Diagnosis. T. immundella is most likely to be confused with the slightly smaller T. serotinella; small specimens found in localities where the hosts of both species occur can only be identified with certainty by their genitalia. Other similar species, T. beirnei and T. squamatella can be identified by their larger size and number of antennal segments. The smaller T. ortneri is more ochreous and has a hair-pencil on the hindwing (in male). The male genitalia differ from T. serotinella by the wider capsule and valva, the presence of anterior processes of the gnathos and longer sublateral processes.
Male. Wingspan: 6.0-8.2 mm. Head: frontal tuft varying from yellowish white to mixed ferruginous fuscous, sometimes almost completely white; collar paler, yellowish white; scape white, usually with few to several brown scales; antenna with 35-43 segments. Forewing pale ochreous grey to greyish brown, irrorated by presence of white and dark tipped scales; colour very variable, extremes from almost completely white to almost completely brown occur; cilia-line irregular, terminal cilia white. Hindwing: greyish white. Abdomen with yellowish anal tufts. Female. Wingspan: 6.4-8.4 mm. Antenna with 34-40 segments.
Male genitalia. Vinculum anteriorly truncate. Uncus truncate, not widened. Gnathos with broadly triangular central element, ending in a rounded tip; with narrow diverging anterior processes. Valva with margins at base parallel, beyond middle margins suddenly narrow towards slightly curved tip; dorsally excavated over more than half its length at base; transtilla with straight transverse bar. Aedeagus slender, tip bifurcate; with one large straight, spine-like cornutus and two large curved cornuti with large base. Female genitalia. T VIII covered with many scales and several setae; anal papillae each with 18-23 setae; apophyses curved. Vestibulum small, folded. Ductus bursae with folds, long; corpus bursae elongate, covered with minute spines and pectinations; signa long and narrow, margins irregular, often with incomplete cells. Ductus spermathecae with 4½ convolutions.
Host plants: Cytisus scoparius (= Sarothamnus scoparius), in eastern Austria (Hundsheimer Berg) also reared from Chamaecytisus cf. austriacus. Egg: deposited on branch, between ridges. Larva: very long and slender, deeply yellow, hardly visible in mine. Mine : a long almost straight gallery, appearing black, at first going downwards, often passing under the ridges and changing direction; the mine later fills the space between two ridges completely. The larva is difficult or not to be seen in the mine. A plant with many mines can often be recognized from a distance by the black stained branches. Cocoon: ochreous brown.
Widespread in Denmark. In Sweden local in the south (Sk. and Vg.). Not in Finland and Norway. - Widespread in Ireland, Britain, France, Netherlands, Belgium, West and East Germany, West and southern Poland and Switzerland. Also known from Spain, Portugal, Teneriffe, Italy and Austria. In southern Europe several undescribed species with similar external features occur.
Voltinism: univoltine with a long period of larval feeding. Larvae start feeding from September onwards. Full-fed larvae can be found from October until April or May; in mild winters most have left their mines by January, but frost can extend the period of larval feeding considerably. Adults on the wing from June until mid September, in southern Europe sometimes later (October). Life-history and larva were described in detail by Sich (1917).
Description based on van Nieukerken and Johansson (1990)