Quantifying time-mortality relationships for nuclear polyhedrosis viruses when survivors are present

The time-mortality relationship for nuclear polyhedrosis viruses (NPVs) is 1 of the more important factors to be considered in the development of NPVs as biological control agents. This factor has become of particular interes t in recent years with the advent of NPVs that have been genetically modifi ed to kill insects faster. However, much information published on time-mort ality relationships is quantified in such a way that the results may be mis leading. In quantifying time-mortality relationships for NPVs, many researc hers calculate median lethal times (LT(50)s), or median survival times (ST( 50)s), using probit or similar analyses. However, some researchers include insects that survive the treatment in their calculations, whereas other res earchers do not include survivors. Simulated data are presented to show tha t when survivors are included, treatment effects on LT(50)s and ST(50)s are inherently influenced by any treatment effects on percent mortality. Examp les of real data from bioassays of the NW of the celery looper, Anagrapha f alcifera (Kirby), against the corn earworm, Helicoverpa zea (Boddie), and t he beet armyworm, Spodoptera exigua (Hubner), that illustrate the same patt ern are also presented. Rationale is presented that if survivors are simply not infected, data on them represents no useful information on the time-mo rtality relationship, and so survivors should generally be excluded from ca lculation of LT(50)s or ST(50)s, as well as from alternative methods of qua ntifying this relationship. Data on percent mortality data should be presen ted as well to allow full evaluation of the experiments.