|Insects and damage
More information about aphids
- Aphids are most prevalent on pre-emergent to flowering cereals in late winter and early spring. High numbers often occur in years when there is an early break in the season and mild weather in autumn and early winter provide favourable conditions for colonisation and multiplication.
- Adults and nymphs suck sap and produce honeydew. Very high numbers may cause plant stress (yellowing or wilting in extreme infestations); these symptoms are more common in moisture-stressed crops. Direct feeding damage may occur when colonies develop on stems, heads, leaves. Aphids can affect root development, the number of tillers, seed set and grain size.
- Aphid species:
- Oat aphid – basal leaves, stems and back of ears of wheat, barley, oats
- Corn aphid – inside the leaf whorl of the plant – cast skins indicate their presence – seldom on wheat or oats
- Grain aphid – colonises the younger leaves and ears of wheat, oats and barley
- Rose grain aphid – underside of lower leaves and moves upwards as these leaves die.
- Rice root aphid (Qld) – below ground parts of cereals – noticeable when the bases of plants are exposed – often during periods of moisture stress.
- Aphids can transmit barley yellow dwarf virus (BYDV). Significant yield losses may occur if transmission is in the first 8-10 weeks after emergence.
- Whilst rare, aphid infestations in filling heads have the potential to directly compete with grain for resources.
- Monitor all crop stages from seedling stage onwards. Look on leaf sheaths, stems, within whorls and heads, and record the number of adult and juvenile aphids, beneficials (including parasitised mummies), and the impact of the infestation on the crop. Check at least 5 points in the field and sample 20 plants at each point.
- Stem elongation to late flowering is the most vulnerable stage; frequent monitoring is required to detect rapid increases of aphid populations.
- Populations may be patchy – aphid infestations may initially be detected on crop edges. Winged aphids will disperse throughout the field and colonise, creating hotspots across the field. As populations grow, infestations become more uniform across the field.
- Average number of aphids per stem/tiller samples gives a useful measurement of their density. Repeated sampling will provide information on whether the population is increasing (lots of juveniles relative to adults), stable, or declining (lots of adults and winged adults).
- Beneficials (including parasitic wasps, lacewings, hoverflies, ladybird beetles) can exert effective control of small to moderate populations of aphids, however they may not arrive early enough to prevent the build-up of aphids to above threshold. They are useful in controlling individuals and small colonies that may survive an insecticide application. For this reason, the use of soft options (e.g. pirimicarb) for aphid control should be considered, particularly if the aphid infestation is being treated in the early stages of crop development (prior to grain fill) when there is the potential for aphid infestations to resurge.
- The presence of bloated aphids with pale gold/bronze sheen (mummies) indicates parasitoid activity in the crop.
- Aphid fungal diseases can cause a rapid reduction in aphid population in wetter seasons. Fungal infection is detected by the presence of white, fluffy growth on aphids, particularly those located on lower leaves and stems.
- Aphids, and BYDV, survive over summer on self sown cereal and perennial grasses. Control weeds and volunteers to minimise early infestation of crops. In some seasons, aphid movement may occur over large distances and local weed management will have little impact.
- Encourage beneficial populations through the preservation of native vegetation which provides a refuge for beneficials.
- Early infestations (emergence to booting): When 20% of tillers have 10 or more aphids
- Later infestations (flag – soft dough) : When 50% of tillers have 15 or more aphids
Aphid populations can decline rapidly, which may make control unnecessary. In many years aphid populations will not reach threshold levels.
The risk to robust crops with adequate moisture is less than that to moisture stressed crops. The impact of significant moisture stress on yield loss is considerably higher than of aphid infestations. In a stressed crop, the relative benefit of controlling aphids is less than in a crop with a greater yield potential.
- Delay chemical control if heavy rain is forecast. Heavy rain can reduce aphid populations.
- Early control of infestations around the edge of the crop (using a border spray) may delay or prevent more widespread infestation of the crop.
- Use aphid-selective products (e.g. pirimicarb) to preserve beneficial insects and potentially reduce the need for follow-up applications.
- Seed treatments may be effective in minimising the spread of Barley Yellow Dwarf Virus and delay the build up of aphids in the crop.
- The prophylactic use of synthetic pyrethroids is not recommended as it encourages build up of resistance in aphids and other non-target insects. Broad spectrum insecticides kill natural enemies and increase likelihood of subsequent aphid infestations later in the season. Refer to the pesticide impact on beneficials table to identify products least likely to impact on non-target beneficials.
||An Area Wide Management approach to the control of weed and volunteer hosts may be useful.
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