When Woolly Apple Aphid gets ‘wired’

Henry Schneider and Kim Andrews

Department of Primary Industries (DPI) researchers are collecting electrical data to better understand the feeding behaviours of the Woolly Apple Aphid (WAA), in the hope of understanding how to manage the pest insect and prevent costly crop losses to apple growers. The project is funded by Horticulture Australia and based at DPI Rutherglen Centre. The WAA is a major pest in apple orchards and controlling the pest is often difficult and costly. An infestation can be detrimental to the fruit quality of the current season, and may also impact the following season’s flowering capacity. This project aims to interpret WAA feeding behaviour on a range of apple trees in an effort to better understand what makes apple trees attractive to WAA (image 1). Individual WAA will be ‘wired’ by attaching a very fine gold wire, about the diameter of a human hair, onto the back of the insect. A metal probe is inserted into the soil of a potted apple tree (image 2). When the aphid penetrates the plant surface – like a mosquito puncturing human skin – an electrical circuit is completed and the aphids feeding behaviour is electronically recorded. This ‘wired’ technique is known as Electrical Penetration Graph (EPG). EPG has previously been used to examine the feeding behaviour of a wide range of sap-sucking insects. The unique signals recorded by EPG allow for the interpretation of WAA feeding behaviour, providing knowledge of the apple trees’ resistance mechanism against insect damage. Dr Kim Andrews is undertaking this research to determine why WAA may find certain apple sap tastier than others and a list of possible feeding responses for WAA have been identified (table 1).

Image 1: Woolly Apple Aphid infestation on Granny Smith

Electrical signal	Feeding behaviour	Resistance rating (possible mechanism)
None	No feeding	Strong resistance (repellent)
Cell penetration patterns only	Plant surface penetration but no feeding	High Resistance (structural)
Cell penetration and short periods of feeding patterns	Plant surface penetration followed by multiple “tasting” events	Resistant (nutritional)
Cell penetration and extended periods of continuous feeding patterns	Plant surface penetration followed by prolonged feeding events	Susceptible (no structural or nutritional blockers to WAA feeding behaviour)

Studying insect feeding behaviour, on a range of apple trees, under controlled conditions, will allow us to predict WAA infestations under different management conditions in the field,” Dr Andrews said. Dr Andrews will initially monitor WAA feeding behaviours on Granny Smith apple plants, and then compare the EPG signals with a range of apple rootstocks, including Northern Spy and MM106. Future research projects could utilise EPG to investigate WAA feeding behaviour under a range of management regimes to interpret the impact of fertiliser and water application on the pest insect.

Image 3: Dr Kim Andrews preparing apple trees to be wired with WAA at DPI Rutherglen Centre