Making decisions about control of Cereal Aphids
This post is an update on cereal aphid management following a number of enquiries from growers and agronomists over the last week or so.
Which species in crops?
There are two species of aphid you are most likely to encounter in winter cereals (oats, wheat and barley). They are the oat aphid (Rhopalosiphum padi) and the corn aphid (R. maidis). The oat aphid is found around the base of the tillers, and the corn aphid in the whorl and under leaves higher on the plant. Both aphid species are greenish to black with rusty red-purple areas on the rear end around the ‘tail’.
(Line drawings from DPI Victoria’s Insectopedia)
See the Northern Region Ute guide for more detailed descriptions and pictures – or see August 23 posting on the Beatsheet Blog
How much damage can aphids cause?
The key question about aphids is “will the population of aphids in my crop cause damage to the crop, and yield loss?”
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.
West Australian 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).
Overseas research (Canada, US) suggests that significant yield loss occurs when aphids are present from flowering through to milky grain. The data also suggests that yield loss does not occur when infestations are present earlier or later than this period.
Making control decisions
Corn aphids may disappear by themselves
Corn aphids, the species that lives in the whorl, generally disappears when the crop comes into head. This is because their preferred site is no longer there, and they tend not to survive as well on leaves. If you have the corn aphid in crops, consider delaying a control decision until the crop starts to head.
Qld and WA threshold recommendations are comparable, with the WA recommendations based on the most recent research that has been undertaken in Australia.
Recommendation are to check crops regularly from late tillering, and consider control if the aphid population exceeds 15 aphids/tiller on 50% of tillers.
Other considerations when making a decision about cereal aphids
- Natural enemies (lady beetles, hoverflies, parasitic wasps) can have a big impact on aphid populations, reducing them to very low levels in many instances.
- Dimethoate and synthetic pyrethroids (registered for cereal aphid control) are highly disruptive to natural enemies. The application of these insecticides early (e.g. during the vegetative and early tillering stages) 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 (Pirimor®) is a soft option for cereal aphid control, compare its price with that of dimethoate when making a decision – it may be competitive. Pirimicarb has some systemic activity.
- Oat aphids, at the base of the plant, can be difficult to contact in a dense crop. Dimethoate will kill aphids by contact, but its systemic activity is by translocation upwards, so its efficacy against oat aphid is unclear.
- The systemic activity of both pirimicarb and dimethoate may be reduced in moisture stressed crops.
- Rain will reduce aphid populations by knocking/washing individuals of plants, and the aphids tend not to get back on the plants. Often ground predators, like carabid beetles, ants etc will eat aphids on the ground. Check populations of aphids again if you get more than 20 mm rain.