Hymenoptera

Ward, Anna K. G., Busbee, Robert W., Chen, Rachel A., Davis, Charles K., Driscoe, Amanda L., Egan, Scott P., Goldberg, Bailey A. R., Hood, Glen Ray, Jones, Dylan G., Kranz, Adam J., Meadely-Dunphy, Shannon A., Milks, Alyson K., Ott, James R., Prior, Kirsten M., Sheikh, Sofia I., Shzu, Shih-An, Weinersmith, Kelly L., Zhang, Linyi, Zhang, Y. Miles & Forbes, Andrew A., 2022, Borneosa aspera, Zoological Studies 61 (57), pp. 1-30 : 6-8

( Hymenoptera : Cynipoidea : Cynipidae : Synergini )

7,227 individuals (mean = 104.7, range 1–3,228) reared from 69 gall types ( Table S1).

Summary of Natural History: Synergus ( Hymenoptera : Cynipidae : Synergini ) are usually professed to be inquilines, but are perhaps more accurately described as gallers of galls ( Askew 1961). Synergus induce additional growth in existing galls, including the formation of larval chambers, and their developing young feed on the tissue of the gall ( Evans 1965). Though gall inducing Synergus have been documented in Japan ( Abe et al. 2011; Ide et al. 2018), gall induction in Synergus is a derived habit ( Ide et al. 2018) and not known from the Nearctic. In some galls, the presence of Synergus is fatal to the developing gall inducer, but in other cases food may be sufficient such that both may emerge ( Pénzes et al. 2012). In some galls, Synergus develop and emerge as adults within a matter of weeks, while others can take one or even two years to emerge ( Evans 1965; Busbee 2018; Ward et al. 2020).

Multiple species of Synergus can be associated with the same gall type ( Askew 1961; Pénzes et al. 2012; Bird et al. 2013; Forbes et al. 2016; Weinersmith et al. 2020), while other galls have no known Synergus associates despite large collection efforts (e.g., Joseph et al. 2011). There has been some previous suggestion that two other genera of cynipid inquilines ( Ceroptres and Euceroptres ) of Nearctic gall wasps may not co-occur with Synergus ( Brookfield 1972) , but curated rearing records (e.g., Krombein et al. 1979) and our own data presented here show that this is not universally true.

Relationship to galler phylogeny: Synergus wasps were reared from gall types across most of the Nearctic gall wasp phylogeny ( Ward et al. 2022), with some exceptions. In only two cases were Synergus reared from gall types produced by gallers in the large clade that includes genera Melikaiella Pujade-Villar , Loxaulus Mayr , and most of the Neuroterus Hartig ( Fig. S1 ; gallers # 2-15 in Fig. 1a ). Both of these Synergus / Neuroterus associations were from Pacific coast galls. The reduced apparent association of Synergus with gall inducers in this clade might reflect that the Synergus association with oak gall wasps originated in the clade represented by the lower two-thirds of the tree. However, this hypothesized relationship is belied by records of Synergus being associated with the topmost clade of Palearctic gall wasps in fiugre 1a (though these a) b)

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could represent secondary colonizations). Ultimately, assessment of coevolutionary relationships of Synergus with oak gall wasps requires a phylogeny of the Holarctic Synergus .

Biogeography and oak tree section: Synergus were reared from galls in all three North American oak floristic regions and from galls on trees across all three sampled oak sections ( Fig. 1b c ). Of the three sections, Synergus were least often reared from galls on section Lobatae .

Tree organ and gall size: Synergus were reared from galls developing on leaves, stems, buds, acorns, and petioles ( Fig. 1d ). Among organs from which we sampled galls, only flower galls did not have apparent Synergus associates. Synergus in our collections were most commonly reared from medium sized galls (61%) and least commonly reared from small galls (38% of galls smaller than 5 mm) ( Fig. 1e ). Though these differences are not large, they comport with observations of Palearctic Andricus Hartig galls which suggest that small bud and catkin galls were less likely to host Synergus ( Stone et al. 1995) . Alternatively, reduced association of Synergus with small bud and catkin galls could be related to their earlier temporal occurrence: 25 of the 27 putative Synergus species in Ward et al. (2020) were reared from galls developing in June or later. However, we again raise the caveat that small galls may desiccate in the lab causing associated insects to die before emergence and leading to apparent non-associations.

