Research Article |
Corresponding author: Angela Chuang ( achuang1@vols.utk.edu ) Academic editor: Jose Fernandez-Triana
© 2019 Angela Chuang, Michael W. Gates, Lena Grinsted, Richard Askew, Christy Leppanen.
This is an open access article distributed under the terms of the CC0 Public Domain Dedication.
Citation:
Chuang A, Gates MW, Grinsted L, Askew R, Leppanen C (2019) Two hymenopteran egg sac associates of the tent-web orbweaving spider, Cyrtophora citricola (Forskål, 1775) (Araneae, Araneidae). ZooKeys 874: 1-18. https://doi.org/10.3897/zookeys.874.36656
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We report the discovery of two wasp species emerging from egg sacs of the spider Cyrtophora citricola (
colonial spider, Eulophidae, Eurytomidae, natural enemy, orbweaver
Cyrtophora citricola (Forskål, 1775) is a widespread tent-web spider historically occurring in Mediterranean Europe, Asia, the Middle East, and across Africa (
The ecological impact of C. citricola in its new invasive ranges remains largely unknown; its colonial, group-living behavior results in wide-spanning networks of individual capture webs (
In this study, we report two hymenopteran species reared from C. citricola egg sacs collected from their native Spanish range. We reared the egg predator Philolema palanichamyi (Narendran, 1984) from egg sacs collected from the Iberian Peninsula as well as Tenerife in the Canary Islands. We only found Pediobius sp., a member of the pyrgo (Walker, 1839) species group, in Tenerife egg sacs. It is a suspected hyperparasitoid; these parasitize primary parasitoids. Philolema palanichamyi is one of several related species (formerly latrodecti species group of Eurytoma) that uses spider eggs as a larval host, whereas other species of Philolema attack insects as primary or secondary parasitoids (
The discovery of a Philolema species in Spain with C. citricola as a host is interesting for several reasons. Firstly, Philolema has not been documented in Europe, having previously been recorded with an Afrotropical, Neotropical, and Oriental distribution (
Members of the Pe. pyrgo species group are most often primary parasitoids of Lepidoptera or hyperparasitoids through primary hymenopteran parasitoids. Spider associations have been previously documented in this genus:
Here, we describe our observations of Ph. palanichamyi emerging from C. citricola egg sacs in the wild and present preliminary data collected on the prevalence of egg parasitism, mean parasitoid emergence, and mean spiderling emergence in parasitized egg sacs from wild-collected egg sacs. We also report on the morphological variation within both sexes, provide mean body size measurements, and describe sex ratio variation. In laboratory settings we test whether wasps can infect C. citricola egg sacs without intermediate hosts. Importantly, we also re-describe the female holotype and for the first time a male paratype of Ph. palanichamyi.
We hand-collected C. citricola egg sacs between 30 May and 16 June 2016 and between 4 October and 1 November 2018 on the Iberian Peninsula. These areas experience hot summer Mediterranean (Cádiz and Málaga provinces) and cold semi-arid steppe (Murcia and Valencia provinces) Köppen climates of south and east coastal Spain. We further collected egg sacs from Tenerife, Canary Islands, between 29 May and 16 June 2018, in mountainous habitats of the north and dry, scrub habitats of the south (Figs
In the field, we transferred egg sacs to 59.1 mL clear polypropylene containers with clear polyethylene lids. These were then transported to the University of Tennessee (Knoxville, Tennessee, USA) and stored in the laboratory at 21.0–23.5 °C and a 14 (light): 10 (dark) hour photoperiod. We misted the egg sacs weekly with water for up to eight weeks after the collection period. The wasps were confined to cups as they emerged, and we killed them by freezing and made post-mortem wasp counts and egg sac dissections afterwards.
We estimated Ph. palanichamyi variation in body size and sex ratio from those individuals reared from 11 egg sacs. In two cases, two egg sacs were conjoined and the wasps emerged from each of the pairs into their one shared container. The remaining seven egg sacs were kept in seven separate containers. Hence, we used nine batches of wasps, seven originating from single egg sacs and two from pairs of egg sacs. After we froze them, wasps and egg sacs were allowed to air-dry. We dissected egg sacs by teasing the looser silk-domed surface from the firmer silk flat ‘floor’. All of the wasps that had emerged, or were still inside the egg sac, were sexed and measured provided that they were in suitable condition. Wasps without a gaster or that had failed to emerge completely from the pupal case were not measured. We measured the distance from the front of the pronotum to the tip of the metasoma to estimate body size using a squared graticule in a microscope eyepiece at uniform magnification. The head was omitted from the body length measurement because it had become detached from a number of specimens. In total, 665 wasps were sexed and 576 were measured.
