Further results of an investigation into the resistance of basket willows to button gall formation

Summary. 1. Twelve commercial varieties of S. triandra have been proved, under experimental conditions, to be very susceptible to attack by the button top midge (R. heterobia). 2. Three varieties of S. purpurea, one variety of S. viminalis, three hybrids of S. viminalis and purpurea and S. alba var. vitellina have proved to be totally immune. Notwithstanding eggs sometimes being laid and the larvae hatching, no galls or side-branching have occurred. 3. From field observations a closely allied gall midge, R. terminalis, is known to attack S. alba var. vitellina, but has not been known to attack any variety of S. triadra. 4. Reference has been made to a similar phenomenon with regard to attacks on willows by the Chrysomelid beetles; viz. to Galerucella lineola which appears to be confined to varieties of S. triandra; and to Phyllodecta vitellinac which is known to attack S. alba var. vitellina and S. purpurea and its varieties, while it is not known to attack any variety of S. triandra. 5. It is suggested that hybridisation of S. triandra?S. purpurea or ?S. viminalis or ?S. alba should be attempted. Mention is made of three aspects of host plant, preference needing special attention in connection with willow gall midges. In conclusion it is desired to express the indebtedness the writer owes to Mr Hutchinson, who was responsible for the choice of the varieties of willows to be tested in the various trial beds and the supply of sets. Dr Walton, of the Long Ashton Research Station and Mr Kearns of Bristol University have taken part in several very helpful conversations. Thanks are also due to Dr Imms for the generous aid he has given throughout the period of the investigations.