Density dependent prey-feeding time of Stethorus bifidus (Coleoptera : Coccinellidae) on Tetranychus lintearius (Acari : Tetranychidae)

The gorse spider mite, Tetranychus lintearius, a biological control agent i ntroduced into New Zealand to control gorse, is often attacked by the endem ic coccinellid, Stethorus bifidus. Predation by S. bifidus has been suggest ed as a reason why T. lintearius colonies collapse. For S. bifidus predatio n to regulate T, lintearius populations, at least one component of its nume rical or functional response must result in an increased proportion of mite s being killed as mite density increases. Laboratory experiments showed tha t feeding time (a sub-component of the functional response) decreased marke dly with increased T. lintearius density. An increase in available prey den sity from 3-25 mites/177 mm(2) led to an exponential decrease in mean feedi ng time from 870 s to 100 s. Furthermore, despite S. bifidus killing more m ites, it extracted progressively smaller proportions of the contents of eac h mite killed as mite density increased.