WATCH FOR ARMYWORMS IN BARLEY AND OATS

With the current high value of barley, growers should closely monitor armyworm infestations as crops approach maturity. Armyworms are important pests in southern Queensland where they attack winter cereals, particularly barley and oats, in September and October. Larvae appear in plague proportions in some years, and are patchy in others. Head cutting by large larvae can lead to serious losses in barley.

Description and Life History

The common armyworm moths are light brown-reddish. Wings are speckled with black and a have a white spot in the centre. Moths have a wingspan of 30 to 40 mm. Moths become active in spring, sometimes moving long distances on suitable winds from inland areas where they breed on grasses, to more easterly cropping areas. Female moths lay small, white spherical eggs in irregularly-shaped masses in leaf litter, on dead leaves at the base of the plants, in folded blades or under the sheaths of the upper leaves. Eggs hatch in as little as 3 to 4 days depending on temperature.

Larvae can grow up to 35 mm in length with conspicuous white, pink and brown stripes running the whole length of the body. Larvae are distinguished from similar larvae (helicoverpa, cutworm) by the presence of three pale stripes just behind the head, and by their smooth skin, without any hairs or bumps. Larvae mostly feed at night and shelter on the ground during the day. Larvae consume about 90% of their total food intake in the last larval stage (20-35 mm). Mature larvae leave the plant and burrow below the soil surface, where they transform into pupae in earthen cells. Moths subsequently emerge from these pupae.

Damage
During the vegetative growth phase, plants can tolerate considerable leaf feeding. Leaves may look tattered from the eaten-out leaf margins. Faecal pellets around the base of plants are another indication of armyworm infestation. Armyworm generally do not require control during the vegetative stage.

The most serious armyworm damage in cereal crops occurs when larvae feed on the upper flag leaf and stem node as the crop matures. 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. Head cutting begins at this time. One large larva can sever up to seven heads of barley a day. One larva a square metre 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, so the crop is susceptible to maximum damage for this period. The current high value of barley (over $400/tonne) would suggest keeping a close watch on armyworm infestations in maturing crops.

Armyworm populations in ripening crops in excess of 1 large larva per square metre will usually warrant spraying. For insecticide application to be economic, check or scout the crop and assess the problem before head cutting starts. Check for larvae on the plant and in the soil litter under the plant. Late in the day, when the larvae are becoming active, use a sweep net (or swing a bucket through the crop) to make a quick assessment of whether armyworm larvae are present in the crop. Infestations are often patchy, so check a number of sites across the field.

Early recognition
It is essential to recognise the problem early and be prepared to spray when economic damage is imminent. A barley crop can be almost destroyed by armyworm in just a few days. Whilst large larvae do the head lopping, controlling smaller larvae that are still leaf feeding may be more achievable.

Control
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 leaf litter 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. Always read the label.

Biological control agents may be important in some years. These include parasitic flies and wasps, predatory beetles and diseases.

Further information:
Common, northern and sugar-cane armyworms in pasture, and winter cereals, maize and sorghum. http://www2.dpi.qld.gov.au/beef/3250.html

Cereal Aphid Update

Making decisions about control of Cereal Aphids

This post is an update on cereal aphid management following a number of enquiries from growers and agronomists over the last week or so.


Which species in crops?
There are two species of aphid you are most likely to encounter in winter cereals (oats, wheat and barley). They are the oat aphid (Rhopalosiphum padi) and the corn aphid (R. maidis). The oat aphid is found around the base of the tillers, and the corn aphid in the whorl and under leaves higher on the plant. Both aphid species are greenish to black with rusty red-purple areas on the rear end around the ‘tail’.













(Line drawings from DPI Victoria's Insectopedia)
See the Northern Region Ute guide for more detailed descriptions and pictures – or see August 23 posting on the Beatsheet Blog

How much damage can aphids cause?
The key question about aphids is “will the population of aphids in my crop cause damage to the crop, and yield loss?”

Direct aphid damage, as a result of feeding, is difficult to detect. In moisture stressed crops you may see yellowing with high aphid populations. Otherwise, there are generally no early signs of how much impact the aphids are having on the crop.

West Australian research showed yield losses of up to 10%, and reduction in seed size, with aphid infestations (this was without any impact of barley yellow dwarf virus).

Overseas research (Canada, US) suggests that significant yield loss occurs when aphids are present from flowering through to milky grain. The data also suggests that yield loss does not occur when infestations are present earlier or later than this period.

Making control decisions

Corn aphids may disappear by themselves
Corn aphids, the species that lives in the whorl, generally disappears when the crop comes into head. This is because their preferred site is no longer there, and they tend not to survive as well on leaves. If you have the corn aphid in crops, consider delaying a control decision until the crop starts to head.

Control thresholds
Qld and WA threshold recommendations are comparable, with the WA recommendations based on the most recent research that has been undertaken in Australia.

Recommendation are to check crops regularly from late tillering, and consider control if the aphid population exceeds 15 aphids/tiller on 50% of tillers.

http://www.agric.wa.gov.au/content/pw/ph/dis/cer/bydv_aphidfeeding.htm

Other considerations when making a decision about cereal aphids

1. Natural enemies (lady beetles, hoverflies, parasitic wasps) can have a big impact on aphid populations, reducing them to very low levels in many instances.
   
2. Dimethoate and synthetic pyrethroids (registered for cereal aphid control) are highly disruptive to natural enemies. The application of these insecticides early (e.g. during the vegetative and early tillering stages) may result in a later reinfestation of the crop because small numbers of surviving aphids are no longer controlled by natural enemies. The impact of these products on natural enemies can persist for some days.
   
3. Pirimicarb (Pirimor®) is a soft option for cereal aphid control, compare its price with that of dimethoate when making a decision – it may be competitive. Pirimicarb has some systemic activity.
   
4. Oat aphids, at the base of the plant, can be difficult to contact in a dense crop. Dimethoate will kill aphids by contact, but its systemic activity is by translocation upwards, so its efficacy against oat aphid is unclear.
   
5. The systemic activity of both pirimicarb and dimethoate may be reduced in moisture stressed crops.
   
6. Rain will reduce aphid populations by knocking/washing individuals of plants, and the aphids tend not to get back on the plants. Often ground predators, like carabid beetles, ants etc will eat aphids on the ground. Check populations of aphids again if you get more than 20 mm rain.