Phytophagous mite performance on intact plants and leaf discs with different defence levels

Plant quality for herbivores is usually evaluated with molecular and physiological techniques and with herbivore performance bioassays. These performance assays are usually done either on intact plants or on detached plant parts, such as leaves, leaflets, and leaf discs. It is thus far unclear how performance experiments on intact plants and on plant parts compare. Plant quality may change in detached plant parts because of the mechanical wounding inflicted, which may induce plant defences. Moreover, plant defences are often systemic, i.e., involving the entire plant, and may not be fully expressed in leaf discs. We investigated the effects of plant quality on the performance of a phytophagous mite on intact plants and on leaf discs. Plant quality was manipulated by infesting intact tomato plants (Solanum lycopersicum L., Solanaceae) either with a strain of the spider mite Tetranychus urticae Koch (Acari: Tetranychidae) that induces plant defences, or with Tetranychus evansi Baker & Pritchard, which suppresses defences, and using uninfested tomato plants as control. We subsequently evaluated the oviposition rate and survival of individual females of T. evansi on intact plants and on leaf discs as a measure of herbivore performance. The mites produced fewer eggs on detached leaf discs than on intact plants. However, the oviposition rate of T. evansi was lower on plants infested with the inducer T. urticae than on plants infested with the suppressor T. evansi, and than on uninfested plants, both on leaf discs and on intact plants. We conclude that both intact plants and leaf discs can be used to evaluate the effects of plant quality on performance of this herbivore.