Abstract
Terrestrial parasitengone mites (Prostigmata: Parasitengona) are known for their complex life cycles, with active forms confined to larvae, deutonymphs, and adults, and quiescent proto- and tritonymphal stages. Knowledge of the host range of parasitic larvae of most species is still limited, but it is likely that host choice and suitability may influence intraspecific variation of subsequent developmental instars. In this study we assessed the variation of morphometric traits in unfed deutonymphs of Allothrombium fuliginosum which developed from larvae that had parasitized four different aphid hosts: Acyrthosiphon pisum, Aphis sambuci, Macrosiphum rosae and Hyadaphis sp. Analysis of 48 morphometric traits in 80 deutonymphs revealed significant differences between host groups in 19 traits, with M. rosae contributing the most to observed variability. Principal component analysis showed some distinct clustering of deutonymphs according to the host species parasitized by larvae. The smaller, with respect to basic body measurements, deutonymphs developed from larvae that parasitized M. rosae, despite the overall larger body size of this host. Thus, factors other than host size influence the morphology of mites. The findings enhance our understanding of host-parasite interactions and highlight the need for further insight into sources of intraspecific variability within terrestrial Parasitengona.
Keywords: Terrestrial Parasitengona, Aphididae, Host-parasite interactions, Experimental rearing, Morphometric analyses
Introduction
Terrestrial Parasitengona, traditionally comprising three superfamilies (Calyptostomatoidea, Erythraeoidea, Trombidioidea), are among the most diverse groups of actinotrichid mites. The complex life cycle, heteromorphic life instars (larvae versus active postlarval forms), and diverse lifestyles are influenced by different selection pressures exerted at particular stages of development of these mites. There are seven stages in the life cycle of terrestrial Parasitengona (Wohltmann 2000). Four stages are inactive: egg, prelarva, protonymph, and tritonymph. Active stages include the larva, deutonymph, and adult. Deutonymphs and adults of most parasitengone species prey on arthropods, whereas larvae predominantly parasitize them. Notable exceptions include vertebrate-associated larval Trombiculidae s. l. (Kudryashova 1998), and a few taxa that have non-parasitic larvae (Wohltmann 2000).
Little is known about the extent of intraspecific variation of morphological traits of most terrestrial parasitengones. Taxonomic descriptions are often confined to single instars, such as larvae, adults, or, to a lesser extent, deutonymphs, and usually rely on a limited number of specimens. In many cases, haphazardly collected samples are used to determine the range of metric traits applied in species identification. Data on host range, host specificity, and host preferences (for definitions see: Lymbery 1989 ), based predominantly on the results of field observations, are scanty not only in the case of Trombidioidea (Felska et al. 2018), but also in the case of Erythraeoidea and, to a lesser extent, Calyptostomatoidea (Wohltmann 2000). Furthermore, drawing conclusions based on field-collected and preserved material does not provide clear information on the success of the parasitism, as measured by the transformation of a larva into subsequent developmental instars.
An insight into literature (e.g., Mąkol 2005) indicates that the deutonymph instar of terrestrial parasitengones is notably under-researched in terms of morphological variation, with most species documented primarily in their larval or adult stages. Despite this, several species, among them some representatives of Allothrombium, Caenothrombium, Trombidium (Trombidiidae), Podothrombium (Podothrombiidae) but also of Balaustium (Erythraeidae), Sphaerotarsus (Smarididae), and Haplothrombium (Trombellidae) have been described and remain known only by deutonymphs (Southcott 1946; Willmann 1950; Gabryś 2000; Mąkol 2005). Many published descriptions, especially older ones, refer to postlarval instars without clearly distinguishing between deutonymphs and adults. This lack of clarity may arise from challenges in distinguishing these instars, especially due to allometric growth and particularly in groups that show minimal differences between deutonymphs and adults (Mąkol et al. 2019). A key characteristic that aids in the correct identification of postlarval instars is the number of genital papillae. In most parasitengone taxa, except for Erythraeoidea, which have reduced in number or modified genital papillae (Witte 1995), deutonymphs usually possess two pairs of papillae, while adults have three pairs (Wohltmann et al. 2007). However, in the case of Calyptostoma, both deutonymphs and adults have only two pairs of papillae (Vistorin-Theis 1977). In contrast, three pairs of papillae have been observed in adults and at least some deutonymphs of Allothrombium, including A. fuliginosum (Mąkol 2007).
