egg-laying in relation to egg substrate in gryon bolivari , an egg parasitoid of the golden egg...

Journal of Insect Behavior, Vol. 20, No. 2, March 2007 ( C© 2007)DOI: 10.1007/s10905-007-9072-5

Egg-Laying in Relation to Egg Substrate in Gryonbolivari, an Egg Parasitoid of the Golden Egg Bug(Phyllomorpha laciniata)

David Carrasco,1,3 Asa A. Borg,1,2 and Arja Kaitala1

Revised February 15, 2007; accepted January 12, 2007

Published online: March 6, 2007

KEY WORDS: Gryon bolivari; scelionidae; Phyllomorpha laciniata; egg carrying; host loca-tion; oviposition; parasitoid.

INTRODUCTION

Insect female reproductive success is determined by the number of eggsshe lays during her life and the survival of her offspring. However, forfemale parasitoids host availability may limit their reproductive success.Female parasitoids must first locate hosts and then oviposit in them in or-der to be reproductively successful. In general, this parasitization processhas been divided into four behavioral steps: host habitat location, host lo-cation, host recognition and host acceptance (Vinson, 1976; Vinson, 1998).

Wasps within the Scelionidae family are egg parasitoids of insects aswell as other arthropods (e.g. spiders) (see review: Austin et al., 2005).For Scelionids host search and location should be important since the eggsof the host species are relatively small and non-mobile. Gryon bolivari(Giard) (Hymenoptera: Scelionidae) is an egg parasitoid of the golden eggbug, Phyllomorpha laciniata (Villers) (Heteroptera: Coreidae) (Bolivar,1894; Mineo, 1984; Kaitala, 1996). G. bolivari and P. laciniata coexist inseveral populations within the Mediterranean area, for instance in Italy(Mineo, 1984) and Spain (Kaitala, 1996; Katvala and Kaitala, 2001).

1Department of Biology, University of Oulu, P.O. Box 3000, FIN-90014, Oulu, Finland.2Department of Biology, Norwegian University of Science and Technology, 7491 Trondheim,Norway.

3To whom correspondence should be addressed at Department of Biology, University of Oulu,P.O. Box 3000, FIN-90014, Oulu, Finland. E-mail: [email protected].

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308 Carrasco, Borg, and Kaitala

P. laciniata females mainly use conspecifics, both males and otherfemales, as oviposition substrates (Kaitala, 1996). Occasionally they layeggs on their food plant, Paronychia argentea (Kaitala, 1996; Reguera andGomendio, 2002). It has been suggested that development of egg-carryingbehavior may be an adaptation against predators and parasitoids. In somefield experiments survival of P. laciniata’s eggs is higher when eggs are car-ried by individuals than when deposited on plants due to predation andparasitization by parasitoid wasps (Kaitala, 1996; Reguera and Gomendio,2002). Moreover, it has been claimed that P. laciniata females more oftencarry parasitized clutches than males (Kaitala, 1996).

Studies on the host-parasitoid interaction have always been carried outfrom the host species point of view (Kaitala, 1996; Reguera and Gomendio,2002), whereas studies focusing on the behavior of Gryon bolivari arelacking.

This is the first study concerning the behavior of Gryon bolivari duringhost location and oviposition. We investigated whether eggs that P. laciniatafemales deposited on the food plant (P. argentea) were discovered fasterthan the eggs carried on the backs of P. laciniata individuals. Because bugsare mobile and plants are not, it is likely that the possibilities for the para-sitoids to locate the host and oviposit in them differ depending on the hostsubstrate.

MATERIALS AND METHODS

P. laciniata individuals were collected from the field at the beginningof June, in the locality of Llanca situated in north-eastern Spain. Bugs werekept in laboratory conditions (16:8 light:dark photoperiod; 25 ± 2◦C) insideplastic boxes (23 × 39 × 24 cm) which contained soil and live greenhousefood plants (P. argentea).

P. laciniata females were placed in small boxes (13 × 10 × 6 cm) forone day and were provided with fresh inflorescences of the food plant. Pre-viously, we removed all the eggs that they could have received during thetime they were kept in the holding boxes. Some females were kept in pairsto oviposit on each other’s backs, and some were kept alone to force themto lay on the inflorescences of the food plant. When either a bug or an in-florescence had one egg attached, they were used in the experiment.

