High levels of aphid parasitism a nice surprise for some

Several crops have not needed to be sprayed due to high levels of aphid parasitism.

Where have they been reported?

High rates of aphid parasitism are being seen in some cereal crops around Manangatang in the Victorian Mallee.

Close monitoring of aphid numbers and parasitism levels has been crucial in the past few weeks with numbers of cereal aphids building up; in some cases levels of parasitism (seen as aphid ‘mummies’) were as high as 50% in cereal crops with moderate aphid numbers of up to 10 per tiller.

There were instances where growers had planned to spray paddocks for aphids but held-off due to the high levels of parasitism. These growers continued to monitor, each time re-assessing ratios of aphids to aphid ‘mummies’.

Brown lacewings were also recorded in sweep net catches in low to moderate numbers within these crops; hoverfly larvae have also been seen amongst the aphids.

A vetch crop west of Kerang in the Victorian Mallee has also had high levels of aphid parasitism. This crop had not been sprayed with insecticides during winter, which had allowed parasitoids to persist. In patches where aphid numbers were highest, parasitism levels were greater than 50% and control was not required.

In central and western Victoria, both winged and wingless aphids are being seen within cereal crops. Overall, colonies do not seem to have built up. Beneficial insects, including hoverflies, parasitic wasps and brown lacewings are present though in low numbers. 

About aphid parasitoids

There are many wasp parasitoids that attack aphids in Australia, the most common of these come from the genera Aphidius, Aphelinus, Diaeretiella, Lysiphlebus, and Trioxys. Each parasitoid species attacks one or a few species of aphid. There is often overlap in their preference so that for each species of aphid there may be several parasitoids that attack it. Aphid parasitoids are small (less than 5 mm long) and usually dark in colour.

Aphid parasitoids can be effective at controlling low to medium density populations of aphids within crops. However, this assumes that parasitoid populations have been allowed to persist undisturbed by broad spectrum insecticides during late winter when aphid populations are beginning to build up. Softer chemicals, such as pirimor, can be used against aphids with minimal impact on beneficials.

Crops that are not moisture stressed have a greater ability to compensate for aphid damage and will generally be able to tolerate far higher infestations than moisture stressed plants before a yield loss occurs. This will be a factor when assessing relative numbers of aphids and aphid ‘mummies’.

The presence of aphid ‘mummies’ in crops is an indicator that parasitism has been occurring for sometime and that many more aphids are also likely to have been parasitised.

Our advice

We recommend close monitoring of aphids, and the ratio of aphids to ‘mummies’ as an indication of whether parasitoids are keeping on top of aphid populations over the following weeks.

Acknowledgements

Sources of field reports

Kris Dixon – Agronomist, Landmark (Victorian Wimmera)

Neil Hives – IPM consultant covering central, southern and western Victoria (based in Central Victoria)

Andrew McMahen – Agronomist, Ag Advantage (New South Wales Central West Slopes & Plains)

Cover image: Photo by Andrew Weeks, Cesar Australia