Spring sprays
Rosy apple aphid populations should be carefully monitored in spring at the green cluster to pink-bud growth stage and again during and after blossom.
The rosy apple aphid is easier to control shortly after eggs have hatched and before colonies develop at the green cluster growth stage. It is very difficult to control later when colonies have developed which are protected in curled, mature leaves.
- A spray of an approved aphicide should be applied as soon as infestation is detected.
- If only rosy apple aphid is to be controlled, then flonicamid (Mainman) is likely to be a good choice as it is a selective aphicide.
- A full approval for spirotetramat (Batavia) on apples for the control of sucking insect pests will control rosy apple aphid. It must be used after flowering and works best when pests are moving from brown wood to green tissue. It will prevent population build-up but does not offer pest ‘knockdown’.
- A recent EAMU for Flipper (fatty acids) has increased the available options should growers wish to reserve other insecticides for conrol of pests later in the season. It is known to complement the use of Batavia as it provides a quick ‘knockdown’.
- The neonicotinoid acetamiprid (Gazelle) is also effective against rosy apple aphid. Its activity against other apple pests has not been explored sufficiently widely although it is known to control mussel scale very effectively when applied at the correct time for the pest, at 90% crawler emergence.
- Earwigs are important natural enemies.
Note that these materials are largely ineffective against winter and tortrix moth caterpillars.
- The synthetic pyrethroid insecticide deltamethrin (Decis) is also approved for control of aphids on apple but its use should be avoided as it is harmful to predatory mites and other insects.
It is important to apply the above insecticides in warm weather conditions at the full recommended dose and in a sufficient spray volume to give adequate cover.
It is also important to apply the insecticide early, before large colonies form which are difficult to control once surrounded by distorted mature leaves.
Autumn sprays
Rosy apple aphid can readily be controlled in the autumn by one or two sprays of an aphicide in October to control males and the females (oviparae) before egg laying commences.
The advantages or spraying at this time are that the aphids, which are present on the undersides of the leaves, do not cause leaf curling at this time and are more readily directly intercepted by sprays. Another advantage is that if insecticides are used at this time, they are unlikely to result in residues on fruits.
The main disadvantage with this approach is that it is difficult to determine whether or not sprays are necessary. This decision would require careful inspection of the trees in early October to quantify the numbers of oviparae on the undersides of leaves.
- Rosy apple aphid oviparae are generally green in colour and are difficult to distinguish from oviparae of the apple grass aphid, Rhopalosiphum insertum, which can be very numerous. Distinguishing the species requires expert entomological examination.
- Thresholds have not been determined, though it has been observed that very heavy infestations of rosy apple aphid can result the following spring if 10% of shoot leaves are infested in the autumn.
- The outcome is, in effect, that autumn treatment has to be applied as a routine.
- If only one autumn spray is applied, it may not completely preclude the need for treatment the following spring.
- However, when autumn sprays are not used, the need for routine treatments for rosy apple aphid in spring, often with more than one spray, is an important consideration.
Insecticide resistance
Resistance of rosy apple aphid to conventional insecticides has been shown to occur and is a significant problem in Italy and Switzerland and is probably widespread in all apple producing regions of southern/central Europe.
- Resistance has not been shown to occur in the UK but no testing has been done. It is possible that the increased problems of control that growers have faced in the UK in recent years are due to resistance or a loss in sensitivity.
- However, an equally likely alternative explanation is that poorer control can be attributed to the loss of all systemic OP insecticides (e.g. chlorpyrifos) for use in orchards.
- The chance of resistance developing should be reduced by avoiding repeated use of the same insecticides and by treatment only when necessary.