Laboratory validation of rubidium marking of herbivorous insects and their predators.

A simple and effective marking technique is an invaluable tool for tracking the movement of invertebrates in the field. In order to do this, the technique must be validated in a controlled setting. In the first study, two soft bodied herbivores with different feeding strategies and two of their insect predators, also with different feeding strategies, were marked with rubidium (Rb), an elemental marker previously shown to be effective in marking herbivorous invertebrates. The naturally occurring concentration of Rb was determined for both the herbivores and their predators. Rb concentrations were determined in (1) the herbivore species over several days of feeding on rubidium chloride (RbCl) marked plants; (2) in the predatory species immediately after consuming prey marked at different rates with Rb; and (3) in the predatory species for several days after consuming a single meal of Rb marked prey. The concentration of Rb within the marked herbivores was found to be significantly higher than the naturally occurring concentration after eating RbCl marked plants for to up 8 days. A significantly higher concentration of Rb was found in both predators when fed prey that had been on the RbCl sprayed plants for 2 or 8 days compared with naturally occurring concentrations. There was also a significant difference between the Rb concentrations in the controls and each of days 5, 10 and 15 since feeding in both predators. A second smaller study was carried out to examine the effect of feeding Rb marked prey to spiders. The concentration of Rb in the spiders fed marked prey was significantly higher than that of the controls. This suggests that Rb is an effective way of marking both herbivores and their insect and arachnid predators enabling them to be tracked in the field and to determine their movements across habitats.