DOI: 10.5061/dryad.rfj6q57m6
title: Data from: Host identity, nest quality, and parasitism strategy: influences on body size variation in parasitoid bees and wasps

description: Body size determines mobility and fitness across taxa in various ways.
 Yet, drivers of body size in higher trophic invertebrate, especially
 parasitoids, including intra- and interspecific variations, are poorly
 understood due to complex interactions between parasitoid behaviour, the
 environment, and their hosts. We measured the body size of 393 individuals
 of four parasitoid species (collected from 2220 parasitized brood cells)
 sampled with trap nests for cavity-nesting bees and wasps in the Southern
 Black Forest, Germany. We related parasitoid body size to the size of 15
 host species and the diameters of their nests along four environmental
 gradients (proportion of conifers, canopy cover, structural complexity,
 herb cover, and deadwood diameter). Host identity, nest diameter, and to a
 lesser extent, size differences within host species, were primary drivers
 of parasitoid body size, albeit responses varied among parasitoid species.
 For instance, when the host Black Wood Borer wasp, Trypoxylon figulus
 doubled in size, the Ichneumon wasp, Nematopodius debilis (parasitizing
 the host directly) increased by 37% in size, while the Blue Cuckoo wasp,
 Trichrysis cyanea (parasitizing food resources) increased by only 8%.
 Across host-parasitoid species combinations, parasitoid size corelated
 weakly with host size, and environmental gradients did not significantly
 influence host or parasitoid body size. Our findings highlight the primary
 factors influencing body size, with host identity and nest diameter
 emerging as influential factors within and between parasitoid species,
 although not uniformly. In contrast, the relationship between the top
 trophic level (parasitoids), the lower trophic level (hosts), and host
 size, with environmental gradients were less influential. Considering the
 environmental variables that directly affect body size, such as
 microhabitat conditions and biotic interactions, may further clarify the
 dynamics shaping the variation in body size at higher trophic levels and
 should be considered in future studies addressing how land management
 influences multitrophic interactions.
description: <strong>Study Area</strong> The
 study was conducted in the Southern part of the Black Forest,
 Baden-Württemberg, Germany, as part of the Research Training Group 2123 -
 Conservation of Forest Biodiversity “ConFoBi” (Storch et al. 2020). This
 part of the Black Forest consists of mixed temperate forests within a low
 mountain range, spans roughly 5,000 km² (75% forests) and rises from 120
 to 1,493 metres above sea level (m.a.s.l.). Norway spruce (<em>Picea
 abies </em>(L.) H. Karst.) account for about 42.8% of trees in the
 forest, and is especially prevalent in the northern and eastern regions as
 well as higher elevations. In the Southern and Western region of the Black
 Forest, silver fir (<em>Abies alba</em> Mill.) and European
 beech (<em>Fagus sylvatica</em> L.) comprise greater
 proportions of forest stands (18.5% and 15.3%). The forest has been
 managed under a close-to-nature forest management system since the 1990s.
 However, an increasing focus on conserving biodiversity led to the
 introduction of a retention forest program in 2010 (ForstBW, 2016),
 requiring state-owned forests and encouraging private forests to integrate
 deadwood and habitat tree groups—around 15 trees per 3 hectares—into their
 management strategies (ForstBW, 2016). In the Black
 Forest, 134 one-hectare plots (mean distance between plot centres is 750m)
 were distributed across strictly protected forest reserves, and
 multi-functional forests actively managed by the State Forest Service
 (Figure 2). The selection of study plots was guided by two design
 gradients: (1) landscape-scale forest connectivity, determined by the
 percentage of forest within a 25 km² area surrounding the plot centres,
 and (2) retention-related forest structure at the plot level, including
 the richness of habitat tree species and deadwood per hectare. This
 approach ensured the representation of a broad spectrum of conditions
 prevalent in European montane forests. Plots were pre-selected based on
 criteria such as topography, forest stand age, absence of water bodies,
 and human infrastructure (Storch et al. 2020) to minimise variation due to
 confounding factors. Further verification was conducted to confirm the
 absence of forest operational activities, such as harvesting and road
 construction, during the ConFoBi funding periods.
 <strong>Study species</strong> The
 Wild Carrot wasp, <em>Gasteruption assectator</em> (Linnaeus,
 1758) (Gasteruptiidae), parasitizes solitary bees that nest in cavities
 (Johansson and van Achterberg 2016). Larvae of <em>G. assectator
 </em>is also known as predator-inquiline, initially consume the host
 larva of the Yellow-Faced bee <em>Hylaeus</em> spp., and
 subsequently feed on the host’s provisions (similar to idiobiont
 kleptoparasitoid behaviour), such as pollen and nectar (Bogusch et al.
