Insect pest management in winter cereals

Wheat crop
© Queensland Government

Winter cereals include wheat, barley, oats, canary and triticale. Insects are not normally a major problem in winter cereals but there will be times when they build up to an extent that control may be warranted. For current chemical control options refer to the Pest Genie or (Australian Pesticides and Veterinary Medical Authority) websites.

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Cutworm

Several species of cutworms (Agrotis spp.) attack establishing cereal crops in Queensland and New South Wales. As their name suggests, cutworm larvae sever the stems of young seedlings at or near ground level, causing the collapse of the plant. Damage usually shows up as general patchiness or as distinct bare areas in a very short time. Controlling weeds in the fallow prior to planting will assist in reducing cutworm population and reduce crop damage - at least 3-4 weeks prior to sowing. Chemical control may be warranted if larval numbers exceed 1 larva/square metre in emerging crops. The best time to monitor is late afternoons and evenings when larvae feed. During the day, scratch away soil around damaged plants to find larvae sheltering in the soil. For more information read how to recognise and monitor for soil insects.

Aphids

Aphids are usually regarded as a minor pest of winter cereals, but in some seasons they can build up to very high densities. Four different species of aphid can infest winter cereals:

  • Oat or wheat aphid (Rhopalosiphum padi) is one of the most common aphid-infesting winter cereals. Typically this species colonises the base and lower portions of the plant.
  • Corn aphid (Rhopalosiphum maidis) is also a common species found in winter cereals. It generally colonises the upper parts of the plant, particularly the rolled up terminal leaf.
  • Rose-grain aphid (Metopolophium dirhodum) generally colonises the undersides of the leaves, high in the canopy.
  • Rice root aphid (Rhopalosiphum rufiabdominalis) colonises the roots of the plants under the soil surface, and colonies may extend up from the roots to the base of the plant.

Aphids can impair growth in the early stages of crop and prolonged infestations can reduce tillering and result in earlier leaf senescence. Infestations during booting to milky dough stage, particularly where aphids are colonising the flag leaf, stem and ear, result in yield loss development, and aphid infestations during the grain fill period may result in low protein grain. As aphids may compete for nitrogen (N) with the crop, crops grown with marginal levels of N can be more susceptible to the impact of an aphid infestation. In barley, aphids can spread Barley Yellow Dwarf Virus. While this can have a large effect on barley yield in some areas, it is not considered a major problem in Queensland in most seasons. In virus-prone areas, use resistant plant varieties to minimise losses due to Barley Yellow Dwarf Virus.

Inspect for aphids throughout the growing season by monitoring leaves, stems and heads as well as exposed roots. Choose six widely-spaced positions in the crop and at each position examine five consecutive plants in a row. Research is currently underway into damage thresholds and control options for cereal aphids. Some research indicates that aphid infestations can reduce yield by around 10% on average. Current notional thresholds suggest control is warranted when there are 10-20 or more aphids on 50% of the tillers.

The decision to control aphids on winter cereals depends on both the size of the aphid population and the duration and timing of the infestation. Controlling aphids during early crop development generally results in a recovery of the rate of root and shoot development, but there can be a delay. Aphids are more readily controlled in seedling and pre-tillering crops which are less bulky than post-tillering crops. Corn aphids in the terminal leaf tend to disappear as crops come into head, and other species generally also decline in abundance about this time as natural enemy populations build up. Note that the rice root aphid feeds below ground and can not be effectively controlled by non-systemic foliar treatments.

Prophylactic seed dressings may be effective in delaying the build up of aphid populations in a crop, but because aphids are sporadic (not occurring every season), it can be difficult to decide if a seed dressing is warranted. A locally wet summer and autumn is generally a precursor to an aphid outbreak, as there are abundant alternative hosts to breed up on. Delay any planned chemical control if rain is forecast and check again after rain as intense rainfalls can reduce aphid infestations by dislodging aphids from the plants. Foliar insecticides registered for aphid control are generally broad spectrum, meaning they kill natural enemies (beneficial insects such as ladybird beetles and larvae, hover fly larvae, lacewing larvae or parasitic wasps) as well as aphids. Preserving natural enemies is important in managing aphid populations long-term. Natural enemies can exert effective control on small to moderate aphid infestations. Large aphid populations can also be controlled, but often not until the crop is maturing, which may be too late to prevent impact on yield. Natural enemies can also be effective in suppressing aphid numbers that may survive post-treatment, preventing the need for subsequent treatments.

