Persistence of a commercial codling moth granulovirus product on apple fruit and foliage

Journal of the Entomological Society of British Columbia, Dec 2004 by Cossentine, J E, Jensen, L B M

ABSTRACT

Codling moth, Cydia pomonella (L.), larval bioassays were carried out on apples and leaves collected from trees treated with the commercially available codling moth granulovirus, Virosoft CP4®, to estimate the persistence of the product over time. The virus had a significant effect on survival of laboratory derived codling moth larvae placed on apples collected up to five and eight days post-treatment. Larvae died with virus symptoms after feeding on treated foliage and the leaf bioassay was easier to count than the apple bioassay. A combination assay, exposing larvae to leaf discs and fruit may more accurately account for potential exposure of wild neonate codling moth to virus in treated orchards. The addition of fish, soybean or mineral oils to Virosoft CP4® treatments did not significantly increase the efficacy or persistence of the viral insecticide on apples in this study.

Key Words: Virosoft CP4®, leaf discs, Cydia pomonella

INTRODUCTION

The codling moth granulovirus (CpGV) (Baculoviridae) is found in wild and colonized codling moth (Cydia pomonella (L.), Lepidoptera: Tortricidae) (Tanada 1964; Eastwell et al. 1999) which is a major pest of apples and pears throughout most of the temperate world (Cross et al. 1999). CpGV is noted for its high virulence when ingested by this host, particularly in the neonate stage (Sheppard and Stairs 1976; Tanada and Leutenegger 1968). Commercial formulations of the virus have been registered for use against the codling moth in Europe since 1988 and in the U.S.A. since 1995. In 2000, Virosoft CP4®, produced by BioTepp Inc., Quebec, became registered for use on apples and pears in Canada.

Commercial formulations of CpGV require application of aqueous suspensions of the virus onto the apples and foliage of treated trees. There is a relatively short time when a wild codling moth can be effectively exposed to CpGV treatment in an orchard. Most wild codling moth eggs are oviposited on leaves (Jackson 1979). Neonates move over leaf surfaces before finding a fruit and chewing through the surface where they remain, feed and develop through to the last instar. Ballard et al. (2000) found that CpGV was ingested by codling moth neonates browsing on CpGV treated leaf surfaces, therefore there is the potential for codling moth neonates to encounter and ingest lethal levels of CpGV on both treated foliage and fruit surfaces.

Orchard trials of various commercial preparations of CpGV have generally shown good early suppression of neonate codling moth (Glen and Payne 1984; Jaques et al. 1987). However, like other viral insecticides, the CpGV is susceptible to inactivation and dilution due to temperature, exposure to sunlight, and precipitation (Jaques 1975, 1985; Glen and Payne 1984). CpGV has been found to be 50% inactivated in 2-8 d on apples (Glen and Payne 1984; Jaques et al. 1987; Arthurs and Lacey 2004).

The goal of this study was to determine the persistence of Virosoft CP4® on apples and foliage under the orchard conditions in the interior of British Columbia (BC), Canada. We also evaluated whether various oils, added to the spray mixture, could increase or extend the duration of the Virosoft CP4®'s efficacy under orchard conditions, by possibly improving coverage and increasing the penetration of the virus through the leaf surface.

MATERIALS AND METHODS

All treatments were applied to a high density (Im tree spacing within rows) orchard of Macintosh apple trees at the Pacific Agri-Food Research Centre, Summerland, BC.

Virulence of Virosoft CP4 over time on apples and leaves. Two blocks of five rows of high density apple trees were treated with Virosoft CP4® at 239 ml/ha (original preparation: 4x10^sup 13^ occlusion bodies/946.34 ml) on 12 June, 2003 using an air-blast sprayer set to deliver a volume of 2,347.6 L/ha. The Virosoft CP4® was stored refrigerated. An untreated block of trees separated by ≥ 30 trees in the same orchard was used as the control. Two hours after application of the virus, 10 leaves and apples were randomly collected from the treatment and the control. Similar collections were made 1, 4, 6, 8, and 12 days after application of the treatments. The entire trial was replicated on different blocks of trees within the same orchard on 3 July, 2003. Mean ± SE daily temperatures during the orchard collection (12-24 June: 17.6 ± 3.0 °C; 3-15 July: 22.5 ± 2.3 °C) and the accumulated daily rain that occurred during the June (0.6 ±1.1 mm/d) and July replications (0 ± 0 mm/d) were similar to daily temperatures (18.8 ± 4.1 °C) and precipitation (0.7 ± 2.3 mm/day) for this area, from 1 June to 15 July, averaged over a 5 y period (1999 - 2004) (Anon. 2004).

Codling moth neonates used in the assays emerged on the same day that the leaves and apples were collected, from egg sheets obtained from the colony of the Okanagan-Kootenay Sterile Insect Release Program (Osoyoos, BC). Five neonate codling moth were placed on the stem end of each collected apple. The apples were sealed in plastic cups and incubated at 24 °C, 16:8 h L:D before codling moth survival was assessed by cutting open the fruit on days 7 and 14. The total number of live larvae in the apple bioassays was a more accurate assessment of the impact of the virus than percentage mortality as larvae occasionally could not be found. Some neonates naturally move off the apple and die by dessication without feeding in apple assays (Laing and Jaques 1980); others disintegrate beyond recognition due to a viral infection or are difficult to locate.