The Large Pine Weevil Hylobius abietis (L.)(Coleoptera: Curculionidae) is economically one

of the most important insect pests in Northern Europeanconiferous forests (Eidmann 1974; Nordlander et al.1986). The larvae develop under the bark of fresh coniferstumps, whereas adults feed on the phloem of youngconifer plants inflicting substantial damage. In the U.K.,H. abietis annually kills on average 33% of all untreatedrestock plants, although in some plantations no plant

survives (Heritage et al. 1988). The problem is mostacute where spruce or pine stands has been felled andimmediately restocked with transplants, because thecloseness of fresh stumps with young plants. In thesecases, the seedlings can often be at high risk of H. abietisdamage until the third season after planting. Femalesof H. abietis utilise the stumps of many conifer speciesas breeding sites. After mating females lay their eggs onthe under-ground part of stumps (Elton et al. 1964).Hatched larvae feed in solitary galleries in the phloembeneath the bark. Once fully grown, larvae constructpupal chambers where they overwinter until June, whenthey pupate. The new adults start to emerge in August

Abstract – The Large Pine Weevil, Hylobius abietis, is one of the most important pests in European conif-erous forests.The ectoparasitoid Bracon hylobii is known to cause around 50% larval mortality. Chemicalvolatiles are among the most important stimuli involved in the host location. In the present study, a 4-armolfactometer and a 2-choice wind-tunnel were used to test the behavioural responses of B. hylobii femalesto four odour sources from the host-plant/host-insect complex. For each odour, the behaviour of twentyvirgin females was individually recorded in an olfactometer, whereas twenty virgin and twenty matedfemales were individually tested in a wind-tunnel. The following behavioural parameters were analysed:a) time spent walking; b) time spent stopped; c) total time spent in each arm; d ) total speed; e) net speed;f ) the rate of turning per unit distance walked; g) rate of turning per second. Comparison between odoursshows clear differences in the behavioural parameters analysed. In particular, the bark disc/larva complexis the only odour source able to elicit behavioural responses of both naive and mated females. As strongresponses were obtained only using experienced females, this suggests that previous searching activi-ties increase the ability of B. hylobii to find its host.

Résumé – Localisation de l’hôte par des stimuli chimiques chez Bracon hylobii (Ratzeburg)(Hymenoptera: Braconidae), parasite larvaire d’Hylobius abietis (L.) (Coleoptera: Curculionidae).– Le charançon du pin, Hylobius abietis, est économiquement un des insectes les plus importants desforêts de conifères en Europe. Parmi ses ennemis naturels, le parasitoïde Bracon hylobii peut causerjusqu’à 50 % de mortalité. Cependant, les composés chimiques volatils jouent un rôle très important dansla localisation des hôtes potentiels. Le but de l’étude présentée ici était de déterminer les effets des compo-sés chimiques volatils qui stimulent une réponse dans le comportement de recherche des femelles deB. hylobii. Un olfactomètre et un tunnel de vol ont été utilisés pour tester les réponses des femelles duparasitoïde (taux de rotation, temps passé à marcher ou voler, temps passé a l’arrêt, vitesse de marcheou de vol) à plusieurs combinaisons du complexe insecte hôte – plante hôte (écorce de pin avec larve deH. abietis; larve seule; écorce seule; écorce avec dégâts d’alimentation larvaire). L’examen du compor-tement des adultes testés dans l’olfactomètre et dans le tunnel de vol a révélé que presque tous les para-mètres mesurés étaient significativement différents selon les composés chimiques volatils testés, avecune réponse croissante de la femelle en fonction des situations suivantes: écorce avec larve; écorce seule;écorce endommagée et larve seule.

Host location by chemical stimuli in Bracon hylobii (Ratzeburg) (Hymenoptera: Braconidae), a larval parasitoid of Hylobius abietis (L.) (Coleoptera: Curculionidae)

Massimo FACCOLI * (1) & Cliff James HENRY (2)

(1) Department of Environmental Agronomy and Crop Productions,Entomology, University of Padua, via dell’Università 16, I-35020 Legnaro, Italy

(2) School of Environmental Studies, Faculty of Sciences, University of Ulster (UK)

Ann. Soc. entomol. Fr. (n.s.), 2003, 39 (3) : 247-256. ARTICLE

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* Corresponding author. E-mail: massimo.faccoli@unipd.itAccepté le 03-10-2003.

