You are here: SARDI Home  / Pests & Diseases  / Horticulture  / Horticultural Pests  / Integrated Pest Management  / Resources  / Greenhouse pests  / Aphids

Green Peach Aphid, Cotton Aphid, Potato Aphid

Family Aphididae, HEMIPTERA

Description and life cycle

Aphids are small (2-3mm) soft bodied insects, pear shaped, ranging in colour from a light olive green and pinkish to dark green and black.

Aphids have unusual lifecycles. Some aphids have primary and secondary hosts, others will produce eggs for a winter diapause in cold climates. In more temperate climates and especially in protected environments where the autumn temperatures are not extreme and suitable hosts are available, continuous reproduction by females alone is possible. Females give birth to live female young at the rate of 4-6 per day.

There are usually 4 nymphal stages all a smaller version of the adult but sometimes lighter in colour. Some colonies can have aphids of two colour forms. There are also winged and wingless adult forms which occur in the same colony. Several environmental factors trigger the production of winged forms that can fly off to start new colonies. Aphids often enter a crop in large numbers in flights, which then reproduce large numbers of wingless colonies.

A female will give birth to more than 100 live young over her life of one to four weeks. These young can mature into adults in 4-7 days in summer and immediately begin producing live offspring. A colony can produce many generations in a very short period of time. The ideal temperature for aphids is about 22^oC with most activity occurring during the warmer months. No development generally occurs below ~5^oC or above ~33^oC.

Crops attacked and problems caused

Aphids are sup sucking insects, most species of which occur world wide. They feed on a large range of crops including vegetables, tree crops, broad acre crops and ornamentals. Herbaceous weeds are also favoured hosts of aphids. Aphids are usually found on the soft growing points of plants living in colonies. When aphid numbers are very high they will be found on any growth over the whole plant. If a winged aphid is found alone it has usually just arrived.

Aphids can be high risk in any crop if not treated. These sap sucking insects reduce the total plant vigour and the production and fruit quality suffers. Their feeding usually distorts the new growth affecting the leaves, flowers and fruit. In some crops feeding aphids can transmit viruses which can often have more impact on the crop than the actual feeding itself. The sticky honeydew excreted onto the plant by aphids while feeding encourages sooty mould growth which covers the foliage and fruits, retarding growth and reducing the market value of the fruit.

Reducing the threat of invasion and attack

Aphid numbers can be greatly reduced by observing the following practices:

• Control weeds on field edges if scouting indicates aphids are present and control weeds within crops
• Destroy old crops immediately after harvest has finished
• Use a fallow period, if possible, when no crop is grown to clear pest populations
• Do not plant new crops near or close to neighbouring infested weeds or old crops (greenhouse and open field)
• Use seedlings that have been grown away from infested areas, i.e. start with a clean crop.
• Avoid moving aphids around the crop on staff moving from infested to clean areas. Do not work in old crops on windy days, especially when prevailing winds are blowing towards new plantings
• Mesh screening has been shown to effectively exclude aphids in covered/protected crops and use of reflective plastic soil mulch can be beneficial in reducing aphid numbers. Rolls of yellow sticky tape may be useful in some greenhouse designs if placed near entry points and hot spots.
  

Monitor pest populations for early control. Use yellow cards to alert you of new infestations and scout plants by turning leaves to work out were they are. Control is simpler and less expensive when plants are young and spray coverage is not an issue.

Crop monitoring

MAKE INSPECTIONS ROUTINE AND KEEP GOOD RECORDS!

• Get into the habit of walking right through your crops in a set pattern (a M or Z)
• Check about (about 1%) of your plants very carefully
• Hunt for aphids and symptoms of virus disease where this is an issue
• Check underside of new leaves for feeding nymphs and adults
• Keep good records of pest levels and treatments used
  

Monitoring with sticky traps (only adults with wings are caught on traps):
For insecticide-based control programs economic thresholds need to be worked out from monitoring and spray records using sticky trap and plant leaf checks. Decide on a threshold level for aphids in your crop, above which you must spray and below which you can safely withhold spraying.

• Plan the layout of traps to identify hot spots and estimate overall aphid levels together with plant scouting
• Place traps just above the plant tops
• Do a weekly count of aphids on each trap and look for infected plants. Mark infected plants with tape for removal and replace sticky traps.
• Note any aphid hot spot and check aphid numbers on nearby plants to find the size of hot spots. Check for biological control activity.
• Record trap, scouting details
• If aphid numbers are above the threshold you must spray. Think about using chemicals compatible with bio control
• Check numbers on marked plants 1-2 days after spraying to check results
  

Note:

• If you can spot isolated hot spots early you may only need to spray a small area!
• Plan to introduce biological control agents as soon as thrips are found
  

Chemical control

Chemical control of aphids is not as difficult as some of the other pests, though some aphid species are resistant to some chemicals. All stages are susceptible but small droplet size is much more effective and will make better contact. Only spray for aphids when you need to, but don't wait till you've got large numbers, spray early and always look for 'hot spots" in your monitoring.

Because of the aphids habit of colonising the growing points, they can be easy targets, but are more difficult to control when large populations move to the underside of leaves. For this reason the systemic type of insecticides are more effective. When using the contact products you need to make sure you spray the entire plant to achieve control. Remember that only a few females will recolonise a plant in a week in summer.

Include a resistance management strategy into your spray program to reduce the chance of aphids becoming resistant. If monitoring indicates the need to spray earlier, then insecticide resistance, inappropriate spray application or inadequate farm hygiene should be suspected and expert advice sought.

Relevant beneficial insects

Natural enemies have been investigated and biological control programs using a number of naturally occurring insects is possible. There is a parasitic wasp, hoverfly larvae, several ladybird beetles and lacewings that will attack aphids, but they are all very easily killed by insecticides and their residues. Naturally occurring beneficial insects usually lag behind the build up of aphids and some damage can occur before effective control is achieved.

As with WFT you can also boost the numbers of these wild beneficial insects in your crop naturally by holding back on broad spectrum insecticides, providing safe plant species as habitat near the crop and maintaining higher levels of organic soil carbon.

Only a parasitic wasp and the green lacewing are commercially available for aphid control. The brown lacewing and a ladybird beetle are currently under development.

The female adult wasp is about 0.6mm long and black in colour, with a thin slender body. A female wasps lays its egg into the aphid, the wasp egg then hatches and feeds on the insides of the aphid. The aphid dies and the larvae of the wasp pupates inside the aphid body swelling it and turning it into an "aphid mummy" usually a golden or silvery appearance depending on the aphid species. An adult wasp then emerges from a small circular hole it cuts in the shell. At 25^oC a wasp lifecycle takes 10 days.

Green lacewings are generalist predators and the larval stage will eat most small insects and eggs it encounters. It particularly likes aphids and can consume 60 aphids in an hour. Adults feed on nectar and pollen.

Commercial suppliers of bio-control agents in Australia can be found listed at the Goodbugs website (external link).  The suppliers on this page will help you develop an IPM program suitable for your crop and situation. Many also provide IPM monitoring services.   

Download the Aphid factsheet (PDF)

Information on other greenhouse pests:
• Western Flower Thrips
• Greenhouse Whitefly
• Two Spotted Mites
• Broad mites