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Plant architecture is the spatial arrangement of plant parts, such as leaves, branches and stems. The number of leaves and branches primarily determines plant architecture. There are many plants on the market with complex architectures because consumers often prefer selections with dense plant canopies. That plant architecture can influence the effectiveness of contact insecticides in managing insect pest populations.

Procedures that change the plant architecture or canopy complexity, such as pruning or applying plant growth regulators, create dense plant canopies that can influence the effectiveness of insecticide spray applications. An increase in the plant architecture or plant canopy density can negatively affect foliar applied insecticide spray applications by reducing penetration into the interior plant canopy, resulting in fewer insect pests killed.

Plant architecture may negatively affect insecticide spray applications by decreasing penetration into the interior plant canopy. Insecticide applications are generally more effective in managing insect pest populations when plants are small or have a sparse canopy. When spray coverage is not sufficient within the interior plant canopy, insect pests that reside within and feed on stems or branches in the interior plant canopy avoid exposure to insecticide spray applications and subsequent residues.

Effect of leaf size on insecticide spray applications

Leaf size may reduce the effectiveness of insecticide spray applications. Plants with large leaves obstruct insecticide spray applications from penetrating the interior plant canopy, resulting in insufficient spray solution contacting insect pests residing and feeding within the interior plant canopy. Consequently, insect pests will continue to feed and reproduce. Hence, the insecticide spray application will not provide effective management of insect pest populations to prevent subsequent plant damage. For example, hydrangea leaves, because of their size, can inhibit insecticide spray penetration, reducing the volume of spray solution entering, which will decrease exposure of insect pests to insecticide spray residues.

Photo © Raymond Cloyd

How to overcome effects of plant architecture

Spraying plants from the bottom of the canopy at a 45-degree angle upward and/or increasing the spray volume will increase spray penetration into the interior plant canopy. Use water-sensitive paper to verify spray coverage, and determine if adjustments in spray technique (e.g. slowing down or increasing nozzle distance from plants) or equipment (e.g. changing spray nozzles or flow rate) are required.

In conclusion, it is important that producers understand how plant architecture can affect insecticide spray applications, which will allow for adjustments to ensure sufficient coverage of all plant parts, especially within the interior plant canopy, thus resulting in high mortality of insect pests.

Raymond A. Cloyd is a professor and extension specialist in horticultural entomology/plant protection in Kansas State University’s Department of Entomology.