SOME SPECIAL FEATURES OF THE LEAF-ROLLING TECHNIQUE OF BYCTISCUS POPULI L. (COLEOPTERA, RHYNCHITINI)

by

A. DAANJE1) (With 4 Figures) (Rec. xs-VI-1974)

I. INTRODUCTION

In my paper on the life-cycle and the leaf-rolling technique of Deporaus betulae L. (DAANjE, 1964) I compared the behaviour of this species with that of Byctiscus populi. Some special features of the leaf-rolling technique of the latter species were hardly dealt with there, especially the rolling of more than one leaf in a roll. The females of Byctiscus populi generally construct rolls of single leaves, in which the eggs are deposited and the larvae develop. In a few cases, however, more than one leaf is used (DnarrJE, 1964; VON LENGERKEN, 1954), which necessitates a kind of behaviour different from the rolling of a single leaf; this special kind of behaviour was not described up till now. In this respect it would have been important to make a comparison with the leaf-rolling behaviour of Byctiscus betulae L., because in the latter species rolls are generally constructed from more than one leaf and only exceptionally from a single leaf. However, my observa- tions on this species are still very scanty, because it its sparsely distributed in my area of observation and, moreover, hardly reproduces in captivity.

II. THE ACTIONS BY WHICH THE FEMALE GENERALLY CONSTRUCTS A ROLL FROM A SINGLE LEAF As in the leaf-rolling weevils Deporaus betulae L., Deporaus tristis F., Byctiscus betulae L., Attelabus nitens Scop. and Apoderus coryli L. the female of Byctiscus populi constructs leaf rolls in which the eggs are de- posited. The inner coils of the rolls serve as food for the developing larvae. - Leaves of poplars are used exclusively; in my area of observation the - south-eastern part of the province of Noordbrabant these are mainly

i) Author's address: Petrus Dondersstraat 37, Eindhoven, Netherlands. 286 leaves of aspen, Populus tremula L. and to a much lesser extent leaves of white poplar, Populus alba L. and of canadian poplar, Populus canadensis Moersch. When a female becomes motivated to roll, a young poplar leaf is searched for. After having reached a leaf over the petiole, one of the edges is fol- lowed about half-way or slightly further, and then a little is eaten from the upperside of the leaf. As in Deporaus betulae, by eating from it, the suit- ability of the leaf for rolling is tested. Leaves of inadequate species, or too aged leaves of the above mentioned poplar species, are left immediately after eating, after which the same procedure generally begins on another leaf. When the leaf is accepted the beetle returns to the petiole, in general over a secondary nerve and the main nerve, but sometimes along the edge. Only the last part of the walk to the petiole is walked in the normal manner. - During the first part, each time that the weevil has moved three legs as in walking these are the front and hind leg at one side and the middle leg at the other side - the tarsus-claws are driven into the leaf, and then all six legs are firmly drawn up. Meanwhile, it often bites into the leaf with the mandibles. Because these actions result in the appearance of small holes in the epidermis and upper cells of the leaf - also when no biting occurs - I called this behaviour "perforating". The very same way of perforating is shown by females of Byctiscus betulae, Deporaus tristis and Deporaus betulae; in the latter species, however, it is never accompanied by biting. Being returned to the petiole, close to the leaf, the beetle straddles the petiole and clings firmly to it with its legs, with the head turned away from the leaf. Then with the mandibles it bores a hole in the petiole, and this to such an extent that only a thin skin is left of it. After having hollowed out the petiole the beetle moves to one of the leaf- halves and starts perforating near the edge. Perforating can occur both on the under- and upperside, but also on only one side. Moreover, on that half of the leaf, the female perforates the thickest secondary nerve with the mandibles; in leaves of poplars this is one of the nerves that arise from the base. Generally, this nerve is treated either on the upper- or on the under- side. The dotted area in Fig. la broadly shows the perforated area on the leaf. Perforating is not carried out according to fixed curves as in Deporaus betulae. Because of the frequent perforating, the turgor decreases more quickly in that part than in the remainder of the leaf, by which it is made flexible and more easy to wrap up.