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Aphid monitoring and thresholds

We have received numerous reports of aphids present in high numbers in crops in parts of Victoria and New South Wales. These reports, along with the onset of warmer temperatures as we move into spring, should serve as a timely reminder to commence aphid monitoring in crops if you have not already begun to do so. Aphids move into crops in autumn from alternative host plants, usually from weeds found on roadsides and verges where they over-summer. They build up on crop edges before moving throughout the crop. Population ‘explosions’ can occur within a very short period of time (i.e. just a few weeks) when favourable conditions prevail.

Several growers and agronomists have recently expressed some concern regarding thresholds for controlling aphids in cereals and canola. Economic thresholds should be used as a guide to implementing a control strategy when the cost of the potential damage outweighs the cost of control. However, for aphids it can particularly difficult to determine thresholds as there are many factors that will affect the damage caused by a given aphid density. For example, local weather conditions, the growth stage of the crop, the crop’s yield potential, the time remaining until harvest and the potential transmission of aphid vectored plant viruses can all be important considerations. For these reasons, economic thresholds for aphids have the potential to differ substantially between regions and over time.

When monitoring crops for aphids it is important to check representative parts of the entire paddock. Check at least five points in the paddock and look for aphids on a minimum of 20 plants at each point. Typically, cooler weather conditions and/or significant rainfall will slow the rate of aphid development, whereas warm and relatively dry conditions will favour the rapid build up of populations. Greater aphid build-up leading to higher yield losses is also more likely when crops suffer moisture stress. Click here for further information on insect pest control thresholds.

Aphid monitoring should also include looking for beneficial insects, and these should be considered before deciding on control strategies. Beneficial insects include parasitic wasps, ladybirds, hoverflies and lacewings. They are usually prevalent in spring and provide a reliable form of biological control when low to moderate numbers of aphids are present. If chemical control is warranted, selective insecticides (such as pirimicarb) are available, which are aphid specific and less harmful to beneficial insects.

A reminder that many insecticides applied to broad-acre crops can also have negative impacts on honeybees, particularly when they are applied to flowering crops. Bee poisoning can occur in several ways including direct spraying, foraging on sprayed foliage, bees accessing water containing pesticide residues, and spray drift onto bees, their hives or flowering plants. Click here for further information, including steps that both landholders and apiarists alike can take to help manage the risk of bee poisoning.

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