Oobius minusculus Triapitsyn & Petrice

Triapitsyn, Serguei V., Petrice, Toby R., Gates, Michael W. & Bauer, Leah S., 2015, Two new species of Oobius Trjapitzin (Hymenoptera, Encyrtidae) egg parasitoids of Agrilus spp. (Coleoptera, Buprestidae) from the USA, including a key and taxonomic notes on other congeneric Nearctic taxa, ZooKeys 498, pp. 29-50 : 35-37

Taxon classification Animalia Hymenoptera Encyrtidae

Oobius minusculus Triapitsyn & Petrice sp. n. Figures 11a, 12-16, 25, 31, 33-34

Avetianella sp.: Petrice et al. 2009: 179-180 (egg parasitoid of Agrilus subcinctus in Livingston Co., Michigan, USA).

Type material.

Holotype female [UCRC] on slide (Fig. 11a) with following four labels: "USA: Michigan, Clinton Co., near Bath, 42.812°N, 84.410°W, 255 m, parasitized Agrilus subcinctus Gory eggs collected 12.vii.2013, T.R. Petrice, emerged 16-22.vii.2013 in laboratory (Lansing, MI)", "Mounted by V. V. Berezovskiy 2014 in Canada balsam", [magenta] " Oobius minusculus Triapitsyn & Petrice HOLOTYPE ♀", [database label] "Univ. Calif. Riverside Ent. Res. Museum UCRC ENT 142420". The holotype is in good condition, complete, dissected under 3 coverslips.

Paratypes: USA, Michigan: Clinton Co. (same data as the holotype), 2 ♀ on points [MSUC, UCRC] and 1 ♀, 1 ♂ on slides [UCRC]. Ingham Co., Michigan State University Tree Research Center, 42°40'12"N, 84°28'12"W, 267 m, 14.viii.2014, T. R. Petrice, emerged in laboratory (East Lansing) from parasitized Agrilus egenus Gory eggs on black locust, Robinia pseudoacacia , twigs: emerged 22.viii.2014 [3 ♀ on points, MSUC, UCRC, USNM]; emerged 29.viii.2014 [3 ♀ on points, MSUC, UCRC, USNM, and 1 ♂ on slide, UCRC]; emerged 6.ix.2014 [1 ♀ on point, UCRC]; emerged 17.ix.2014 [1 ♂ on slide, UCRC].

Description.

FEMALE (holotype). Body dark brown to black except scutellum and propodeum brown; scape and pedicel brown, flagellum light brown; legs whitish or pale yellowish with wide brown bands on coxae, femora, and tibiae.

Frontovertex and mesonotum with faint mesh-like or lineolate sculpture [very difficult to see in dry-mounted specimens, best observed in slide-mounted ones (as in Fig. 25)]. Pronotum, mesoscutum, axillae, and scutellum with short, dusky setae; scutellum also with a pair of long, fine setae near posterior margin.

Head (as in Fig. 15, collapsed when air-dried) with ocelli in an obtuse triangle, posterior ocellus a little less than its diameter away from eye margin. Transfacial and inner orbital sutures present. Mandible 3-dentate, the inner tooth with two denticles; maxillary palpus 4-segmented, labial palpus 1-segmented (i.e., palpal formula 4-1).

Antenna (Fig. 12) inserted below lower eye margin. Radicle about 0.3 × total scape length, rest of scape slender, 4.5 × as long as wide, a little wider in the middle, with faint longitudinal sculpture. Pedicel longer than any funicle segment; F1-F5 slightly transverse, F1-F4 subequal in length, F5 a little longer and slightly wider than long; F6 the longest funicle segment, longer than wide; F1-F5 without mps, and F6 with 2 mps. Clava 3-segmented, about 2.3 × as long as wide and almost as long as funicle; first claval segment with 1 mps, second and third segments each with 3 mps.

Mesosoma a little shorter than gaster (Fig. 13). Mesoscutum about 1.7 × as wide as long. Scutellum a little wider than long, a little shorter than mesoscutum; scutellar placoid sensilla closer to the posterior margin of scutellum and close to each other.

Wings (Fig. 31) not abbreviated, forewing extending far beyond apex of gaster. Forewing 2.1 × as long as wide, hyaline; marginal setae very short; disc densely setose, linea calva interrupted posteriorly by an irregular row of setae, filum spinosum present. Hindwing 4.2 × as long as wide, hyaline; longest marginal seta 0.3 × maximum wing width.

Mesotibial spur a little longer than mesobasitarsus.

Ovipositor occupying a little more than 0.5 × length of gaster, exserted markedly beyond gastral apex (by 0.2 × own length) (Fig. 13); ovipositor length:metatibia length ratio 1.2:1. Outer plate of ovipositor with two subapical setae.

Measurements of the holotype (mm, as length or length:width). Body (of the dry-mounted specimen prior to slide-mounting): 0.462; mesosoma: 0.233; gaster: 0.245; ovipositor: 0.173. Antenna: radicle: 0.03; rest of scape: 0.103; pedicel: 0.045; F1: 0.012; F2: 0.012; F3: 0.011 (0.012); F4: 0.012; F5: 0.015; F6: 0.03; clava: 0.103. Forewing: 0.495:0.234; longest marginal seta: 0.021; hindwing: 0.357:0.085; longest marginal seta: 0.025.

