Managing Insect Pests in Winter Cereals - Aphids

Corn aphid

Managing insect pests in winter cereals

Insects are not normally a major problem with wheat. However, there will be times when they build up to an extent that control may be warranted.

Aphids

Aphids are usually regarded as a minor pest, although they can build up to very damaging levels of 100-200 per plant on occasions. They may be found at the base of cereal plants, on leaves or on roots below the soil surface. In barley, aphids can spread Barley Yellow Dwarf Virus which is considered to have a large effect on barley yield in some areas. This is not considered a major problem in Queensland.

Four different species of aphid can infest winter cereals:

Oat or Wheat Aphid (Rhopalosiphum padi)

The oat aphid is brown to muddy green with rusty red patches at the base of the tubes at the rear end of the body. It normally occupies the base and lower portions of the plant. This is generally the most common aphid attacking winter cereals.

Corn Aphid (Rhopalosiphum maidis)

The corn aphid is green to dark olive-green with a purplish area at the base of the tubes at the rear end of the body. It normally lives on the tops of the plants particularly within the rolled up terminal leaf.

Rose-grain Aphid (Metopolophium dirhodum)

The rose-grain aphid is pale green with a darker green stripe along the middle of its back. It normally occupies the undersides of the leaves. It colonizes the lower leaves and moves up the plant as leaves senesce.

Rice Root Aphid (Rhopalosiphum rufiabdominalis)

The rice root aphid is a honey-brown colour with a rusty red area at the base of the tubes at the rear end of the body. It normally occupies the roots of the plants under the soil surface.

The most critical issues we face in managing cereal aphids currently is the lack of local knowledge about the likely impact of infestations on yield and quality (the damage thresholds). Literature, largely from North America and Europe, indicates that there can be significant differences in the way different cultivars respond to the impact of aphids. For this reason, it is important to use this information as general information that may assist in understanding how your crop may be responding to an aphid infestation. In the absence of local data, it is a useful starting point.

Early aphid infestations (from seedling)

Root and shoot growth may be impaired as a result of aphids competing for Nitrogen (N). Inadequate N for the crop may make the crop more vulnerable to the impact of an aphid infestation. There is no impact on yield after grain has filled and is maturing (soft-hard dough).

Aphids have a requirement for N to complete development and reproduce. Honeydew is a by-product of their feeding. Essentially aphids compete with the plant for available N, which can impact on the plant in at different stages of crop development.

Prolonged infestation can reduce tillering and result in earlier leaf senescence. Controlling aphids generally results in a recovery of the rate of root and shoot development, but there can be a delay.

Late aphid infestations

Infestations that occur during booting to milky dough, particularly where aphids are colonising the flag leaf, stem and ear, result in yield loss. Generally, the distal grains in the head fail to fill. Infestations at this stage in which aphids colonise the leaves, particularly lower in the canopy, tend to result in grain with reduced N (protein) rather than a loss in yield. Aphids are intercepting the N being relocated from leaves to the filling grain.

The relative impact of timing and location of infestation makes sense if you review it along with what is known about the contribution of different parts of the crop to yield. See Figure 1 for the contribution of the upper leaves, stem and ear to the yield of wheat and barley (GRDC Winter Cereal Crop Growth Guide 2005)

Making decisions

The decision as to whether controlling aphids on winter cereals will provide an economic return is often complex, and is not just dependent on the size of the aphid population.

Several other factors could influence the control decision. Aphids are more readily controlled in seedling and pre-tillering crops which are less bulky than post tillering crops. Aphids tend to disappear as crops come into head.

Prolonged infestation of moisture stressed crops can exacerbate the effect of moisture stress. Yield potential, value of grain and cost of control are important considerations, but anecdotal evidence suggests that direct feeding by very large numbers of aphids is needed to impact on yield.

Natural enemies (ladybird beetles, ladybird larvae, hover fly larvae, lacewing larvae or parasitic wasps) can exert effective control on small to moderate aphid infestations.

A general recommendation is to check for aphids by choosing six widely-spaced positions in the crop and at each position examine five consecutive plants in a row. If 27 out of 30 plants are covered with aphids and if there are less than two natural enemies per plant on each of the infested plants, then consider treatment. Delay any planned chemical control if rain is forecast and check again after rain.

Dimethoate at 500 mL/ha of 400 g/L product is the recommended chemical control. It has a withholding period of 28 days for harvest and one day for grazing.

Dimethoate has a contact action but is also a systemic insecticide taken up through the leaf and then translocated through the upper portion of the plant. Aphids are subsequently controlled when they feed on the plant.

The rice root aphid feeds below ground and can´t be effectively controlled by spraying.

Dimethoate may be tank-mixed with certain broadleaf herbicides. Check the label before use. Also check water quality as high pH can affect performance of dimethoate.

Dimethoate will kill natural enemies, increasing the possibility of subsequent aphid infestations later in the season.

Other than pirimicarb, foliar insecticides registered for aphid control are broadspectrum in their activity. This means that they will kill the natural enemies (beneficials) as well as aphids. Preserving natural enemies is important in managing aphid populations long-term. Natural enemies can be effective in suppressing aphid numbers that may survive post treatment, preventing the need for subsequent treatments.

Seed dressings are effective in delaying the build up of aphid populations in a crop. Because aphids are a sporadic pest (not occurring every season) deciding when a prophylactic, like a seed dressing, is warranted can be difficult. Aphids tend to be more abundant, and more likely to be in damaging numbers, when the preceding season has been wet and there have been abundant hosts available for them to breed up on. In the case of winter cereals, a locally wet summer and autumn is generally a precursor to an aphid outbreak, and in these seasons a seed dressing is a good option.

Ongoing research

There is currently research being conducted on damage thresholds for cereal aphids, by DEEDI and control options by the Northern Grower Alliance (NGA).

In 2008, initial trial work by DEEDI and NGA showed different results (see the GRDC Update, Goondiwindi, 2009, 2010 papers for NGA results). Briefly, NGA trial work showed an overall yield benefit of around 10% from using seed dressings containing imidacloprid.