (see also Zuccoloto, 1976).

In 445 flowers examined, 345 dead insects were found (97.0% Meliponinae ¾ mostly S. postica; others included 1.7% Diptera and Vespidae, 1.0% Formicidae and 0.3% Orthoptera ¾ the Hymenoptera and Diptera had small length, 0.5-1.0 cm, and Orthoptera about 2 cm length). Additional collections in other trees in the Ribeirão Preto region also revealed many dead S. postica in the flowers.

The highest mortality was found just after anthesis (334 dead insects 96.8%); on the second day after the flowers were opened only 8 dead insects (2.3%) were found, and only one (0.3%) from the third to fifth days. All dead S. postica had pollen on their bodies. One individual of S. postica was observed perforating and entering in a large bud. In open flowers, a large number of S. postica were observed collecting nectar; no deaths and no behavior modification was seen. The analysis of pollen grains by acetolysis (see Kearns & Inouye, 1993) from a colony of S. postica five meters far from the trees did not show any S. campanulata pollen, showing that pollen was not collected in open flowers. The hummingbird Eupetomena macroura (Trochilidae) was observed also frequently visiting Spathodea flowers. That bird showed two types of nectar-collecting behavior: (1) introducing their bill into the open flower, and (2) more frequently, perforating the base of the corolla (robbing nectar). Bioassays with 25% mucilage reduced the survival of freshly-emerged S. postica by 52.9% in relation to the sucrose control and the pure mucilage 95.2% (Table 1). The latter data is ambiguous, since there is no evidence that the insects consumed the diet; if the insects did not feed on pure mucilage, their survival is the same of the distilled water (94.9% reduction).

The large number of dead insects found in the frst day after anthesis, the perforation and penetation behavior of S. postica, nectar collection by bees without death or change in their behavior, and the lack of Spathodea pollen in the S. postica colony, suggest that Spathodea flowers might have a defense system that protects the buds from the pollen and nectar robbers. Janzen (1975) and other authors reported tropical bees as illegitimate resource removers (see Roubik 1989), but Scaptotrigona were not observed as such.

If Spathodea did not have some kind of defense, the pollen and nectar would be robbed before the anthesis by Scaptotrigona or other efficient pollen robbers, reducing or preventing pollination by vertebrates. Since diets containing Spathodea mucilage drastically reduced the survival of insects, the defense system in this mucilage might be chemical (perhaps toxical substances), or mechanical; the mucilage on the buds might "suffocate" the bees. The chemical protection could also not be related to the pollen and/or nectar robbers, but to the protection against herbivory in flowers, since the production of these are costly to the plant (see Zangerl & Bazzaz, 1992).

As the plant is introduced in Brazil, a comparative study in its original region is necessary for further elucidation of these possibilities.

Acknowledgments ¾ We thank Prof. Dr. R. Zucchi for the suggestions about the research and for the supply meliponine bees for the experiments, Mrs. A. J. Calusso and S. Mateus for technical assistance, and to Prof. Drs. João Semir, W. W. Benson, T. Lewinsohn, K. Brown Jr. and Mr. E. Borba for suggestions and criticism.

• AYENSU, E.S., 1974, Plant and bat interactions in West Africa. Ann. Missouri Bot. Gard., 61:702-727.
• GENTRY, A.H., 1974, Coevolutionary patterns in Central American Bignoniaceae. Ann. Missouri Bot. Gard., 61:728-759.
• JANZEN, D.H., 1975, Ecology of plants in the tropics. Edward Arnold Ltd., London.
• KEARNS, C. A. & Inouye, D.W., 1993, Techniques for pollination biologists University Press Colorado, Niwot, CO.
• NOGUEIRA-NETO, P., 1970, A criação de abelhas indigenas sem ferrão (Meliponinae). Ed. Tecnapis, São Paulo.
• PORTUGAL-ARAUJO, V., 1963, O perigo de dispersão da Tulipeira do Gabão (Spathodea campanulata Beauv.). Chácaras e Quintais, 107:562.
• ROUBIK, D.W., 1989, Ecology and natural history of tropical bees. Cambridge Tropical Biology Series, Cambridge University Press, Cambridge.
• SUSSMAN, R.W. & RAVEN, P.H., 1978, Pollination by lemurs and marsupials: an archaic coevolutionary system. Science, 200:731-734
• ZANGERL, A.R. & BAZZAZ, F.A., 1993, Theory and pattern in plant defense allocation. In: R. S. Fritz & E.L. Simms (eds.), Plant resistance to hervbivores and pathogens, University of Chicago Press, Chicago.
• ZUCCOLOTO, F.S., 1976, Valor nutritivo de alguns carboidratos para Nannotrigona (Scaptotrigona) postica (Hymenoptera, Apoideae). Ciência e Cultura, 28:193-194.

Publication Dates

• Publication in this collection
  23 Feb 2001
• Date of issue
  Aug 2000

History

• Accepted
  26 July 1999
• Received
  18 Nov 1998