(Novák 1965) and feed extensively on any coniferspecies, although they show feeding preferences for pine(Ohnesorge 1953; Långström 1982; Leather et al. 1994).

Among the natural enemies of H. abietis, the para-sitoid Bracon hylobii (Ratzeburg) (Hymenoptera:Braconidae) is known to cause around 50% larvalmortality in Sitka spruce stumps (Henry, 1995),although Hanson (1943) reported 90% parasitism inpine stumps. Bracon hylobii has a very wide distributionin Europe and further east (Hedqvist 1958; vonWaldenfels 1975; Gerdin 1977). Parasitoid adults aresexually mature on emergence and may mate immedi-ately (Henry 1995). The female probes the bark withher ovipositor and, after injecting the host larva with aparalysing venom, lays 6-7 eggs (Henry & Day 2000)onto the body of the host or just a few millimetres closeto it (Henry 1995). B. hylobii is an ectoparasitoid andits larvae begin to feed by burying the head into the skinof the H. abietis larva. The parasitoid larvae progressrapidly through five stages often leaving only the headcase and the empty body cuticle of their host (Munro1917; Henry 1995). Development through the larvalstages is usually quite closely synchronised for all theindividuals in the same clutch, so that they all start tospin cocoons on the same day. The pupal period lastsfor about seven days, then the mature adults start tochew their way out through their cocoons and the bark(Henry 1995).

A possible method to reduce the breeding success ofH. abietis is to enhance the parasitism caused by B. hylo-bii (Munro 1914) increasing the natural number ofparasitoids by inundative release program (Henry &Griffin, unpublished data). Munro (1917) was the firstto consider the use of B. hylobii as a potential controlagent. He released 517 females onto what He describedas a “recent felling” of pine stumps, in an attempt toenhance the impact of the parasitoid by increasing itsnumbers in the target area at a critical time. Parasitismrate after Munro’s release was considerably improvedreducing at the same time the damage caused by weevilsto the plantation. This technique of biological controlrequires to breed a large numbers of parasitoids in labo-ratory, but at the present the only method of rearingthe B. hylobii is on actively feeding H. abietis larvae(Henry 1995). Thus, a H. abietis breeding would bealso necessary to get host larvae where B. hylobii femalescan lay eggs. This is a potential obstacle to the use ofthis parasitoid as a biological control agent if they cannotbe reared at a reasonable cost. Attempts were made toreduce costs in many mass release programmes by rear-ing parasitoids on unnatural hosts (Loke & Ashley 1984)or artificial media (Guerra 1992, Tilden & Ferkovich1988). However, such method of producing B. hylobii

will not be possible without knowledge of its host-find-ing cues. But which are the most important sources ofthese cues?

Successful parasitism by insect parasitoids resultsfrom a series of behaviours that lead to the location andacceptation of suitable hosts for the development ofprogeny (Doutt 1959; Vinson 1984). In general, longdistance location by parasitoids seems to be essentiallyguided by information from the food of the host (Vinson1981), like the products of stump degradation in thecase of B. hylobii. At shorter distance information fromthe host, such as volatiles from tissue damaged by larvae,larval frass or the larva itself, are the best cues (Vinson1976). In this respect, chemical volatiles are among themost important stimuli involved in the host location(Solinas 1993, Mills et al. 1991). If identified and synthe-sised, these chemicals might be useful to increase theparasitism rate on natural hosts or to force B. hylobiifemales to lay in substitute hosts or in artificial diet howalready tested in other braconids parasitoids (Loke &Ashley 1984). Beside, synthetic volatiles could beemployed for attracting parasitoids into crops at riskfrom pests and for stimulating and prolonging search-ing behaviour (Lewis et al. 1975), as they show atendency to disperse after release (Lewis & Nordlung1985). For example Loke & Ashley (1984) found thatspraying corn seedling with frass extract of Spodepterafrugiperda increased parasitism from Cotesia marginiven-tris by 26% as a result of increased retention.