Among trombidioid genera, Allothrombium Berlese has attracted significant attention from researchers due to the widespread distribution and common occurrence of its members (e.g., Pagenstecher 1860; Zhang and Xin 1992). Allothrombium species have been also recurrently indicated as having potential in biological control of pests of plants, with considerable attention devoted to their impact on reducing populations of various aphid species (e.g., Davis 1961; Anonymous 1983; Dong 1991, 2001; Zhang and Xin 1989; Zhang 1992; Zhang et al.1993 ). Allothrombium includes over 70 nominal species worldwide. One of the most common and extensively studied representatives of this genus is Allothrombium fuliginosum (Hermann), widely distributed across the western Palearctic (Mąkol and Wohltmann 2012; Mąkol et al. 2019). It is also one of approximately 10% of the members of the genus for which both larval and postlarval instars (deutonymphs and adults) have been described (Mąkol 2005; Mąkol and Wohltmann 2012).
Larvae of Allothrombium spp. have been reported to parasitize various aphid species (Hemiptera: Aphididae), and– to a lesser extent - other arthropods, including Lepidoptera, Orthoptera, Hymenoptera, and Araneae (Felska et al. 2018), however, the degree of host specificity remains unknown (Wohltmann and Mąkol 2009). Deutonymphs and adults prey on small arthropods, many of which are regarded as economically significant pests (Welbourn 1983; Zhang 1991). The parasitic phase of larval A. fuliginosum lasts an average of six days, with a range of 5 to 8 days, at a daytime temperature of 22 °C and a nighttime temperature of 15 °C; during this phase, the larvae create feeding canals known as stylostomes, which extend deep into the body cavity of the host (Felska et al. 2020). According to Shatrov et al. (2014), who studied stylostomes in Leptotrombidium (Trombiculidae), the canals enable larvae, which possess short chelicerae, to reach the content of liquified host tissues, and extract a sufficient portion of nutrients to continue the development within a relatively short time.
Aphids, which serve as primary hosts for Allothrombium spp., are specialized phytophagous insects, exhibiting varying levels of host specificity, from strict monophagy to polyphagy. Adaptation to phytophagy is completed by an extremely ductile reproduction system that can alternate biparental and parthenogenetic generations (Guerrieri and Digilio 2008). They are economically significant pests, but also a key food source for natural enemies, including predators and parasites such as larvae of Allothrombium. For parasitic larvae, the host represents the whole nutritional and physiological environment during immature development. Several scientific studies have shown that the morphology of post-larval stages in various arthropod species can be significantly influenced by the characteristics and conditions of their larval hosts (e.g., Lepidoptera (Cook 1961; Rodrigues and Moreira 2004), Mesostigmata (Lachaud et al. 2016). However, such effects remain largely unexplored in terrestrial Parasitengona mites.
Our aim was to ascertain the range of morphological variation in deutonymphs of A. fuliginosum that parasitized various hosts as larvae: the pea aphid Acyrthosiphon pisum (Harris), the elder aphid Aphis sambuci L., the rose aphid Macrosiphum rosae (L.) and the honeysuckle aphid Hyadaphis sp. Of those, the first three species have been previously reported as hosts of A. fuliginosum (Henking 1882; Felska et al. 2018, 2020). We hypothesize that the host species, particularly its body size, affects the metric traits of deutonymphs whose correct identification and adequate description constitute an important part of species diagnosing.
Materials and methods
Sampling and rearing. To obtain larvae of A. fuliginosum, we collected active postlarval forms at weekly intervals from early March till the end of April 2022. The mites were collected directly from litter, the upper soil layer, and lower parts of the tree trunks at the campus of Wrocław University of Environmental and Life Sciences, Wrocław, Poland (51°05’54” N, 17°05’42” E). Specimens were transferred individually into glass rearing vials (⌀ 20 mm × 35 mm) filled to 1/3 with plaster and charcoal (9:1) pre-soaked in distilled water, and sealed with a plastic, semi-transparent, non-perforated lid. The vials were kept in a Sanyo MRL-351 H climate chamber under conditions of 22 °C (day), 15 °C (night), 80% humidity, and a 12 h/12 h day-night cycle. Each vial was inspected every 2–3 days to maintain appropriate substrate moisture and to record changes such as oviposition, the transition of eggs into prelarvae, and the emergence of larvae. Mites were confirmed to be A. fuliginosum based on Mąkol (2005) and through morphological comparison with specimens originating from the same population and used in a previous study (Mąkol et al. 2019).
Of the aphids used in the experiment, the pea aphids originated from laboratory culture carried out at the Department of Invertebrate Systematics and Ecology, Wrocław University of Environmental and Life Sciences. Pea aphids were fed on young seedlings of garden pea (Pisum sativum L.), specifically the ‘Cysterski’ edible variety from the Poznańska Hodowla Roślin, Poland. The aphids of the remaining three species were field-collected from their plant hosts in May 2022: Macrosiphum rosae from ornamental rose bushes, Hyadaphis sp. from honeysuckle in a private garden in Kiełczów near Wrocław, and Aphis sambuci from elderberry (Sambucus nigra L.) shoots growing in Czarna Woda Municipal Park in Wrocław. The choice of particular aphid species as potential hosts was primarily based on their availability in the field, which needed to align with the timing of larval emergence. Furthermore, the ability to maintain the aphids in laboratory conditions using small fragments of host plants was also considered.