Eggs received by P. laciniata in the field were carefully removed andplaced in small vials where they were kept until hatching. Eggs were rearedin the same laboratory conditions as bugs. The development was monitoredtwice a day to check which eggs were parasitized. Whether an egg is para-sitized or not can easily be determined since parasitized eggs turn black incoloration. This is due to the mucous sheath secreted by the mature larvae

Egg-Laying in Relation to Egg Substrate in Gryon bolivari 309

(Mineo, 1984). Adult parasitoids hatched normally inside the vials. Newlyhatched G. bolivari‘s males and females were placed for two days in smallboxes (13 × 10 × 6 cm) and they were provided with a solution of honeyand water ad libitum. We assumed that during this time a female parasitoidwould sexually mature and mate. We used only naıve females i.e., femalesthat had no previous experience of P. argentea plants, P. laciniata or hosteggs, and that had not oviposited before.

Naıve G. bolivari females were given the chance to parasitize hosts de-posited either on P. argentea or on the backs of P. laciniata females. We putone branch of a greenhouse food plant and one golden egg bug female, bothholding one egg each, in small plastic boxes (13 × 10 × 6 cm) covered witha net. Thus, each wasp could oviposit in a maximum of two host eggs dur-ing the trial. Only one-day-old eggs were used for the experiment. Oldereggs may have been rejected by the female parasitoid due to changes inthe physical and chemical properties of the corion (Austin, 1984; Godfray,1994). Each set-up (i.e., box, plant, bug and wasp) was used only once. Welet the bug acclimate in the box for approximately five minutes before in-troducing a parasitoid. P. laciniata females were after this time usually onthe plant, some feeding on it. A naıve female parasitoid was placed abouteight centimeters from the possible targets. The behavior of 22 individualwasps was monitored during 25 min after release into the study box. BothP. laciniata eggs in each set-up were collected and reared (see methods forparasitoid rearing above) after the experiment to assess parasitization.

RESULTS

Three out of 22 wasps tested during the experiment did not move fromthe initial point, and were therefore excluded from the analyses. The re-maining 19 G. bolivari females started to walk and drum with the antennaetouching every surface they walked on, shortly after being introduced tothe boxes. The wasps rarely flew more than a few centimeters, and mainlyspent all the time walking. Of the two egg substrates, wasps first approachedthe plant in 18 cases out of 19 and only one wasp first approached the bugfemale. The approach took place between after a few seconds and after sev-eral minutes from parasitoid introduction (mean ± SE = 3 min 56 s ± 67 s;min = 32 s, max = 9 min 21 s). Once on the plant, wasps continued drum-ming with the antennae. Usually, they walked all over the inflorescence withback and forth movements. Twelve of the wasps secondly approached P.laciniata females.

Only two out of 19 wasps did not find any egg during the experiment.Egg-drumming started immediately when G. bolivari females found an egg.


Substrate of the hosts

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Fig. 1. Number of P. laciniata eggs that were first found by G. bolivarifemales depending on the substrate where eggs were oviposited on.

It was characterized by the movement of the antennae up and down andalso to the sides touching and rubbing the egg over its surface. While drum-ming they were walking slowly over the egg. Female parasitoids insertedtheir ovipositor after drumming. Oviposition was characterized by the spe-cial posture of the wasp: the posterior part of the abdomen faced the eggand the hind legs were always in contact with the egg surface. During ovipo-sition the body and the antennae remained immobile. G. bolivari femalesnearly always first found the egg that was deposited on the plant (Fig. 1;χ2 = 7.12, df = 1, p = 0.008). Twelve out of the 17 eggs which female par-asitoids tried to oviposit in the first attempt ended in parasitization. All theeggs that were successfully parasitized occurred on plants, resulting in four

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Fig. 2. Number of P. laciniata eggs that were found and parasitized duringthe experiment depending on the substrate where eggs were oviposited on.


G. bolivari males and eight G. bolivari females. None of the eggs carriedby bugs was parasitized in the first attempt. On the other hand, only fourout of 17 parasitoids found and oviposited in the second egg available. Inall these cases the egg was carried by the P. laciniata female, and three outof the four parasitoids succeeded in parasitizing the eggs (Fig. 2).

DISCUSSION

This is the first study looking at the behavior of Gryon bolivari. Thedrumming, walking and host handling of G. bolivari females that we ob-served are common behaviors during the oviposition process within the Sce-lionidae family (Higuchi and Suzuki, 1996; Wiedemann et al., 2003; Austinet al., 2005). However, we revealed that the substrate where P. laciniata’seggs are deposited may play a role in the host search and location processof G. bolivari.

