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Absence of Learning and Local Specialization on Host Plant Selection by Heliconius erato

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There is considerable interspecific variation in larval host plants (Passifloraceae) used among Heliconius erato (Lepidoptera: Nymphalidae) populations. This study evaluates the role of learning and the influence of interspecific variation in host
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   Journal of Insect Behavior, Vol. 18, No. 3, May 2005 (  C   2005) DOI: 10.1007/s10905-005-3701-7 Absence of Learning and Local Specializationon Host Plant Selection by  Heliconius erato Solange M. Kerpel 1 and Gilson R. P. Moreira 2 , 3  Accepted July 26, 2004; revised November 15, 2004 There is considerable interspecific variation in larval host plants (Passiflo-raceae) used among  Heliconius erato  (Lepidoptera: Nymphalidae) popula-tions. This study evaluates the role of learning and the influence of interspe-cific variation in host plant attributes on such local specialization in  H. erato host plant choices. Experiments were conducted under laboratory, insectary,and field conditions, with the two most widely used host plants in south-ern Brazil (  Passiflora suberosa  Linnaeus and  Passiflora misera  Humbold,Bonpland et Kunth). Larval feeding preference and induction were evalu-ated through choice tests for all instars. Oviposition was evaluated in relationto host plant preference, Hopkins host-selection principle, and conditioningtime (for 3, 7, 11, or 15 days). Also, oviposition choice was tested regardingdensity, intemode length, and presence of intact terminal bud on  P. suberosa and  P. misera  shoots. Both larvae and adults of   H. erato phyllis  showed preference for   P. misera  compared to  P. suberosa  , under all conditions. Lar-val feeding preference could not be induced for most instars. The Hopkins’effect was not detected and oviposition choice could not be conditioned. Fe-males alternated use of host plant species as a function of variation in either density or presence of terminal buds on shoots. Thus, our data indicate host  plant preference in  H. erato phyllis  is not learned but innate. Therefore, weconcluded that variation in local use of host plant by this butterfly in southern 1 PPG Ecologia, UFRGS, Avenue Bento Gonc¸alves, 9500, Bloco Pr´ edio 43422, Porto Alegre,RS 91501-970, Brazil. 2 Departamento de Zoologia, UFRGS, Avenue Bento Gonc¸alves, 9500, Bloco IV, Pr´ edio43435, Porto Alegre, RS 91501-970, Brazil. 3 To whom correspondence should be addressed. E-mail: gilson.moreira@ufrgs.br. 433 0892-7553/05/0500-0433/0  C  2005 Springer Science + Business Media, Inc.  434 Kerpel and Moreira Brazil results from qualitative and quantitative variation of the passion vine species. KEY WORDS:  heliconians; passion vines; induction of preference; Hopkins’ host effect;postimaginal conditioning. INTRODUCTION Heliconius  butterflies are chemical generalists with respect to  Passiflora ,but host specialists within a given habitat thus suggesting local specializa-tion at the species level (Gilbert, 1975). For example, oviposition choice of  Heliconius erato phyllis  varies geographically (Benson, 1978; Brown, 1979).In Brazil, a similar situation is observed in S ˜ ao Paulo (SP) and Rio Grandedo Sul (RS) States, where there are changes in host plant use for thisspecies depending on location. In Rio Grande do Sul State, use of   Passiflora suberosa ,  Passiflora misera ,  Passiflora capsularis ,  Passiflora elegans ,  Passi- flora actinia  by this nymphalid butterfly has been recorded (Menna-Barretoand Ara´ ujo, 1985; P´ erico, 1995; Rodrigues and Moreira, 2002). The first twoare the most frequently utilized host plants, but their use varies spatiallyand temporally (Rodrigues and Moreira, 2002, 2004). However, contrary towhat was observed in other locations, there are no records of immatures onother passion vines, as for example on  Passiflora alata  and  Passiflora edulis ,which are used in S ˜ ao Paulo State (Brown, 1979; Ramos and Freitas, 1999).The underlying mechanisms that maintain such local specialization are stillunknown. Thus, the main goal of this study is to identify ecological and be-havioral aspects that influence the choice of host plant species by  H. erato phyllis , and that in turn would explain the spatial variation in host plant useby this butterfly.Variation in host plant use can be a result of individual differenceswithin insect populations, as caused by differences in age, physiologicalstate,andbehavior(Scriber,1984;Jaenike,1990;RenwickandChew,1994).Some behavioral phenomena such as induction of food preference, theHopkins host-selection principle, and associative learning or conditioning,can mediate variation in the spatial use of certain host plant species. In thecaseofinductionoffoodpreference,larvaealtertheirpreferenceinfavorof the most recently experienced host plant. This type of learning probably in-volves several physiological processes (Bernays and Weiss, 1996). Accord-ing to Hopkins’ principle of host-selection (Hopkins, 1917) larval feedingexperience is transferred to the adult stage (van Emden  et al. , 1996), thusadultfemalesovipositpreferentiallyontheplanttheyfeedonaslarvae(Foxand Morrow, 1981; Turlings  et al. , 1993). In the case of conditioning, or as-sociative learning, a plant that was used previously for oviposition tends to  Learning and Local Specialization on Host Plant Selection by  Heliconius erato  435 be subsequently preferred for oviposition by that individual (Bernays andChapman, 1994).Herbivore population size does not necessarily reflect food