Modifying the landscape to reduce spotted wing drosophila populations in Michigan cherry orchards

Project goal: To investigate cultural strategies that make orchard landscapes less suitable for SWD

Project description:  Data suggest that SWD prefer high humidity microhabitats that are conducive for their development.  We conducted experiments to modify the orchard landscape with: 1) pruning strategies, 2) under tree canopy manipulations, and 3) between row modifications.  Pruning strategies included no pruning, 25% more pruning than a ‘normal’ year, and 25% less pruning than a ‘normal’ year.  Under tree canopy manipulations included: wood chips, weed fabric and clean cultivated/herbicide.  Lastly, we looked at the following between row modifications: grower standard sod row centers mowed every other week from 1 May – 15 August; sod row centers not mowed throughout the season; clean cultivated row middles using herbicide to maintain bare ground; and clean cultivated row middles using herbicide followed by cultivation every two weeks.  Temperature and humidity dataloggers were placed in each treatment, two in the tree canopy and two at ground level.  SWD populations were monitored weekly with pheromone baited traps and fruit was sampled for larvae at harvest.  In 2018, we repeated the pruning strategy and between row modifications and started collecting yield per tree data.

The pruning strategy treatments were repeated 2017 – 2020.  This data has helped us establish annual pruning regimes that minimize SWD infestations and maintain annual yields over time with every other year pruning strategies.

In 2019, to test how SWD behave in different canopy conditions, we conducted a small-scale mark and recapture study in a 6-year-old block of Montmorency tart cherry trees. The block was used for a cherry leaf spot efficacy trial, and at the time of SWD release, half of the block was defoliated due to high levels of leaf spot infection and the other half had trees with a full canopy as the trees were treated with fungicides. On 4 Sept., we released 3800 lab-reared SWD adults marked with fluorescent dye from a central point in the block between defoliated trees and full canopy trees. Twenty-four red sticky panel traps baited with Scentry lures were placed on diagonal transects to capture flies as they dispersed Traps were left in the orchard for 30 hours. Traps were brought back to the lab and flies were counted under a dissecting microscope with the use of a black light. We captured 19 SWD in trees with full canopies, and 6 SWD in defoliated trees.

The second objective 2019 tested light penetration and fruit temperature and the effect on SWD infestation.  We measured light, temperature, and relative humidity in five trees in each of the aforementioned pruning treatments. Canopy temperature and relative humidity was measured from 15 June-15 August using Onset Hobo Pro v2 data loggers. Additionally, we collected fruit temperature measurements 3x/week in the four areas of the tree canopy: 1) inside high, 2) inside low, 3) outside high, and 4) outside low. We also evaluated the attenuation of photosynthetic photon flux (PPF) through the canopy by taking radiation measurements with the midpoint of the line sensor aligned with the tree trunk and in two directions within and across row. We also evaluated fruit for natural SWD larval infestation by collecting 100 fruit weekly from 15 June-15 August from each of the treatments and each region of the tree canopy.

Results: Pruning strategies to help reduce SWD populations in MI cherry orchards 2019

Project lead/collaborators: Nikki Rothwell, Emily Pochubay, Larry Gut, Julianna Wilson

Funding sources:  Project GREEEN, Michigan Cherry Committee

Start year: 2017

End year: 2020