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Oviposicin de las preferencias y las tasas de crecimiento de las larvas de Caligo memnon (Nymphalidae: Brassolinae)
Oviposition preference and larval growth rates of Caligo memnon (Nymphalidae: Brassolinae)
Caligo memnon caterpillars specialize on plants in three families, Heliconiaceae, Marantaceae, and Musaceea. These families are in the Order Zingiberales. Heliconiaceae and Marantaceae are native to the Neotropics, but Musaceae is an introduced family. I studied oviposition preference and larval performance of C. memnon on four host plants: Heliconia latispatha and Heliconia stricta (Heliconiacea), Calathea insignis (Marantaceae), and Musa acuminata (Musacea). Results showed that preferred host plants for oviposition did not correspond to the host plant that provided the fastest growth rate for caterpillars. Females preferred to oviposit on M. acuminata even though larval growth was lowest on this species, though not significantly. It may be that C. memnon is exhibiting maladaptive oviposition behavior and the introduced M. acuminata may be confusing the coevolved mechanisms for host plant choice.
Estudi la preferencia en el sitio de oviposicin y el desarrollo de la larva de C. memnon en cuatro plantas hospederas: Heliconia latispatha y Heliconia stricta (Heliconiacea), Calathea insignis (Marantaceae) y Musa acuminata (Musaceae).
Text in English.
Tropical Ecology 2006
Ecologa Tropical 2006
t Monteverde Institute : Tropical Ecology
1 Oviposition preference and larval growth rates of Caligo memnon Nymphalidae: Brassolinae Emily Webb Department of Biology, Indiana University Abstract Caligo memnon caterpillars specialize on plants in three families, Heliconiaceae, Marantaceae, and Musaceea. These families are in the Order Zingiberales. Heliconiaceae and Marantaceae are native to the Neotropics, but Musaceae is an introduced family. I studied oviposition preference and larval performance of C. memnon on four host plants: Heliconia l atispatha and Heliconia stricta Heliconiacea, Calathea insignis Marantaceae, and Musa acuminata Musacea. Results showed that preferred host plants for oviposition did not correspond to the host plant that provided the fastest growth rate for caterpi llars. Females preferred to oviposit on M. acuminata even though larval growth was lowest on this species, though not significantly. It may be that C. memnon is exhibiting maladaptive oviposition behavior and the introduced M. acuminata may be confusing the coevolved mechanisms for host plant choice. Resumen Las orugas de la especie Caligo memnon se especializan en plantas de tres familias, Heliconiaceae, Marantaceae, y Musaceea. Estas familias son del Orden Zingiberales. Heliconiaceae y Marantaceae so n nativas de los Neotropics, pero Musaceae es una familia introducida. Yo estudiÃ© la preferencia en el sitio de ovoposiciÃ³n y el desarrollo de la larva de C. memnon en cuatro plantas huÃ©sped: Heliconia latispatha y Heliconia stricta Heliconiacea, Calath ea insignis Marantaceae, y Musa acuminata Musacea. Los resultados mostraron que la planta que la especie C. memnon prefiriÃ³ para ov o positar no coincidiÃ³ con la planta huÃ©sped que proporcionÃ³ el crecimiento mÃ¡s rÃ¡pido de las orugas. Las hembras prefieren la ovoposici Ã³n en M. acuminata aun cuando el crecimiento de la larva fue menor en esta especie. Es posible que C. memnon estÃ¡ mal adaptada en su comportamiento de ovoposiciÃ³n y M. acuminata este conf undi endo sus mecanismos de coevolu ci Ã³n para la selecciÃ³n de la planta huÃ©sped.
