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Sampling for California oakworm on landscape oaks

C a l i f o r n i a oakworm populations periodically erupt, defoliating both deciduous and evergreen oaks over widespread areas of coastal California. More often this insect occurs at low to only moderate densities and in some years can hardly be found at all in any one locality. Causes of oakworm population fluctuations are not well understood, but population declines have been variously attributed to natural enemies, naturally occurring diseases, starvation, and changes in genetic “quality” of larvae in the outbreak phase. The common name of this insect, California oakworm (Phryganidia californica Packard), was an unfortunate choice, because it invites the opportunity to consider any “worm” that occurs on oak to be termed an oakworm. Several dozen lepidopterous larvae commonly feed on oak, but in most parts of the state only the California oakworm is able to defoliate trees. Nevertheless, many property owners have their oak trees sprayed with insecticides every year on the basis that the “worms” present must be California oakworms. Actually, a tree in a given area probably needs protection against oakworm damage in only two or three consecutive years out of every eight or nine. Satisfactory criteria for judging when an oak needs protection are lacking. Sampling oak shoots from various parts of the tree for larval counts, a procedure designed for use by highway crews responsible for large numbers of trees, requires equipment for sampling foliage high in the crown. Because of the difficulty of sampling the few trees on the average homeowner’s property, oaks may be routinely sprayed without determining the need for treatment. Such practices may result in unnecessary pesticide use. Apart from destroying foliage, the oakworm produces fecal material that drops from infested trees, fouling outdoor furniture, patios, and swimming pools. Most lepidopterous larvae produce rather hard, ovoid to cylindrical fecal pellets whose appearance and shape are often quite specific to insect species, genus, or family, and whose size increases as larvae grow. It occurred to us that timely collections of larval feces on sticky cards placed beneath trees might accurately indicate oakworm activity above and facilitate control decisions. Variations of the proposed method of pellet collection and analysis have been used before, more in Europe than in this country, and primarily for forest insect research. Fecal or frass characteristics are routinely used in identification or detection of wood-infesting insects such as drywood termites and powderpost beetles. The sampling card selected was circular, 13.7 cm (5% inches) in diameter, with backing that could be peeled off to expose its adhesive surface. It was placed in an open 14 cm (5% inches) plastic petri dish with its sticky surface upward. The underside of the dish was attached horizontally to a suction cup affixed to a short metal rod, which was pushed into the ground beneath the oak tree.