Incorporation of Rubidium into Pin Oak and Gypsy Moth (Lepidoptera: Lymantriidae)

Abstract

The feasibility of marking gypsy moths, Lymantria dispar (L.), with rubidium was assessed in all life stages and in frass and pupal exuviae. Rubidium, as rubidium chloride (RbCl), was injected into pin oak, Quercus palustris Muenchhausen, at three doses (100, 200, and 500 g RbCl/tree) and one control dose (0 g RbCl/tree), and moths were reared on foliage from injected trees. All samples except eggs were collected in all treatments. Eggs were only collected in the 500- and 0-g treatments. Rubidium concentrations (ppm dry wt) in leaves and all gypsy moth life stages and products, and rubidium contents (µg Rb+/sample) in all life stages, were directly proportional to the dose injected into the tree. In postlarval life stages, sex influenced rubidium levels. The slope of rubidium content as a function of injected dose was significantly different between sexes in pupae, pupal exuviae, and adults. Accounting for dry weight (i.e., using ppm rubidium as the dependent variable) removed the difference in slope in pupae, but not in pupal exuviae and adults. This resulted from a sexual dimorphism in the proportion of total rubidium retained by the adult at ecdysis. Sex also influenced rubidium concentrations in pupal exuviae and adults reared on control trees. Eggs from the highest injected dose contained rubidium concentrations three orders of magnitude higher than controls. More than 82% of the gypsy moths reared on foliage from the 100-g RbCl treatment, and more than 95% of those moths reared on foliage from the 200- and 500-g treatments, carried the rubidium mark. Thus, the rubidium-enrichment technique should prove useful in studies of gypsy moth dispersal.