2 Introduction The host plant species on which female butterflies lay their eggs can be important to the growth and survival of their offspring. Eggs are vulnerable to predation and the larvae are relatively immobile, so the plant on which larvae hatch will often serve as their food source until they pupate Rausher 1997. Because of this it is very important that adult butterflies develop an oviposition preference that facilitates fast growth rates, adequate nutritional sequ estration, and high survival. Ovipositing butterflies encounter a variety of possible larval host plant species with different physical and chemical characteristics. It is likely that there is a combination of cues that influence host plant preference, bu t some of the main factors seem to be secondary plant compounds, water and nutrient content of leaves, and visual cues like the color, size, shape, and texture of the leaves Chew and Robins 1989. Human caused introductions of closely related plants may present novel, but useful host plants. Since exotic plants did not coevolve with the species in their new environments, they may not have defenses against herbivores. Butterfly species may thrive on exotics as a result. However, exotics can also confuse butterflies and disrupt their normal behavior. Many butterflies have tightly adapted evolutionary histories to the plants in their habitats, but may respond to cues of exotics that are similar to their host plants, even if the behavior is maladaptive. I studied the oviposition preference and larval performance of Caligo memnon Nymphalidae: Brassolinae on four potential host plants, to determine if ovipositing females select their ovipoition sites in an adaptive manner. In this experiment, three of its major host plants are native, and one is exotic. I hypothesize that there will be a
3 difference in host plant preference for oviposition and that there will be a difference in growth rates for caterpillars feeding on different host plants. Females sh ould prefer to oviposit on plants that result in faster growth rates for their larval. Methods Natural history of the study system Caligo is one of the largest butterfly genera in the Neotropics. They are commonly known as Owl butterflies because of the large ocelli on their hind wings. The eggs are round and can be laid singly or in clusters De Vries 1983. The exact incubation time for the eggs is unknown, but it is believed that it takes one or two weeks for the eggs to hatch. The exact number of instars for the larvae also is unknown, but it is thought that they have six or seven. Host plants Observational studies show that the major host plants of Caligo spp. are in the families Heliconiaceae, Marantaceae, and Musaceae De Vries 1987. These f amilies share many similarities and belong to the same Order, Zingiberales Berry and Kress 1991. They all have large and colorful bracht inflorescences and long petioles Stiles 1983. Their leaves are large, stiff, and contain large amounts of cellulo se. Heliconiaceae and Marantaceae are native to the Neotropics, while Musaceae originated in South East Asia Vandermeer 1983. Heliconia have very low nutrient value and are therefore eaten by only a few herbivores Stiles 1983. Studies found that Cal igo caterpill ars that feed on Heliconiaceae grow very slowly. This is thought to be due to a low nitrogen concentration in the leaves De Vries 1987. I could find no information regarding the nutrient content of Marantaceae and Musaceae. My study invo lves two species from the
4 Heliconiaceae family , Heliconia latispatha and Heliconia stricta , one species from the Marantaceae family, Calathea insignis , and one family from the Musacea family, Musa acuminata . Study site My experiment was conducted in Montev erde, Costa Rica, in a garden enclosed by a net tent. The garden is used to rear various types of butterflies for the Selvatura Butterfly Garden in Monteverde. The butterflies in the garden were captured in the Monteverde area and allowed to freely ovipos it on plants in the garden. There were approximately fifty female Caligos in the garden at the time of my study. The garden is fumigated every few months to control for pests, and had been fumigated ten days prior to my study. As a result, all the larva e in my study were recently hatched and of similar size. Measuring oviposition preference My project consisted of two experiments conducted simultaneously. In the first experiment I counted the number of eggs on twenty randomly chosen leaves of each host plant. I did these on seven different days over a twelve day period, each day choosing twenty leaves randomly. Studies of other butterfly species have shown that there may be preference for individual stalks within a plant species because of age, phonological stage, condition, or size Ladner and Altizer 2005. Choosing random leaves eliminated the pattern of preference of butterflies for certain stalks within a host plant species. I circled the egg clusters with a pen after I counted them, to ensure tha t I would not recount them on another study day. I also took observations on the placement of eggs on the leaves and the number of eggs in each cluster.