At the present, among the several studies carried outon different species of parasitoids and their ability inthe host-location, no one concerns B. hylobii. To deter-mine the source of chemicals that elicit host-findingcues in B. hylobii, the behaviours of females in responseto “host-complex odours” were compared. Variouschanges in the behaviour of parasitoids are in factconsidered as indicators of attraction towards the host(Kennedy 1977), as found also by Waage (1978) whichobserved that adults of the parasitoid Nemeritis canescenswalking out of an area containing host odour into cleanarea, turn more per unit time and walk faster.

The aim of the present study was to determine ifvolatile chemicals released by the host plant and by thecomplex of host-plant/host species may act as a cue forhost location and host acceptance by the B. hylobii.

Materials and methods

General techniques – A 4-arm olfactometer and a 2-choicewind-tunnel were used to test the behavioural responses of femaleparasitoids to various combinations of its host and host-plantcomplex. The olfactomer is employed to test the behaviour of

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walking parasitoids, whereas the wind-tunnel allows to evaluatethe behaviour of both walking and flying insects.

In a 4-arm olfactometer four distinct field of odour can becreated in a chamber in which individual parasitoids can freelywalk (Vet et al. 1983). The apparatus includes an exposure cham-ber composed of a 10 mm thick white and opaque Plexiglas floor,with a 5 mm thick glass ceiling. The four pointed star-shapedwalls of the exposure chamber are constructed by from four greyPlexiglas crescents (90° arc, radius 170 mm, depth 5 mm), gluedto the floor. A thin strip of foam insulation tape was fixed ontop of the Plexiglas arc to make an airtight seal when the glassceiling was in place. Each point of the star ran into an aluminiumtube (inside radius 5 mm) which came through from the under-side of the floor at an angle of 45°. The glass ceiling was helddown firmly on four sides by clamps.

The 2-choice Plexiglas wind-tunnel was constituted by a10 mm thick white floor and 10 mm thick transparent walls andceiling (200 cm long, 100 cm large and 100 cm high). An inter-nal Plexiglas wall 100 cm long glued in the middle of the wind-tunnel and running parallel to its largest side created two symmet-ric open chambers inside to the apparatus. Thus, each chamberwas delimited by three walls and by the wind-tunnel floor andceiling. The open side of each chamber was finally communi-cating with a common area provided by a vial where the para-sitoid was singly released (releasing arm). In each of the two openchambers there was a glass either containing the source odour(odour arm) or empty (control arm).

All coming air passed through an activated carbon filter(Whatman “Carbon-Cap 75”), before being branched off to sepa-rated but identical lines leading to each corner of the olfactometeror to the wind-tunnel lacing. The airflow passed through aflowmeter (0-2 litres/minutes, Gapmeter Type GT). The amountof airflow was determined by fine adjustments to the flowmeter.From the flowmeter air passed into a 300 ml glass jar contain-ing distilled water to increase humidity, and from here into asecond 300 ml jar containing the odour source. After this jar, airwas drawn directly to the corners of the exposure chamber (olfac-tometer) or to open chambers (wind-tunnel). Finally, air flowsexited through a tube placed either in the centre of the olfac-tometer floor or the opposite side of the wind-tunnel. In bothcases, the flow of air was created by a small vacuum connectedwith the exit tube. This tube, which could be disconnected, wasprovided with a Plexiglas vial containing a test insect. The vialwas open at the top and had nylon gauze on the bottom, whichallowed a through-flow of air when the vacuum tube wasconnected. Before starting the experiment it was essential to checkthat the air flows in the olfactometer or in the wind-tunnel createdsharp borderlines between adjacent fields. This was determinedby first covering the floor with black tape, and “sucking smoke”from P.H. “Smoked Pellets” separately through each arm or cham-ber of the apparatuses. The sharpest boundaries, with no inter-mingling, were produced with a flow rate of 400 ml/minute.

Fluorescent tubes were attached on all the sides of both olfac-tometer and wind-tunnel providing homogeneous and diffuseillumination.

To prevent possible disturbing effects of movement aroundthe apparatuses, as noted by Vet et al. (1983), all the walls werecovered by white sheets and a video camera positioned centrally

above the chambers was linked to a video monitor and recorded.The videotapes were analysed later.

