Aprosthema tardum (Klug, 1814) is a sawfly that belongs to the family Argidae (Hymenoptera: Tenthredinoidea). Until 2018 little was known about its biology. Our research of the biology of Aprosthema tardum began with the discovery of an, at the time, undescribed ichneumonid wasp of the genus Thibetoides (Thibetoides aprosthemae Shaw, 2018), emerging from a summer cocoon made by an Aprosthema larva in 2014.
This Aprosthema has three to four generations, from mid-March to mid-August. Aprosthema tardum hibernates in a winter cocoon. The Aprothema female lays eggs between the two cell layers in the leaf of its only hostplant Lathyrus latifolius (and possibly the closely related L. sylvestris). The larvae emerge after about 8 days and feed only on the leaves and stems of the host plant. Development takes between 14 and 16 days during which the larvae moult four times. The stages between moults are called instars. Males make their cocoons when around 12-14 mm, about 1 to 2 days after their last moult, while females feed for another 3 to 4 days, until they are about 16 mm.Larvae hatched from the eggs of the first generation sawflies mostly make summer cocoons, from which females or males emerge after about 10 days. Once emerged, Aprosthema tardum males fly directly to the canopy. Since mating has never been observed in rearings and males have hardly been seen in the wild, we assume that mating takes place in a kind of Master tree where males and females meet, as is known from various butterflies such as the brown hairstreak (Thecla betulae). Adult Aprosthema tardum have never been observed nectaring on flowers. The larvae of the second and third generation mainly make winter cocoons.
Although the larvae are predated by many different insect species, such as wasps of the genus Polistes and various predatory bugs, carnivorous ant species are one of the main predators of Aprosthema tardum larvae; but Formica fusca ants are not aggressive towards them. When Formica fusca is dominant, Aprosthema tardum has a better chance of survival because Formica fusca protects its territory from other ant species that would attack the larvae.
Aprosthema tardum larvae are parasitized by at least four parasitoids from the family Ichneumonidae of which two are endoparasitoids in the subfamily Ctenoppelmatinae: Lathrolestes erythrocephalus (Gravenhorst, 1829) and Ischyrocnemis goesi Holmgren, 1858. These endoparasitoids lay internal eggs in the body of the host larva. The other two are ectoparasitoids in the subfamily Tryphoninae: Thibetoides aprosthemae Shaw, 2018 and quadrum Perkins, 1962. These ectoparasitoids lay external eggs on the body of the host larva. The host larva can continue its development after attack (parasitoids allowing that are termed koinobionts). Once a cocoon is made by the host larva, the parasitoid larva kills its host in order to accomplish its own development and makes a second cocoon inside the host cocoon.
PDF - The life-history of Aprosthema tardum (Klug, 1814) (Hymenoptera, Tenthredinoidea, Argidae)
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PDF - A new species and first rearing record of the genus Thibetoides Davis (Hymenoptera: Ichneumonidae, Tryphoninae) from France
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Shaw, M.R., Kan, P., Kan-van Limburg Stirum, B. & Wahl, DB. (2022) - Biological and morphological studies on the parasitoids (Hymenoptera, Ichneumonidae) of Aprosthema tardum (Klug) (Hymenoptera, Argidae, Sterictiphorinae) in Var, southern France. Journal of Hymenoptera Research.
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