 2018). This species is commonly found across Europe and thrives in a wide
 range of habitats, from agricultural landscapes to forest ecosystems
 (Bogusch et al. 2018). Ichneumon wasp,<em>
 Nematopodius debilis</em> (Ratzeburg, 1852) (Ichneumonidae) is an
 idiobiont ectoparasitoid species found in the nest of Crabronid wasp
 species (<em>Trypoxylon </em>spp.) specialised in hunting
 spiders. The adult female paralyses the host during oviposition;
 subsequently, the parasitoid larva consumes the host and develops from
 outside its body. This species is distributed across Europe, but there is
 a lack of information on the specific habitat types in which it occurs
 (Broad et al. 2018). Two Cuckoo wasp species,<em>
 Omalus aeneus</em> (Fabricius, 1787) and <em>Trichrysis
 cyanea</em> (Linnaeus, 1758), both members of the Chrysididae
 family, typically parasitize solitary wasps (Paukkunen et al. 2015,
 Wiesbauer 2020). <em>Omalus aeneus</em> and <em>T.
 cyanea</em> are kleptoparasitoids, meaning the parasitoid larvae
 appropriates the host's food resources (Wiesbauer 2020). Although
 they share similar parasitism types, each species targets distinct groups
 of host species. In our study, <em>O. aeneus</em> parasitized
 two species from the genus <em>Passaloecus</em> spp.
 (Crabronidae), while <em>T. cyanea</em> parasitized a broader
 range of hosts, including four species from the genus
 <em>Trypoxylon</em> spp. (Crabronidae), and two species from
 the genus <em>Deuteragenia</em> spp. (Pompilidae). Both
 species are distributed across central Europe in various habitats, from
 forests to open areas such as agricultural landscapes, parks, and gardens
 (Paukkunen et al. 2015, Wiesbauer 2020). It is
 important to note that, while all host and parasitoid species involved in
 this study are broadly distributed across the Palaearctic region according
 to the Global Biodiversity Information Facility (GBIF), specific
 information on their native, non-native, endemic, or invasive status
 within our study area is not available. Based on their broad distribution,
 these species are likely native to the Palaearctic region, but precise
 information on their origin is lacking.
 <strong>Collection, identification, and measurements of
 parasitoids and their hosts</strong> Solitary
 cavity-nesting bees and wasps were collected using trap nests between
 March and October 2020. Each trap nest was constructed using hollow reed
 internodes (<em>Phragmites australis</em> (Cav.) Trin. ex.
 Steud). The length of hollow reeds was approximately 20 cm and were fitted
 into a PVC tube with a diameter about 11 cm. Each trap exposed an average
 of 150 cavities per trap side, with a wide diameter range between 1 mm to
 10 mm. Traps were attached in pairs on 1.5 m high wooden poles, placed in
 open ground spaces halfway between plot centres and the Northwest and
 Southeast corners (within a radius of approximately 5 m), totalling four
 traps per plot. Each trap was oriented facing Southeast and Northwest, to
 promote nesting via sunlight exposure. After being occupied with nests,
 traps were removed and refrigerated at ~4°C during October-February to
 simulate winter diapause. Nests were then exposed to room temperature, and
 hatched bees, wasps and parasitoids were collected for species
 determination. Species identifications were done with identification keys
 for each taxonomic group (e.g. (Dahl et al. 2007), for host species;
 (Bogusch et al. 2018), for parasitoid species, see Rappa et al. 2023,
 2024). The parasitoid and host specimens used in this study are a subset
 of specimens used in previous investigations on solitary bees and wasps by
 Rappa et al. (2023, 2024). The authors investigated the importance and
 effectiveness of structural elements within forest ecosystems in enhancing
 biodiversity conservation through retention forestry practices.
 Body size (measured as body length) of both parasitoids and their
 hosts were measured from the top of the head to the end of the abdomen,
 excluding the ovipositor protrusion, as the metric for body size in this
 study. Each parasitoid that emerged from a trap was measured. Similarly,
 only hosts that emerged from similar nests (not from other nests in the
 same trap) as the parasitoids were measured. Severely damaged specimens
 were excluded from the measurements to enhance data reliability and ensure
 accuracy. Variables such as intertegular distance (ITD) and forewing
 length were difficult to measure for nearly one third of our samples in
 our case due to damage incurred during the pinning preparation process,
 such as being stabbed by pins, over glued, or having folded wings. As a
 consequence, we did not include these variables in our
 measurements.  We measured a total of 393 individual
 parasitoid specimens (n = 25 for <em>G. assectator</em>, n =
 151 for <em>N. debilis, </em>n = 45 for <em>O.
