Aphids are small (0.5-2mm), soft-bodied insects that feed by sucking sap from plants. They are one of the most important groups of insect pests in the world.
Aphids can cause direct feeding damage to plants when large numbers build up and remove sap, which can cause wilting. Aphids also cause indirect damage as they are responsible for spreading plant viruses, which they take up and pass on when sucking sap. Viruses spread by aphids in potato crops include Potato Leafroll Virus (PLRV) and Potato Virus Y. In Western Australia, PLRV is the most important virus to seed potato production. This virus is persistently spread, meaning that aphids transmitting the virus need to colonise the host plants to spread the virus. Only a few aphid species are able to spread PLRV. Once a colonising aphid acquires the virus, it can theoretically spread that virus for the rest of it's life.
Surveys of the south-west potato growing areas of Western Australia indicate that the green peach aphid (Myzus persicae) is the most common aphid of potato crops in Western Australia. This aphid is a particularly important pest for the following reasons:
Aphids range in colour from green, to yellow, to pink. Immature forms are wingless and go through 4 sizes before maturing into adults that are winged or wingless. Most aphids have a pair of cornicles which protrude from the rear end (abdomen) of the aphid.
Midges and Leafhoppers are a similar size and shape to aphids, however, these insects moved very quickly when disturbed, either jumping or flying away rapidly, whereas aphids are slow moving and tend to remain on the leaves when disturbed. For instance, when you check lower leaves of a potato plant by turning the leaves upside down, any aphids that are present will stay put. Any insects that rapidly jump or fly away before you can get a close look are unlikely to be aphids.
Under the mild climatic conditions in Australia (i.e. snow is not experienced during winter in most cropping areas), most aphids reproduce asexually (clones). This means that adult female aphids give birth to live young called nymphs. These nymphs go through 4 sizes (instars) before becoming adults, and can mature in 8-10 days under ideal conditions (eg. spring). In cooler weather they take longer to mature. They look alike at all stages, except that they grow larger from one stage to the next. After the 4th instar stage, the immature aphid will grow into either a winged or wingless adult. These adults will start to reproduce after 2-3 days. Throughout Australia over 99% of aphid populations are solely female. This enables the aphids to keep reproducing without having to find a mate, and as they do not lay eggs, their lifecycle is completed very quickly. Under this reproductive system, a single aphid can give rise to a population of thousands of aphids within 4-6 weeks.
As the population grows, the aphids will move between plants in the crop, and if there is a source of virus present, they can potentially transmit the virus from infected to healthy plants.
Aphid survival and population growth is largely influenced by environmental conditions, particularly temperatures. Being small and soft-bodied, they do not survive very well under the hot conditions of summer. Our surveys have determined that aphid numbers in potato crops in Western Australia are most abundant during autumn and spring. In potato crops planted in October/November, you may notice aphids infesting young crops. However, numbers usually drop right back in December and January (Australian summer). Toward the end of February when conditions become milder, aphid numbers usually start to rise again.
Infestations of aphids start when a few winged aphids fly into your crop from elsewhere. These flying aphids can originate from numerous sources, ranging from a few hundred metres away to several hundreds of kilometres. Only a few aphids (even a single aphid!) are needed to start an infestation (see above section on reproduction).
In potato crops, the green peach aphid is always found on the lowest leaves of the plant. If infestations of green peach aphids become large and crowded, the aphids will move upwards onto the younger leaves. The other species of aphids found in potatoes usually start their infestations by colonising the new growth.
Avoid spraying on a calendar schedule, as aphids are not always present in your crop. Monitoring potato crops for aphids before spraying can save time and money. Spray only if you find aphids. The monitoring method used is very important. Select a monitoring system at the start of the season, and consistently use this method throughout the season so that your records are comparable. For instance, you may choose to look at 2-3 sites within a crop, checking 20 plants/site. Walk into the crop in a diagonal pattern, stopping every 2-3 steps to check a plant for aphids. Write down the numbers of aphids you see on each of the 20 plants you check. Alternatively, walk into the crop in a zig-zag pattern, again stopping every 2-3 steps to check a plant for aphids.
Remember the "edge-effect rule" that is, larger numbers of aphids are found at the borders of crops, because they start colonising the edges first. This is because more flying aphids land at the edge of crops than in the middle. Remember that it is most important to plant your seed crop with healthy (virus free) stocks.