Aprosthema tardum - Pea Twinhorn
One of three very similar Aprosthema species found in the British Isles. The only confirmed records are from the New Forest from 1907. However, there remain further specimens of Aprosthema that need to be redetermined since Vikberg (2004) established new identification criteria for the genus (Musgrove, 2022). The separation of the three species has been subject to much study and conjecture over the years. This species is bivoltine and shows seasonal dimorphism adding to the confusion. Head shape differs in that the summer generation has a short head with strongly narrowing temples and longer flagellae, whereas the overwintering spring generation has a longer head with parallel or posteriorly broadening temples and shorter flagellae. Vikberg postulates that the head shape corresponds to the muscle requirements needed for emergence from the cocoon. The summer spun cocoon is weakly formed from soft plant material, whereas the overwintering cocoon (spring generation) is stronger and constructed underground.
Aprosthema tardum can be distinguished from Aprosthema fusicorne and melanurum by less erect and more flattened or lateral directed hairs on the mesoscutellum.
Larvae feed on Broad-leaved Everlasting-pea.
Size: Approx. 5 - 7mm.
IUCN Status: Data Deficient
GB Rarity Status: Unclear
Flight period: Bivoltine, May to August
Plant associations: Lathyrus latifolius. Broad-leaved Everlasting-pea (Liston et al, 2014)
Benson, R.B., 1952. Handbooks for the Identification of British Insects. Hymenoptera, Symphyta, Vol 6, Section 2(a-c), Royal Entomological Society, London
Liston, A.D., Kan, P. and Kan-van Limburg Stirum, B., 2018. The life-history of Aprosthema tardum (Klug, 1814)(Hymenoptera, Tenthredinoidea, Argidae). Beiträge zur Entomologie, 68(1), pp.97-106.
Liston A, Knight G, Sheppard D, Broad G, Livermore L (2014) Checklist of British and Irish Hymenoptera - Sawflies, ‘Symphyta’. Biodiversity Data Journal 2: e1168. https://doi.org/10.3897/BDJ.2.e1168
Musgrove, A.J. 2022. A review of the status of sawflies of Great Britain - Phase 1: families other than Tenthredinidae. Natural England, unpublished
Vikberg, V. (2004). Seasonal head dimorphism and taxonomy of some European species of Aprosthema (Hymenoptera: Symphyta: Argidae). Beiträge zur Entomologie= Contributions to Entomology, 54(1), 107-125