The Plant Pest Diagnostic Clinic (PPDC) received a stem sample of common lilac (Syringa vulgaris) in April 2024. The homeowner removed the lilac because it was dying and would not survive. The curious homeowner split open one of the stems and found several black pupal cases and two wood-boring larvae. The larvae appeared to belong to the family Cerambycidae.
Borer larvae inside the lilac stem (left) and pupal cases of the wasp (right)
The homeowner sent the PPDC the lilac stem, the larvae, and the pupal cases. At that time, it was unclear what the pupal cases were, but due to the shape of the head and mouth parts, they looked like wasps in the superfamily Chalcidoidea. Rearing of the two larvae was unsuccessful because they arrived at the PPDC attached to sticky tape and died shortly thereafter. All the pupal cases the homeowner collected were also empty.
The lilac stem was kept inside a sealed chamber to observe any additional activity or insect emergence. As time passed, several wood dust piles accumulated under the stem.
Several weeks later, dozens of tiny wasps emerged and then died after a few days. Two weeks after the emergence of the wasps, two longhorned beetles (Coleoptera: Cerambycidae) emerged from the stem.
1. Lilac stems with dust piles where the parasitic wasp emerged 2. Emergence of two longhorn beetles
The wasps were identified as belonging to the genus Horismenus (Chalcidoidea: Eulophidae), and the beetles were identified as spined oak borer, Elaphidion mucronatum (Coleoptera: Cerambycidae).
Spined oak borer - Elaphidion mucronatum (Say, 1824), (Coleoptera: Cerambycidae)
This cerambycid beetle is a major wood-boring insect of dead or dying oaks. The beetle is about 1.2 to 2 cm long. It is brownish-yellow with mottled dark spots on the elytra and pronotum. Antennae are longer than their body, sometimes twice their body length, like most cerambycids. The apical end of the meso and meta femora have a relatively long spine, and the third, fourth, and fifth antennal segments have long spines, accounting for the species name.
mucro = ‘a sharp point’; natus = ‘arise or born by nature’ in Latin.
There are 59 species of Elaphidion reported in the western hemisphere. The majority of these species (47 species) are reported in the Antilles Archipelago (West Indies includes the Antilles and the Lucayan Archipelago) (Josef and Silva, 2021), while 8 species of Elaphidion are recorded from the United States.
E. mucronatum is widespread in the eastern part of the United States, ranging from Minnesota to Texas and also reported from Ontario, Canada. Host trees of this species are not limited to oak, but include mostly dead or dying eastern hardwood and shrub trees. Adults do not live long. After mating, the female lays eggs on the crevices in the bark. After the larvae hatch, they spend the first year feeding in the sapwood and then feed deeper in the tree during their second year of development.
1. Just emerged spined oak borer adult from the lilac stem and its pupation chamber 2. Adult spined oak borer
The wasp - Horismenus Walker 1843 (Chalcidoidea: Eulophidae)
Horismenus is a parasitoid genus (Chalcidoidea: Eulophidae). The genus can be identified based on the characteristics of the propodium and with assistance from a chalcid parasitoid expert.
Four hundred species of Horismenus are reported worldwide, of which the majority are distributed in tropical Central and South America with few species occurring in temperate North America.
There are 15 species of Horismenus reported in North America. Two species of Horismenus are associated with ants.
1. Pupal cases of Horismenus wasp. 2-3. Horismenus wasp adults. 4. Characteristic identification features of the genus, Horismenus - propodeum with broad, smooth, raised median strip joined by sunken channels.
The genus Horismenus has a wide host range and has been reported to parasitize vegetable crop pests, mantid oothecas, beetle larvae, leaf minors, and egg sacks of spiders. These Horismenus species have been reported developing either as primary parasitoids on the above host organisms or as secondary parasitoids in braconid wasps.
After the lilac stem was carefully split open, more pupal cases of the wasps, three active cerambycid larvae, a parasitized egg, and one parasitized cerambycid larvae were found. We believe these active cerambycid larvae belong to spined oak borer and are left inside the stem until emergence. One spined oak borer adult also emerged from its pupal case. Among the stem debris, a few remains of beetle larval exuviae along with remains of wasp pupal cases were also found, indicating the parasitic activity of the wasp.
Management and control of spined oak borer
Management and control of spined oak borer is not necessary since it is not a pest of economic importance. However, Miller et. al., (2017) indicated that this species is one of the most polyphagous cerambycids in North America, and the species can be destructive in some cases as it has been reported damaging rustic furniture and other finished wood products. In addition, E. mucronatum has the potential to be established in other regions of the world because the larvae are readily exported accidentally to other continents along with timber and wood products.
Adult E. mucronatum are known to be attractive to fermented baits, ethanol, or odors from freshly cut host trees.
Field bioassay data conducted in Portland, Oregon found that the male-produced aggregation sex pheromone attracts females significantly. Also, this chemical can be synthetically produced in several straightforward steps, which could lead to the development of a lure for the beetle in the future, if necessary (Miller et. al., 2017).
References
Bezark, Larry G., 2024. Checklist of the Oxypeltidae, Vesperidae, Disteniidae and Cerambycidae, (Coleoptera) of the Western Hemisphere 2024 Edition (updated through 31 December 2023)
Eulophid genus key: genus list & links - http://www.faculty.ucr.edu/~heraty/Eulophidae/genus_list.html
How to recognize a Eulophid - http://www.faculty.ucr.edu/~heraty/Eulophidae/recognize.html
How to ID Horismenus - http://www.faculty.ucr.edu/~heraty/Eulophidae/Horismenus_page.html
Josef, Vlasak; Antonio Santos Silva. 2021. A new species of Elaphidion Audinet-Serville from the Antilles (Coleoptera, Cerambycidae, Cerambycinae, Elaphidiini).. Faunitaxys, 2021, 9 (34), pp.1-6. ffhal03453175f. https://hal.science/hal-03453175/document
Kenyon SG, Buerki S, Hansson C, Alvarez N, Benrey B. 2015. Uncovering Cryptic Parasitoid Diversity in Horismenus (Chalcidoidea, Eulophidae). PLoS One. 2015 Sep 9;10(9):e0136063. doi: 10.1371/journal.pone.0136063 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4564207/
Lingafelter, Steven W., 2007. Illustrated Key to the Longhorned Woodboring Beetles of the Eastern United States. Systematic Entomology Laboratory, Plant Sciences Institute, Agriculture Research Service, USDA. Published by: ©2007 The Coleopterists Society.
Millar, Jocelyn G., Robert F. Mitchell, Linnea R. Meier, Todd D. Johnson, Judith A. Mongold-Diers & Lawrence M. Hanks, 2017. (2E,6Z,9Z)-2,6,9-Pentadecatrienal as a Male-Produced Aggregation-Sex Pheromone of the Cerambycid Beetle Elaphidion mucronatum. Published online: 18 November 2017 # Springer Science+Business Media, LLC, part of Springer Nature 2017. https://pubmed.ncbi.nlm.nih.gov/29151153/
Universal Chalcidoidea Database - https://www.nhm.ac.uk/our-science/data/chalcidoids/introduction.html
Yanega, Douglas, 1996. Northeastern Longhorn Beetles (Coleoptera: Cerambycidae). Illinois Natural History Survey, October 1996, Manual 6.
