Emergence of overwintered larvae of eye-spotted bud moth, Spilonota ocellana (Lepidoptera: Tortricidae) in relation to temperature and apple tree phenology at Summerland, British Columbia

Journal of the Entomological Society of British Columbia, Dec 2004 by McBrien, Heather L, Judd, Gary J R

ABSTRACT

We recorded daily appearance of overwintered larvae of eye-spotted bud moth (ESBM), Spilonota ocellana (Denis & Schiffermüller) in spring 1992, 1994, and 1996 in an unsprayed apple orchard at Summerland, British Columbia, to relate larval emergence to degree-day (DD) accumulation and apple phenology. In all years the first larva was found between mid-March and early April, and none appeared after late April. Median emergence of larvae occurred when McIntosh apple trees were at early, tight-cluster stage of fruit-bud development. Larval head capsule measurements showed that ESBM usually overwinter as fifth and sixth instars, with a small proportion (≤6%) as fourth-instar larvae. In the laboratory we monitored emergence of non-diapausing overwintered larvae from apple branches incubated at 8.8, 9.4, 12.9, 15.0, 18.0, and 20.9 °C. A least-squares linear regression described emergence over this temperature range relatively accurately (r^sup 2^ = 0.57, P

Key Words: Spilonota ocellana, Tortricidae, larval development, phenology, degree days

INTRODUCTION

The eye-spotted bud moth (ESBM), Spilonota ocellana (Denis and Schiffermüller), is a pest of apple (Gilliatt 1932, MacLellan 1978), blueberry (Gillespie 1985), cherry (Oatman et al. 1962), and prune (Madsen and Borden 1949) throughout fruit-growing areas in the northern hemisphere (Weires and Riedl 1991). ESBM is univoltine and larvae overwinter in hibernaculae on branches of host plants. Larvae crawl from hibernaculae in early spring to feed on leaves and blossoms. Pupation occurs in a nest of dead leaves and blossoms held together with silk. Adults emerge in early summer and lay eggs singly on leaves (Weires and Riedl 1991, McBrien and Judd 1998). Summer-generation larvae arising from these eggs often feed on fruit surfaces causing damage and discolouration beneath leaves attached to fruit with silk (Gilliatt 1932).

In North America, ESBM has a history of sporadic outbreaks in apple orchards (MacLellan 1978) because insecticides applied in summer against codling moth, Cydia pomonella (L.) often control it indirectly (Madsen and Downing 1968, British Columbia Ministry of Agriculture, Fisheries and Food 2004). As non-insecticidal methods like sterile male technique (Dyck and Gardiner 1992) or pheromone-based mating disruption (Judd et al. 1996, Judd and Gardiner 2004) have been implemented to control codling moth and leafrollers (Tortricidae) in British Columbia (B.C.), feeding damage by ESBM has increased (GJRJ unpublished data), mirroring reports from The Netherlands (Deventer et al. 1992). Therefore, control of ESBM in spring has become more critical. Insecticides applied in spring are often timed to control leafrollers and green fruit worms (Noctuidae), providing control of ESBM only indirectly (Madsen and Downing 1968, British Columbia Ministry of Agriculture, Fisheries and Food 2004). Therefore, strategies to control ESBM specifically need to be developed.

The ability to predict when overwintered larvae of ESBM appear in spring would be a useful tool in designing an integrated management programme. The phenology of ESBM larval emergence in spring has been related to apple phenology in other areas (Gilliatt 1932, Madsen and Borden 1949, Oatman et al. 1962) but this approach has not been validated in B.C., Canada, and may not provide consistent prediction of emergence on different species of host plant and on different varieties of fruit trees across different years. A temperature-based model to predict emergence of overwintered larvae may be a more useful approach as this technique has been applied successfully against other species of leafrollers in the Pacific Northwest (Brunner 1991). We describe emergence of overwintered larvae of the ESBM in relation to degree-day (DD) accumulations and apple tree phenology.

MATERIALS AND METHODS

Collecting and Handling Prunings. Several hundred 30-cm branch sections were pruned from a mixed block of McIntosh, Delicious, and Spartan apple trees in an experimental apple orchard at the Pacific Agri-Food Research Centre (PARC) in Summerland, B.C. on 2 February 1992. No insecticides were applied to this orchard for at least five years preceding or during this study and it was heavily infested with ESBM larvae in 1991. Prunings consisting mainly of fruit-spur wood and excluding previous years' growth were transported to the laboratory, stored in cardboard boxes filled with moist sawdust, and held in darkness at 0.4 ± 0.5 °C until required.

Diapause Termination. Before assessing temperature-dependent emergence of overwintered larvae, it was important to ensure they had completed diapause so that some portion of diapause development was not included in any estimates of post-diapause development time. Prunings collected on 2 February and 1 March 1992, when apple buds were still dormant, were placed in a controlled-environment chamber at 19 °C under a 13:11 h L:D photo-regime provided by Daylight fluorescent tubes. On each collection date, seventy 30-cm-long prunings were placed in a plastic basin (35 cm × 35 cm × 16 cm), covered with polyester organza and held in place with an elastic band that prevented larva from escaping but permitted air circulation. On 1 March, an equivalent length of pruned branch sections was removed from laboratory cold storage (0.4 °C) and set up identical to other pruning samples. Every 24 h, prunings were removed from their basin and tapped sharply to dislodge active larvae onto a white cloth. The number of larvae collected daily was recorded and sampling was terminated when larvae went undetected for seven consecutive days after they began appearing.