EFFECT OF MECHANICAL PROCESSING AND LONG-TERM STORAGE ON BIOLOGICAL ACTIVITY OF VIRTUSS

Abstract

The effect of long-term storage on the activity of biological control products, particularly those containing nuclear polyhedrosis viruses (NPVs), has not been extensively studied. Most investigations involving viral insecticides have dealt with their thermal stability (Stuermer and Bullock 1968; Dulmage and Burgerjohn 1977; McLeod et al. 1977; Ignoffo and Couch 1981). Freeze-dried and air-dried powdered preparations of gypsy moth, Lymantria dispar L., NPV show little reduction in potency after storage at 4°C for 10 months and 3 months, respectively. Consequently, it was recommended that gypsy moth NPV be stored in either state at 4°C to maintain acceptable activity (Lewis and Rollinson 1978). TM BioControl-1, the viral product for control of the Douglas-fir tussock moth, Orgyia pseudotsugata (McDunnough), registered and used in the United States, has been shown to have a shelf life of 5 years if stored in a cool, dry place (Martignoni 1978). Virtuss is the NPV product manufactured in Canada, in the whitemarked tussock moth, O. leucostigma (J.E. Smith), since 1975 by the Forest Pest Management Institute, Forestry Canada, Sault Ste. Marie, Ont., for the control of the Douglas-fir tussock moth. It is routinely stored at 4°C in anticipation of tussock moth outbreaks and has been successfully used in the control of outbreaks of Douglas-fir tussock moth in Canada (Otvos et al. 1987). If these, or any other viral products are to be successful commercially, more information is required on the effect of storage on viral potency. Similarly, little information is known of the cumulative effects of the mechanical processes of lyophilization and milling on the biological activity of viral products. This note reports on the effects of mechanical processing and storage on Virtuss. Our results show that, although there is no immediate effect of mechanical processing, there can be substantial loss of biological activity of this product after 2 years of storage.