White, cotton-like masses represent a serious pest of apples in Australia: the woolly aphid.
(continued from last month)
Insecticide field trials
Soil drenching
Failure to control aerial infestations can result in underground infestations on susceptible rootstocks.
In order to control root colonies, insecticides registered for use as a soil drench should be applied.
A soil drench can be used on trees up to seven years old.
To obtain efficient woolly apple aphid control, it is important that the active roots take up a sufficient quantity of product.
This is achieved by ensuring the soil is moist prior to application (to prevent the product from running away from the treated area) and ensuring the application zone is weed free.
The most commonly used insecticide for this purpose is imidacloprid. Imidacloprid has favourable physical-chemical properties such as sufficient soil half-life, and dissipation which allows it to last and work longer in soils.
This means the winter cycle of woolly aphid will be broken along with high leaching potential/soil adsorption coefficient, which allows penetration to root galls.
On blocks where we applied root drenching with systemic insecticides, imidacloprid and spirotetramat, the efficacy on control of woolly aphid on the canopy was 100%.
Eradication
To achieve eradication, woolly aphid needs to be eradicated from all parts of tree.
After inspection of the trunk base, we discovered that some colonies survived. Therefore, this method of controlling woolly aphid has not provided eradication.
Combined foliar plus drenching
As woolly aphid has a life cycle on the tree and in the ground, we combined the two control methods in order to cover all stages and colonies.
The result of the combined application was 100% efficacy. Therefore, foliar application of systemic insecticides with root drenching provides eradication of this pest.
Conclusion
We need to ask, do we need 100% efficacy in the control of woolly aphid; do we need to achieve eradication at all; and what are the economic thresholds?
To answer those questions, similar trials will need to be conducted and other parameters to be observed such as predators and costs.
Complete eradication would be the most expensive method and would not leave any food for predators. Therefore, future trials, the emphasis should be on predator monitoring and costs of woolly aphid control program.
See this article in Tree Fruit April 2017