Variation (paratypes). Body length 0.43-0.46 mm (dry-mounted specimens from Agrilus subcinctus , Fig. 33) or 0.46-0.53 mm (critical-point dried specimens from Agrilus egenus , Fig. 34). In the latter specimens, legs (except tarsi) are somewhat darker (mostly brown), scape (minus radicle) of the female antenna is about 5.0 × as long as wide, and clava is about 2.5 × as long as wide. Mandibles are identical for specimens reared from both host species, and there is no doubt that they are conspecific. In all specimens, F6 is sometimes slightly paler than other flagellomeres but not contrastingly, still almost concolorous or often concolorous.

MALE (paratype from Agrilus subcinctus ). Head dark brown, mesosoma and gaster dark brown to black except mesoscutum with a brownish tinge, base of gaster whitish; antenna with scape and pedicel brown to dark brown, flagellum light brown. Antenna (Fig. 16) with scape minus radicle 2.9 × as long as wide; F2-F4 more or less subequal in length, F1 and F5 slightly longer, F6 the longest funicle segment; F2-F4 without mps, F1, F5, F6 and clava with mps; flagellar segments with very long setae (slightly longer than each funicle segment’s width); clava entire, 2.6 × as long as wide, a little wider than funicle segments. Mesosoma (Fig. 25) about as long as gaster. Forewing 2.0 × as long as wide, hyaline. Genitalia (Fig. 14) typical for the genus.

Variation (paratypes from Agrilus egenus ). Body length 0.4-0.5 mm (critical-point dried specimens).

Diagnosis.

This species is similar to the European Oobius zahaikevitshi Trjapitzin (Figs 19, 30), whose type locality is Zhuravlivka, Vinnytsia Oblast, Ukraine, where it was reared from eggs of Agrilus viridis (Linnaeus) on Carpinus betulus ( Trjapitzin 1963). Oobius zahaikevitshi was recently well illustrated by Gumovsky et al. (2013). It was recorded from several European countries and Agrilus spp. hosts, listed by Trjapitzin and Volkovitsh (2011) and Noyes (2014). However, we are not absolutely confident that all these records are correct: it is quite possible that they might represent a complex of more than one cryptic species that are difficult to distinguish without supporting molecular data and thorough morphological studies based on good quality slide-mounted specimens. Proportions of funicle segments of the female antenna seem to be somewhat different between the specimens of Oobius zahaikevitshi from Ukraine illustrated by Trjapitzin (1963) and Gumovsky et al. (2013), in which F5 is about as long as wide, and the examined specimens from Volgograd Province of Russia, in which F5 is a little wider than long (Fig. 30).

Oobius minusculus differs from Oobius zahaikevitshi in having the palpal formula 4-1, a relatively smaller F5 of the female antenna and also by F6 being longer than wide and almost concolorous or often concolorous with other flagellomeres (Fig. 12). In contrast, the palpal formula for Oobius zahaikevitshi is 3-1, F5 is relatively larger, and F6 is about as long as wide and contrastingly lighter than other flagellomeres (Fig. 30), as also described and illustrated in Trjapitzin (1963) and Gumovsky et al. (2013).

Oobius minusculus is the only described native Nearctic species of Oobius s. str., as characterised by Noyes (2010) in having the outer plate of the ovipositor being relatively short and apically rounded with paired subapical setae (one long and one short), in which this new taxon fits well. In the key by Trjapitzin and Volkovitsh (2011) to the world species of Oobius (s. str.), it keys to Oobius zahaikevitshi . In Noyes (2010), Oobius minusculus tentatively keys (although it really does not key to any of the included Neotropical species) to the same couplet with Oobius xochipili Noyes and Oobius zagan Noyes from Costa Rica, from both of which it differs by F5 of the female antenna being much less transverse, just slightly wider than long (Fig. 12) whereas in Oobius xochipili and Oobius zagan F5 is anelliform, much wider than long ( Noyes 2010).

Etymology.

The name of this new taxon is an adjective referring to its small size.

Hosts.

Agrilus subcinctus on ash ( Fraxinus spp.) and Agrilus egenus on black locust ( Robinia pseudoacacia ).

Notes on biology.

Originally reported by Petrice et al. (2009) as Avetianella sp. that parasitized Agrilus subcinctus eggs. The second author has never found this parasitoid to overwinter in Agrilus subcinctus eggs. However, collections of Agrilus egenus eggs found overwintering Oobius minusculus larvae in eggs. This species likely attacks other Agrilus spp. in North America, and has multiple generations per year.

Comments.

The following specimens of Oobius zahaikevitshi were examined: Bulgaria, Plovdiv Prov., Klisura, 27. vi– 8.vii.1975, A. Atanasov, from eggs of Agrilus cuprescens ( Ménétriés) on Rosa sp. [1 ♂, BMNH] (det. V. A. Trjapitzin 1977). Russia, Volgograd Prov. ( oblast’), Krasnoarmeyskiy District (rayon), environs of Volgograd, vi.1971, A. M. Makhmadziyoev (Makhmadzieev), from eggs of Agrilus viridis on Acer tataricum [1 ♀, BMNH; 5 ♀, UCRC] (det. V. A. Trjapitzin 1977 and 1975, respectively).