Last modified: Sept. 13, 2019, 5:40 p.m.
S. vespiformis is a not so rare species throughout Belgium and, in certain places, it can be straight out very numerous.
S. vespiformis is a medium-sized species (15–26 mm) that can easily be recognised by the clearly defined yellow bands on the abdomen, the large transparent areas on the wings, the bright red discoidal spot on the forewing and the strikingly hairy yellow hind-legs with a sharply defined black band. There is also a striking sexual dimorphism: the anal tuft is predominantly black in the male while it is almost entirely yellow in the female. The proboscis is, as in all species of this genus, well developed and functional.
The males are very well attracted to the pheromone developed for this species and are best lured from early afternoon till late in the evening but the optimal time span is very weather dependent.
The caterpillar constructs irregular galleries between wood and bark –mostly into the bark and not into the wood– and often at the transition zone between dead and living tissue. Where many larvae occur together their galleries often run into each other.
The eggs are deposited in crevices of the bark on cancerous swellings or near damaged parts of the trunk of the hostplant. In the case of felled –but still living– trees, the females can oviposit a large amount of eggs between the exposed transition zone between wood and bark along the entire circumference of the tree stump. The newly hatched caterpillars bore into the tree and make irregular galleries. After a while, no longer used corridors are stuffed with frass that, in most cases, turns black and becomes moist and mouldy during the hibernation of the larva. In ideal circumstances this species seems to be able to complete the complete life-cycle in one year but in most cases the caterpillar overwinters a second time. Often, small and substantially larger caterpillars can be found at the same time although they can both develop to imago the same year, most likely explaining the remarkably long flight period of this species.
Eventually, the caterpillars construct an exit gallery to the surface of the bark leaving a thin layer of wood as a separation with the outside world. Pupation takes place in a cocoon made from gnawed plant particles spun together and with a silken lining at the inside. On hatching, the pupa works its way out through the cocoon and remaining thin bark layer.
The adults have a strikingly long flight period that extends from mid-May till well into September.
The larva bores into the stem or branches of mainly Quercus robur but also Quercus rubra and other Quercus species as well as a variety of deciduous trees such as Fagus, Salix, Populus, Prunus, Castanea sativa, ... are accepted as hostplant. Even coniferous trees as Juniperus communis and Abies alba are mentioned as occasional hosts.
S. vespiformis prefers forest(edges) with predominantly Quercus spp. but also trees along a road, in parks or even solitary ones can be infested.