Cereal aphid numbers have increased rapidly over the past 3 weeks as the temperatures have increased. Whilst low numbers of aphids have been present in many crops (wheat and barley) for some time, it was not until a couple of weeks ago that numbers reached levels of concern to agronomists and growers.
Until the last few seasons, cereal aphids have not been considered a major pest in winter cereals. However, higher grain prices mean that the value of any yield loss is higher than it was and control may be economic at the densities we are experiencing.
Which species are in crops this season?
There are several species of aphid that occur in winter cereals (oats, wheat and barley). The most abundant, and the species that has been present in low numbers through winter are the oat aphid (Rhopalosiphum padi – it sounds like Row-pal-o-si-fum pad-i). This species tends to colonise the lower portion of the plant, mature adults are a dark green and rounded. Juveniles are paler and smaller.
On the Downs, the oat aphid is currently the dominant species, with infestations extending from around the base of plants up on to leaves and stems as the crop starts elongation. Smaller number of the rose-grain aphid and corn aphid are also present.
The rose-grain aphid (Metopolophium dirhodum – sounds like meto-pal-o-fee-um di-road-um) is a large, pale aphid with a dark stripe down the midline of the back. This species tends to colonise leaves higher on the plant, and is often very obvious. Clusters of juveniles are common on upper surfaces of leaves.
The corn aphid (Rhopalosiphum maidis – sounds like Row-pal-o-si-fum may-dis) is rectangular in shape rather than round. Legs and antennae are typically dark, the body green-blue, and they may look waxy.
(line drawings from “Insectopedia” Agriculture Victoria, 2000)
In northern NSW, the corn aphid is abundant higher in the canopy, particularly in crops that are booting. Corn aphid is reputed to decline in number as the crop comes out into head.
The photo illustrates a typical corn aphid infestation in a crop of barley prior to head emergence.
How much damage can aphids cause?
There has been surprisingly little work done on cereal aphids in Australia to establish the relationship between aphid numbers, the duration/timing of infestation, aphid species, and ultimately the impact on yield.
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.Western Australian recommendation are to check crops regularly from late tillering, and consider control if the aphid population exceeds 15 aphids/tiller on 50% of tillers.
The WA 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).
Queensland DPI&F recommendations have been to: Check 5 plants at 6 sites within the field. If 27/30 (90%) of plants are covered with aphids, and there are less than 2 natural enemies per plant, then consider treatment.
A 90% infestation level would be indicative of a well established population. Early infestations tend to be patchy, and become more uniform as the population builds up.
Checking a crop for aphids Sample away from the edge of a field. Aphid numbers tend to be higher around field margins because this is where initial infestations start. The rest of the field will be more representative of the infestation in the majority of the field.
It is simpler to base estimates of infestation on tillers rather than whole plants. It can be difficult to determine where an individual plant starts and stops, and the number of tillers per plant can be variable.
Taking a representative sample of individual tillers from across a field will provide information on the number of aphids, and the proportion of the tillers infested. The lower the infestation the more tillers you will need to sample (e.g. 30 per management unit). The more established the population the more uniform the infestation will be and the number of tillers sampled can be reduced (e.g. 10-20 tillers may be sufficient). Record the number of aphids per tiller and see how consistent numbers are as you go. Lots of zeros means the population is patchy.
If numbers are high, you may want to use a rating system for estimating density rather than actually counting aphid numbers. For example: 0= no aphids, 1= 1-10 aphids, 2= 10-20 aphids, 3= 20-50 aphids, 4= more than 50 etc. Once you have your eye in, a rating system is quicker than counting aphids.
It may be useful to rate the number of aphids above and below the flag leaf separately. This will be particularly useful for assessing how effective a spray has been, and determining if surviving aphids are those that may have simply not been contacted.
Information from overseas research (Canada, US) suggests:
that significant yield loss occurs when aphids are present from the flag leaf stage through to milky grain – no yield loss occurs with infestations later than milky grain
infestations of aphids on the flag leaf, and upper portions of the crop, including on the heads, cause greater yield loss than infestations lower in the canopy
Other considerations when making a decision about cereal aphids
Corn aphids may disappear by themselves. Corn aphids, the species that colonises the upper canopy, reputedly decline in number when the crop comes into head. This may be because they tend not to survive as well on leaves as they do on the flag leaf or in the whorl.
Natural enemies (lady beetles, hoverflies, parasitic wasps) can have a big impact on aphid populations, reducing them to very low levels in many instances. This is particularly important in managing the resurgence of any aphids that survive a spray.
Dimethoate and synthetic pyrethroids (e.g. Bulldock®) are highly disruptive to natural enemies. The application of these insecticides early 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.
Pirimicarb (e.g. Pirimor®) is a soft option for cereal aphid control, but be aware of the with-holding period.
there is no Australian data on resistance to any of the registered insecticides in cereal aphid populations.
Oat aphids, at the base of the plant, can be difficult to contact in a dense crop, and with aerial application.
Rain will reduce aphid populations by knocking/washing individuals of plants, particularly if the rain is high intensity (storm) rain. When washed off, aphids tend not to get back on the plants. Often ground predators, like carabid beetles, ants etc will eat aphids on the ground. It may be worth re-checking numbers if you get a storm between checking and applying a spray.