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Armyworm

Armyworm is the caterpillar stage of certain moths, and can occur in large numbers, especially after good rain follows a dry period. Larvae shelter in the throats of plants or in the soil and emerge after sunset to feed on the leaves of all winter cereals, particularly barley and oats, generally during September and October. Leafy cereal plants can tolerate considerable feeding and control in the vegetative stage is seldom warranted unless large numbers of armyworms are distributed throughout the crop or are moving in a 'front', destroying young seedlings or completely stripping older plants of leaf. The most serious damage occurs when larvae feed on the upper flag leaf and stem node as the crop matures, or in barley when the older larvae start feeding on the green stem just below the head as the crop matures.

The most common species are common and northern armyworm (Leucania convecta and L. separata), and lawn armyworm (Spodoptera mauritia). Infestations are evident by the scalloping on margins of leaves caused by feeding of the older larvae. Larvae target the stem node as the leaves become dry and unpalatable, and the stem is often the last part of the plant to dry. One large larva can sever up to seven heads of barley a day. One larva/m2 can cause a loss of 70 kg/ha grain per day. A larva takes around 8-10 days to develop through the final, most damaging instars with crops susceptible to maximum damage for this period.

Check for larvae on the plant and in the soil litter under the plant. The best time to check is late in the day when armyworms are most active. Alternatively, check around the base of damaged plants where the larvae may be sheltering in the soil during the day. Using a sweep net (or swing a bucket), check a number of sites throughout the paddock. Sweep sampling is particularly useful early in an infestation when larvae are small and actively feeding in the canopy. One full sweep with a net samples the equivalent to a square metre of crop.

The following table shows the value of yield loss incurred by 1 and 2 larva/m2 per day, based on approximate values for wheat and an estimated loss of 70 kg/ha per larva. Based on these figures, and the relatively low cost of controlling armyworm, populations in ripening crops in excess of 1 large larva/m2 will warrant spraying.

Value of grain ($/t) Value of yield loss ($) per day
1 larva/m2 2 larva/m2
$140 $9.80 $19.60
$160 $11.20 $22.40
$180 $12.60 $25.20
$200 $14.00 $28.00
$220 $15.40 $30.80
$250 $17.50 $35.00
$300 $21.00 $42.00
$350 $24.50 $49.00
$400 $28.00 $56.00

Early recognition of the problem is vital as cereal crops can be almost destroyed by armyworm in just a few days. While large larvae do the head lopping, controlling smaller larvae that are still leaf feeding may be more achievable. Prior to chemical intervention consider how quickly the larvae will reach damaging size and the development stage of the crops. Small larvae take 8-10 days to reach a size capable of head lopping, so if small larvae are found in crops nearing full maturity/harvest no spray may be needed, whereas small larvae in late crops which are still green and at early seed fill may reach a damaging size in time to significantly reduce crop yield.

Control is warranted if the armyworm population distributed throughout the crop is likely to cause the loss of 7-15 heads per square metre. Many chemicals will control armyworms. However, their effectiveness is often dependent on good penetration into the crop to get contact with the caterpillars. Control may be more difficult in high-yielding thick canopy crops, particularly when larvae are resting under soil at the base of plants. As larvae are most active at night, spraying in the afternoon or evening may produce the best results. If applying sprays close to harvest, be aware of relevant withholding periods.

Biological control agents may be important in some years. These include parasitic flies and wasps, predatory beetles and diseases. Helicoverpa NPV (Nucleopolyhedrovirus) is not effective against armyworm.

Helicoverpa

Helicoverpa are frequently found in winter cereals, usually at levels too low to warrant control, but occasionally numbers may be sufficiently high to cause economic damage. Virtually all helicoverpa present are H. armigera, which has developed resistance to many of the older insecticide groups. As it is not unusual to find both helicoverpa and armyworm in cereal crops, correct identification of the species present is important. Helicoverpa do not cause the typical head-cutting damage of armyworms. Larvae tend to graze on the exposed tips of a large number of developing grains, rather than totally consuming a low number of whole grains, thus increasing the potential losses. Most (80-90%) of the feeding and crop damage is done by larger larva (the final two instars).

Check for larvae on the plant throughout the growing season (monitoring can be done in conjunction with sampling for armyworm). Using a sweep net, check a number of sites throughout the paddock (read more about insect sampling methods). Larger larvae are more difficult to control than are small larvae, and NPV (helicoverpa nucleopolyhedrovirus) is most effective when larvae less than 13 mm in length are targeted.

While there are no thresholds developed for helicoverpa in winter cereals, using a consumption rate determined for helicoverpa feeding in sorghum (2.4 g/larva), one larvae per m2 can cause 24 kg grain loss/ha. The following table shows the value of yield loss incurred by a range of larval densities, using the estimated consumption of 2.4 g/larvae and a range of grain values for wheat. Note that larval damage is irrespective of the crops's yield potential (i.e. each larva will eat its fill whether it is 1 t/ha crop or a 3 t/ha crop).

Cereal price ($/t) Value of crop loss ($/ha)
4 larvae/m2 6 larvae/m2 8 larvae/m2 10 larvae/m2
150 14.4 21.6 28.8 36
200 19.2 28.8 38.4 48
250 24.0 36.0 48.0 60
300 28.8 43.2 57.6 72
350 33.6 50.4 67.2 84
400 38.4 57.6 76.8 96
450 43.2 64.8 86.4 108

Based on the preceding information, a crop worth $250/tonne will incur a loss of $6/ha from each helicoverpa larvae. If chemical intervention costs $30/ha (chemical + application costs) the economic threshold or break-even point is 5 larvae/m2. These parameters can be varied to suit individual costs, and can incorporate a working benefit:cost ratio. A common benefit:cost ratio of 1.5 means that the projected economic benefit of the spray will be 1.5 times the cost of that spray. Spraying at the break even point (benefit:cost ratio of 1) is not recommended.

Small larvae (<7 mm) can be controlled with biopesticides (e.g. NPV). Biopesticides are not effective on larger larvae. Helicoverpa armigera has historically had high resistance to pyrethroids and control of medium-large larvae using pyrethroids is not recommended. Predators of helicoverpa eggs and larvae include spined predatory bug, glossy shield bug , damsel bug and bigeyed bug.

Where winter cereals have previously been treated with broad spectrum insecticides to control aphids, fewer natural enemies may be present and survival of caterpillar pests could be greater than in an untreated field.

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Mites

  • Brown wheat mite (Petrobia latens) damage is only severe in dry seasons.
  • Blue oat mite or BOM (Penthaleus species) are important pests of seedling winter cereals, but are generally restricted to cooler grain-growing regions (southern Queensland through eastern New South Wales, Victoria, South Australia and southern Western Australia).

Adults and nymph mites pierce and suck leaves resulting in silvering of the leaf tips. Feeding causes a fine mottling of the leaves, similar to the effects of drought. Heavily infested crops may have a bronzed appearance and severe infestations cause leaf tips to wither and can lead to seedling death. Damage is most likely during dry seasons when mites in large numbers make moisture stress worse and control may be warranted in this situation.

Check from planting to early vegetative stage, particularly in dry seasons, monitoring a number of sites throughout the field. Blue oat mites are most easily seen in the cooler part of the day, or when it is cloudy. They shelter on the soil surface when conditions are warm and sunny. If pale-green or greyish irregular patches appear in the crop, check for the presence of blue oat mite at the leaf base.

Where warranted, foliar application of registered insecticide may be cost-effective. Check with the most recent research to determine the likely susceptibility of BOM to the available registered products. Cultural control methods can contribute to the reduction in the size of the autumn mite population (e.g. cultivation, burning, controlling weed hosts in fallow, grazing and maintenance of predator populations). Since eggs laid in the soil hibernate throughout the winter, populations of the mite can build up over a number of years and cause severe damage if crop rotation is not practised. The use of control tactics solely in spring will not prevent the carry-over of eggs into the following autumn.

Predators of BOM include spiders, ants, predatory beetles and the predatory anystis mite and snout mite. BOM are also susceptible to infection by a fungal pathogen (Neozygites acaracida), particularly in wet seasons.