The severity of infestation of each apple orchard by apple grass aphid should be determined when the pre-blossom pest assessment is done at the green cluster to pink bud growth stage.
- Inspect at least two blossom trusses on each of at least 25 trees per orchard for infestation by the aphid.
- Presence of the aphids in a particular truss is often indicated by slight leaf distortion and/or the presence of cast skins on the surface of the rosette leaves.
- An insecticide treatment for the aphid is justified if the economic threshold of 50% of blossom trusses infested with 5 or more aphids is exceeded.
- Counts of the wingless aphids on apple trees in late October give a good indication of the likely infestation the following spring.
- An average of one aphid or fewer per leaf from samples of 20 leaves examined on 8-10 trees across the orchard indicates a potentially light spring infestation.
Forecasting
Useful forecasting models have not been developed for apple grass aphid. However, the pest is usually most abundant when there has been plentiful rainfall in summer and autumn the previous year to encourage growth of grass, the aphid’s summer host.