Blastobasis (Blastobasis decolorella (Wollaston))

Blastobasis is a highly damaging, but local, pest of apple. The life cycle involves one main mid summer generation with a very small partial second flight of adults in the autumn and early winter. The caterpillars feed on the flesh of ripening fruit around the stalk or where two or more fruits are touching.

Apple varieties with short stalks, notably Bramley and Egremont Russet, are highly susceptible. Cox is moderately susceptible.

Look out for signs of damage at harvest.  The purplish-brown caterpillars remove large areas of skin and flesh, the wounds tending to weep and are sometimes covered by a sticky mass of black frass.  The caterpillars form shelters for themselves by tying leaves together with silken webbing, often to the surface of a fruit. Sometimes caterpillars penetrate more deeply into the flesh.

Crop losses can be very high, approaching 100%. The damage is sometimes overlooked or misidentified because of failure to distinguish the damage from that caused by tortrix moth caterpillars.

If damage is seen one year, even at a low level, insecticidal treatment to prevent the pest increasing should be applied the next season.

Beech hedges sometimes harbour the pest and can act as a source of infestation. Hand thinning fruits to singles will reduce damage.

Recognition is relatively straightforward.  Adult blastobasis are 9-11 mm long at rest and have pale ochreous yellow forewings each with four darker spots and scattered darker scales.  They occur in June and July, about the same time as codling moth.  To monitor numbers  they can be sampled using the beating method as when dislodged from the vegetation they do not fly but fall onto the beating tray, where they characteristically either lie still or scuttle around on their backs. Beating should be used to determine the flight period.

Control

• There are a number of chemical control options.
• Chlorantraniliprole (Coragen) offers incidental control of blastobasis when applied for codling moth control. For best fruit protection, it should be applied during egg-laying at the ‘ovicide’ timing, before egg-hatch and fruit penetration occurs.
• Pyriproxyfen (Harpun) is approved for use on apples to control codling moth and may offer incidental control of blastobasis. It inhibits egg hatch, metamorphosis of nymphs to adults and reduces the fecundity of adult females. However, as a new product to the UK in 2020, further experience of its use is required to inform growers and agronomists of its efficacy at controlling blastobasis.
• The insecticidal protection should be maintained continuously by spraying a suitable insecticide at 2-3 week intervals from 1 week after the start of the flight period until 2-3 weeks after the end of the flight period, adhering to the maximum number of applications of any insecticide permitted.
• Bacillus thuringiensis has little activity against blastobasis.
• Synthetic pyrethroids are highly effective but their use should be avoided because they are harmful to the orchard predatory mite Typhlodromus pyri.

Insecticides approved for control of codling, tortrix moths or caterpillars on apple and pear that are known or likely to be effective against Blastobasis caterpillars – Download 

Control in organic orchards

Blastobasis is potentially a devastating pest in organic apple orchards and could possibly make organic apple production impossible.

• If the pest starts to occur, cultural control measures, especially hand thinning of fruits to singles should be given priority.
• A programme of sprays of Bacillus thuringiensis may be applied but this approach is likely to have limited efficacy.

Further reading

Blastobasis moth - additional information