 aeneus</em>, and n = 172 for <em>T. cyanea</em>) (Figure
 S1, Table S1). For the hosts, we were unable to obtain an equal number of
 measurements as we did for their parasitoids, because some hosts had been
 consumed by the developing parasitoids. <em>Omalus aeneus</em>
 parasitized two crabronid wasp species: <em>Passaloecus insignis
 </em>(Figure S1)<em> </em>(n = 24) <em>and P.
 corniger </em>(Figure S1)<em> </em>(n = 1); <em>T.
 cyanea </em>parasitized four species of
 <em>Trypoxylon</em> and two species of
 <em>Deuteragenia</em> wasps (Figure S1), with
 <em>Trypoxylon figulus </em>being the most parasitized host
 species (n = 85). There are three bee species from genus
 <em>Hylaeus</em> that were parasitized by <em>G.
 assectator </em>(Figure S2)<em>, </em>only one
 individual being available for the measurement of body size.<em>
 Nematopodius debilis</em> parasitized five
 <em>Trypoxylon</em> species, with <em>T. clavicerum
 </em>(Figure S1)<em> </em>(n = 17) and <em>T.
 figulus</em> (Figure S1) (n = 34) being the two dominant hosts
 parasitized (Table S1). All measurements were conducted using a Leica
 stereo microscope M165 C supported with a Leica Application Suite 3.8
 (LAS) imaging software system using a 10x magnification.
 <strong>Nest quality and forest
 structures</strong> We selected a variety of nest
 diameters to have enough choices for the hosts and parasitoids to select
 their preferred sizes. The quality of the nests, i.e. the diameter of the
 hollow reeds, was measured with digital callipers. The diameters of the
 nest entrance (one side of an internode only) each of 383 hollow reeds
 were measured. For forest structures, the proportion of coniferous trees
 were calculated from the inventory basal area of coniferous trees carried
 out between 2016 and 2018 (Storch et al. 2020). The mean canopy cover was
 calculated in ImageJ using overhead hemispherical photos obtained at each
 trap location in early fall 2020. The remotely sensed indices stand
 structural complexity index (SSCI), were obtained from terrestrial laser
 scanners performed at the northwest and southeast corners, as well as the
 centres of the plots. Diameters at breast height (DBH) of standing
 deadwood above 7 cm in diameter were obtained during plot-level
 inventories in 2017 and 2018. There were five decay stages applied in
 classification of deadwood following Hunter (1990): recently dead or raw
 wood (i), solid deadwood (ii), rotten wood (iii), mould wood (iv), and
 duff wood (v). As the host species for parasitoids in our study preferred
 fresh and/or moderately decomposed deadwood, only decay stages i-iii were
 used to calculate the cumulative diameter of standing deadwood at plot
 level. Herbaceous covers were estimated from 5x5 m subplots during
 2017. <strong>Statistical
 analyses</strong> We analysed the relationship
 between parasitoid body size as a response variable, and host identity and
 nest diameter as fixed factors, with study plot as random factor using
 linear mixed-effects models (type III sums of squares). For a subset of
 host-parasitoid interactions, in which we had at least 10 replicates (for
 two host species in <em>N. debilis</em>, and for one host
 species in each of <em>O. aeneus </em>and <em>T.
 cyanea</em>), we also tested if differences in the body size of host
 individuals explained variance in the body size of parasitoids. Here, we
 used nest diameter, host size and, in the case of <em>N.
 debilis</em>, host identity and the interaction of host identity and
 size, as fixed factors in the analysis. Because <em>G.
 assectator</em> fed on host larvae, we did not perform this
 analysis, as we could not obtain data on the size of larvae from any host
 species that <em>G. assectator</em> parasitized (see Table
 S1). We used separate models for each parasitoid species because (1)
 parasitoid species parasitized different host species, (2) parasitoid
 species and parasitoid-host combinations strongly differed in their number
 of replicates, and (3) data on the length of hosts could not be collected
 for all host-parasitoid species combinations (e.g. <em>G. assectator
 </em>and its hosts). Together, these factors led to a strong
 context-dependency of the studied host-environments effects on parasitoids
 and the associated underlying data structure, making it impossible to
 analyse all four parasitoid species in one global model.
