Feeding by Cassida rubiginosa (Coleoptera: Chrysomelidae) and the Effects of Defoliation on Growth of Musk Thistles

Abstract

Plant response studies were conducted to determine the effects of feeding by Cassida rubiginosa Müller and mechanical defoliation on the growth and reproduction of musk thistle (Carduus thoermeri Weinmann). Mortality of thistles was highest with 75% mechanical defoliation at the rosette stage; defoliation of thistles after bolting had little effect on growth and reproduction. Thistle growth was more adversely affected by multiple than single defoliations, particularly when 50% or more of the leaf tissue was removed. Single defoliation produced little reduction in plant growth except at the 75% level. Defoliation by C. rubiginosa produced effects on thistles that were intermediate to those resulting from 50 and 75% multiple mechanical defoliations, even though only 23% of the leaf tissue was removed. Laboratory studies of leaf consumption showed that C. rubiginosa larvae consumed more thistle leaf tissue as they matured and that their feeding rates were temperature dependent.