Handling of the experimental subjects – Adult parasitoidswere obtained from a previous breeding experiment carried outin laboratory conditions (Henry 1995). Insects were stored at15 °C, 16L: 8D photoperiod, and fed ad libitum on a 1: 1 honey:water solution before being tested. Four odour sources from thehost-plant/host-insect complex were tested:

1) bark disc (6 cm diameter) with feeding host larva (B + L);

2) bark disc with host feeding damage, but no host (BD);

3) healthy bark disc (B);

4) host larva only (L).

In the olfactometer, for each odour complex, the behaviourof twenty virgin females was individually recorded. In the wind-tunnel, twenty virgin and twenty mated females (having alreadylaid eggs in the laboratory breeding) were individually tested foreach odour complex. All the virgin females were 3-days-old,whereas the mated females were 6-days-old.

In all tests the activity of each insect was recorded for tenminutes and each insect was used only once.

Procedure for testing olfactory responses – During eachtest carried out with the olfactometer only one arm contained atest odour (arm 1), while the other arms were empty. Leadingin a clockwise direction from arm 1 the other arms were arms 2,3 and 4. Similarly, in the wind tunnel only one of the two openchambers contained the test odour (arm 1 or odour arm). Toavoid any distortion of the results due to the effects of possibledifferences in the shape or intensity of the air flows running indifferent arms or chambers, after every five insects tested, thewhole apparatuses were washed and the test complex moved toa different arm or chamber. All the experiments were carried outunder laboratory conditions (21 ± 1°C, 60% U.R.)

Data analysis – Video tape of the activity of each insect in theolfactometer was analysed using the PC program “MicroMeasure“ (Wye College Programs)”. Using a video mixing desk,it was possible to overlay a computer-generated map of the olfac-tometer on the video image. Borderlines of the odour fields weredrawn on the computer image and labelled as zones. Using acursor, all the flying or walking tracks of the insect were tracedfrom the video image. For each insect tested and for each armof the olfactometer, the program can calculate the followingbehavioural parameters: a) time spent walking (sec); b) timespent stopped (i.e. stationary) (sec); c) total time spent in eacharm (sec); d ) total speed (mm/sec); e) net speed (i.e. excludesstationary periods) (mm/sec); f ) the rate of turning per unitdistance walked (deg/mm); g) rate of turning per second(deg/mm). Because of the three-dimensional space of the windtunnel, the measurements concerned only the time spent walk-ing, flying or stopped and total time spent in each chamber.

Statistical differences were tested using analysis of the varianceand the means compared by test t (Tukey test). The homogeneityof the variance was assessed by the Cochran test and if necessarythe data were log-transformed (X' = Log (X + 1)). The signifi-cance of all tests was fixed at P ≤ 0.05.

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Figure 1Means of the parameters measured in eacharm of the olfactometer for the four testedodours ± S.D.

RESULTS

Olfactometer responsesThe presence of the bark disc/host larva complex in theolfactometer (B + L) appeared to have no major effecton the behaviour of female parasitoids. Analysis of vari-ance showed that there were significant differencesbetween arms only in the amount of time stopped(d.f. = 3, 76; F = 3.24; P < 0.05) and the total time spent(d.f. = 3, 76; F = 3.46; P < 0.05). However, comparisonof the mean values for these two parameters shows thatthe significant differences are due to the considerablylower valves in arm 2 (Tukey test, P < 0.05) than in theother arms (fig. 1). On average more time was spent bothwalking and stopped in arm 3 than in the others (fig. 1).

Considering the bark disc (B), the only significantdifference between arms showed by the analysis of vari-ance was for rate of turning per unit distance

(d.f. = 3, 76; F = 3.28; P < 0.05), which was highest inthe arm 2 (Tukey test, P < 0.05) (fig. 1). Time stopped,total time, net speed and turning per unit distance wereinstead lowest in arm three, although there were nostatistical differences between arms (fig. 1).

