Obtainment of specimens

This study was carried out in the Sierra Fría of Aguascalientes, Mexico, in the municipality of San José de Gracia, where plant communities of oak and pine-oak are principally found (Figure 1) (^{Siqueiros, 1989}; ^{de la Cerda, 1999}). The collection of insects was made in stands with Pinus leiophylla Schl. et. Cham. and Pinus teocote chl. Et Cham., which presented light or moderate infestations of D. mexicanus. These two pine species are the most abundant in the Sierra Fría of Aguascalientes (^{Siqueiros, 1989}).

Figure 1 Location of the study area in the Sierra Fría, municipality of San José de Gracia, Aguascalientes, México, where plant communities of oak and oak-pine grow. 

The first specimens of Siricidae were captured in December of 2005, in a 12-unit Lindgren trap, baited with Frontalin + alfa-pinene (ChemTica Int., San José, Costa Rica), installed for monitoring D. mexicanus. A second specimen was captured in September of 2012 in a Lindgren trap baited with Frontalin + blend of monoterpenes (Synergy, Canada) close to a stand of P. leiophylla infested with D. mexicanus, that had received a sanitation cutting for the control of the bark beetle. The specimens collected after September of 2012 were obtained during the revision of trees recently invaded by D. mexicanus, which allowed to define the association between the woodwasp and the bark beetle attack. The recent specimens of Siricidae were obtained through direct capture while they were ovipositing in standing pines currently infested by D. mexicanus, in a tree recently felled as part of a control measure, and from a tree killed by the invasion of the beetle, where the emergence of a female was observed. Of the latter, a chainsaw was used to cut trunk sections 30 to 40 cm long, which were brought to the Forest and Agricultural Health Laboratory of the Pabellón Experimental Field, INIFAP. The trunk sections were placed in entomological cages in which two specimens of Siricidae and three parasitoids emerged.

At each collection site, the date, geographic coordinates (with a Magellan eXplorist XL GPS unit, utilizing the Datum WGS84), altitude, hour of capture and host species were registered. Photographs were taken of females depositing eggs and of a specimen in the process of emergence. The behavior of the females was observed during the oviposition process and the diameter and height of the hosts were measured. After capture, the specimens were preserved in alcohol at 70 % and were later dry mounted and placed in the Reference Collections.

Identification and preservation of specimens

The taxonomic determination of the specimens of Siricidae was made using the taxonomic keys and illustrations of ^{Schiff et al. (2012)}. The identification process was based on observation with a stereoscopic microscope of the external morphological characteristics of adult females: minimum distance between the eyes, size and abundance of punctuations in the gena, mesoscutum and mesoscutellum, form and size of the second metatarsomere, coloration and veining of the fore wing, form of tergum 10 (cornus) and especially the form and size of the pits of the ovipositor in the middle section, where the basal and apical parts converge, given that each species of Sirex of the western hemisphere presents an ovipositor with pits of particular size and shape, characteristic which is key for the determination of the species (^{Slippers et al., 2012}; ^{Gouler, 2012}; ^{Schiff et al., 2012}).

Eleven voucher specimens were deposited in the Forest Insect Collection of the Pabellón Experimental Field of INIFAP (km. 32.5 of the Aguascalientes-Zacatecas highway, in Pabellón de Arteaga). The specimens were labeled and the taxonomic and biological information was documented according to the protocol of the structure of data for scientific collections contained in the Sistema Biótica 5.0 (^{CONABIO, 2007}). In addition, other three specimens were deposited in the U.S. National Museum of Natural History (Smithsonian Institution, Washington, D.C.). Five specimens were destroyed in the process of identification.

Description of the species

The female of S. obesus is a wasp whose body, legs and antennae are black, with dark blue metallic reflections (Figure 2A and B). It has dark tinted fore wings, without the presence of the 3A vein. Body length is variable. The wasps in our study measured between 1.3 and 2.8 cm. This variation in body size within the species is common in the genus Sirex (^{Middlekauff, 1960}). In the head, the gena had relatively large pits; dense, with diameters between 0.3 and 0.5 times the size of the lateral ocelli; separated between 0.0 and 0.5 times their diameter (Figure 2 C). In the thorax, the mesoscutum and mesoscutellum presented relatively large and dense pits (Figure 2 D). The ovipositor was comprised of a dorsal lance and two ventral lancets, which presented various annuli which were barely perceptible, but delimited by conspicuous pits (Figure 2 E and F). In repose, the ovipositor was covered by two sheaths, which opened laterally to allow it to come out at the moment of oviposition (Figure 2 B, E, and F). Pits in the middle annuli about the base of apical section of sheath, were semi-triangular in shape, and measured nearly 0.35 times the length of an annulus (^{Schiff et al., 2012}) (Figure 2 F). Metatarsomere 2, distinctive of the species, was short and robust, approximately 1.5 times as long as wide in lateral view and of equal or shorter length as the sum of metatarsomeres 3 + 4 (Figure 2 G and H).