To test whether Ph. palanichamyi reared in the laboratory could infect C. citricola egg sacs, we placed a freshly laid egg sac (produced under laboratory conditions) in a container with multiple adult male and female wasps. After seven days, we removed the egg sac and placed it in a separate container at room temperature (20±2 °C) for four weeks after which we carefully opened the egg sac to reveal its contents.
For specimen preparation, we preserved the wasps in 80% ethanol and dehydrated them through increasing concentrations of ethanol before transferring them to hexamethyldisilazane (HMDS) (
We took scanning electron microscope (SEM) images with a Hitachi TM3000 (Tungsten source). We adhered body parts of disarticulated specimens to a 12.7 × 3.2 mm Leica/Cambridge aluminum SEM stub by a carbon adhesive tab (Electron Microscopy Sciences, #77825-12). We used a Cressington Scientific 108 Auto to sputter coat stub-mounted specimens with gold-palladium from multiple angles to ensure complete coverage (~20–30 nm coating). To capture the habitus image of the holotype and recently reared female we used a Macropod Pro 3D system (Canon 6D Mark II body) with a Canon EF 70-200 mm telephoto with affixed 10× objective lens (Macroscopic Solutions, LLC). Our image series were merged into a single in-focus, composite image with the program Zerene Stacker (ver. 1.04). Post-imaging processing was completed with built-in editing tools in Zerene Stacker, Photoshop CS4 and InDesign CS5.
Specimens from the Smithsonian Institution National Museum of Natural History and borrowed holotypes of Ph. palanichamyi and Ph. lankana Narendran, 1994 were compared with our reared specimens by MG. RA provided independent confirmation of chalcidoid identity. We used keys in
The hymenopteran terminology we use for surface sculpture follows
We use the following abbreviations for collections:
In the field, we observed Ph. palanichamyi emerging from C. citricola egg sacs on 11 June 2016 in Málaga, Andalusia. Owing to the central location of the egg sacs within a web, dozens of these wasps were immediately snared in the surrounding web or consumed by nearby C. citricola colony members.
Philolema palanichamyi emerged from 43 of 103 groups of 1–5 (mean = 1.86 ± 0.11) conjoined egg sacs collected in Cádiz (21 of 37 groups), Málaga (14 of 21 groups), Murcia (8 of 34 groups), but not in Valencia (0 of 11 groups) (Figs
Parasitism rate by Ph. palanichamyi for each of four collection locations. We describe parasitism rate per string of egg sacs ranging from 1–5 egg sacs per female.
Ph. palanichamyi parasitism of C. citricola egg sacs by location | |||
---|---|---|---|
Location | # Egg sac strings | Parasitism rate (%) | |
With wasps | Total | ||
Cádiz | 21 | 37 | 56.8 |
Málaga | 14 | 21 | 66.7 |
Murcia | 8 | 33 | 24.2 |
Valencia | 0 | 12 | 0 |
Total | 43 | 103 | 41.7 |
We sexed 665 adults from 11 egg sacs. Between 5 and 151 wasps emerged from single egg sacs (median = 59; mean = 67.29). Wasps emerged from up to eight exit holes made without an observed preference in either the domed surface or the silken floor of the egg sac. The sex ratio varied greatly among egg sacs from 5.0% to 75.0% males (median = 20.0%; mean 24.6% males) (Table
Female and male average lengths and sex ratios for each group of measured egg sacs.
ID | # Egg sacs | # Females | # Males | # Wasps | Sex ratio (% males) | Female av. length | Male av. length |
---|---|---|---|---|---|---|---|
1 | 1 | 4 | 1 | 5 | 20 | 2.16 | 1.98 |
2 | 1 | 5 | 15 | 20 | 75 | 1.71 | 1.69 |
3 | 1 | 21 | 1 | 22 | 4.5 | 1.82 | 1.56 |
4 | 1 | 55 | 4 | 59 | 6.8 | 1.71 | 1.29 |
5 | 1 | 60 | 20 | 80 | 25 | 1.97 | 1.8 |
6 | 2 | 50 | 33 | 83 | 39.8 | 1.47 | 1.14 |
7 | 2 | 84 | 26 | 110 | 23.6 | 1.6 | 1.35 |
8 | 1 | 109 | 25 | 134 | 18.7 | 1.52 | 1.35 |
9 | 1 | 119 | 32 | 151 | 21.2 | 1.75 | 1.57 |
A variable primary sex ratio (i.e., that of deposited eggs) that favors females is usual in Chalcidoidea, enabled by their haplodiploidy. Several factors can influence this ratio in favor of an increased proportion of males (unfertilized eggs), including smaller hosts, less quality hosts, or more numerous hosts at increased density (
When we opened the egg sac presented in the laboratory to adult Ph. palanichamyi, several live wasp larvae were visible, confirming the association of Ph. palanichamyi with C. citricola. Furthermore, each larva appeared considerably larger than a single spider egg, suggesting that a single larva might feed on multiple eggs within the egg sac. This and the lower ratio of wasps to spiderlings found in parasitized compared with unparasitized egg sacs also suggests that this species is an egg predator, not an egg parasitoid. The first wasp offspring eclosed from its pupa seven weeks after the fresh egg sac was introduced to adult wasps.
