I Differences in age structure among field cricket populations (Orthoptera; Gryllidae): I possible influence of a sex-biased parasitoid Anne-Marie Murray and William H. Cade Abstract: This study examined age structure in adult populations of three species of field cricket, Gryllus veletis, G. pennsylvanicus, and G. integer. Adults were aged by counting growth layers in cross sections of tibiae. The study species differ in several life-history traits including the likelihood of parasitism by Ormia ochracea, a tachinid that orients to calling males. Gryllus integer is parasitized whereas G. veletis and G. pennsylvanicus are not. Such differences between the species should result in different age patterns. Data from field collections demonstrated that adult G. veletis and G. pennsylvanicus had similar maximum life-spans of about 4 weeks, and males were similar in age or slightly older than females. The maximum age for female G. integer was also about 4 weeks, but few males >20 days old were encountered. Moreover, male G. integer were significantly younger than females in five out of six samples. This pattern in G. integer, evident in 2 successive years, could be consistent with sex-biased mortality by Ormia ochracea. The results are discussed in relation to differential longevities and the intensity of sexual selection on male mating behaviour. Resume : Nous avons ktudik la structure selon l'ige de populations adultes de trois espkces de grillons, Gryllus veletis, G. pennsylvanicus et G. integer. L'ige a kt6 dktermink par dknombrement des couches de croissance dans des coupes transversales de tibias. Les espkces ktudikes different par plusieurs aspects de leur biologie, notamment par leur susceptibilitk au parasitisme d'Ormia ochracea, un tachinide qui recherche les miles chanteurs. Gryllus integer sert d'h8te au tachinide, alors que les deux autres espkces ne sont pas parasitkes. I1 est lkgitime de s'attendre alors ce que ces espkces diffkrent aussi par la structure selon 1'8ge de leurs populations. Les rksultats des rkcoltes sur le terrain ont For personal use only. dkmontrk que les adultes de G. veletis et de G. pennsylvanicus ont des longkvitks maximales semblables d'environ 4 semaines et que les miles sont d'iges kquivalents et lkgkrement plus igks que les femelles. L'ige maximal des femelles de G. integer a kgalement kt6 kvaluk a 4 semaines, mais peu de miles de plus de 20 jours ont kt6 rencontrks. De plus les males de G. integer se sont avkrks significativement plus jeunes que les femelles dans cinq des six kchantillons. Cette tendance, observke chez G. integer au cours de 2 annkes conskcutives, est compatible avec la mortalitk causke par 0. ochracea qui affecte un sexe plus que l'autre. Ces rksultats sont examinks a la lumikre des effets de la longkvitk diffkrentielle et de l'intensitk de la sklection sexuelle sur le comportement reproducteur des miles. [Traduit par la Rkdaction] Introduction Considering its significance to a wide range of theoretical and applied biology, relatively little is known about age Knowledge of age structure is important in many ecological structure in insect populations. This is due in large part to and behavioural studies. Differential mortality rates within a lack of quantitative methodology. Qualitative methods Can. J. Zool. Downloaded from www.nrcresearchpress.com by University of Lethbridge on 07/13/13 and between populations can select for different life history abound (reviewed in Southwood 1978), but allow only broad patterns (Stearns 1976, 1992). Comparisons of age distribu- age categories to be defined. Neville (1963, 1965) was the tions between males and females can detect sexual bimatur- first to use a simple but accurate way to determine adult ism, an important component of sexual selection (Thornhill insect age. He described growth layers in locust cuticle that and Alcock 1983) and age has also been implicated as an were deposited daily in response to circadian rhythm and important factor in female choice (Halliday 1978, 1983). photoperiod. Similar growth layers have since been documented in representatives of &any orders (reviewed by Received November 11, 1994. Accepted March 3 1, 1995. Neville 1983). This work was undertaken to examine age structure within A.