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Apple grass aphid (Rhopalosiphum insertum (Walker))

Apple grass aphid attended by black ants

Apple grass aphid is the most common aphid species on apple but it only causes minor damage to commonly grown apple varieties.  Some apple varieties are more sensitive e.g. Jonathan.  The life cycle involves two hosts.  Apple, pear and quince are the winter hosts of apple-grass aphid and grasses (especially annual meadow grass) are the summer hosts. The migration to the summer host in May-June is obligate and so infestations do not persist on fruit trees during the summer.

It is widespread and abundant in apple orchards in spring, especially when the previous summer has been wet so favouring the growth of grasses.

The aphid hatches at bud-burst to early green cluster from overwintered eggs on the bark and develops on the rosette leaves and amongst blossom trusses causing minor curling damage of leaves if large populations occur in early spring before or during early blossom. Damage is rarely important unless populations are very high. There is little risk of damage from this pest after mid blossom as winged forms develop which migrate to grass during and after blossom.

Green apple aphid and common green capsid both may be present and causing damage but are readily distinguished.

The severity of infestation by apple grass aphid should be determined in each orchard by monitoring  in spring when the usual pre-blossom pest assessment is done.

  • At least two blossom trusses should be inspected on at least 25 trees per orchard.
  • 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.

 

Chemical control

An insecticide treatment for the aphid should be applied before blossom if the economic threshold of 50% of blossom trusses infested with 5 or more aphids is exceeded. Insecticide sprays should only be applied for this pest if strictly necessary as it provides an early food source for natural enemies of other more damaging species.

  • Several insecticides are approved for control of aphids on apple  but if apple grass aphid is the only pest that needs to be controlled, pirimicarb (Aphox etc.) or flonicamid (Mainman) are the preferred choices in conventional orchards as they are selective and partially systemic.
  • The neonicotinoids thiacloprid (Calypso) and acetamiprid (Gazelle) are also approved for control of aphids on apple though they can be a little slow acting in cool conditions.
  • They are systemic and are highly effective against aphids. They will give excellent control of apple grass aphid and incidental control of several other pests such as capsids and sawfly.
  • They have little effect against caterpillars and thiacloprid (Calypso) and acetamiprid (Gazelle) are ineffective against woolly aphid.
  • Use of synthetic pyrethroids, which are harmful to natural enemies, should be avoided.

 

Insecticides approved for control of aphids on apple

Choice of insecticides – efficacy factors

Active ingredient Trade name (examples) Class Selectivity Approved for control of Safety to Typhs 
acetamiprid Gazelle neonicotinoid broad-spectrum, systemic  Aphids safe
cypermethrin various pyrethroid broad spectrum Aphids, capsids,caterpillars, codling & tortrix moths, sawflies, apple sucker harmful
deltamethrin Decis etc. pyrethroid broad spectrum Aphids, codling & tortrix moths, capsid, sawfly, apple sucker harmful
dodecylphenol ethoxylate Agri 50E physical acting insecticide broad spectrum Aphids, leafhoppers, mealy bugs, spider mites harmful
fatty acids Savona soap broadspectrum Aphids, scale insects, leafhoppers harmful
flonicamid Mainman neonicotinoid selective Aphids and woolly aphid safe
maltodextrin Majestik polysaccharide broad spectrum Aphids, spider mite harmful
pirimicarb Aphox etc. carbamate selective aphicide, trans-laminar Aphids safe
pyrethrins Spruzit extract from pyrethrum broad spectrum Aphids, apple blossom weevil, caterpillars, spider mites, scale insects harmful
thiacloprid Calypso neonicotinoid broad-spectrum, systemic Rosy apple aphid. (Also likely to control capsids and sawfly, though not caterpillars or woolly aphid) safe

 

 

Choice of insecticides – Safety factors
  Hazards Harvest interval(days)  Max. no. sprays Buffer zoneWidth (m)
Anticholin-esterase?  Humans Fish & aquatic life Bees
acetamiprid no u h u 14 2 20
cypermethrin no h, i ed d 0 4 18
deltamethrin no h, i ed d 0 u 50
dodecylphenol ethoxylate no u u u 0 u u
fatty acids no u h u 0 u sm
flonicamid no u h u 21 3 sm
maltodextrin no i d d 0 20 sm
pirimicarb yes t, c h 3 u sm
pyrethrins no hi ed d 0 4 50
thiacloprid† no h, i h h 14 2 30
h=harmful, i=irritant, d=dangerous, ed=extremely dangerous, t=toxic, c=closed cab required for air assisted sprayers, sm=statutory minimum of 5 m for broadcast airassisted sprayers u=uncategorised/unclassified/unspecified

 

Control in organic orchards

Chemical control of apple grass aphid is usually not attempted in organic orchards because the pest only causes minor damage which is tolerated.

  • Emphasis should be placed on cultural control measures such as the provision of artificial refuges for earwigs and lacewings and of flowering herbs in and around the orchard to encourage predatory insects.
  • High volume sprays of fatty acids (potassium soap) (Savona) could be applied when damaging infestations develop if necessary on more sensitive varieties.
  • Pyrethrins (Spruzit) is approved for use in organic orchards but permission should be sought from certification bodies before use.

 

Further reading