resourceavailability (Solomon, 1981). Host plant use is also a consequence of otherfactors, such as quality, distribution, growth type, nutritional value, suitabil-ity of tissues, concentration of secondary chemical compounds (Thomas,1987; Bernays and Chapman, 1994), and the action of natural enemies(Thompson and Price, 1977). These factors also influence the spatial andtemporal distributions of herbivorous insects.Studies on behavioral aspects of host plant choice by  H. erato phyllis showed that females are able to discriminate shoot size, leaf area, presenceof conspecifics, and presence of terminal bud on shoots (Mugrabi-Oliveiraand Moreira, 1996a,b). In addition, Rodrigues and Moreira (1999) found adependence of   H. erato phyllis  larvae on young tissues. The authors sug-gested that such dependence is a component of high adaptive value, as leaf age affects survivorship. Rodrigues and Moreira (2002, 2004) evaluated ge-ographical and seasonal variation in host plant use by  H. erato phyllis  anddetected consequences in size of individuals, which is correlated with fecun-dity. Thus, such characteristics influence larval performance and reproduc-tive success of adults. These studies provide a baseline for determining localspecialization on host plant selection by  H. erato .Here we identified the passion vines used by larvae in the field, andthen the oviposition choice made by females in relation to  P. suberosa  and P. misera , and respective plant phenotypic attributes. Larval feeding pref-erence and its respective induction in the larval stage was studied in labora-tory. Under insectary conditions, oviposition was evaluated regarding hostplant preference, conditioning of females, and influence of the plants usedas food by larvae in female choice (Hopkins’ host effect). Female choiceregarding variation on shoot density and quality was also tested for both Passiflora  under insectary conditions. MATERIALS AND METHODSInsect and Plants Heliconius erato phyllis  females used in experiments were captured atCampus do Vale, Universidade Federal do Rio Grande do Sul (UFRGS),Porto Alegre, RS (30 ◦ 04  S 51 ◦ 06  W). Females were individually kept inan outdoor insectary divided in 2 . 0 m × 2 . 0 m × 2 . 7 m compartments, con-taining shoots of   P. suberosa  and  P. misera  for oviposition. Adults were feddaily with a seminatural diet. At the same time, eggs, when present, were  436 Kerpel and Moreira removed from shoots, and kept on plastic Petri dishes in laboratory for in-cubation.  Passiflora suberosa  and  P. misera  were transplanted from HortoFlorestal Barba Negra, Barra do Ribeiro, RS (30 ◦ 23  S, 51 ◦ 12  W) intoplant pots, containing organic soil. They were kept in an outdoor screenedcage (Mugrabi-Oliveira and Moreira, 1996a), at the Ecology Department(UFRGS), and were watered daily. Learning Tests Larval Feeding Preference and Induction Larvae used in the experiment were individually reared on intact P. suberosa  or  P. misera  shoots ( n  =  40/species). Each shoot was main-tained in a bottle of water protected by a thin mesh cloth (Ferro, 1998).Leaf disk choice tests on preference and respective food plant inductionwere carried out in a Biomatic ® chamber (25 ± 1 ◦ C) according to the modelshown in Fig. 1A. Each larva was removed from the host shoot and placedin the middle of the pot, which contained leaf disks of the two species of passion vines. Double choice tests were individually conducted over all fiveinstars. Change in choice regarding a given host plant species was consid-ered as induction of preference (Hanson, 1983). Feeding periodicity andconsumption rates were taken into account in each larval instar during thetests (Rodrigues and Moreira, 1999). After 5 h, larvae were returned totheir respective rearing shoots, being tested again in the following instar.The remaining leaf disk portions were glued to a paper sheet in order todetermine food consumption. The consumed area was measured by placingthe leaf disks against a graph paper on a glass table equipped with reflectedlight. Choices made by larvae were registered in favor of   P. suberosa  or P. misera , or as neutral, following leaf area boundaries adopted by Thomas(1987). Hopkins Host-Selection Principle Larvae were individually reared either on  P. suberosa  or  P. misera  asdescribed above. Tests were performed according to the model presentedin Fig. 1B. Following emergence, females ( n  =  20, for each passion vine)were marked with a felt-tip pen and kept in groups in an insectary with fieldcollected males (ratio of two males: one female). After 5 days, inseminatedfemales were individually placed on insectary compartments containing fiveshoots of   P. suberosa  and five-shoots of   P. misera . The number of eggs laidby each female on each plant species was recorded during 2 days.  Learning and Local Specialization on Host Plant Selection by  Heliconius erato  437Fig. 1.  Experimental scheme adopted for testing: A) larval feeding preference andrespective induction; B) Hopkins’ effect; C) female oviposition conditioning, s: P. suberosa  and m:  P. misera . Conditioning of Females Field collected females were kept individually in the described outdoorinsectaries during 3, 7, 11, or 15 days (conditioning times) in contact witheither  P. suberosa  or  P. misera  shoots ( n  =  20; five females/passion vinespecies/conditioning time). On the subsequent day, they were submitted todouble choice oviposition tests on  P. suberosa  or  P. misera . Five shoots of each passion vine species were offered for oviposition. After 24 h, the num-ber of eggs laid on each passion vine species was counted (Fig. 1C).
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