The relationship between host and parasitoid body length was slightly curvilinear on linear scales of body length, but more or less linear on log-log scales. On linear scales, the variance in parasitoid size increased with increasing host size, but not on log-log scales. Thus, log-log scales were used for analyzing the relationship between host and parasitoid body lengths.

Primary parasitoid species belonging to the genus Aphelinus were generally much smaller than, and different in shape from, members of the other primary parasitoid genera analyzed here. They tended to attack small stages of the aphids and also took much longer to develop than parasitoids within the Braconidae. By visual inspection of Fig. 1, observations involving the genus Aphelinus were outliers with respect to the other primary parasitoids (circles located in the lower part of the scatter of secondary parasitoid observations in Fig. 1). Therefore, we excluded 34 observations involving primary parasitoid Aphelinus abdominalis and one observation involving primary parasitoid Aphelinus varipes from the regression analyses of the data in Fig. 1a. We also excluded four links involving A. abdominalis and one link involving A. varipes from the regression analyses of the data in Fig. 1 b and c. Secondary parasitoid species Syrphophagus mamitus sometimes behaved as a hyperparasitoid and sometimes as a mummy parasitoid; seven observations involving this species (dots in Fig. 1) were excluded from the regression analyses in Fig. 1, where hyperparasitoids were distinguished from mummy parasitoids, but were included in the analyses of all secondary parasitoids.

For the reasons mentioned above, we excluded 34 observations involving primary parasitoid Aphelinus abdominalis from the analyses of the effect of primary parasitoid species identity on the regression slope. Only species with >10 data points (individual observations of a parasitoid emerging from an aphid) were included in the analysis.

Comparison of Secondary Parasitoids: Hyperparasitoids Vs. Mummy Parasitoids. The biologically important distinction between secondary parasitoids, called hyperparasitoids, that attack the still living aphid before mummification (in this web, members of the Alloxystinae genera Alloxysta and Phaenoglyphis) and the other secondary parasitoids, so-called mummy parasitoids, that attack the aphid after mummification (see Methods) influenced the relation between final aphid host size and the size of the emergent parasitoid. The slope was significantly <1 (P < 0.001) for hyperparasitoids (b = 0.6862, r²~0.87) and mummy parasitoids (b = 0.6374, r²~0.64) analyzed separately. These slopes were also significantly less than 3/4 and were significantly different (P = 0.025) from one another.

Comparison of Primary and Secondary Parasitoids. Overall, individual body length measurements presented this picture. When emerging from aphids of similar final size, primary parasitoids were, with a few exceptions, larger than secondary parasitoids and similar in size to their hosts. Small aphid mummies gave rise to primary parasitoids slightly longer than themselves, and large aphid mummies produced primary parasitoids slightly shorter than themselves. Emerging secondary parasitoids were smaller than their aphid hosts. Parasitoids emerging from larger aphids were larger absolutely but smaller relative to their aphid hosts than parasitoids emerging from smaller aphids.

Effects of Parasitoid Type and of Species of Plant, Aphid, and Parasitoid. Within each type of parasitoid (primary, hyper-, and mummy parasitoid), analysis of covariance showed that the relationship between parasitoid log body length and aphid log body length was affected by the species identity of the plant (P < 0.001 for both slope and intercept among primary, hyper-, and mummy parasitoids), aphid (P < 0.001 for both slope and intercept among primary, hyper-, and mummy parasitoids), and parasitoid species (P < 0.001 for both slope and intercept among primary, hyper-, and mummy parasitoids). Table 3 gives the slope, intercept, and degrees of freedom of the linear least square regressions of log₁₀(parasitoid size) as a function of log₁₀(aphid size) for different species of parasitoid. Among different species of primary parasitoids, the slopes ranged from 0.3684 to 0.8938 (a mean of 0.6523). A multiple comparison showed that the slopes of 8 of 12 species of primary parasitoids were significantly different from the slope of at least one other primary parasitoid and no single species was responsible for the difference among species. Among hyperparasitoids, the slopes ranged from 0.2038 to 0.8478 (a mean of 0.5324), and one species (Alloxysta tscheki) was the main contributor to the overall significant difference among slopes. The slope for this species was significantly different from the slopes of five (species Alloxysta halterata, Alloxysta macrophadna, Alloxysta ruficollis, Alloxysta victrix, and Phenoglyphis villosa) of the other seven hyperparasitoid species. In addition, the slope of Alloxysta brevis was significantly different from that of species Alloxysta ruficollis. Among mummy parasitoids, the slopes ranged from 0.5053 to 0.7648 (a mean of 0.6214), and one species (Asaphes vulgaris) was the only contributor to the overall significant difference among slopes. The slope for this species was significantly different from the slopes of three (species Asaphes suspensus, Coruna clavata, and Dendrocerus carpenteri) of the other five mummy parasitoid species, and no other slopes were significantly different from any other.

The effect of aphid species identity on the regression slope within different parasitoid species was difficult to analyze. Most individuals in any given species of parasitoid emerged from only one species of aphid (not necessarily the same species of aphid for different parasitoid species), preventing an analysis of differences in regression slopes among different aphid species within one species of parasitoid. For the few parasitoid species (Asaphes vulgaris, Coruna clavata, and Dendrocerus carpenteri) where individuals emerged in sufficient numbers from several species of aphids, an analysis of covariance in combination with a multiple comparison showed that only for parasitoid species Coruna clavata did aphid species identity significantly affect the slope of the relationship between aphid and parasitoid size (P = 0.0474). For this parasitoid species, the slope of aphid species Microlophium carnosum was significantly different from the slope of aphid species Sitobion fragariae/Sitobion avenae.

