Why is it a problem?
Larvae burrow into developing apples, resulting in early fruit drop. Dissection is generally needed to confirm that they are the cause of early fruit drop. This is rarely done, so losses due to this insect are probably underreported.
Where and when is it a problem? It is a serious problem in most high-altitude apple-growing areas (> 2,000 m asl). It is widespread in Bumthang, Haa, Thimphu, and Paro.
Adult moths are tiny (6 mm in length) and can be recognized by the white spot on the head and thorax. Larvae are less than 1 cm long when fully grown, much shorter than other apple-boring caterpillars in Bhutan.
Infested fruits are riddled with tunnels, and the seeds are destroyed. The larval entry point is a tiny pinprick mark, often with a slight depression on the skin of the fruit. Entry points are hardly visible except to the experienced eye, so attacks are frequently overlooked until just before harvest, when it is too late to act. The emergence hole is quite large and conspicuous. Infested fruit can drop prematurely (September to October) and will soon rot.
Confusion with other pests: The small size of the caterpillars (less than 1 cm long) means that they can’t be confused with other apple borer in Bhutan.
Lifecycle: Females lay eggs on developing apples from mid-June to early July. Larvae hatch about two weeks later and immediately burrow into the fruit to feed. Several larvae are usually present in a single infested fruit. Larvae dig tunnels to feed on the seed. Fully grown larvae emerge from the fruit to pupate in the soil or plant debris. Adults emerge the next season. There is only one generation per year.
Dispersal: Adult moths are active flyers. They may spread into apple orchards from their primary hosts, which are frequently found nearby.
When can damage be expected? Damage begins in early July but is frequently discovered only at harvest.
Hosts: Wild trees such as Malus baccata (Tepcho in Dzongkha), Malus sikkimensis (Tipsi) and Pyrus pashi (Leechum or Leepzhay) are probably the primary hosts, with apple only a secondary host.
Fruit losses to the apple fruit borer can often go unnoticed. Monitoring and assessing the cause of early fruit drop is therefore necessary to establish whether it is a problem, and to guide management activity. Community-based apple orchard hygiene is normally sufficient for restricting this pest.
Monitoring of fruit loss (including dissection of dropped fruit to establish the cause of early drop) is essential from July to September to determine the effectiveness of control measures, and whether control efforts need to be increased in the following season.
Effect of variety
- Dropped fruit should be collected on a regular basis and destroyed by burying in a deep pit, or feeding to livestock. This will gradually reduce moth populations over the years, depending on the distribution and abundance of wild hosts locally and the cooperation of neighbors.
- Community participation is critical for this to be effective as the moth can fly long distances.
The use of insecticides is not recommended. Please contact the NPPC during outbreaks
Image acknowledgements: NPPC.