Experimental design. To observe the interactions between the mite larvae and their potential hosts, we used glass containers (⌀ 90 mm × 35 mm) as experimental arenas (Fig. 1). The bottom of each container was lined with a thin layer of coconut fiber, pre-moistened with distilled water. Approximately 60 adult aphids of a given species, along with part of the host plant, were introduced to each arena and combined with A. fuliginosum larvae originating from the egg clutch of approximately 300 eggs laid by a single female. Aphids were exposed to larvae not later than three days after the eclosion of the mite larvae. The containers were covered with a fine mesh screen (pore size < 70 µm), and sealed with an elastic rubber band to allow ventilation and prevent larvae from escaping. For each host species, the experiment was set up in three separate arenas, thus the suitability of each host species was assessed with larvae from three different females of A. fuliginosum. The mite larvae were provided with continuous access to live aphids, allowing them to parasitize ad libitum. Nymphs that were born from parthenogenetic aphid females during the experiment, were not removed from the arenas, thus increasing the pool of potential host specimens. At the completion of the parasitic phase, engorged and immobile larvae that had reached the onset of the protonymphal instar were transferred individually to rearing vials (⌀ 20 mm × 35 mm). Vials were checked every two days to record the appearance of deutonymphs or the death of specimens. The number of successful transitions to inactive protonymph (PN) and active deutonymph (DN) was recorded separately for each group of mites exposed as larvae to different host species. Deutonymphs were preserved in 96% alcohol not later than two days after emerging from the preceding instar.
Fig. 1.
The generalised experimental setup. Plant, aphid, and larvae not to scale
Morphological measurements. Twenty deutonymphs were selected in a haphazard manner (i.e., without a formal randomization process) from each of the four groups exposed to different hosts and were dedicated to microscopic study. The mite idiosoma was punctured with a fine entomological needle, and the specimens were cleared for several minutes in c. 10% KOH, then rinsed in distilled water, and mounted on microscope slides using Hoyer’s medium. The material was examined and measurements were taken using a Nikon Eclipse E600 microscope equipped with a Nikon DS-Fi1 camera and NIS Elements software. Morphological terminology follows Mąkol (2005) and Gabryś (1999) with the addition of the abbreviation aPr (the anterior process of crista metopica). To compare the approximate surface area accessible for mite larvae on different host species, ten adult aphids of each species were mounted in a dorso-ventral plane on microscope slides, following the protocol used for mites. We then measured their body length and width (Table 1). All measurements are given in micrometers (µm).
Table 1.
Body measurements of adult female aphids selected from groups exposed to Allothrombium fuliginosum larvae
|
Ac. pisum
(n = 10) |
Ap. sambuci (n = 10) |
Hyadaphis sp. (n = 10) |
M. rosae
(n = 10) |
|
|---|---|---|---|---|
|
mean (min.–max.) body length |
3338 (2690–3898) |
3025 (2625–3476) |
1850 (1588–2234) |
3237 (2773–3824) |
|
mean (min.–max.) body width |
1371 (1020–1572) |
1734 (1478–1890) |
897 (692–1068) |
1469 (1012–1993) |
Statistical analyses. The analyses carried out using the STATISTICA 13.3 software package (StatSoft 2017) and R Statistical Software (R Core Team 2024) aimed at assessing differences between groups of A. fuliginosum that parasitized different aphid species. The Shapiro-Wilk test revealed that the distributions of most variables deviated from normality. Consequently, the Kruskal-Wallis test was applied separately to all dependent variables, with the false discovery rate controlled using the Benjamini–Hochberg stepwise adjustment. This was done to test the null hypothesis that the metric traits of the deutonymphs do not differ significantly (p = 0.05), regardless of the host species parasitized by larvae. Following the Kruskal-Wallis test, a post-hoc Dunn’s multiple comparison procedure was performed for each statistically significant variable to identify which host species contributed to the observed differences in specific metric traits (p = 0.05).
A principal components analysis (PCA) was performed to reduce the dimensionality of the original metric data concerning the morphological traits of deutonymphs. This analysis aimed to classify the deutonymphs according to the groups defined by the host species infested by larvae. The input variables for the PCA were chosen based on the results of the Kruskal-Wallis test, which identified traits that exhibited significant variation among the host groups. Furthermore, only those morphological traits that differentiated at least three pairs of host groups, as determined by Dunn’s post hoc test, were included in the analysis.