Figure 2 Morphological characteristics of the female of Sirex obesus, present in the Sierra Fría of Aguascalientes, Mexico; A) body in dorsal view; B) body in lateral view; C) head in lateral view; D) thorax in dorsal view; E) terminal part of the abdomen showing the ovipositor and the sheaths that cover it in repose; F) close up of the ovipositor in the middle part, showing the characteristics of the annuli and pits of the lancet; G) metatarsus in ventral view, with emphasis on the characteristics of metatarsomere 2. 

The female of S. obesus is similar to that of S. californicus, but is differentiated by its short and robust metatarsomere 2, given that the metatarsomere of S. californicus in lateral view is more than twice as long as wide, and its length is larger than the sum of metatasomeres 3 + 4. Furthermore, the pad of metatarsomere 2 of S. obesus had a length of at least 0.6 times the ventral length of the tarsomere (Figure 2 G), and in S. californicus, the pad of metatarsomere 2 has a length of 0.5 times the ventral length of the tarsomere. Another marked difference occurs in the punctuations of the gena, which in S. californicus are smaller and not as dense, with a separation among them greater than twice the size of their diameter (^{Schiff et al., 2012}).

The observations of the female of S. obesus showed that it is also similar to the females of Sirex nigricornis Fabricius and Sirex nitidus (T.W. Harris), but it is differentiated from the former because it presents a metatarsomere 2 of greater length, and they surpass the sum of the length of metatarsomeres 3 + 4, and from the latter because the wings of this species present the vein 3A. A detailed description of the external morphology of the Sirex species of the western hemisphere is presented by ^{Schiff et al. (2012)}.

Hosts

In the Sierra Fría of Aguascalientes it was confirmed that S. obesus utilizes adult trees of P. leiophylla and P. teocote as hosts, with diameter at breast height (DBH) between 18.0 and 45.0 cm and height between 8.0 and 12.0 m. These data comprise new records, as previously only Pinus ponderosa Dougl. Ex. Law had been identified as host of S. obesus (^{Schiff et al., 2012}).

Other pine species genetically close to the hosts grow near the study area, such as P. chihuahuana Engelm. and P. durangenis Mtz. which can also be potential hosts of S. obesus; and it will be necessary to prove this premise.

Biological observations

The adults of S. obesus emerge from October to December and present daytime habits. On October 2 of 2012, at coordinates 22º 10' 57" N and 102º 38' 21" W, at 2637 masl, at 12:03 h two females of S. obesus were observed depositing eggs on the bark of a tree of P. leiophylla recently invaded by D. mexicanus (Figure 3 A) and a female depositing eggs on the stem of a recently felled tree of P. leiophylla, with still green foliage (3B). The oviposition was observed at 3.0 and 5.0 m height on the bark.

Figure 3 Female of Sirex obesus under natural conditions in the Sierra Fría of Aguascalientes, Mexico. A) depositing eggs in the stem of a tree of Pinus leiophylla attacked by the bark beetle Dendroctonus mexicanus; B) incrusting the ovipositor (in a position perpendicular to the body) in a section of Pinus leiophylla; notice the small reddish particles of sawdust that fall on the abdomen of the insect during the process of perforating the bark; C) female recently emerged from a tree of Pinus teocote, killed by the attack of Dendroctonus mexicanus; note the exit hole of the insect. 

In another site (22º 11' 43" N, 102º 38' 09" W, at 2708 m altitude), on October 2 and 19 of 2012, females of S. obesus were observed depositing eggs on P teocote. Oviposition occurred between 12:10 and 14:00 h, on the bark of the tree, at a height between 1.0 and 1.20 m above the ground level, and separation between females of 25.0 cm. In this last site, on November 13 of 2012, at 12:50 h, the emergence of an adult of S. obesus was also observed from a tree of P teocote killed by D. mexicanus (Figure 3 C). The tree presented abundant circular orifices on the bark from 1.20 m height above the ground level. Samples of stem sections were taken from this tree and brought to the Forest and Agricultural Health Laboratory of the Pabellón Experimental Field, where two females of S. obesus emerged, one on November 13 and the other on November 15 of 2012.

Natural enemies associated with Sirex obesus

From the same sections of P. teocote from which adults of S. obesus emerged, three specimens of Ibalia leucospoides Norton (Hymenoptera: Ibaliidae) emerged: one on November 12 and two on November 20, 2012. According to ^{Pietrantuono et al. (2012)} and ^{Barnes et al. (2014)}, I. leucospoides is a parasitoid of various species of Siricidae, and in Australia it was imported for the biological control of S. noctilio. Thus, its presence in Aguascalientes as a natural control agent of S. obesus could be relevant.