Like the bark discs with feeding larva, the analysis ofdata from bark disc with feeding damage (BD) showedthat there were significant differences between arms fortime stopped, total time and turning per unit distance(df = 3, 76; F=4.88; P < 0.05; F = 4.96; P < 0.05; F=2.89;P < 0.05, respectively), with the highest values always inthe arm 4 (Tukey test, P < 0.05) (fig. 1). Time spentwalking, total speed and rate of turning per second werehighest in arm 1, although without significant differ-ences with the other arms (fig. 1).

Finally, analysis of variance of the data observed usinglarva only (L) showed similarities with the bark disc andlarva complex (B + L) as there were significant differ-ences between arms only for time stopped (d.f. = 3, 76;F = 3.57; P < 0.05) and total time spent in each arm(d.f. = 3, 76; F = 4.63; P < 0.01), with the highest valuesstill in the arm 4 (Tukey test, P < 0.05) (fig. 1). However,this time arm 3 showed a non-statistically lower meanof time spent walking and time spent stopped than theother arms (fig. 1).

If the insects were responding to the odour, a differ-ence would be expected between the odour-containingarm (arm 1) and the arm farthest from the odour source(arm 3), with the arms 2 and 4 (separating arms 1 and 3)to be intermediary for these parameters since they bothborder the arm containing the odour source.Unfortunately, for any source of odours tested in theolfactometer, the arm containing the odour source(arm 1) never shows behavioural parameters with valueshigher than other arms.

Although there were very few significant differencesbetween the arms in the behaviour of the females testedin the olfactometer with different odour sources, whenthe effect of the different odours on the behaviour ofall individuals was considered, the differences were moredramatic (fig. 4). Analysis of variance showed signifi-

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Host location in Bracon hylobii, a parasitoid of Hylobius abietis

Figure 2Means of the parameters measured in each arm of the wind-tunnel for thefour tested odours ± S.D., testing naive females

Table 1 – Statistical differences between the four tested odours for eachmeasured behavioural parameter.

Parameter measured d.f. F-value P-value

Time spent walking 3; 76 6.66 < 0.001Time spent stopped 3; 76 6.31 < 0.001Total speed 3; 76 14.36 < 0.001Net speed 3; 76 1.19 N.S.Rate of turning per distance 3; 76 11.95 < 0.001Rate of turning per second 3; 76 5.79 < 0.001

cant differences between the four odours tested(P < 0.001) except for the net speed (table 1) (fig. 4).From comparison of the mean values of the summedactivity of each insect, there appears to be a gradationbetween the test odour complexes. For the parameterstime spent walking, total speed and turning rate persecond, the lowest values are recorded for bark disc withlarva (B + L), progressively higher for bark disc only(B), bark disc with feeding site (BD) and larva only (L)(fig. 4). However, the opposite gradation appears fortime stopped and turning per mm travelled, where thehighest values were found for bark disc with larva (B + L)and the lowest for larvae only (L) (fig. 4).

Wind-tunnel responsesIn the case of naive females, using the bark disc/larvacomplex (B + L) the tested insects stay a time signifi-cantly longer (total time spent) in the arm containingthe odour source (arm 1) than in the other arms of thewind-tunnel (d.f. = 2, 77; F = 4.4; P < 0.01) (fig. 2).Nevertheless, the other odour sources tested in the wind-tunnel (B, L and BD) do not statistically affect the testedbehavioural parameters (total time spent, time spentwalking, stopped or flying in each arm). In other hands,the only odour source able to elicit behavioural responsesof naive females is the bark disc/larva complex (B + L).

Similarly, for mated females exposed to the odourcoming from the bark disc/larva complex (B + L), timespent walking, stopped and total time spent in the armcontaining the odour source (arm 1) were higher than

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Figure 3Means of the parameters measured in each arm of the wind-tunnel for thefour tested odours ± S.D., testing mated females.

Figure 4Behaviour of B. hylobii females (total in all four arms) when the air plumein one arm of the olfactometer carries an odour associated with its host larvaH. abietis.

for the other odour sources (d.f. = 2, 77; F = 5.22;P < 0.01; F = 3.4, P < 0.03; F = 4.14; P < 0.01, respec-tively) (fig. 3). Time spent flying did not differ signifi-cantly between arms or tested odours (fig. 3). Also inthis case, the bark disc/larva complex (B + L) is the onlyodour source influencing the female behaviour.

