Initial test of a semiartificial diet for the thistle biocontrol beetle, Cassida rubiginosa

Abstract

The thistle biocontrol beetle, Cassida rubiginosa is established in New Zealand, but often not sufficiently abundant to achieve control of the weed, Cirsium arvense (Californian thistle). Mass production of the beetle could enhance biocontrol efforts through supplemental and inundative releases. We carried out an initial test of a semiartificial diet (containing host plant material) designed for laboratory mass production of the beetle. Larval survival rates were tested on diets with three different concentrations of preservatives (full, half, and no preservative), and compared to a positive control (leaf disc of Cirsium arvense), and a negative control (water). Only larvae on the leaf disc developed to the adult stage. Of the diets, the longest survival time was on the full preservative diet, with a mean mortality time of 8.8 ± 0.6 days, and a maximum survival time of 21 days. Although no larvae completed development on the diets, some important progress was achieved: (1) Neonate larvae were mobile on the diet; (2) larvae fed on the diet; and (3) there was adequate control of microbial contamination without being acutely toxic to the larvae. Further development of a diet for Cassida rubiginosa should focus on nutritional components for larval development.