 In addition, one host species (<em>T. figulus</em>)
 was parasitised by <em>N. debilis</em> and <em>T.
 cyanea</em> simultaneously. Here, we investigated if relationships
 between the size of <em>T. figulus </em>and the two
 parasitoids differed between the parasitoid species. We used a linear
 mixed effect model with the size of both parasitoids as response
 variables, with the identity and size of <em>T. figulus</em>
 and its interaction as fixed factor, and study plot as random
 factor. To test the relationship between the size of
 the four parasitoid species and their 15-host species, we averaged the
 size values of hosts and parasitoids at the species level. This resulted
 in a total of eleven host-parasitoid size combinations, as data for some
 parasitoid-host combinations could not be retrieved. We adjusted body
 sizes of parasitoids for differences between the parasitoid species and
 their host combinations using a linear mixed intercept model, with
 parasitoid species as a random variable and parasitoid body size as
 response variable. The residuals from this model were then used in a
 standard major axis regression (SMA) with parasitoid and host body sizes
 as dependent variables. To test for host-environment
 interaction effects on parasitoid size, we averaged values of body length
 for host and parasitoid individuals at the plot-level. First, we analysed
 relationships between forest variables (stand structural complexity,
 canopy cover, standing deadwood diameter, and herb cover) and host body
 length using linear models. Then, we tested relationships between
 parasitoid length and the forest variables. Because differences in nest
 quality and host size explained differences in the size of parasitoids, we
 also included host-related factors at the plot-level (nest diameter, host
 identity, host size and its two-way interactions) as fixed factors in
 these models. In <em>T. cyanea</em>, <em>O.
 aeneus</em>, and <em>G. assectator</em>, host size and
 nest diameter were correlated at the plot-level, and host-environment
 interactions could only be analysed for one host species. Therefore, we
 included only host size as a co-variable in their respective models (Table
 S2 a, b). Note that we did not find effects of sex on the parasitoid-host
 interactions, nor differences in the sex ratios across forest variables.
 Consequently, the sex variable was excluded from all our
 analyses. The linear mixed models were performed using
 the package ‘glmmTMB’ (Brooks et al. 2017). To study assumptions of
 homogeneity and normality of residuals, and to check for overdispersion,
 we used the package ‘DHARMa’ (Hartig 2024). We further used Wald-χ² test
 from the package ‘car’ (Fox and Weisberg, 2019) to assess the statistical
 significance of each term in the model. For the standard major axis
 regression, we used the package ‘lmodel2’ (Legendre 2018). All analyses
 were conducted using R (4.2.1) within the RStudio environment (R Core Team
 2022).
description: # Data from: Host identity, nest quality, and parasitism strategy:
 influences on body size variation in parasitoid bees and wasps
 [https://doi.org/10.5061/dryad.rfj6q57m6](https://doi.org/10.5061/dryad.rfj6q57m6) ## Description of the data and file structure This README file describes the data package accompanying the associated publication. The folder **OIK-11052.R1_data.zip **containing six-data files in .csv format that were used in analysis, and folder **OIK-11052.R1_R_script.zip ** containing R files to reproduce the analysis of above mentioned publication. ### Files and variables #### File: OIK-11052.R1\_data.zip **Files list (files found within OIK-11052.R1_data.zip):** canopycover.csv deadwood.csv nest_data.csv prop_conifer.csv ssci&alttude.csv understorey%.csv **Description:**  1\. canopycover.csv: data for thee mean value of canopy cover calculated in ImageJ using overhead hemispherical photos obtained at each sampling site. 2\. deadwood.csv: data for the measurement of diameters at breast height (DBH) of standing deadwood above 7 cm in diameter were obtained during plot-level inventories. 3\. nest_data.csv: data includes list of parasitoid species, parasitoid sex, body length of parasitoid in mm, list of host species, and the body length of host in mm. NA in the host length variable justify for non availale speciments for a direct measurements. 4\. prop_conifer.csv: data for the proportion of coniferous trees were calculated from the inventory basal area of coniferous trees at each sampling site. 5\. ssci&alttude.csv: The remotely sensed indices stand structural complexity index (SSCI) data, were obtained from terrestrial laser scanners performed at the northwest and southeast corners, as well as the centres of the sampling site. 6\. understorey%.csv: Includes data on herbaceous covers and understory plant species richness, which were estimated from 5x5 m subplots. Missing data: NA #### File: OIK-11052.R1\_R\_script.zip **File list files found within OIK-11052.R1_R_script.zip):** Revised_main_code_OIK-11052.R Revised_main_plot_OIK-11052.R **Description:**  7\. Revised_main_code_OIK-11052.R: This is R file reproduces the analysis of the above publication. 8\. Revised_main_plot_OIK-11052.R: this is R file reproduce the plot and visualisation of the above publication. **NOTE:** all data in OIK-11052.R1_data.zip were measured at plot level and those each of the data contains "plot_ID" indicating the number of plots which data were collected. ## Code/software **Primary software:** R programming language software, ideal for processing and visualizing the data.