However, similarly to the trials carried out by theolfactometer, considering the mean values calculatedon all tested insects, it is possible to observe a grada-tion in the behavioural responses of B. hylobii in rela-tion to the different tested odours (fig. 5). Testing naivefemales, time spent walking and time spent stoppedshow values lowest in bark disc with larva (B + L) andprogressively increasing using bark disc only (B), barkdisc with feeding damage (BD) and larva only (L) (fig.5). The same gradation also occurred in mated femalesbut only for time spent stopped (fig. 5). Finally, the flyactivity of the parasitoids, similar both in mated andnaive females, decreases in the following order: barkonly (B), bark with larva (B + L), larva only (L) anddamaged bark (BD) (fig. 5).

DISCUSSION

Both in the wind tunnels and in the olfactometer, thebehaviour of the naive females never shows statisticaldifference among arms (fig. 1 and 2). In this respect,there are several possible reasons for the apparent lack

of difference in female behaviour between arms contain-ing or not certain odours. It has been demonstrated thatbehaviour of parasitoids is affected by the odours whichthey encounter emerging from the host (Hérard et al.1988a). In the natural situation emerging B. hylobiiadults would be surrounded by relatively large amountsof host frass, in a degrading stump which may containactively feeding host larvae close by. This is in starkcontrast to the relatively clean conditions of the labo-ratory apparatus containing a small amount of old frass.Similarly, the presence of certain kairomones elicits host-finding behaviour in some parasitoids. In the absenceof kairomones, laboratory-emerged test parasitoids mayhave been trying to disperse instead of finding hosts.From this point of view, the response of the laboratory-emerged adults could be substantially impaired incomparison to stump-emerged adults. If the test insectshatched in conditions more closely reflecting the natu-ral situation, they may have shown stronger responses.

The intensity of the odour to which a parasitoid isexposed has an important influence on its activity (Elleret al. 1988; Noldus et al. 1990). Several parasitoid speciesfail to respond to low or high concentrations of akairomone (Jones et al. 1971; Vinson 1977). As theodours tested with B. hylobii females did not cause asignificant alteration in behaviour compared to cleanair, it is unlikely that the odours were strong enough tobe repellent.

Many parasitoids are known to mark their hosts(Rabb & Bradley 1970; Greany & Oatman 1972), orthe areas they have searched (DeLeon 1935; Price 1972).Host marking may reduce the response of females tokairomones (Vinson 1976). In our experiment, the firstfemales could have left marking pheromones, whichmay have made the test odour less attractive to subse-quent test insects reducing their response. However,only five B. hylobii females were tested consecutively inthe olfactometer and in the wind tunnel before the wholeapparatus were washed and the new odour-sourcesplaced in another arms.

However, when mated females, which have alreadylaid eggs, were tested in the wind tunnel their behav-iours show interesting differences between arms. Matedfemales are clearly stimulated by the bark/larva complex(arm 1), with significantly shorter time spent walkingor stopped in the air plume with no odour (arm 2 and 3)(fig. 3). Also the flight activity was longer in thebark/larva odour plume. As strong responses wereobtained only using experienced females, this suggeststhat previous searching activities increase the ability ofB. hylobii to find its host, which is similar to the behav-iour found in other braconid parasitoids (Faccoli, unpub-lished data). Following the first host-searching period,

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Figure 5Behaviour of B. hylobii females in a wind tunnel air-plume containing anodour associated with a larva of H. abietis.

females respond more readily to the kairomonal cuesreleased by potential hosts (Cortesero et al. 1993). Theoccurrence of similar mechanisms in young B. hylobiifemales could explain the lack of response toward thechemical stimuli tested in the olfactometer and windtunnel. In addition, the status of the tested females(virgin or mated) is the only variable differentiating thetwo groups of parasitoids.

