Codling moth granulovirus
Now that the codling moth granulovirus is available and approved for use in the UK, it should be used wherever possible, bearing in mind the following limitations:
- It only controls codling moth and not tortrix moths, Blastobasis or other pests.
- For Madex Top, a maximum of ten sprays each giving 8-14 days protection is allowed per season (a maximum of six sprays per generation)
- Application should be made just before egg hatch is expected to ensure larvae pick up virus particles as they move from the egg to the apple entry point. Application a little bit too early is preferable to a little bit too late.
- Heavy attacks may result in some superficial sting injury as the young larvae hatching from eggs do not die immediately.
Key aspects of the virus are:
- The virus is highly selective and virulent. In orchards, only codling moth can be infected. A single virus particle is sufficient to kill a first instar codling moth larva. The virus is totally safe to man, plants and the environment, though the formulants included in the product may be harmful. No ‘pesticide residues’ occur on fruits at harvest.
- The virus has to be ingested by the newly hatched larva when feeding on the skin of the apple before it penetrates the flesh. The larva continues feeding for a few days before the virus acts. This results in small, shallow, larval feeding holes in the surface of the fruit, known as ‘sting’ injury. Although sting injury is superficial, it can result in downgrading of fruit to a lower quality class. This injury is the main disadvantage of the virus.
- The virus is sensitive to UV light and high temperatures which limit its persistence. In hot/sunny weather, protection lasts about 8 sunny days. In cooler, cloudy weather protection lasts up to 14 days.
- A programme of sprays of the virus at 8-14 day intervals, starting from the onset of egg hatch (see ‘Chemical control’) and continuing until hatch is complete is usually required, remembering that there is a maximum of ten sprays per annum.
- The virus is compatible with most fungicides. Higher volume sprays are considered to be more effective as they offer improved crop coverage. A feeding stimulant, such as molasses, sugar or milk powder may be added to the spray solution to increase the amount and rate of larval feeding and hence improve the efficacy of the treatment.
- First generation larvae that overwinter can carry over sub-lethal infections of the virus to the following season, so the granulovirus can also be considered as a multiple season population management tool.
- Users of the granulovirus against the second generation of codling moth in late July or August as opposed to using insecticides will reduce the likelihood of pesticide residues occurring on fruits at harvest. However, in the UK, the second generation larvae do not usually complete their development, so mature larvae and pupae carrying sub-lethal infection will not be carried over to the next season.
- Using granulovirus products to target the second generation close to harvest also risks superficial ‘sting’ damage which affects the grade-out of the apples where high populations of the pest are experienced.
Bacteria
Sprays of Bacillus thuringiensis (Bt) at the egg-hatch period may give a limited degree of control of codling moth. However, the newly hatched larvae only feed for a short time before boring into the fruit and do not usually ingest an adequate dose.
Nematodes
A spray of the entomopathogenic nematode Steinernema carpocapse applied to the trunk and main branches in September or early October can give over 80% control of cocooning larvae.
- However, it is vitally important that the weather conditions are suitable at the time of application and after application.
- The air temperature must be above 14 °C and the surface of bark must remain wet for at least 24 hours during and after application..
- The spray should be applied at high volume. Nematodes are exempt from pesticide registration requirements, but they must be a native strain or a strain licensed for release in the UK.
Parasitic wasps
Egg parasites (Trichogramma sp.) are available from biological suppliers and can be introduced but efficacy is poor even if huge numbers are introduced making the method uneconomic.