Effect of Aphid Life Stage and Parasitoid Sex. Female parasitoids tended to be larger than male parasitoids, and parasitoids emerging from adult aphid mummies were larger, on average, than those emerging from nymphal aphid mummies. Specifically, within primary parasitoids, hyperparasitoids, and mummy parasitoids, the relationships between log size of aphid host and log size of emerging parasitoid were in most cases significantly different among female and male parasitoids emerging from nymphal and adult aphid mummies (F-test of four regression lines, against the null hypothesis of no difference, within primary parasitoids: P_slope = 0.0037, P_intercept<0.001, P_overall<0.001; within hyperparasitoids: P_slope = 0.0018, P_intercept<0.001, P_overall<0.001; and within mummy parasitoids: P_slope = 0.08534, P_intercept<0.001, P_overall<0.001). Within each category of parasitoids (primary, hyper-, and mummy parasitoids), the slope was different (P < 0.05, Tukey’s test) between female parasitoids emerging from nymphal aphids and female parasitoids emerging from adult aphids. The slope tended not to be different between female and male parasitoids emerging from nymphal aphids (with the exception of hyperparasitoids, P = 0.049) and between female and male parasitoids emerging from adult aphids (with the exception of mummy parasitoids, P = 0.041).

When the intercepts, but not the slopes, differed significantly between female and male parasitoids emerging from nymphal aphids and/or between female and male parasitoids emerging from adult aphids, then the sexual size dimorphism in parasitoids was constant for hosts of any length. For example, the difference in intercepts yielded a ratio of male primary parasitoid body length to female primary parasitoid body length of 0.88 (for primary parasitoids emerging from nymphal aphids) and 0.87 (for primary parasitoids emerging from adult aphids).

Hyperparasitoid species developed differently. Female hyperparasitoids emerging from nymphal aphids had a significantly steeper slope than males (but not a significantly different intercept). The ratio of male hyperparasitoid body length to female hyperparasitoid body length was close to 1 for a 1-mm nymphal aphid mummy and decreased as the size of the nymphal aphid mummy increased. For hyperparasitoids emerging from adult aphid mummies, neither slope nor intercept differed significantly between males and females, implying a constant sex ratio of body length close to unity.

Intercepts, but not slopes, differed significantly between male and female mummy parasitoids emerging from nymphal aphid mummies, yielding a constant ratio of male parasitoid body length to female parasitoid body length of » 0.88. For mummy parasitoids emerging from adult aphid mummies, both slope and intercept differed significantly between males and females, yielding an increasing ratio of male parasitoid body length to female parasitoid body length (because males showed a significantly steeper slope than females).

In summary, the relationship (across species of parasitoids) between aphid host and emerging parasitoid body length was affected by the life stage of the aphid when mummified (nymph or adult) and the sex of the emerging parasitoid, in addition to being affected by the species identity of the plant, aphid and parasitoid. Within parasitoid species, the relationship between host and parasitoid body length was in general too weak (because of too few data points) to allow an analysis of the effect of life stage of the aphid when mummified and of the sex of the emerging parasitoid.

Few parasitoid species had 10 or more observations of a parasitoid emerging from a particular aphid species in each of the four categories: (i) female parasitoid emerging from larval host, (ii) male parasitoid emerging from larval host, (iii) female parasitoid emerging from adult host, and (iv) male parasitoid emerging from adult host. In three of the four combinations of aphid and primary parasitoid species with enough observations (³ 10), the intercepts were significantly different but not the slopes. Among hyperparasitoid species, the intercepts were significantly different, but not the slopes in the one combination of aphid and parasitoid species with enough observations. Among mummy parasitoids, no combinations of aphid and parasitoid species had enough observations to allow an analysis of the effect of life stage of the aphid when mummified and the sex of the emerging parasitoid.

Discussion of Results. Previous studies found a relationship between the size of the aphid and the size of the emerging parasitoid and inferred that developing parasitoid larvae were resource limited. Parasitoid-host choice models assumed that female wasps selectively placed fertilized eggs (i.e., daughters), in large (i.e., high-quality) hosts, and unfertilized eggs (i.e., sons), and in small (i.e., low-quality) hosts. If parasitoid larvae were resource-limited, female parasitoids should be larger than male parasitoids because the aphid size of emerging female parasitoids would be larger, on average, than the aphid size of emerging male parasitoids. However, whether female parasitoids were larger than male parasitoids, on average, when emerging from hosts of similar size, this would imply that female parasitoid larvae used resources more efficiently than male parasitoid larvae. The latter alternative appears to hold in the data analyzed here because female primary parasitoids were larger, on average, than male parasitoids when emerging from hosts of similar size.

In most species of parasitoid wasps, females are larger than males, although the range of male and female sizes overlaps in many species (reviewed by Hurlbutt; ref 1). In a few groups of parasitoid wasps, the sexual size dimorphism was reversed: males were larger than females. The ratio of male parasitoid body length to female parasitoid body length ranged from 0.56 to 1.56, with a mean of 0.89, across all 361 species analyzed by Hurlbutt (1). In two species of parasitoid wasps, the size ratio changed with the size of the host. The size ratio Hurlbutt (1) found was remarkably close to the values found here. The data analyzed here suggested that the ratio of male parasitoid body length to female parasitoid body length changed with the length of the host in hyperparasitoids and mummy parasitoids, but not in primary parasitoids. Female hyperparasitoid and mummy parasitoid larvae may use resources more efficiently than male hyperparasitoid and mummy parasitoid larvae, respectively.

1. Hurlbutt, B. (1987) Biol. J. Linn. Soc. 30, 63-89.