Although both in olfactometer and wind tunnelnaive females do not show statistical preferences for thearm containing the volatile cues (arm 1) (fig. 1, 2), fromthe comparison of the four odours tested in the presentstudy emerge clear differences in the behavioural param-eters analysed. In particular in the olfactometer, timespent walking, total speed and turning rate per second,show the lowest values for bark disc with larva (B + L)and progressively higher for bark disc only (B), barkdisc with feeding site (BD) and larva only (L) (fig. 4).Whereas the opposite gradation is observed for timestopped and turning per mm travelled (fig. 4). Similarly,comparing the data obtained from the four odours testedin the wind-tunnel, statistical differences appear amongsome behavioural parameters (time spent walking andstopped) of both naive and mated females with the samegradation observed in the olfactometer (fig. 5). Inconclusion, the bark disc/larva complex (B + L) is theonly odour source able to elicit behavioural responsesof both naive and mated females. But how to interpretthese behavioural responses?

When a parasitoid contacts a host chemical trace, itusually either slows its walking speed or stops, and itsrate of turning may also increase (Waage 1978). FemaleB. hylobii exposed to bark discs with larvae comparedto the other odour complexes walked slower, stoppedmore and turned more per mm walked. In this respect,Price (1970) found increased walking speed in responseto a repellent. The high measured rate of turning pertime is a clear indicator of attraction to this arm asdemonstrated also by Rowlands & Chapin (1978).Stopping is usually considered to be a response to anattractive odour, but could again reflect a tendency tostay in an odour field, which does not contain a repel-lent. All these parameters indicate that the host-plant/host-insect complex (B + L) is the most attractivecue for B. hylobii as demonstrated for many otherbraconid species (Kudon & Berisford 1981; Turlings etal. 1991; Petit et al. 1992).

However, during the searching activity of B. hylobii,the other odours tested in the present study can play amarginal role in the host-finding. Obviously a chemicalcue coming from freshly damaged bark would be a goodindicator of the presence of a host larva. Host mandibu-lar secretions, often deposited on plant tissue during

feeding, have been found to be attractive for some para-sitoids (Vinson 1968; Corbet 1971). In addition, thefrass resulted by the host feeding activity are commonlyreported as one of the most attractive cue (Gross et al.1975; Weseloh 1981; Hérard et al. 1988b; Thibout etal. 1993). Beside, odours from the bark disc alone wouldnot indicate to the parasitoid if a host larva was presentor not, although they are an attraction to the habitat oftheir host (Nishida 1956). Finally, in our trials larvaealone appear to be the least attractive component of thehost-plant/host-insect complex, as found also in severalother parasitoids (Turlings et al. 1991). It might havebeen expected that larvae would be more attractive thaneither bark discs alone or bark discs with feeding damage,because any chemical from the host larva are a real goodindicator of its presence. However, observations carriedout in other experiments (Henry 1995) showed thatfemale B. hylobii will not attack exposed larvae.

Since this experiment was designed to determine thedifferences in behaviour between air plumes with andwithout odours, it is not possible to state which volatilepart of the tested complexes is attractive. In some caseskairomones from several parts of the host-plant/host-insect complex have been demonstrated to be attractiveto parasitoids, but not the host plant itself (Corteseroet al. 1993). However, as using the B + L complex thebehavioural parameters are significantly different tothose observed with other odours, it suggests that thepresence of at least one of the volatiles spreading fromthis source causes host-searching activity in matedfemales of B. hylobii.

At the moment, the mass rearing of B. hylobii onartificial media seems to be possible only on substratecontaining simultaneously chemicals obtained fromboth bark and host larva as, when together, they are theonly known attractive cues. On the base of the dataacquired with the present study, the next experimentswould be addressed to research the kairomones able tostimulate electrophysiological responses in the para-sitoid antennae starting from the B + L complex. Thesekairomones could be potentially very important encour-aging parasitism of unnatural hosts or in artificial mediain a mass rearing program (GUERRA 1992). However,field application of kairomone may retain parasitoidsfor a longer period increasing the rate of parasitisation.

Acknowledgements – We would like to thank the ForestryCommission Research Agency for part-funding of the projectand for the use of facilities and technical support at the NorthernResearch Station, Roslin. We especially thank Stuart Heritageand staff of Entomology Branch at the Northern Research Stationfor their assistance. We would also like to thank the School ofEnvironmental Studies at the University of Ulster, Colerainecampus for use of their facilities.

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