Last modified: May 1, 2019, 10:39 p.m.
Very common throughout Belgium.
S. apiformis is, together with S. bembeciformis, the largest clearwing species in Belgium having a wingspan ranging from 30 mm to 50 mm. Females are in general considerably larger than males. It resembles a big hornet (Vespa crabro) really well. Both yellow 'epaulettes' on the thorax are very striking and allow to distinguish this species easily from S. bembeciformis, its closest relative. The proboscis is strongly reduced and non-functional.
The males are very well attracted to the pheromone developed for this species and are best lured in the morning hours till noon. It's a day-active moth.
The female deposits the reddish brown eggs on the lower part of the trunk of the hostplant (mostly poplar trees).
When heavily infested, the base of the tree-trunc can show numerous emergence holes since these remain visible for years. Except for very old ones, these exit holes are sharply delineated and have a diameter of about 1 cm. Most of these holes are located in the lower 20 cm of the trunk.
The mines always descend into the root system of the tree, making the exit hole the highest point of their gallery-system.
The cocoon, constructed of wood-particles spun together, is thick and strong and located lower than the exit hole. This is an important difference with S. bembeciformis, its closest relative, where the cocoon is positioned higher than the exit hole and the pupa is oriented head down.
After hatching of the imago, the empty cocoon remains present inside the trunk behind the emergence hole for a rather long time after the imago hatched.
Eggs are laid on the bark of the tree-trunk. Young larvae subsequently bore into the tree and migrate towards the root. This species hibernates two or three times as a larva. Before the last wintering, the larva prepares the future exit hole which is absolutely invisible form the outside. Also the cocoon is already constructed in autumn, just underneath the thin layer of bark that conceals the exit hole and since it is positioned lower than the prepared exit hole, the pupa inside is always oriented head up.
On hatching, the pupa breaks the thin bark cover and works its way out till about half of the pupa is visible. The imago then hatches, unfolds the wings and crawls up the tree-trunk.
Most of the time, females already start luring males from that moment on.
The adults have been observed from mid May till mid August. They are often observed sitting on the tree-trunks in early morning sunshine.
This clearwing occurs everywhere where poplar trees are growing. Typical habitats are poplar lanes, river banks, parks, ...
The only condition is that the base of the trunk may not be overgrown with other plants e.g. long grasses, nettles, blackberries,... obstructing free access of sunlight and -warmth to the tree-base.