ECOLOGY AND POPULATION BIOLOGY Sex Ratio of Inquiline Wasps on Cynipid Oak Galls in Relation to Gall Characteristics N. IKAI AND N. HIJII1 Laboratory of Forest Protection, Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Japan Ann. Entomol. Soc. Am. 100(6): 869��875 (2007) ABSTRACT We examined the effects of host quality (as measured by gall size) and local mate com- petition (LMC) on the sex ratio of inquilines emerging from cynipid oak galls. In the galls of Trigonaspis sp. (Hymenoptera: Cynipidae), which were solitarily parasitized by the inquiline, the proportion of males decreased signi��cantly with increasing gall size. Large galls produced large inquiline females and large numbers of offspring, but gall size did not affect mortality rate. In the galls of Cynipidae sp. 1 and sp. 2, which were parasitized by the inquilines either solitarily or gregariously, the overall sex ratio was female biased, and it did not change with increasing number of inquilines emerging from a single gall. Thus, in these galls, LMC is likely to have little effect on the sex ratio of the inquiline. The inquiline seems to manipulate its offspring sex ratio only in accordance with gall size. KEY WORDS galling cynipid, gall size, inquiline, LMC, sex ratio Attack by parasitoids is the main mortality factor for many herbivorous insects, and these mortality rates change with the population density of the parasitoids (e.g., Hajek 2004). Parasitoid population density is affected by various factors, such as searching ef��- ciency, handling time, host abundance, and the sex ratio of parasitoid progeny (Hassell 1986). Although a one-to-one sex ratio has frequently been observed in various animal species, many parasitoid wasps are known for large variations in sex ratio among and within host species (Wrensch and Ebbert 1993, God- fray 1994, West et al. 2002). Therefore, the sex ratio of parasitoids is a factor of critical importance in under- standing the dynamics of host��parasitoid systems. On oaks (Quercus spp.), many galling cynipids (Hy- menoptera: Cynipidae) are attacked by various para- sitoid and inquiline species, which take over the galls, and some of which kill the cynipids at oviposition (Askew 1961, 1975 Wiebes-Rijks and Shorthouse 1992 Schonrogge �� et al. 1995, 1999, 2000 Stone et al. 1995 Ito and Hijii 2000). The mortality rates of many galling cynipids as a result of gall invasion by parasitoid wasps or inquiline wasps are often very high (Wash- burn and Cornell 1979, 1981 Wiebes-Rijks and Short- house 1992). Although many previous studies have examined the species composition of parasitoid com- munities on cynipid galls and the mortality rates of galling insects from parasitoid attack (Askew 1961 Washburn and Cornell 1979, 1981 Ito and Hijii 2000 Schonrogge �� and Crawley 2000), there is little infor- mation on the sex ratios of parasitoids and inquilines on cynipid galls in the ��eld (Hails 1989 Schonrogge �� et al. 2000). Two major factors that can affect the sex ratio are host quality (Charnov et al. 1981) and competition for mating in a natal patch (local mate competition: LMC) (Hamilton 1967). If females can bene��t more than males from developing in high-quality hosts and if mating structure of the population is panmictic, then parasitoids will lay more male eggs into hosts of low quality and more female eggs into hosts of high quality this prediction has been supported in many solitary parasitoid species, in which only one individual grows in or on a single host (e.g., King 1987, 1989). However, in gregarious parasitoids, in which many individuals emerge from a single host, many studies have sup- ported the LMC hypothesis (e.g., Godfray 1994): brothers and sisters frequently pupate together in or on their hosts, and they remain in their natal patches before mating. Thus, brother��sister mating and com- petition for mating among brothers (LMC) are very likely to occur. If one son can fertilize many daughters, the number of daughters in a single host will increase with the clutch size (the number of offspring in a single host), whereas the number of sons will not increase enough to reduce LMC thus, the proportion of males decreases with increasing clutch size. A com- parative study of the sex ratios of parasitoid species across the family Bethylidae (Grif��ths and Godfray 1988) revealed that sex ratio declines signi��cantly with increasing clutch size. On oaks, Ito and Hijii (2000) found that some inquiline species parasitize cynipid galls either solitarily or gregariously (faculta- tively gregarious species). Female inquilines may ma- 1 Corresponding author, e-mail: hijii@agr.nagoya-u.ac.jp. 0013-8746/07/0869��0875$04.00/0 2007 Entomological Society of America

