Resource utilization and sex allocation in response to host size in two ectoparasitoid wasps on subcortical beetles

Abstract

Patterns of host resource utilization and sex ratio manipulation in relation to host size were investigated for two solitary ectoparasitoid wasps, Atanycolus initiator and Spathius brevicaudis (Hymenoptera Braconidae). Both species parasitize subcortical beetles on the trunks of Japanese pine trees. A. initiator is on average 8 times larger in body weight and has an ovipositor that is 3.7 times longer than that of S. brevicaudis. In both parasitoids, the size of emerging wasps was positively correlated with host size, but the host/wasp size regressions were linear for all three major host species in A. initiator, whereas in S. brevicaudis the regression was logarithmic for a relatively large host species. The sex ratios (proportion of males) of both parasitoids emerging from different host species decreased with increasing host size, but the overall sex ratio on each host species was male‐biased in A. initiator, while female‐biased in S. brevicaudis. How the proportion of host consumed changed in response to host size, differed between the two parasitoids for the same host species. In the field survey, the size and sex ratio of the emerging two parasitoids from a dead tree were closely related to host size. However, the spatial distribution of the two parasitoids depended on the bark thickness of the trunk. The data suggest that differences in the relative evaluation of host size and in ovipositor length may enable the coexistence of the two parasitoid wasps.

The developmental strategy of an idiobiont ectoparasitoid, Dendrocerus carpenteri : influence of variations in host quality on offspring growth and fitness

“…Host size, however, may not be a suitable proxy of host quality for idiobiont parasitoids under two conditions. Available resources will be overestimated if the parasitoid larva does not completely consume all host tissues, because they are in surplus to parasitoid requirements (Urano and Hijii 1995) or cannot be digested. More important perhaps, shape parameters are likely to confound estimates of the amount of available host resources derived from length measurements and, possibly, lead to erroneous conclusions about qualitative dierences between hosts.…”

“…7). Urano and Hijii (1995) reported that larvae of Spathius brevicaudis and Atanycolus initiator, two braconid ectoparasitoids of various beetles, did not consume all resources if host larvae were relatively large, as opposed to small. The quantity of host remains increased as a percentage of host size in a parasitoid sexspeci®c manner, with the smaller males generally consuming less of the host larva than did the larger females, especially in S. brevicaudis.…”

“…For idiobiont parasitoids, quality is often indexed by host size, which is estimated by body length or a correlate of body length such as head-capsule width (van den Assem et al 1989;O'Neill and Skinner 1990;Harvey and Thompson 1995;Urano and Hijii 1995) or hind-tibia length (Bai et al 1992;Rosenheim and Rosen 1992). Although body mass is some function of length ( Fig.…”

“…Insect eggs undergo physiological changes during embryogeny (Strand 1986) and represent a fast-diminishing resource for the parasitoid. The same may hold for insect pupae (King 1990), especially if the immature parasitoid is constrained in the kind or amount of host tissues it can digest (Urano and Hijii 1995;Chow and Mackauer 1999).…”

“…It ®ts growth and developmental patterns of solitary koinobionts, including aphidiine wasps (Sequeira and Mackauer 1992a,b, 1993, 1994Mackauer et al 1997) and the icheumonid Venturia canescens (Harvey et al 1994(Harvey et al , 1995Harvey and Vet 1997). Because most studies of parasitoid growth and development in dierent host species have examined each variable separately (Salt 1940;Arthur and Wylie 1959;Legner 1969;Smilowitz and Iwantsch 1973;Beckage and Riddiford 1983;Liu 1985;Bai et al 1992;Urano and Hijii 1995;Harvey and Vet 1997), rather than as components of an integrated system, models 1 and 2 are based on limited information. In particular, the importance of host size relative to host age (or developmental stage) as a measure of host quality in idiobiont parasitoids is not clear.…”