-M. Murray1 and W.H. Cade. Department of Biological three species of field crickets (Orthoptera, Gryllidae). We Sciences, Brock University, St. Catharines, ON L2S 3A1, used the method originally described by Neville and first Canada. applied to field crickets by Zuk (1987a, 1987b, 1988). I Author to whom all correspondence should be addressed. d~llusveletis and G. pennsylvanicus are common in the Can. J. Zool. 73: 1207 - 1213 (1995). Printed in Canada / ImprimC au Canada Can. J. Zool. Vol. 73, 1995 Niagara region. Both are univoltine with an obligative dia- and around the campus of Brock University, St. Catharines, pause. Gryllus veletis overwinters as a nymph and the pri- and in various locations in the Niagara region of southern marily micropterous adults are found from late May to late Ontario. Gryllus veletis adults were collected between 2 June July. Gryllus pennsylvanicus overwinters as an egg and and 8 July 1992. Most males collected were calling just prior adults are evident from late July to October. Macropterous to capture. Samples of G. pennsylvanicus were captured individuals occur infrequently (Alexander and Bigelow between 25 July and 17 September 1991. Males were 1960; Alexander and Meral 1967; Alexander 1968). Gryllus labelled callers if they were calling just prior to capture and integer occurs in central Texas, has a high frequency of noncallers if they were silent. Data from calling males and macropterous individuals, requires no obligative diapause, noncalling males were analysed separately. and seems to have two population peaks a year (Alexander Gryllus integer adults were collected around the Bracken- 1968; Cade 1979a). ridge biology field station in Austin, Texas, over the periods The three species also differ with respect to selection by 25 June - 22 September 1991 and 20 May - 8 October a sex-specific parasitoid. Males of all three species call and 1992. Most individuals were captured by attracting flying attract receptive females for mating. Calling carries a heavy males and females to broadcasts of tape-recorded conspecific cost in the Texas species, G. integer, as calling males also male song (for a description of song broadcasting see Cade attract an acoustically orienting dipteran parasitoid, Ormia 1989). Phonotactic individuals landed near the loudspeaker ochracea. Infested males die within 7 - 10 days and infesta- and were then collected. Calling males in the vicinity were tion rates as high as 80% have been recorded (Cade 1975, also captured. Samples collected using the different methods 1979b, 1984). Female G. integer are very rarely parasitized were analysed separately. (Cade 1979b; A. -M. Murray, unpublished data). Gryllus Three to five samples were taken across the season for veletis and G. pennsylvanicus are not affected by such a sex- each species so that seasonal trends in age structure could be specific parasitoid, and this might influence male-female determined. Each sample consisted of 8 - 100 individuals age patterns among the species. It is predicted that males of captured over a short time interval (1 -6 days). All speci- G. integer will have significantly shorter life-spans than con- mens were immediately labelled and preserved in 70% specific females and that such differential longevities may ethanol until they were sectioned. not be apparent between the sexes in either G. veletis or Ages in days are given as means f 1 SD. Data proved nor- G. pennsylvanicus. mal and homoscedastic for variance, so subsequent data analy- ses were carried out using standard parametric methods. Methods Two-way ANOVAs were used to determine the effects of date and sex on age variation. Post hoc Tukey's tests were Age was estimated by counting chitin layers in cross sections used for multiple comparisons. of tibiae. Individuals of known age from laboratory cultures were first examined to establish the accuracy of the method for the study species. Samples of crickets were then gathered Results For personal use only. across the 1991 and 1992 field seasons and compared. Accuracy of the ageing method Figure 1 illustrates the significant linear, relationships Ageing method between number of growth layers in cross sections of tibiae The hind leg, from the distal end of the femur down, was and actual age in days for laboratory-reared adult G. veletis, excised and held between the fingers, and thin sections of G. pennsylvanicus, and G. integer. Curvilinear relationships tibia were cut using a razor blade under a binocular micro- were also examined but did not improve the relationships. scope. Six to 10 sections were taken per adult. Tibia1 sections Results for males and females were pooled, as there were no were arranged in a drop of water on a slide, allowed to dry, significant differences between the sexes in either slope and then mounted in Canada Balsam. The growth layers visi- (G. veletis, t = 0.24, p > 0.05; G. pennsylvanicus, t = ble in 4-6 of these sections were later counted using a Leitz 0.25,~> 0.05; G. integer, t = 0.24,~> 0.05) or elevation light microscope ( x 400). The maximum number of layers in (G. veletis, t = 0.3 1, p > 0.05; G. pennsylvanicus, t = any one section was used in subsequent analysis. 1.77, p > 0.05; G. integer, t = 0.13, p > 0.05) of lines. The slopes and elevations of the three regression lines do not Accuracy of the ageing method differ among ,the three species (F[2,2361= 0.31, p > 0.05; Can. J. Zool. Downloaded from www.nrcresearchpress.com by University of Lethbridge on 07/13/13 To test the accuracy of the method, large groups of nymphs F[2,236,= 1.87, p > 0.05).