Co-occurrence with other natural enemies: When Synergus were present, two other putative inquiline genera were significantly less likely to be present: Euceroptres (P = 0.005) and Ceroptres (P = 0.0026). Thus while our data disagree with the suggestion that Synergus and Ceroptres / Euceroptres entirely displace one another ( Brookfield 1972), they do appear to co-occur less often than expected. This pattern could be due to competitive exclusion, but also or instead be an indirect result of differential adaptation to dimensions of gall hosts. Notably, different Synergus species do not apparently competitively exclude one another, with as many as five species having been reared from the same host gall ( Pénzes et al. 2012). To the extent that more closely related species are expected to compete more closely when sharing the same habitat ( Miller 1967; Denno et al. 1995), differential adaptation to some dimensions of the gall environment seems the more attractive hypothesis.

Additional notes: Our record of four Synergus wasps reared from galls of Andricus quercuscalifornicus appear to be the first ever, despite much attention having been paid to this particularly large and common Pacific coast gall and its natural enemies ( Joseph et al. 2011). Other efforts to collect and rear insects from large numbers of potential hosts often turn up uncommon associations (Yee 2008; Yee and Goughnour 2008). Given that host shifts have often been implicated in the origins of parasitic insect diversity ( Diehl and Bush 1984; Drés and Mallet 2002; Forbes et al. 2017), evidence of insects occasionally being reared from unexpected hosts suggest that variation in host recognition syndromes may result in insects often “testing” new potential host plants.

Figures

Fig. 1. Summary of data for Synergus inquilines reared from Nearctic galls. a) Associations of Synergus mapped to the Ward et al. (2022) Nearctic oak gall wasp phylogeny (Fig. S1). Numbers at tips of branches refer to those in figure S1. Closed circles at branch tips indicate Synergus was reared from galls of that gall wasp species in this study. Open circles indicate other previously known associations either not studied by us or not recovered in our collections. Blue-colored branches within the phylogeny indicate Palearctic gall wasps. b) Total number of gall types from which Synergus were reared in the three bioregions identified by Hipp et al. (2018) as constituting different assemblages of North American oaks: Californian (blue), Mexican and Central American (orange), and Eastern North American (green) floristic provinces. Numbers in parentheses indicate the total number of gall types collected in each region, excluding gall types from which no insects emerged. For figures c–f, gall types from which fewer than five individual insects were reared were excluded, whether or not a Synergus was reared. c) Associations of Synergus with trees in sections Quercus (Q), Lobatae (L), and Virentes (V). Gray bars and numbers indicate gall types with which a Synergus was associated. White bars and numbers indicate the number of gall types from which a Synergus was not reared. d) Association of Synergus with gall types on different oak tissues. Clockwise from top left: leaf, stem, acorn, flower, petiole, bud (“Bud” includes galls that may be found on both buds or stems. “Petiole” includes galls that may be found on both petioles and stems or petioles and leaves); e) proportion of gall types of three size categories (“small” <0.5 mm; “medium”, “large”> 20 mm) from which Synergus were reared. f) Results of probabilistic co-occurrence analysis (Veech 2013) for Synergus against seven other common associates (Cer = Ceroptres, Euc = Euceroptres, Orm = Ormyrus, Syc = Sycophila, Tor = Torymus, Eur = Eurytoma, Eud = Euderus). Yellow = significantly less likely to co-occur; blue = significantly more likely to co-occur; gray = no difference from probabilistic expectations. g) Synergus lateral habitus.