Green peach aphids are causing some concern in advanced canola crops
- Central agricultural region
- Great Southern region (including Kojonup)
Brad Joyce (ConsultAg) reports that green peach aphids (GPA) have been moving up onto the stems and pods of podding canola crops in Central and Great Southern areas. He said that the GPA survived sprays which were targeting cabbage and turnip aphids and native budworm caterpillars and the GPA may pose a threat to grain fill if populations get higher. However, Brad noted that crops are not moisture stressed given the recent rainfall.
A grower near Kojonup reports that GPA have been moving up onto the racemes of his podding canola crop.
In advanced canola (flowering onwards), GPA tend to colonise the underside of leaves and are typically not expected to have an impact on yield, even with quite high numbers.
Pests do behave out of character from time to time. Known as the ‘plant stress hypothesis’, there is evidence that plant moisture stress can increase insect activity and performance on affected crops in some circumstances. For example, one study showed that in the brassica plant Arabidopsis thaliana, GPA population growth was higher in drought-affected plants. This was partly attributed to the higher nutritional concentration of sap in plants when moisture-stressed. So, it is quite conceivable that aphids are finding canola stems and pods more attractive than usual.
However, recent rainfall in many areas of the WA grainbelt should work to our advantage in reducing moisture stress of crops still at risk, as well as dislodging aphids from plants and increasing the fungal infection of GPA, which is quite common after rainfall events in spring.
Entomologist Dusty Severtson (DPIRD) has seen GPA colonising canola racemes in dry, warm conditions over the years whilst researching canola aphids. According to Dusty, the worst cases of damage he has seen during spring is when GPA is infesting canola along with turnip and cabbage aphid, and the plants are very moisture-stressed.
It is not known what impact GPA feeding on canola pods will have, and there are no threshold guidelines to guide spray decisions under these conditions.
Canola is more vulnerable to aphid damage at the flowering development stage than at podding, however researchers have seen grain damaged as a result of dense cabbage aphid colonies feeding on pods in moisture-stressed crops. Given this, it cannot be assumed the impact of GPA feeding on stems and pods will be negligible.
If GPA is present in canola paddocks, consider the following:
Beneficials are active
During spring, a suite of beneficials are building in crops and will assist with aphid control. These include, but not limited to; parasitic wasps, ladybird beetles, hoverfly larvae, lacewing larvae, damsel bugs, assassin bugs and spiders. Many beneficial species are highly mobile and will move from crop to crop if left unsprayed. Monitor crops regularly enough so you can measure whether the relative rate of increase in beneficial insects (per sweep, per metre etcetera) is faster (or slower) than that of pest populations. If the former is the case, the beneficials are getting ahead.
With respect to parasitic wasp activity, keep in mind that when an aphid host takes on the typical aphid mummy appearance, wasp development is almost complete. So, the level of parasitism in a crop is higher than what can be seen by eye.
Chemical control options are very limited
Due to insecticide resistance, chemical options for GPA control are very limited in late-stage canola crops. cesar research has found that >90% of all GPA populations collected from canola crops across Australia are highly resistant to synthetic pyrethroids (for example, alpha-cypermethrin) and carbamates (for example, pirimicarb). Applications of these chemicals, even at very high rates, will not control GPA if these resistances are present.
The vast majority of GPA populations also possess resistance to organophosphates (for example, dimethoate). However, the mechanism underlying this resistance is unusual in that it can be ‘switched on’ in response to particular stressors. Additionally, the level of resistance is much lower than for pyrethroids and carbamates. Although risky, this means organophosphates can be effective in some instances. Anecdotal evidence suggests GPA numbers can be reduced by up to 50% despite the presence of organophosphate resistance, although complete control failures have been known to occur (so there are no guarantees!).
Sulfoxaflor (Transform®) is effective against GPA, however it cannot be used on canola past full flower. Paraffinic oils are also registered against GPA in canola, however these sprays will only suppress populations. Oils have the advantage of being soft on beneficial populations and have very short withholding periods.
For more information refer to GRDC’s Resistance management strategy for the green peach aphid in Australian grains .
Who is the main culprit?
If you have large colonies of aphids on canola racemes and have a mix of species, it is important to correctly identify the main culprit. Turnip aphid and cabbage aphid are common on stems and pods during spring and aren’t resistant to registered chemistries. If either of these two species are present and in large numbers, we recommend the use of pirimicarb as it is softer on beneficials. The current threshold for cabbage aphid and turnip aphid is >20% of plants infested with colonies, however the research conducted for this threshold was with growth stages prior to advanced podding, and it is critical to consider several other factors before making a decision. Most importantly, the current growing conditions and moisture availability should be assessed. Crops that are not moisture-stressed have a greater ability to compensate for aphid damage and can tolerate higher infestations than moisture-stressed plants before yield loss occurs.
Aphid numbers will crash naturally
Rain, particularly heavy rainfall events, will impact GPA populations by physically dislodging them from plants. This is more likely where aphids are located on the pods and stems (as opposed to the leaves). Rain can also promote the growth of entomopathogenic fungi which kills aphids.
As the canola seed develops and ripens and pods begin to senesce, they quickly become unfavourable for aphids as they are less able to probe the plant’s tissues. At this time, aphids die out very quickly (or move on to other crops) and management actions are not necessary. Field reports suggest GPA numbers have plummeted in some canola crops in the past week.
For more information on identifying and managing canola aphids refer to the DPIRD’s;
- Aphid management in canola crops page
- Diagnosing canola aphids page
- 2017 Protecting WA Crops Issue 3 newsletter Aphids – WA’s insect problem children
- Aphids in your crops YouTube video.
This article was written in collaboration with cesar.