Based on female holotype (Fig.
Head
(Fig.
Antenna
(Fig.
Mesosoma
(Figs
Legs
(Fig.
Forewing
(Fig.
Petiole
(Fig.
Gaster. Smooth dorsally, very faintly alutaceous laterally, just shorter than mesosoma, Gt4 longest tergum, ~2.0× as long as Gt3. Gt1 and Gt2 asetose, Gt3 and Gt4 with a few setae dorsolaterally [some appear to have been abraded], Gt5 and Gt6 and syntergum more densely setose.
Male (Figs
Antenna
(Fig.
Petiole
(Figs
Gaster. Smooth dorsally, very faintly alutaceous laterally, somewhat shorter than mesosoma, Gt4 longest tergum, ~1.25× as long as Gt3. Gt1 and Gt2 asetose, Gt3 and Gt4 with a few setae dorsolaterally [some appear to have been abraded], Gt5 and Gt6 and syntergum more densely setose.
Variation. Specimens vary greatly in size from approximately 0.8–2.5 mm total length. Prominence and extent of morphological characters diminishes with decreasing body size; for example, general body sculpture is less apparent in small specimens.
Holotype, ♀: INDIA: Timadu, Palani, 1992, coll. Palanichamy, Host Cyrtophora cicatrosa (spider); holotype, Desantisca palanichamyi ♀, sp. nov., det. Narendran 1983; B.M. TYPE HYM 5.3060; NHMUK013455729 (
This species can be distinguished from the widespread Philolema latrodecti Fullaway, 1953 by the suberect flagellar setation (females only, adpressed in Ph. palanichamyi) and smaller ventral plaque (males only, less than half the depth seen in Ph. palanichamyi). Usually, the sculpture of the tegula is much more distinct in Philolema latrodecti. Also, Ph. palanichamyi is known only from the eggs of Cyrtophora spp. while Ph. latrodecti is known only from the eggs of Latrodectus spp.
Egg predator (Fig.
We examined 4♀♀ and 3♂♂ of a Pediobius species reared from C. citricola egg sacs collected by AC on Tenerife in the Canary Islands in May 2018. These specimens closely resemble material identified as Pe. pyrgo (Walker) from England and elsewhere in Europe reared from lepidopteran hosts as primary or very often secondary parasitoids. Only small and probably insignificant differences could be found. It seems best to regard the Pediobius material reared from Cyrtophora egg sacs collected in Tenerife as probably Pe. pyrgo until more material is available for morphological and molecular analyses.
As currently understood, Pe. pyrgo has been reported from an unusually broad range of primary hosts (
Pediobius pyrgo is a solitary or slightly gregarious endoparasitoid of larvae and pupae, and it has been described as a koinobiont larva/pupal parasitoid of Leucoptera (Lep., Lyonetiidae) (
Although egg parasitism or predation in spiders has received little attention, case studies suggest it may be common in native ranges (e.g.,
Overall, we found that Ph. palanichamyi was present in about 40% of egg sacs. While wasp presence did not completely preclude spiderling emergence, it was associated with about 60% fewer spiderlings. It thus seems likely that C. citricola eggs in Spain experience predation pressure from the wasplarvae, an issue that is ripe for more detailed examination. Additionally, it would be useful to understand whether wasp predation rates remain stable across C. citricola’s range and breeding season and whether they promote extinction patterns in this spider, as is well-known among spider colonies (
The discovery of these two hymenopteran associates of C. citricola in its native Spanish range has particularly important implications for the multiple introductions of this spider throughout the Americas and Caribbean. No wasp associates have been reported from any of the non-native populations of C. citricola, even though yearly surveys of two expanding populations in Florida from 2014–2017 have been conducted by AC (Chuang, unpublished data). This is notable because in Florida, the range of C. citricola overlaps L. geometricus and its egg sac parasitoid Ph. latrodecti, known only to parasitize the widow spiders Latrodectrus spp. (
We thank Jordi Moya-Laraño for help with permits and tips on where to find the spiders as well as Susan Riechert, Daniel Simberloff, and Yael Lubin for their comments on earlier versions of the manuscript. We are grateful to Jayne Lampley and Vicente Deltoro for identifying the plants on which the spiders built webs. Thanks to Natalie Dale-Skey for the loan of types from the
Table S1. Egg predator and spiderling emergence rates
Data type: species data