Why is it a problem? The woolly aphid is a very conspicuous pest on trees of all sizes, but is mostly a problem on small trees on which it can stunt growth. Galls can also cause splitting and secondary infections, and root galls can affect nutrient uptake. Although it is often identified by farmers as a pest of serious concern, infestations are generally not serious enough to require management.
Where and when is it a problem? It can cause problems in all apple growing regions. Infestations in Bhutan don’t appear to be sufficient to cause nodules, except in roots.
Adults are purplish brown and about 2-3 mm long. Colonies are very characteristic, appearing as white woolly masses due to the covering of sticky, white wool. Colonies range in size from 2 mm to 6 cm.
Galls are most common on the roots, but in severe infestations can also be seen on branches. Galls can then split, providing entry points for disease. Honeydew (sugary excreta) can get infected with sooty mould, fouling fruit. Larger colonies weaken and stunt the growth of small trees. They can also reduce the formation of flower buds, and subsequent fruit set in the following year.
Confusion with other pests: No other apple pests make white, woolly colonies.
Lifecycle: Woolly aphids overwinter in cracks, loose bark and on exposed roots. They become active in March-April and breeding colonies are established under the white sticky wool. Aphids occur on all parts of the plant (twigs, branches to exposed roots). Part of the population remain on roots through the year.
Dispersal: A few winged aphids are produced in June-July which may infest other trees, but most natural spread is by young wingless nymphs which crawl or are blown from tree to tree. Strict orchard hygiene is therefore important in limiting the unintentional spread of this pest.
When can damage be expected? Pest populations generally build up relatively slowly.
Hosts: In Bhutan woolly aphids have only been reported on apples. Elsewhere it utilises many other hosts, but is only known as a serious pest of apples.
This is mainly a pest of younger trees. The use of insecticide cover sprays should never be necessary provided active monitoring and non-chemical management activities are undertaken.
Woolly aphid is a highly visible pest. Monitoring is needed, especially of young trees which are most susceptible.
Effect of variety
- For mild infestations, and infestations that can be reached by hand, aphids can be rubbed off with cloth soaked in a bucket of water and kerosene mix (or just water).
- For large scale mechanical control, colonies can be removed from the tree with the powerful jet from a power-spray filled with plain water.
- Suckers should be removed as they provide shelter for aphids.
- Ensure good orchard hygiene practices are undertaken to minimise the spread of colonies within and between trees.
- Application of TSO in February for San Jose scale will also provide some control of woolly aphids.
- The wasp parasitoid mali could be used as a biological control agent. It has not been reported from Bhutan.
- With the exception of treating infected seedlings, chemical spray is only needed in the case of severe infestations, which should not be occurring if monitoring and non-chemical management recommendation are being undertaken. While spraying, care should be taken to thoroughly soak the colonies as they are well protected by wax nurseries.
- Chemical treatment of infected seedlings (where most serious damage occurs from gall formation in roots) is recommended prior to planting to prevent further spread of this insect. Roots of these seedlings should be dipped in a solution of Chlorpyifos before planting (4 ml per 1 litre of water).
- The use of pesticide cover sprays should only be used in the most extreme cases where non-chemical treatments are insufficient. The ideal time to spray is November, after trees have lost their leaves (allowing the chemical to reach the colonies) but before the insect descends to the roots for winter. Use of the systemic insecticide (dimethoate) will give the best results (2 ml per 1 litre of water), but it can only be applied after harvest has finished.
- To help penetration of the pesticide add a small amount (about a teaspoon per 10 litres) of detergent or soap to the chemical and mix in very well. Spray very thoroughly. Spray pressure should be increased so colonies can be well targeted.
- Contact insecticide (Chlorpyrifos at 4 ml in 1 litre of water or Cypermethrin at 1 ml in a liters of water) is needed if spraying when fruits are on the tree. It needs to be applied at least 7 days before harvest.
Image acknowledgements: NPPC.