Results
Parasitism of A. fuliginosum larvae was visually confirmed for each pairwise set of larvae and aphid species (Fig. 2). The number of specimens transferred to separate rearing vials at the onset of protonymph instar ranged from 96 to 120, depending on the host (Table 2). The success of the transformation from protonymph to deutonymph in specimens that parasitized any of the four offered host species ranged from 49 to 57.5% (Table 2). Approximately 57.5% of specimens completed parasitism on Ap. sambuci, 56% on Ac. pisum, and 49% on both Hyadaphis sp. and M. rosae. At the same time, the hitherto unreported parasitism of A. fuliginosum on Hyadaphis sp. was confirmed.
Fig. 2.
Larvae of Allothrombium fuliginosum on various hosts: 72 h from the start of parasitism on: a) Macrosiphum rosae, female; b) Acyrthosiphon pisum, female; c) Aphis sambuci, female; and d) 48 h from the start of parasitism on Hyadaphis sp., nymphs
Table 2.
Number of deutonymphs (DN) of Allothrombium fuliginosum that developed from protonymphs (PN) and success of transformation depending on the host species parasitized by larvae
| Host species | ||||
|---|---|---|---|---|
| Ac. pisum | Ap. sambuci | Hyadaphis sp. | M. rosae | |
| PN | 96 | 120 | 116 | 102 |
| DN | 54 | 69 | 57 | 50 |
| Success of transformation [%] | 56 | 57.5 | 49 | 49 |
The morphometric data on deutonymphs, which parasitized different host species as larvae, are provided in Table 3. Out of 48 analyzed morphometric features, 19 revealed statistically significant differences between groups (p < 0.05) (Table 4). These morphological traits (referring most of all to the measurements of crista metopica, anus, and particular leg segments) appear to be influenced by the host species. The highest number of significant differences (13) in morphometric data on deutonymphs was observed between the deutonymphs that had fed as larvae on Ap. sambuci and M. rosae, followed by M. rosae and Hyadaphis sp. (8), M. rosae and Ac. pisum (7), Ap. sambuci and Hyadaphis sp. (5), with the lowest number of differences in morphometric traits (2 each) between Ac. pisum and Ap. sambuci and Ac. pisum and Hyadaphis sp. (Table 5). The differences were distributed in a mosaic manner between deutonymphs assigned to four different host groups. However, the deutonymphs that developed from larvae that parasitized M. rosae more often achieved lower values of the traits compared to the other three groups (Table 5).
Table 3.
Morphometric data on deutonymphs of Allothrombium fuliginosum, which developed from larvae parasitizing different host species. Data provided in the format mean (min.–max.)
| Character | Host species | |||
|---|---|---|---|---|
|
Ac. pisum
(n = 20) |
Ap. sambuci
(n = 20) |
Hyadaphis sp. (n = 20) |
M. rosae
(n = 20) |
|
| LB | 733 (564–1026) | 710 (523–919) | 720 (609–817) | 695 (552–761) |
| WB | 556 (377–755) | 517 (398–638) | 527 (449–591) | 504 (418–690) |
| LB/WB | 1.3 (1.2–1.5) | 1.37 (1.13–1.61) | 1.4 (1.3–1.5) | 1.4 (1.2–1.5) |
| PaTa (L) | 61 (43–89) | 62 (45–76) | 58 (40–77) | 60 (50–76) |
| TiCl | 37 (21–61) | 41 (29–51) | 34 (19–52) | 37 (28–52) |
| ChCl | 35 (24–53) | 37 (24–50) | 32 (24–41) | 35 (25–53) |
| CML | 106 (72–147) | 108 (84–141) | 104 (86–132) | 85 (71–106) |
| CMW | 58 (48–76) | 57 (45–72) | 58 (48–74) | 50 (42–62) |
