SWD series #3
Spotted-wing drosophila can develop on a wide range of cultivated and wild, soft-skinned fruits. This complicates pest management because SWD populations can move among several hosts with different ripening times throughout the year, allowing them to survive and reproduce in many environments.
Host susceptibility is influenced by such characteristics as color intensity, sugar content (°Brix), flesh firmness, penetration resistance of the skin, and acidity. Fruits become more susceptible to SWD as they begin to take on color and ripening progresses. Maturation indices predict the risk of infestation. For example, blueberry fruit are often not sprayed until the onset of coloration. As fruit ripens, sugar content increases, whereas penetration resistance, firmness and acidity decrease (Figure 2).
The increase in sugar content is the easiest way to determine fruit susceptibility and can be measured directly in the field with a refractometer. Fruit are usually too firm for SWD egg-laying when sugar levels are below 10°Brix. Above that threshold, the probability of SWD infestation increases by 50%.
Differences in susceptibility between cultivars are influenced by physical characteristics such as texture, firmness, and the force required to penetrate the skin. Selecting thicker-skinned cultivars of cherry, blueberry, peach and grape may reduce SWD infestation. SWD are less successful and spend more time trying to lay eggs in thicker-skinned fruit. In cherries, cultivars that are resistant to fruit cracking also limit SWD infestation and their population buildup.
Multiple studies indicate that SWD do not prefer wine grape as a host. The majority of grape cultivars resist SWD attack during the harvest period because of relatively high skin penetration resistance. Nonetheless, SWD are able to lay eggs in damaged fruit and intact fruit of soft-skinned varieties. SWD feed on damaged berries during the harvest period. When berries are cracked, diseased or damaged by birds, heavy rainfalls or hail, levels of SWD feeding and egg laying increase, along with the possibility of spreading spoilage bacteria and fungi.
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This publication is one of a series of nine publications focused on strategies for controlling spotted-wing drosophila in Oregon. Find them at https://catalog.extension.oregonstate.edu/. The publications in this series include: