Characterizing Spray Penetration Following Insecticide Application in Dense and Sparse Oakleaf Hydrangea 'Alice'

Nursery crop producers often rely on pesticides to manage arthropod pests.  However, cultural practices can influence plant architecture which may, in turn, affect pesticide penetration and pest control.  The objectives of this study were to examine spray penetration in dense and sparse canopies and assess if canopy density affects beneficial insect survival following insecticide application with the goal of better understanding the implication of canopy density on pest management for select container-grown plants.  To characterize spray penetration, water sensitive cards were placed on the exterior, middle and interior of oakleaf hydrangea ‘Alice’ (Hydrangea quercifolia ‘Alice’) plants with dense or sparse canopies.  The exterior position served as a check to ensure that both canopy types received the same amount of coverage.  Water was applied with a CO₂ sprayer and cards were analyzed for percent coverage and droplet density using DepositScan software.  These results were compared to the outside control to get the percentage of the maximum coverage and droplet density received.   The dense and sparse middle positions only received 2% and 17% coverage, respectively, of the total coverage applied. The interiors received 1% and 5% coverage of the maximum, respectively. In the dense canopy, middle and interior coverage was not different, but in the sparse canopy, the interior positions received 71% less coverage of the maximum than the middle position, making the interior more protected from spray than the exterior.  Percent of maximum droplet density paralleled that of coverage.  The middle positions received 3% and 26% of the maximum for dense and sparse canopies, respectively, and the interior positions received 3% and 14% of the maximum.  Again, the middle and interior position of dense hydrangea canopies were not different; however the sparse interior received 46% les droplet density of the maximum than the sparse middle and was not different than the dense middle and interior.  In this study, spray coverage in the interior of plants was low regardless of plant architecture indicating that the interior of a plant may not be sufficiently penetrated for adequate arthropod control but may serve as a refugium for beneficial insects.