In tests of bacteria pathogenic for defoliating insects, previous workers used water suspensions of the microorganisms and contaminated the foliage fed to the insects hy dipping or spraying (1, 2). Where a quantitative result is sought such procedures have some limitations. The waxy cutin present on leaf surfaces, and this is especially true of the foliage of coniferous trees, encourages the formation of surface droplets and makes it extremely difficult to achieve uniform spreading of the bacterial suspension. In addition; the cutin interferes with the adhesion of the dried suspension to the leaf surface. To overcome these difficulties, certain adhesive agents, such as peptones, milk solids, albumin, etc., have been used, but these too are open to objection. If it is desired to reproduce as closely as possible a natural infective process, the use of protein products introduces a new variable and occasionally renders the foliage unpalatable to the insect.