5 There was not an equal abundance of host plants in the garden, giving the butterflies more leaves to c hoose from for oviposition on certain host plants than others. To correct for this potentially confounding factor, I counted the number of leaves for each host plant in the garden and took the width and height of ten leaves of each host plant species in o rder to find the average leaf size. I did this so that I could compare the number of leaves I found per host plant and the density of leaf area of each host plant to the actual number of observed eggs per host plant. Larval growth rate and survival In my second experiment I located twenty Caligo caterpillars on each of the potential host plants under study H. stricta, H. latispatha, C. insigins , and M. acuminata . Caterpillars were found on several different leaves of each host plant, in groups of one t o ten. I labeled all the leaves containing caterpillars in my study as well as the number of caterpillars on it, ensuring that if more caterpillars hatched during my study I would not confuse them with my originals. I measured the body length of the cater pillars using a caliper on seven different days over a twelve day period, and recorded their body lengths. At the end of the experiment I calculated the average growth and survival rates of the larva for each of the host plants. Results Oviposition prefer ence Females oviposited on all four plants studied. In general, I found eggs in clusters of one to eight, with clusters of three found significantly more often than the others Chi squared goodness of fit, X2 = 172.845, df = 7, p < 0.001 table 1. I c ounted a total of 622 eggs on the four host plants in the garden. I counted 130 eggs on Heliconia stricta leaves , 30
6 eggs on C. Insignis , 335 on M. acuminata leaves, and 127 eggs on H. latispatha . More eggs were deposited on M. acuminata and fewer were deposited on C. Insignis than would have been predicted by chance alone Chi squared goodness of fit, X2 = 317.88, df =3, p < 0.001 figure 1. Heliconia stricta accounted for 38 % of the leaves, H. latispatha had 33%, M. acuminata had 21%, and C. Insig nis only accounted for 8% of the leaves. M. acuminata was found with many more eggs than would be expected compared to the relative abundance of its leaves in the garden, and the other three host plants had fewer leaves than would be expected Chi squared goodness of fit, X2 = 29.83, df = 3, p < 0.001. M. acuminata had the highest total leaf area in the garden with 55%, followed by H. stricta with 31%, C. Insignis had 8%, and H. latispatha had 5%. M. acumin ata had an egg count that was close to what was expected according to its relative leaf area, H. latispatha had more eggs, and H. stricta and C. insignis had significantly fewer eggs than were expected for their relative leaf area Chi squared goodness of fit, X2 = 243.97, df = 3, p < 0.001 Table 1: Observed clusters of eggs Groups of 1 Groups of 2 Groups of 3 Groups of 4 Groups of 5 Groups of 6 Groups of 7 Groups of 8 32 41 73 42 15 5 1 1
7 . FIGURE 1 The observed and expected egg distributions over four host plants. The observed number of eggs on H. stricta was close to the expected number that did not take plant abundance or leaf size in the garden into consideration. However, it was much smaller than would be expected taking these factors into consideration. H. latispatha showed similar to H . stricta , but its observed number of eggs was higher than would be expected according to its leaf area. The observed egg number for C. insignis was low for all three expected factors. M. acuminata had many more eggs on them than would be expected by cha nce alone, and the number of leaves i t had in the garden. However, the observed egg number was close to what would be expected if leaf area was important to the butterflies. Larval growth rate and survival The average change in size for a caterpillar on H. stricta was 13.52 mm, on H. latispatha is was 12.79 mm, for C. insignis it was 11.68 mm, and for M. acuminata it was 5.49 mm figure 2. This trend shows that caterpillars have a slower growth rate on M. acuminata . However, statistics do not detect a significant difference in growth rate between M. acuminata a nd the three other host plants Kolmogorov Smirnov test M. acuminata vs. H. stricta P = .2034, M. acuminata vs. H. latispatha P > 0.999, M. acuminata vs. C. 0 50 100 150 200 250 300 350 400 Heliconia stricta Heliconia latispatha Calathea insignis Musa acuminata Caligo memnon host plants Number of eggs Expected if unable to assess size/density Observed Expected if leaf number is important Expected if leaf area is important
8 insignis P = .2034. Survival rates show that caterpillars feeding on H. stricta had a 95% survival rate, on M. acuminata and C. insignis they had a 76% survival rate, and on H. latispatha they had a 70% survival rate. FIGURE 2. Caterpillar growth on four host plants. C. insignis , H. la tispatha , and H. stricta show similar growth rates with nearly parallel slopes. M. acuminata shows a possible slower growth rate than the other three host plants. Discussion Results from the oviposition experiment showed that C. memnon prefer M. acumin ata over the other host plants, even though trends show that caterpillars grow slower on this species. Though there is no statistical difference amongst the growth rates of the larva on the four different host plants, larvae grew more slowly and, over a t welve day period, they were only half the size of larvae feeding on other plants. The observed number of eggs for M. acuminata was very close to the predicted number of eggs 0 5 10 15 20 25 30 0 5 10 Time days Caterpillar length mm Musa acuminata Heliconia latispatha Heliconia stricta Calathea insignis