G. bolivari found the eggs deposited on plants faster and more oftenthan those that were attached on the back of P. laciniata females. Thus,eggs placed on plants seem to be an easier target for parasitoids than thosecarried by P. laciniata. However in natural environments, the occurrenceof host eggs on the food plant is low. In our field site only five P. laciniataeggs were found on plants when inspecting more than 1000 inflorescences(Katvala and Kaitala, unpublished data). In addition, eggs deposited onplants suffer from predation (Kaitala, 1996; Reguera and Gomendio, 2002).These two facts make the proportion of host eggs found on plants very low,and the latter should increase the probability of predation on G. bolivarieggs when using the plant substrate. Thus, G. bolivari females are expectedto primarily parasitize hosts carried by bugs. Doing so, females may re-duce search time (a potential fitness limiting factor) and increase offspringsurvival. It is therefore surprising that, in our study, G. bolivari femalesfound and used the plant eggs more often than the ones carried by the bugs.We suggest three possible and not mutually exclusive explanations for thisseemingly maladaptive host location behavior. First, the fact that G. bolivarifemales used were naıve may have influenced the host searching behavior.Second, there may be a temporal delay by G. bolivari in the coevolutionaryarms race with P. laciniata. Third, oviposition in eggs placed on plants maynot be so costly if some of these eggs survive. We will now shortly discussthese three alternatives.

Since G. bolivari females used in the experiment were naıve, theymight not have had enough experience to assess the quality of the host inrelation with the substrate the eggs were laid on. Some parasitoids exhibitassociative learning when they search for hosts (Van Baaren and Boivin,


1998; Kaiser et al., 2003; Ueno and Ueno, 2005). Vinson (1998) defined as-sociative learning as “a response to a novel cue as a result of its associationwith an innate cue or a cue to which the female is genetically programmedto respond during an experience involving both.” Generally learning im-proves searching efficiency of parasitoids. However, we only exposed naıveG. bolivari females to two eggs (one on a plant and one on a bug) during25 min, and these may not have been sufficient for parasitoids to perceiveand learn the most appropriate substrate to search for.

We assume that the ancestral behavior of P. laciniata females was to layeggs on its food plant. Thus, if we also assume that the host location mecha-nism of G. bolivari females evolved when host eggs were mainly depositedon plants, they may be one step behind in the coevolutionary arms race withP. laciniata. G. bolivari females may still primarily search for hosts that aredeposited on plants. However, G. bolivari females are under natural condi-tions able to parasitize host eggs that are carried by P. laciniata individuals(Kaitala, 1996). Phylogenetic studies should be performed to explore thispossibility.

Parasitizing hosts on plants could be a strategy to ensure some repro-ductive success, if parasitoids are not able to oviposit in hosts carried bybugs. In sibling species G. philippinense, female fecundity ranged from 65eggs over an 11 days lifespan to 122 eggs over a 41 days lifespan (Dasilaoand Arakawa, 2004). If the reproductive capacity of G. bolivari females issimilar, oviposition in the eggs deposited on plants (which are more vulner-able to predation) might not be so costly for them since firstly, not all theeggs might be predated (see Reguera and Gomendio, 2002), and secondly,they potentially can parasitize the eggs carried on bugs.

Besides G. bolivari’s point of view, we show that P. laciniata eggs laidon bugs seem harder to find for parasitoids than those laid on the plant.This is in agreement with other studies suggesting that egg-laying on plantsdecreases offspring survival of P. laciniata (Kaitala, 1996; Reguera andGomendio, 2002). Thus, P. laciniata individuals seen as recipients may givein some way protection to the eggs against parasitoids. However, furtherstudies are needed to assess what kind of behavioral or physical shelter P.laciniata individuals provide to the eggs against parasitoids.

ACKNOWLEDGMENTS

We thank Damia Molina and Hanna Suutari for their help during fieldcollection, and Sami Kivela, Marko Mutanen and Panu Valimaki for theircomments on the manuscript. The study was supported by the Academy ofFinland (Project 53899) to A.K. and CIMO Fellowships to D.C.


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egg-laying in relation to egg substrate in gryon bolivari , an egg parasitoid of the golden egg...

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