Spotted alfalfa aphid — winged adults and nymphs
Spotted and bluegreen aphids
Spotted alfalfa aphids (Therioaphis trifolii f. maculata) are pale yellow with six or more rows of black spots along their backs. Bluegreen aphids (Acyrthosiphon kondoi) vary from pale green-grey to dark green-blue. When fully grown, they are about 2-3 mm long, while spotted alfalfa aphids are 1-2 mm long. In both species, all adults are female and they can be either winged or wingless.
Spotted alfalfa aphids are normally found on the stem and undersides of the lower leaves and have a tendency to jump when disturbed. Bluegreen aphids typically inhabit the growing point and adjacent areas and do not jump as readily as spotted alfalfa aphids.
Both aphids can develop rapidly, with living young born as often as four times a day under ideal conditions. The life cycle can be as short as one week in summer.
The spotted alfalfa aphid is most common in the warmer months from spring to autumn. The bluegreen aphid is most active during the cooler part of the year, with the winged aphids most active in the autumn. There is a dramatic reduction in populations in spring. Both aphids suck juice from the leaves and stems, and inject a toxin into the plants, which can rapidly kill lucerne seedlings.
The first sign of spotted alfalfa aphid is yellowing or whitening of the leaf veins. As populations increase, the lower leaves yellow, curl, die and drop. Eventually, only the terminal leaves remain and these also die if the infestation persists.
Bluegreen aphids cause shortened internodes, stunted plants, and misshapen and yellow leaves. Other effects include lower root reserves and plant death, with subsequent weed invasion. Not only is the yield reduced, but honeydew and associated moulds also affect hay quality. Continuing damage shortens the productive life of a stand.
Bluegreen aphids
Lucerne cultivars with varying resistance to these aphids are available.
Tiny wasps, which are parasites of the aphids, have been imported and are established in all lucernegrowing areas. A wide range of native predators also feed on the aphids. Some of the more important are ladybirds, brown lacewings, hoverfly larvae, damsel bugs, some red mites and spiders. Native and exotic fungal diseases attack the aphids, dramatically reducing their numbers, particularly when the populations are high, and the climate warm and humid.
These natural controlling agents may not halt a serious infestation before crop damage occurs. However, they are important in suppressing populations, holding down light infestations and delaying reinfestation after applications of selective insecticides.
Seedlings should be inspected at least twice weekly. If an average of two aphids on each seedling is found at any inspection site in the fields, action is needed to prevent plant loss. Between a height of 100 mm and the first harvest, five aphids on each plant should also signal a need for applying an insecticidal control. If the cultivar is resistant to the particular aphid species, these levels can be increased.
When aphids infest established stands, control depends on the value lucerne has in the farming system. If each stem carries an average of more than 20 aphids, treatment is necessary to ensure continued lucerne growth.
Pea aphid
Where insecticide is considered necessary in hay production, apply it as soon as practicable after cutting. Where an infestation develops late in the growth period, the best procedure is to cut (or graze) and then spray as soon as possible to ensure rapid growth for the next harvest.
Pea aphid
The pea aphid (Acyrthosiphon pisum) is similar to the bluegreen aphid. It is a shiny, light yellow-green aphid that grows to about 4-5 mm in length. Pea aphids suck juice from the tender parts of the plant and, in large numbers, cause wilted, stunted and misshapen plants. The top leaves turn light green; the lower ones turn yellow and die.
The pea aphid is also a significant carrier of plant diseases. However, pea aphid populations have generally been very low in Queensland lucerne fields and have not reached economic levels.
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