9 adjusted for relative leaf area. So large leaf area may be an important factor f or the butterflies, when choosing to oviposit on M. acuminata . There were fewer eggs on H. stricta than were predicted by all three factors, though larvae grew just as well on it and had the highest survival rate. Females showed a much lower preference f or oviposition on C. Insignis than the other three host plants, even though the larva seemed to grow just as well on it as the two other Heliconia species and better than M. acuminata According to my data it seems the ovipostion behavior of Caligo butter flies is not adaptive to the growth and survival of their larva. If females where acting in an adaptive manner, they should show a preference for H. stricta because caterpillars had the highest average growth on this species as well as highest survival. Females should be laying the fewest number of eggs on M. acuminata , yet they preferred it over the other three host plants. Marantacea is the most recently derived family in the phylogentic lineage of the Order Zingiberales Berry and Kress 1991. It may be that C. Memnon has not had as long an evolutionary history with this plant family. Females may have been confused by M. acuminata because it is phylogentically similar to their preferred native host plants in Heliconiaceae, but have not had enough evol utionary time with the species to realize that it does not facilitate optimum larval growth. Other studies have found similar cases where adult butterflies preferred to oviposit on exotic species rather than their native host plants, even though larval fi tness was decreased on the introduced plants. These exotic plants were biochemically similar to the native hosts, and may have been giving off the same cues to female butterflies Singer 1989. The introduction of exotic plant species is a potential dan ger to many
10 ecosystems, often disrupting the interactions among animals and plants within communities that were formed over long coevolutionary histories DiTommaso and Losey 2003. Further studies are needed involving a longer study time and a larger samp le size to determine if there is a significant difference in larvae growth rates on Caligo host plants. A study in an environment controlled for pests is needed to determine whether the survival rates found in my experiment are due to the plants or to pre dators. Also, the larval growth rates of different sized feeding groups should be studied. Acknowledgements Thanks to Alan for being absolutely fabulous. Te amo. Thanks to Kathy for getting us out of crazy situations and being one of the girls. Tom and Cam, I hope you know how much I appreciated all your help! Karen, your class almost killed me, but I still love you. Thanks to all the boys on our trip for putting up with the girls, there is not many of you, but you re quality. Thanks to the taxi drivers for teaching me dirty words in Spanish. I love my roomies, we are the fabulous four. Special thanks to everyone on the program, I have learned so much from all of you and I feel honored to have spent four months with so many amazing and weird peo ple. I love each and every one of you. Keep in touch. Literature Cited Berry, F. and Kress J. W. 1991. Heliconia : an identification guide. Smithsonian Institution Press, Washington and London. pp. 30 31. Chew, R.S. and Robbins R.K. 1989. Egg laying in Butterflies . In: The Biology of Butterflies , Vane Write, R.J. and P.R. Ackery, eds. Princeton University Press, Princeton, NJ, pp. 65 67. De Vries, P.J. 1983. Caligo memnon Buhito Pardo, Caligo, Cream Owl Butterfly . In: Costa Rican Natural His tory , D. H. Janzen, ed. The University of Chicago Press, Chicago, IL, pp. 703 704.
11 De Vries, P.J. 1987. The Butterflies of Costa Rica and their Natural History . Princeton University Press, Princeton, NJ, pp. 245 255. DiTommaso, A. and Losey J. E. 2003 . Oviposition preference and larval performance of monarch butterflies Danaus plexippus on two invasive swallow wort species. Entomologia Experimentalis et Applica, Vol 108, pp. 205 209. Ladner, D. T. and Altizer S. 2005. Oviposition preference and larv al performance of North American monarch butterflies on four Asclepias species. Entomologia Experimentalis et Applica, Vol. 116, pp. 9 20. Miller, J.C., D. H. Janzen, and W. Hallwachs. 2006. 100 Caterpillars . The Belknap Press of Harvard University Pres s, Cambridge, MA, pp. 4 5. Rausher, M.D. 1979. Larval habitat suitablility and ovipostion preference in three related butterflies. Ecology, Vol. 60, No. 3., pp. 503 511. Singer, M.C. 1989. Butterfly Hostplant Relationships: Host Quality, Adult Choice a nd Larval Success. In: The Biology of Butterflies , Vane Write, R.J. and P.R. Ackery, eds. Princenton University Press, Princeton, NJ, pp. 81 90. Stiles, F.G. 1983. : Heliconia latispatha Plantanillo, Wild Plantain . In: Costa Rican Natural History . D.H. Janzen, ed. The University of Chicago Press, Chicago, IL, pp. 249 251. Vandermeer, J. 1983. Banana Platano, Banano . In: Costa Rican Natural History, D. H. J Janzen, ed. The University of Chicago Press, Chicago, IL, pp. 75 76