| S | 93 (66–122) | 95 (70–126) | 87 (46–121) | 78 (44–144) |
| SB | 43 (36–56) | 44 (33–54) | 45 (36–67) | 41 (33–85) |
| aPr | 33 (22–62) | 31 (16–46) | 28 (19–51) | 20 (15–39) |
| pPr | 12 (4–28) | 14 (8–44) | 12 (6–24) | 10 (5–14) |
| pDS | 34 (29–40) | 33 (31–36) | 37 (33–44) | 43 (30–51) |
| OL | 39 (31–53) | 39 (31–48) | 38 (28–50) | 40 (35–47) |
| An (L) | 50 (33–87) | 39 (31–47) | 61 (45–90) | 49 (30–62) |
| An (W) | 28 (17–49) | 22 (16–32) | 32 (22–46) | 29 (19–41) |
| GOP (L) | 112 (81–159) | 112 (84–132) | 112 (93–133) | 105 (89–145) |
| GOP (W) | 70 (33–111) | 70 (55–87) | 62 (41–98) | 52 (32–83) |
| Cx I | 125 (90–188) | 123 (91–158) | 122 (88–155) | 111 (53–175) |
| Tr I | 76 (47–111) | 75 (52–113) | 75 (49–99) | 67 (43–89) |
| bFe I | 87 (59–128) | 80 (50–106) | 84 (50–112) | 78 (50–114) |
| tFe I | 78 (51–124) | 81 (54–106) | 81 (69–100) | 76 (51–101) |
| Ge I | 114 (82–184) | 110 (73–144) | 109 (94–134) | 103 (65–134) |
| Ti I | 118 (81–177) | 117 (80–156) | 109 (87–137) | 107 (81–138) |
| Ta I (L) | 152 (118–214) | 148 (93–184) | 144 (110–174) | 141 (107–234) |
| Ta I (W) | 67 (52–92) | 65 (46–80) | 70 (54–85) | 61 (47–88) |
| Cx II | 107 (68–143) | 99 (73–176) | 101 (87–127) | 92 (71–110) |
| Tr II | 57 (36–88) | 66 (38–118) | 56 (30–79) | 52 (40–71) |
| bFe II | 58 (38–96) | 58 (31–129) | 57 (29–73) | 48 (20–70) |
| tFe II | 45 (29–72) | 50 (34–117) | 47 (34–69) | 41 (27–53) |
| Ge II | 64 (43–101) | 69 (42–155) | 58 (37–74) | 56 (43–78) |
| Ti II | 73 (45–131) | 76 (44–177) | 68 (50–77) | 62 (48–77) |
| Ta II | 105 (79–150) | 113 (65–263) | 102 (80–125) | 97 (79–126) |
| Cx III | 94 (59–148) | 85 (67–103) | 87 (68–100) | 87 (75–102) |
| Tr III | 52 (37–87) | 51 (31–83) | 49 (39–89) | 44 (30–60) |
| bFe III | 54 (22–90) | 53 (24–81) | 52 (26–74) | 45 (29–66) |
| tFe III | 48 (33–90) | 46 (29–59) | 47 (36–73) | 41 (31–70) |
| Ge III | 61 (39–93) | 63 (48–78) | 58 (42–77) | 53 (39–74) |
| Ti III | 72 (42–110) | 70 (44–94) | 67 (58–78) | 63 (51–93) |
| Ta III | 100 (75–155) | 99 (66–128) | 91 (55–113) | 89 (78–114) |
| Cx IV | 115 (87–61) | 112 (87–151) | 106 (79–121) | 107 (92–135) |
| Tr IV | 53 (25–95) | 60 (36–77) | 49 (32–65) | 52 (35–73) |
| bFe IV | 59 (34–105) | 60 (31–80) | 60 (47–81) | 51 (40–70) |
| tFe IV | 63 (42–98) | 58 (32–70) | 57 (45–84) | 52 (39–84) |
| Ge IV | 88 (62–123) | 87 (60–116) | 79 (52–98) | 75 (55–97) |
| Ti IV | 91 (64–128) | 95 (58–121) | 81 (55–101) | 80 (59–109) |
| Ta IV | 112 (79–153) | 113 (58–154) | 102 (68–125) | 97 (71–124) |
| IP | 852 (633–1160) | 833 (621–1039) | 826 (690–968) | 776 (610–976) |
An—anus; aPr—anterior processes of crista metopica; bFe—basifemur; ChCl—cheliceral claw; CML—length of crista metopica; CMW— width of crista metopica; Cx—coxa; Ge—genu; GOP—genital opening; IP—index pedibus (the total length of legs on one side of the body including coxae); LB—length of idiosoma; OL—length of ocular sclerite; PaTa—palp tarsus; pDS—posterodorsal setae on idiosoma; pPr—posterior processes of crista metopica; TiCl—tibia claw; S—length of sensillum; SB—distance between the bases of sensilla (S); Ta—tarsus; tFe—telofemur; Ti—tibia; Tr—trochanter; WB—width of idiosoma
Abbreviations denote the length or, where indicated, the length and width (L and W, respectively) of specific structures
Table 4.
