Why is it a problem? Caterpillars burrow into developing apples, resulting in early fruit fall and lost production. It is difficult to manage.
Where and when is it a problem? It can be a serious problem in most high altitude apple growing areas (> 2,000 m asl). 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 is probably under-reported. One study found it to be the major cause of fruit drop in Haa and Thimphu, and serious losses have also been observed in Bumthang and Paro.
Adult moths are tiny (6 mm in length) and can be recognised by the white spot on head and thorax. Caterpillars are less than 1 cm long when fully grown, much shorter than other apple-boring caterpillars in Bhutan.
Infested fruit are riddled with tunnels and the seeds are destroyed. The larval entry point is a tiny pin prick 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-boring caterpillars 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. Usually several larvae are present in one infested fruit. Larvae tunnel 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 fliers. They may disperse into apple orchards from their primary hosts which often grow nearby.
When can damage be expected? Damage occurs from early July, but often is only observed 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.
Losses of fruit to the apple fruit borer can often go unnoticed. Monitoring of the extent and 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.
Use of insecticides is not recommended. Please contact the NPPC if outbreaks occur that are considered unmanageable.
Image acknowledgements: NPPC.