FAR has received several reports of heavy aphid infestations in cereal and ryegrass crops around the country. Although the risk of yield losses associated with aphid-vectored viruses is low now, direct feeding damage can still occur when numbers are very high.
Colonies of wingless aphids may be prospering; the good news is that beneficial predator and parasitoid numbers should also be high right now (depending on your choice of sprays). Keep an eye on what is going on in your crop, paying particular attention to the underside of leaves.
The main beneficials to look out for are parasitic wasps, hoverflies and brown lacewings. Although it may be challenging to see parasitic wasps as they are only around 3mm long, it is easy to spot aphids that have been parasitised as their bodies swell up and turn a dull brown colour like in the image below.
New Zealand data is so far limited to glasshouse studies, which appear to show no impact on yield from late (GS59) aphid infestations. Australian studies suggest that aphid direct feeding can lead to yield losses of up to 10% in extreme cases.
If you are not seeing any beneficials or any evidence of their activity, but are seeing significant infestations on 50% or more of inspected tillers, consider using a product that targets aphids specifically (such as those that contain the active ingredient pirimicarb, a Group 1A insecticide that is a good choice at this time of year) so that beneficial insect populations survive to control any further infestations.
