Aphids on strawberry: crop monitoring

As aphids are such common and important pests of strawberry, it is important that growers familiarise themselves with the different species, the damage symptoms they cause and how and where to look for them.

Crops should be checked regularly for aphids, at least once per week during spring and summer.

Aphids can reproduce and build up numbers very rapidly in warmer conditions, and action should be taken as soon as aphid attacks are detected and before excessive damage occurs.

Growers should look for the following signs of aphids, in addition to observing the insects directly:

• Sticky honeydew on tops of leaves indicates aphid colonies at higher points in the canopy
• Cast skins (the pale-coloured, dry cuticles shed each time aphids moult) on leaves or other surfaces
• Ants running over plants (searching for aphids to collect honeydew)
• Distorted or discoloured leaves, indicating effects of aphid saliva or aphid-transmitted viruses on plant growth
• Signs of sooty moulds growing on aphid honeydew

Examine each crop weekly for pests and diseases during the growing season.

As you approach the crop, look across the whole area. Find a high spot in or next to the field or look along the tabletops within the tunnel, searching for patches of stunting or abnormal growth or colouring. Fleece-covered plants will require more active searching and temporary removal of coverings from sections of the crop.

If symptomatic patches are detected, they should be closely inspected and the cause or causes diagnosed. The cause could be aphids, but there are a wide range of other possibilities, including:

• Red core
• Crown rot
• Tarsonemid mite
• Verticillium wilt
• Vine weevil
• Waterlogging

The crop inspection should be continued, covering both edge and middle areas. In the open field, this can be achieved by walking in a zigzag transect, whereas part of every table row should be examined in tunnels.

Stop at a minimum of 10 points per field hectare or tunnel. Aphids are most likely to colonise the edges of new crops initially, whereas it is worth spending more time inspecting middle areas of second-year crops where aphids may have overwintered. In tunnels, ensure that plants in the leg rows are examined particularly closely.  

At each point, all the plants in a 5 m field radius or tabletop length should be overviewed, looking for symptoms of stunting, distortion, poor growth, honeydew and/or sooty mould contamination of foliage or fruits. If present, closely inspect the affected plants to diagnose the cause.  

Select one plant at random from each of the 10 points and closely examine each for the signs of pest or disease damage. Presence or absence should be recorded. Affected plants can be marked (e.g. with a staked flag) so that they can be re-examined after control measures have been applied, allowing the success of interventions to be monitored.

For aphids, it is important to examine the folded youngest emerging leaves in the crown and the undersides of older leaves midway up the canopy, which are both typical refuge sites for overwintered potato aphids. Also check the tips of runners. Shallot aphid may occur singly or in small numbers between the folds of very young, unfurling leaves, which are usually characteristically distorted. Melon and cotton aphid may occur in, or directly below, the flowers. Also examine flowers and fruits, including stalks, for other telltale signs of aphid attack, such as a glistening or stickiness of honeydew or the presence of cast skins. Blackening by sooty mould usually indicates the attack started some time ago.

Look out for the presence of aphids on weeds in and around tunnels, as some of the aphid pests of strawberry are also able to feed on weeds before colonising the crop.

Some growers and agronomists supplement the above strategies with sticky or pan traps to detect the presence of aphids in and around the crop. However, checking these thoroughly is time-consuming and the traps will catch many aphid species, including those posing no risk to strawberry. It is more difficult to identify the winged aphids caught in such traps, compared with those insects observed feeding on plants.