Last modified: May 19, 2019, 9:03 p.m.
A local species, restricted to the southern part of Belgium. The first observation for Belgium was in 1999 (LX).
This is a rather small species (12–21 mm) having, as the name refers to, a yellow belly. Also the very narrow external transparent area on the forewing is typical. The abdomen has 3 yellow bands on the abdomen.
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 in the late afternoon although the optimal time span is very weather dependent.
The egg is elliptically shaped and dark brown to black.
The pupa is bright yellow-brown and is very thin-skinned. The 'horn' on the head is very short which makes it fairly easy to distinguish from the pupa of S. formicaeformis that has a well developed upwards oriented bifurcate 'horn'.
The eggs are deposited on new grown twigs of Salix near a leaf bud or on a nearby leaf. It appears that broad-leaved and rather young Salix trees with strong growing twigs are preferred. The just hatched larva bores into the stem and feeds on the sap. It then hibernates as a young larva. During the following summer, the plant reacts on the presence of the larva by building a typically pear-shaped gall.
This gall is not too difficult to find in the following winter or early spring when leaves do not hinder a clear view. The larva, living in a very short gallery just above the gall, pupates in early summer without constructing a real cocoon. As a consequence, the pupa is always positioned head down.
So, the development takes two years and, rather unique, this species occurs only in odd years. There are only a few very rare observations in even years. Surprisingly, this species flies in the United Kingdom only in even years.
This high-summer species has a rather short flight-period, mainly around mid July and almost exclusively in odd years. Strangely, in England it flies in the even years.
This species lives in different species of Salix (willow). It seems to prefer broad-leaved species and rather young, strong growing plants.
S. flaviventris lives in rather humid –and therefore often cooler– biotopes where also the host plant thrives well. It can be found in marches, along brooks and embankments, on waste grounds, ...
Nevertheless, the warmer south facing side of the trees is often preferred.