nipulate the offspring sex ratio in accordance with both gall size and LMC to increase their performance. Here, we examined the factors affecting the adult sex ratio (i.e., secondary sex ratio) of the inquiline wasp Synergus sp. 1 (Hymenoptera: Cynipidae), which parasitizes cynipids on oak trees either solitarily or gregariously. Our ultimate aim was to determine the in��uence of these factors on the performance of this inquiline species in the ��eld. First, we determined the sex ratio of the inquiline in the galls of four dominant cynipid species (Hymenoptera: Cynipidae): Andricus sp. 1 (Masuda 1996b), Cynipidae sp. 1 (Masuda 1996d) and sp. 2 (Masuda 1996c), and Trigonaspis sp. Second, for the galls of Trigonaspis sp., we examined the effect of gall quality on the sex ratio of emerging inquilines. Finally, for the galls of Cynipidae sp. 1 and sp. 2, we examined the relationship between sex ratio and the number of inquilines emerging from a single gall. Materials and Methods Study Site. Sampling was performed in the Nagoya University Experimental Forest at Inabu, central Ja- pan ( 1,000 m above sea level 35 11 N, 137 33 E). At this site, the annual mean air temperature averaged 8.1 C, and the annual rainfall averaged 2,381 mm over 4 yr (2000��2003). There are two oak species in the forest, Quercus serrata Thunb. (Fagaceae) and Quer- cus crispula Blume [ Q. mongolica Fisher variety grosseserrata (Blume) Rehd. et Wils. Fagaceae] (Ikai and Hijii 2006). BiologyofGallingCynipidsandAssociatedInquilines. Various galling insects are known to induce 50 types of galls on Q. serrata and 30 on Q. crispula (Yukawa and Masuda 1996, Ikai and Hijii 2006). We have recorded 20 types of cynipid galls on both tree species at this site (Ikai and Hijii 2006). Many galling cynipids are cyclically parthenogenetic: the sexual generation develops in spring, whereas the asexual generation develops throughout the summer or autumn of the same year (Stone et al. 2002). Gall characteristics differ among cynipid species and between generations. At this site, we also recorded four types of cecidomyiid gall (Ikai and Hijii 2006), although no cecidomyiid galls were taken over by the inquiline wasps. The sexual generation of Andricus sp., a cynipid species that was dominant at our site, makes a spher- ical gall on the leaf surface in May (Masuda 1996b). Usually, two or more galls are formed on a single leaf. Inquilines attack the galls of Andricus sp. immediately after gall appearance, and the diameter of the galls at this time is small (1.4 0.2 mm [ mean SD]). Adult cynipids emerge from the galls in early June. Cynipidae sp. 1 makes a spherical gall on the leaf surface (Masuda 1996d). Galls of this species form in May and are dominant on oak trees in spring. Usually only one gall is formed on a single leaf. At our site, gall diameter immediately after gall appearance is 1.9 0.5 mm, and all the galls have fallen to the ground by early June (our unpublished data). This galling cynipid is thelytokous. Adults emerge from the galls the follow- ing spring (Masuda 1996d). Cynipidae sp. 2 makes a spherical gall on the shoot (Masuda 1996c). Galls of this species form in May and also are dominant on oak trees in spring. Usually only one gall is formed on a single shoot. Similarly to that of Cynipidae sp. 1, gall diameter immediately after gall appearance is 2.3 0.5 mm. All the galls have fallen to the ground by early June (N.I., unpublished data). We do not know when the adults emerge from these fallen galls. We have not identi��ed Cynipidae sp. 1 and sp. 2 to genus, because no adults have been obtained. The agamic generation of the galling cynipid Trigo- naspis sp. makes a spherical gall on the leaf surface. These galls form from September to October, and they are dominant on oak trees in autumn. Usually only one gall is formed on a single leaf, although there may be more on seedlings and saplings. Gall diameter imme- diately after gall appearance is the largest (3.1 1.5 mm) among the four dominant cynipids. All the galls have fallen to the ground by late October. This species is apterous, and adults emerge from the galls in spring or the next spring. Trigonaspis sp. resembles the cy- nipids recorded by Masuda (1996a) in terms of gall morphology and life history. In these four types of galls, only one larva inhabits a single gall. We identi��ed these cynipid spp. by gall morphology according to Yukawa and Masuda (1996). At our site, we collected Synergus sp. 1 and sp. 2. Synergus sp. 1 was the dominant inquiline. We could easily distinguish between Synergus sp. 1 and sp. 2 by body color. This species attacks only immature galls in which the galling cynipids have not yet begun to grow. Inquiline adults lay one or more eggs into a single gall, and egg hatch occurs in 3 d. Inquiline mothers or immature larvae always kill the galling cynipids before these hosts have begun to grow. Approximately 1 mo after egg hatch, between one and 13 adult inquiline wasps emerge from a single gall. When females and males inhabit a single gall together, they usually emerge from the gall on the same date. Adult inqui- lines live for 2��10 d with water. Mating occurs fre- quently between individuals emerging from the same gall, and mated females do not copulate again within at least 1 wk (N.I., unpublished data). Inquilines of the genus Synergus attack only cynipid galls on oaks (Wiebes-Rijks and Shorthouse 1992), and they can enlarge some types of galls, including those of Cynipi- dae sp. 1 and sp. 2. Galls into which the inquilines lay two or more eggs form membranous walls around the larvae, separating one larva from another. Inquilines may be attacked by many of the same parasitoid spe- cies that parasitize cynipid galls (Askew 1961, 1975 Wiebes-Rijks and Shorthouse 1992 Schonrogge �� et al. 1995, 1996a, 1996b). GallCollectionandInquilineRearing. We selected 24 Q. serrata trees and 17 Q. crispula trees that were 0.5��15 m in height. At intervals of 1 mo from 12 May to 21 October 2004, we collected between 10 and 50 galls from each tree on each census day. Galls were reared in plastic boxes (3.5 by 3.5 by 1.4 cm) in the laboratory at 25 C from May to November 2004 and under outdoor conditions from December to April 2005. We identi��ed species of all adult wasps emerging 870 ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA Vol. 100, no. 6