The differences in morphometric traits between groups of Allothrombium fuliginosum that parasitized various hosts. The result of the Kruskal-Wallis test (p ≤ 0.05)
| Character | Host species | Statistical significance p |
|||||||
|---|---|---|---|---|---|---|---|---|---|
|
Ac. pisum
n = 20 |
Ap. sambuci n = 20 |
Hyadaphis sp. n = 20 |
M. rosae
n = 20 |
||||||
| Mean | s | Mean | s | Mean | s | Mean | s | ||
| LB | 733 | 122.9 | 710 | 112.5 | 720 | 55.4 | 695 | 89.6 | ns |
| WB | 557 | 97.4 | 518 | 66.6 | 527 | 44.3 | 504 | 65.2 | ns |
| LB/WB | 1.32 | 0.1 | 1.37 | 0.1 | 1.37 | 0.06 | 1.38 | 0.1 | ns |
| PaTa (L) | 62 | 10.8 | 62 | 8.3 | 58 | 10 | 60 | 7.2 | ns |
| TiCl | 37 | 8.4 | 41 | 7.2 | 34 | 8.2 | 37 | 6.4 | 0.05 |
| ChCl | 35 | 7.5 | 37 | 7 | 32 | 5.1 | 35 | 7.7 | ns |
| CML | 106 | 17.9 | 108 | 14.8 | 104 | 11.6 | 85 | 10.2 | < 0.001 |
| CMW | 58 | 7.9 | 57 | 6.9 | 58 | 7.2 | 50 | 6 | 0.002 |
| S | 93 | 14.1 | 95 | 16 | 87 | 18.8 | 78 | 27.4 | 0.005 |
| SB | 43 | 6.4 | 44 | 5 | 45 | 6.7 | 41 | 10.9 | 0.02 |
| aPr | 33 | 9 | 31 | 8.5 | 28 | 7.3 | 20 | 5.1 | < 0.001 |
| pPr | 12 | 6 | 14 | 8.2 | 12 | 4.4 | 10 | 2.6 | ns |
| pDs | 34 | 2.7 | 33 | 1.5 | 37 | 2.4 | 43 | 6.8 | < 0.001 |
| OL | 39 | 5.8 | 39 | 4.2 | 39 | 5.9 | 40 | 4.4 | ns |
| An (L) | 50 | 13.3 | 39 | 5.4 | 62 | 12.2 | 49 | 9.6 | < 0.001 |
| An (W) | 28 | 7.4 | 22 | 3.7 | 32 | 7.8 | 29 | 6.3 | 0.002 |
| GOP (L) | 112 | 22.1 | 112 | 13.4 | 112 | 10.6 | 105 | 16.2 | ns |
| GOP (W) | 70 | 24.3 | 70 | 10 | 62 | 16.1 | 52 | 15.3 | 0.005 |
| Cx I | 125 | 26.8 | 123 | 19.4 | 122 | 20.7 | 122 | 0.5 | ns |
| Tr I | 76 | 16 | 75 | 15.3 | 75 | 13.9 | 111 | 26.8 | ns |
| bFe I | 87 | 18.7 | 80 | 16.1 | 84 | 15.1 | 68 | 14.2 | ns |
| tFe I | 79 | 19.2 | 81 | 15.2 | 81 | 7.8 | 76 | 13.8 | ns |
| Ge I | 114 | 24.9 | 110 | 20.5 | 110 | 10.1 | 103 | 17.1 | ns |
| Ti I | 118 | 23.7 | 117 | 22.5 | 109 | 14.6 | 107 | 15.3 | ns |
| Ta I (L) | 152 | 26.2 | 148 | 27.1 | 145 | 14.9 | 141 | 27.4 | ns |
| Ta I (W) | 67 | 12.1 | 65 | 8.3 | 70 | 8.2 | 61 | 9.4 | 0.04 |
| Cx II | 101 | 19.9 | 99 | 24 | 101 | 9.9 | 92 | 9.7 | ns |
| Tr II | 57 | 13.3 | 66 | 18.8 | 56 | 13.4 | 52 | 6.7 | 0.05 |
| bFe II | 58 | 14.7 | 58 | 21 | 57 | 11.8 | 48 | 13.4 | ns |
| tFe II | 45 | 11.5 | 50 | 17.1 | 47 | 7.1 | 41 | 7 | ns |
| Ge II | 64 | 15.6 | 69 | 23.3 | 58 | 10.5 | 56 | 9.1 | ns |
| Ti II | 73 | 20.5 | 76 | 28.7 | 68 | 6.8 | 62 | 6.6 | 0.03 |
| Ta II | 106 | 17.4 | 113 | 39.8 | 102 | 12.2 | 97 | 11.1 | ns |
| Cx III | 94 | 22.9 | 85 | 11.6 | 87 | 10.3 | 87 | 8.4 | ns |
| Tr III | 52 | 12.8 | 51 | 13 | 49 | 11.1 | 44 | 9.1 | ns |
| bFe III | 54 | 15.1 | 53 | 11.2 | 52 | 11.7 | 45 | 11.2 | ns |
| tFe III | 48 | 14.3 | 46 | 8.3 | 47 | 9 | 41 | 11 | 0.02 |
| Ge III | 61 | 13.8 | 63 | 8.5 | 58 | 8 | 53 | 10.8 | 0.01 |
| Ti III | 73 | 20.2 | 70 | 15.5 | 67 | 6.7 | 63 | 11.2 | ns |
| Ta III | 100 | 17.6 | 99 | 18.7 | 91 | 14.3 | 90 | 9.2 | ns |
| Cx IV | 115 | 22.1 | 112 | 19.3 | 106 | 11.7 | 107 | 12.7 | ns |
| Tr IV | 53 | 16.3 | 60 | 11.6 | 49 | 10.6 | 52 | 10.9 | 0.03 |
| bFe IV | 59 | 17.1 | 60 | 13.5 | 60 | 8.9 | 51 | 8.5 | 0.01 |
| tFe IV | 63 | 16.4 | 58 | 9.7 | 57 | 8.6 | 52 | 10.4 | 0.02 |
| Ge IV | 88 | 17.8 | 87 | 16.3 | 79 | 11.2 | 75 | 12.3 | ns |
| Ti IV | 91 | 19.1 | 95 | 18.2 | 81 | 12.8 | 81 | 13.3 | 0.02 |
| Ta IV | 112 | 22 | 113 | 25.8 | 102 | 15.2 | 97 | 15.9 | ns |
| IP | 852 | 146.9 | 833 | 129.1 | 826 | 69.3 | 776 | 105.2 | ns |
Table 5.
