BEETLE

López, J. Manuel Perilla & Johnson, Paul J., 2016, The Ergot Beetle, Acylomus pugetanusCasey (Coleoptera: Phalacridae), is Parasitized by a Species ofMesopolobusWestwood (Hymenoptera: Pteromalidae), The Coleopterists Bulletin 70 (2), pp. 275-278 : 276-277

THE ERGOT BEETLE , ACYLOMUS PUGETANUS CASEY ( COLEOPTERA : PHALACRIDAE ), IS PARASITIZED BY A SPECIES OF MESOPOLOBUS WESTWOOD ( HYMENOPTERA : PTEROMALIDAE )

J. MANUEL PERILLA LÓPEZ AND PAUL J. JOHNSON Insect Biodiversity Lab, South Dakota State University Brookings, SD 57007, U.S.A.

Parasitoids of Acylomus pugetanus Casey ( Coleoptera : Phalacridae ) remain unreported (Lambert and McIlveen 1976; Steiner 1984, 2002; Steiner and Singh 1987; M. Gimmel in litt.) until now. Parasitoids of other phalacrids were reviewed by Steiner (1984). Larvae, pupae, and adults of A. pugetanus were found in the sclerotia of rye ergot ( Claviceps purpurea (Fr.) Tut. , Clavicipitaceae ) on the inflorescences of bromegrass, Bromus inermis Leyss. , quackgrass, Elymus repens (L.), and western wheatgrass, Pascopyrum smithii Rydb. (all Poaceae ), in eastern South Dakota. Here, we report a parasitoid association of a possibly undescribed species of Mesopolobus Westwood , near M. prasinus Walker ( Hymenoptera : Pteromalidae ), with the larva and pupa of A. pugetanus .

During studies on the grass hosts and parasitoid associations of Stenodiplosis Reuter ( Diptera : Cecidomyiidae ) species, rearings of the introduced Stenodiplosis bromicola Marikovskj and Agafonova from B. inermis were made from season-long sequential samples. Stenodiplosis bromicola is an introduced gall midge that is specific to the invasive B. inermis (Gagné and Jaschhof 2014; Perilla López et al. 2015a, b). Samples of B. inermis inflorescences were taken from late May through early August 2014 and 2015 in eastern South Dakota. Rearing trials included mature B. inermis inflorescences with developing and mature rye ergot sclerotia. Inflorescences were held in clear plastic, zip-style bags hung from cords and kept at 22–24°C. Individual florets were periodically dissected to observe insect occurrence and development.

The incidence of S. bromicola on B. inermis decreased through late June and July as the inflorescences of each grass species matured. Acylomus pugetanus increasingly became abundant in proportion to the increased incidence of rye ergot, possibly due to overlapping generations indicated by simultaneous occurrence of multiple life stages and instars. Larvae and adults of A. pugetanus were observed feeding on and within sclerotia with mycelial and cortical tissues consumed. From our rearings, A. pugetanus and M. near prasinus ( Figs. 1–2 ) emerged only from sclerotia-bearing inflorescences collected in July and August. Emergence of M. near prasinus from A. pugetanus developing in sclerotia on P. smithii confirmed the association of this wasp with the beetle. The wasp is likely a larval-pupal parasitoid of A. pugetanus and appears endoparasitic based on the inflated mummy and pupal color change from cream-white color to tan preceding the presence of presumed emergence holes in the abdomen of the host pupa ( Fig. 1 ). Mesopolobus prasinus was previously reared from quackgrass and incorrectly associated with a frit fly ( Diptera : Chloropidae ) by Rosen (1966). Other insects that found in the inflorescences with rye ergot were undetermined aphids and thrips, neither of which are a known host of any Mesopolobus species nor have the body size to support the larva of this wasp.

Both Lambert and McIlveen (1976) and Steiner and Singh (1987) reported A. pugetanus from a species of Claviceps Tulasne , probably rye ergot, from B. inermis and a variety of introduced grain crops and forage grasses. The fungus likely arrived in North America with introductions of rye ( Secale cereale L.) and other Eurasian grain and forage crops during the 1600s or earlier. Bromus inermis is an invasive weed outside of explicit agricultural use, out-competes native grassland plants (Otfinowski et al. 2007), and is a reservoir for C. purpurea in agricultural areas. To date, insect vectoring of the ergot fungus is not demonstrated for A. pugetanus , but was by the handsome little fly, Minettia lupulina (F.) ( Lauxaniidae ) (Lemon 1992). We did not rear this fly or other potential vector, suggesting a different permutation of multitrophic dynamics involving grass hosts, ergot, ergot beetle, and parasitoid occurrences than previously reported.

Acylomus pugetanus adults were also observed on water foxtail, Alopecurus geniculatus L., and barnyard grass, Dactylis glomerata L. (both Poaceae ). However, sclerotia of C. purpurea were not present on these grasses at the time of sampling or otherwise observed. The beetles had fitted themselves between maturing florets of each grass and appeared to be feeding on a shiny material, possibly carbohydrate rich “dry honeydew” from early C. purpurea conidial infection (Mower and Hancock 1975).

The associations noted here form a curious community of mostly introduced and sometimes invasive species. Bromus inermis , S. bromicola and its primary parasitoid Tetrastichus bromi Kostjukov (Perilla López et al. 2015a, b), and C. purpurea and most of its known grain and forage hosts are not native to North America. This pattern suggests the possibility that the Mesopolobus sp. may also be introduced and, if so, may possibly be an undescribed species associated with B. inermis by Rosen (1966). However, there is yet no evidence that A. pugetanus is introduced (W. Steiner, personal communication; M. Gimmel, in litt.).

Figures

Figs. 1–2. Acylomus pugetanus and Mesopolobus near prasinus. 1) A. pugetanus pupal mummy showing parasitoid emergence hole (arrow) on abdominal dorsum; 2) M. near prasinus female reared from an A. pugetanus pupa.