- Adult and nymph aphids.
Photo: P Reid
A wide range of aphid species can affect field crops. Most are small (only a couple of millimetres long) with oval-shaped green, brown or black bodies. Often occurring in colonies, aphids suck on sap, causing loss of vigour, and in some cases yellowing, stunting or distortion of plant parts. Honeydew (unused sap) secreted by the insects can cause sooty mould to develop on leaves. In crops such as cotton, the honeydew affects fibre quality. Aphids can also be vectors (carriers) for viruses.
Their feeding rarely causes major damage in most crops, and control measures are usually not warranted, as a range of parasites and predators keep population numbers down. Exceptions may be where the crop is under moisture stress, or where heavy populations are observed (high populations aggravate the effect of moisture stress). For example, between 30 to 50 aphids per plant could be used as a threshold level for wheat.