Morphometric traits of Allothrombium fuliginosum deutonymphs revealing statistically significant differences (an asterisk refers to p value ≤ 0.05) between groups (results of Dunn’s test of multiple comparisons).
| Character |
Ac. pisum: Ap. sambuci |
Ac. pisum: M. rosae |
Ac. pisum: Hyadaphis sp. |
Ap. sambuci: M. rosae |
Ap. sambuci: Hyadaphis sp. |
M. rosae: Hyadaphis sp. |
Number of times a trait showed differences between pairs of host species |
|---|---|---|---|---|---|---|---|
| TiCl | * [Ap. s.] | 1 | |||||
| CML | * [Ac. p.] | * [Ap. s.] | * [H. sp.] | 3 | |||
| CMW | * [Ac. p.] | * [Ap. s.] | * [H. sp.] | 3 | |||
| S | * [Ac. p.] | * [Ap. s.] | 2 | ||||
| SB | * [H. sp.] | 1 | |||||
| aPr | * [Ac. p.] | * [Ap. s.] | * [H. sp.] | 3 | |||
| pDs | * [M. r.] | * [H. sp.] | * [M. r.] | * [H. sp.] | 4 | ||
| An (L) | * [Ac. p.] | * [H. sp.] | * [M. r.] | * [H. sp.] | * [H. sp.] | 5 | |
| An (W) | * [Ac. p.] | * [M. r.] | * [H. sp.] | 3 | |||
| GOP (W) | * [Ac. p.] | * [Ap. s.] | 2 | ||||
| Ta I (W) | * [H. sp.] | 1 | |||||
| Tr II | * [Ap. s.] | 1 | |||||
| Ti II | * [Ap. s.] | 1 | |||||
| tFe III | * [H. sp.] | 1 | |||||
| Ge III | * [Ap. s.] | 1 | |||||
| Tr IV | * [Ap. s.] | 1 | |||||
| bFe IV | * [Ap. s.] | * [H. sp.] | 2 | ||||
| tFe IV | * [Ac. p.] | 1 | |||||
| Ti IV | * [Ap. s.] | 1 | |||||
| Number of times the deutonymphs from each host species have the larger measurement | Ac. p. vs. Ap. s.: 2 vs. 0 | Ac. p. vs. M. r.: 6 vs. 1 | Ac. p. vs. H. sp.: 0 vs. 2 | Ap. s. vs. M. r.: 10 vs. 3 | Ap. s. vs. H. sp.: 2 vs. 3 | M. r. vs. H. sp.: 0 vs. 8 |
The host for which the higher value of the trait was stated in each host pairwise comparison, is denoted with acronym in square brackets (Ac. pisum – Ac. p., Ap. sambuci – Ap. s., M. rosae – M. r., Hyadaphis sp.– H. sp.)
PCA was finally applied for six variables: aPr, CML, CMW, An (L), An (W), and pDS. The first two principal components explained 70% of the total variability among the four groups of deutonymphs which developed from larvae that fed on different host species. The first principal component (PC1) explained 40.8% of the variability, while the second component (PC2) − 29.2% (Fig. 3). The groups of deutonymphs distinguished by host species overlapped, but isolated subgroups were also observed (M. rosae and Ap. sambuci). The M. rosae host group exhibited the highest variability compared to other groups (shift along the PC1 axis), while the remaining three groups displayed lower within-group variability (shift along the PC2 axis). Based on the component loadings (Table 6), PC1 was primarily influenced by size-related traits such as the length and width of the crista metopica (CML, CMW), whereas PC2 was more strongly influenced by the length of the posterodorsal setae (pDS), suggesting that it captures variation in shape.
