Rosy apple aphid – additional information
- Eggs, laid in bark crevices on smaller branches and spurs and at the bases of buds, hatch in early spring before bloom.
- The aphids feed on buds and outer rosette leaves.
- They increase asexually, giving birth to live young. Numbers increase rapidly in May and June and large colonies may form.
- Winged forms are produced and these disperse to the aphid’s summer host plant, plantains.
- Breeding continues on apple into August, even after winged forms have developed if new growth is available on the trees.
- In the early autumn, winged forms develop on plantains and disperse back to apple.
- The individuals that return first deposit nymphs on apple that develop into females.
- Males then disperse back from plantain to apple and mate with the females which lay eggs on the bark that overwinter.
- An important key pest of apple which can be difficult to control with insecticides.
- The severity of attack varies considerably from year to year.
- A similar species, the pear bedstraw aphid (Dysaphis pyri), is an important pest of pear.
- Apple and related Malus sp. are the only winter host.
- Plantains, Plantago sp., are the summer host.
There are considerable differences in the susceptibility of different apple varieties to rosy apple aphid, but the relative susceptibility has not been adequately characterised.
- Highly susceptible varieties include Bramley, Discovery, Egremont Russet, Golden Delicious and Jonagold.
- Most other commercially grown varieties are moderately susceptible.
- Some new varieties with the Vf scab gene are reputed to be partially resistant.
Widespread and common throughout Europe.
- First signs of damage are in early spring when outer rosette leaves of infested trusses show a slight yellowish discoloration before curling.
- As larger colonies develop, infested leaves are severely curled downward and distorted.
- They sometimes turn yellowish brown, but never red (red distorted leaves are caused by the rosy leaf curling aphid).
- Damaged fruits are often small, malformed with uneven, wrinkled and sometimes waxy skin and often develop a rosy colour.
- Sometimes, larger damaged fruits have sunken darker areas.
Shiny black, and found on the bark of apple trees, mainly on older wood.
Visual distinction between the eggs of different aphid species is impossible.
Pink to dark bluish grey, with a mealy powdering of white wax.
Other pests with which the rosy apple aphid may be confused
Rosy leaf curling aphid
Very similar in appearance to the rosy apple aphid, but damaged shoots turn a distinct bright red colour.
Pear bedstraw aphid
Only occurs on pear and not on apple. Rosy apple aphid does not occur on pear.
- Orchards should be inspected regularly, preferably fortnightly, from the green cluster growth stage (April) through to mid-summer (end of June) for signs of infestation and or damage.
- Critical growth stages are late green cluster (so that a pre-blossom aphicide may be applied) and during blossom when populations are often increasing rapidly.
- On Bramley, where flowering is often very protracted when frosts occur, pre-blossom treatment is especially important as it can be several weeks before another opportunity to spray while the crop is not in flower.
- Examine at least 25 trees per orchard for signs of damage.
- The threshold for treatment increases as the season progresses but presence of the aphid at the green cluster stage, even on only one of the 25 trees inspected, is enough to warrant treatment.
- Rosy apple aphid populations are particularly damaging between bloom and early fruitlet development when fruit cell division is occurring.
- Infestations that occur after this time (more than a month after full bloom) are less damaging.
Work in Switzerland showed that winter eggs of rosy apple aphid hatch between 100 and 180 degree days, 50% emergence being reached at 130 degree-days above a thermal threshold of 4.5 °C.
- Temperature sums are started from 1 January. Rosy apple aphid eggs hatch a week or so after those of apple grass aphid.
- Rosy apple aphid begins to reproduce approximately 230 degree-days after emergence.
- This suggests that the optimum period for pre-bloom population monitoring for control measures extends from 180 (end of hatching) to 230 (beginning of reproduction) degree-days.
- These developments can be used for forecasting, but predictions have not been validated in the UK.
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.
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.
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.
Organic growers are sometimes forced to physically destroy colonies to prevent them spreading.
- This is sometimes done when hand-thinning fruitlets.
- This is very labour intensive and small colonies are often missed which flare up subsequently.
Fostering natural enemies
The main cultural control approach is to foster populations of natural enemies, especially predators (see below). This can be done in several ways.
- Tolerating less harmful aphid species such as the apple grass aphid attracts predators in early spring.
- Flowering plants (e.g. corn marigold, corn camomile and mayweed) can be established in or around the orchard to provide alternative food sources, mainly nectar and pollen, for adult hover flies.
- These may then lay their eggs in aphid colonies.
- Ground herbage under the tree may also become infested with other aphid species (e.g. grasses can become infested with bird-cherry oat aphid) which can provide an alternate food source for aphid predators (e.g. ladybird adults and larvae) and parasites.
- Artificial refuges should be used to foster earwigs and other natural enemies such as lacewings.
- Ideally, a refuge should be provided in each tree. This may simply be some extra lengths of hollow tree tie round the stake.
- In orchards with high tree densities, it is likely to be impractical to provide more elaborate refuges such as half of a plastic drinks bottle containing a roll of corrugated cardboard.
A wide range of predators and parasites are natural enemies of rosy apple aphid. However, when weather and tree growth conditions are favourable in spring, population increase by the aphid is too rapid for natural enemies to prevent population increase and damage. Natural enemies have a greater effect when plant growth slows or ceases in summer.
Predation of aphids and their eggs in the autumn may be more significant as the number of overwintering eggs may be substantially reduced resulting in a smaller number of colonies developing in spring. This may be the reason why more colonies often develop in spring in insecticide treated than untreated orchards.
Predatory insects and spiders
- A wide range of predatory insects prey on rosy apple aphid including earwigs, predatory anthocorid and mirid bugs, lacewing and hoverfly larvae, predatory midge larvae, ladybird adults and larvae and spiders.
- Ephedus persicae is the most important species in spring but contributes little to natural control of the aphid. Ant attendance and hyperparasitism decrease the impact of parasitic wasps.
- Outbreaks of fungal pathogens (Entomophthora sp) have been reported from rosy apple aphid colonies occasionally causing population crashes but their importance has not been investigated adequately.
Effective biological control approaches for this pest have not been developed.
- Artificial introduction of predatory insects is uneconomic.
- Biopesticides based on the use of entomopathogens need to be developed.
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