By David Jones, CropNuts Broad Acre Agronomist
This week, we take a look at:
Identifying and controlling specific aphid to avoid control failures
What thresholds to use to make decisions on farm crop spraying
Why spraying an insecticide on some aphid populations can INCREASE their numbers
Aphid pest pressure at the moment is very high in many farm crops especially in cereals and canola, with warm favourable conditions for multiplication. One of the main reasons is that Drought-stressed farm crops have elevated levels of secondary metabolites in the phloem, which increases aphid pest growth and multiplication.
More aphids lead to even more stressed farm crops… the nightmare positive feedback loop!!!
Know your aphid species in Canola crucial
There two major species of aphid in Canola; Myzus persicae, the Green Peach Aphid (below, on a canola pod), and Brevicoryne brassicae, the Mealy aphid.
Green Peach aphids are a concern throughout the crop’s life with canola; they spread viruses such as Turnip Yellow Virus in the early stages up to early stem extension, and cause direct feeding damage later on the leaves and particularly on the pods when in high concentrations.
There are no effective thresholds in my opinion; if colonies of adult winged aphids and wingless offspring are found in large number on the leaves, take them out without delay. Once they build up to feeding on the pods they are very hard to control and will have already caused damage.
Neonicotinoids (thiamethoxam, acetamiprid) are effective on this species as is sulfoxaflor, but resistance to pyrethroids such as lambda-cyhalothrin and to Carbamates such as pirimicarb is very widespread.
The second aphid species that we are likely to encounter from flowering onwards is the Mealy Cabbage aphid, which builds as thick colonies on the tops of the plants. These look very visible and prominent but in reality damage is often minimal.
A threshold of 13% of plants infested is applied in Europe, but I have carried out several trials and found minimal response to crop spraying once the pods are fully formed. In fact, we were finding more damage is caused by travelling through the farm crop with the crop sprayer at this late stage so unless you are finding them in very large colonies do not worry about them.
In Australia, advice in low rainfall areas is to crop spray when ‘most’ plants have colonies over 2.5cm long on the top of the main raceme, but in medium to high rainfall areas economic damage is very rare.
They also feed on the top of the meristem (growing point) which is perhaps why the pods never seem starved of nutrients.
If you do need to crop spray for Mealy Cabbage aphids, use pirimicarb in the heat of the day. Adding a non-ionic surfactant will improve control in very high populations with dense colonies.
Finding both species in the Canola crop presents a challenge
Where it is particular challenge is fields that have both Green Peach AND Mealy Cabbage aphids; for dense colonies of Mealy Cabbage, Neonicotenoids are not always very effective. Sulfoxaflor is very expensive, and when you do the maths it can be cheaper to use a mix of acetamiprid or thiamethoxam with pirimicarb.
And remember, if Diamond Back Moth or Bollworm are present it may be necessary to add a THIRD insecticide.
Cereal Grain Aphid
I have found several cases of Cereal Grain Aphids (Sitobion avenae) feeding on the ears of barley recently. Feeding on the leaves is very common and rarely yield affecting, but feeding on developing grains can be very damaging indeed.
You might also see colonies of dark coloured aphids on the stems and lower leaves; these tend to be Bird Cherry Oat aphids; a similar species but they tend to do less damage and feed lower down the plant.
Thresholds vary but when they are being found feeding on the grains I always advise crop spraying without delay. Pirimcarb is the most selective and effective chemical pest control method, with its fumigant activity in thick, dense farm crops.
A fine spray quality is a must in cereals, and I think that it is another argument for wide rows in dry seasons like we are experiencing at present; better moisture utilisation as well as insecticide penetration.
Do not be tempted to use pyrethroids at this late stage in cereals; yes they are cheap, but not a very effective chemical control method and they are less selective on beneficial insects that will be predating the aphids.
Why Crop spraying with a pyrethroid may INCREASE aphids
I attended the Association of Independent Crop Consultants conference in the UK last week, where they discuss the results from their field trials program and issues that they are seeing in the field.
Among the many facts that I learnt about the ‘fitness penalty’ associated with evolution of a species.
Scientists at Rothamsted Research Centre found that Green Peach Aphids which have developed resistance to pyrethroids show under-developed Siphunculus; two tubes on the rear section which emits a pheromone to warn others that they are under attack from predators.
They examine two populations, one pyrethroid resistance and one not, in a controlled environment to see if this made them more susceptible to predators, and it did!!! Around 40% high mortality was seen in the aphids that were not able to warn others of predator attack.
This has major implications for field practice; spraying a pyrethroid on a resistant population will not only achieve NO control of the aphids, but may also kill the beneficial insects which are achieving more control than they ever have been.
David Jones is the Broad Acre Specialist at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). David has a keen interest in soils and no till farming systems where he has undertaken work looking into weed levels and changes in soil structure, and has extensive experience in field trials and in the development of precision farming techniques. In his spare time he enjoys playing rugby.