Fig. 3.
Results of principal component analysis based on six most significantly differentiating morphological characters of deutonymphs that developed from larvae which parasitized four different aphid species. Abbreviations for variables are provided in Table 3 annotations
Table 6.
Loading scores of morphological traits in the first two principal components
| PC1 | PC2 | |
|---|---|---|
| CML | 0.528 | 0.198 |
| CMW | 0.526 | 0.103 |
| aPr | 0.362 | 0.385 |
| pDS | −0.157 | −0.540 |
| An (L) | 0.397 | −0.493 |
| An (W) | 0.362 | −0.517 |
Discussion
Few studies have focused on enhancing our understanding of both intra- and interspecific variation in the morphological traits of terrestrial Parasitengona mites, as well as exploring the factors that drive this variation. Although the development of molecular techniques has supported morphological analysis (e.g., Costa et al. 2019, 2024; Mąkol et al. 2019; Derdak et al. 2021; Zajkowska and Mąkol 2023; Mąkol and Felska 2023; Zajkowska et al. 2023), the multiapproach analyses aiming at defining species limits, which are essential for drawing inferences about their biology and ecology, are still scarce. Investigating factors that contribute to intraspecific variation in these mites is further complicated by their complex life cycles and the need to consider the intra-stadial variability of several diagnostic instars, including larvae, deutonymphs, and adults. Analyzing field-collected specimens poses additional challenges compared to laboratory-reared specimens due to the wider array of factors—such as specimen age—that can contribute to observed variations. Our research, based on specimens of the same age obtained from laboratory rearing, has allowed us to limit the number of variables and focus on the effect of host species as one of the primary sources of variation.
The morphometric data on deutonymphs collected during the present study confirmed statistically significant differences between host groups, indicating varied impacts from different host species. Nineteen morphological traits appear to be influenced by the host, possibly due to ecological or physiological factors associated with each aphid species. Results from the Kruskal-Wallis test aligned with those from principal component analysis (PCA). To date, the impact of host species on the variability of morphological traits commonly used for species diagnosis has not been analyzed for terrestrial Parasitengona. Felska et al. (2020) examined the effect of three aphid hosts (Ac. pisum, M. rosae, and Elatobium abietinum (Walker) on the morphology and development of stylostomes in A. fuliginosum. While the feeding tubes found in the tissues of various hosts shared a similar shape, notable differences in their total lengths were observed. This variation occurred in stylostomes of the same age that were produced in different hosts and was not positively correlated with the body size of the hosts. Interestingly, the longest stylostomes were found in the tissues of Ac. pisum, which is an intermediate size compared to the slightly larger M. rosae and the smaller E. abietinum (Felska et al. 2020). Similar to the findings of Felska et al. (2020), we could not confirm that parasitism on the larger host resulted in consistently larger deutonymphs. PCA analysis indicated that while M. rosae is among the largest aphid hosts analyzed, deutonymphs that developed from larvae parasitizing this species were smaller in several traits compared to those from other hosts.
The morphological traits that most distinctly differentiate the four groups of deutonymphs, which developed from larvae parasitizing different host species, are related to measurements of the crista metopica (Pr, CML, CMW), the anus (An (L) and An (W)), and the opisthosomal setae (pDS). These measurements were taken from freshly emerged, unfed deutonymphs, which minimizes the influence of environmental factors on the observed traits in this developmental stage. As a result, the findings suggest that the morphological differences between deutonymphs likely stem from factors that affected the larvae. Additionally, the metric data obtained for deutonymphs support the concept of allometric growth observed in terrestrial parasitengone mites.
The results of this study represent an initial step toward understanding the impact of host species on parasite performance in terrestrial Parasitengona. Further research, encompassing a wider array of factors that contribute to overall species fitness, should be conducted to address the still-unanswered questions related to the biology and ecology of A. fuliginosum and of terrestrial parasitengones in general.
Acknowledgements
We are grateful to anonymous reviewers for their valuable comments which improved the quality of the manuscript.
Author contributions
MP: sampling, laboratory experiments, material preparation, measurements, statistical analysis, editing. JM: conceptualization, writing the original draft, editing. MF: sampling, conceptualization, study design, laboratory experiments, writing the original draft, statistical analysis, editing. All authors read, commented on, and approved the final version of the manuscript.
Funding
This work was supported by the Wrocław University of Environmental and Life Sciences (Poland) as the Ph.D. research program “Innovative Scientist”, no. N060/0001/21.
Data availability
We declare all data is being provided within this manuscript.
Declarations
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Data Availability Statement
We declare all data is being provided within this manuscript.