INCREASING NURSERY CROP CANOPY DENSITY: IMPLICATIONS FOR SUSTAINABLE INSECT PEST MANAGEMENT

Consumers are attracted to woody ornamental plants that have symmetrical, dense canopies. In order to get the desired canopy density and symmetry, growers often manipulate growth by pruning or applying chemical plant growth regulators. Another method of acquiring a dense plant canopy is for growers to purchase in vitro-propagated liners instead of traditional cutting-propagated liners. This work analyzed the validity of all three methods on several woody ornamental species. Liners from Cutting-propagated (CP) and in vitro-propagated (IVP) sources were purchased and treatments of pruning and PGRs were applied. Pruning only increased the canopy density of rhododendron (Rhododendron L. ‘Roseum Elegans’) and was even more effective when IVP plants were pruned. PGRs were generally ineffective on all species with the exception of blueberry (Vaccinum corymbosum L. ‘Duke’). IVP clethra (Clethra alnifolia L. ‘Hummingbird’) and rhododendron had greater canopy density than their CP counterparts.

A dense plant canopy attracts customers more easily than a sparse canopy. However, as canopy density increases, the grower’s ability to achieve adequate spray penetration within the canopy decreases, causing insecticide application to be ineffective at controlling pests within the interior of the canopy. If insect pest populations within the plant canopy are not decreased by chemical application events, it is possible that natural enemy populations within the plant canopy will also be unaffected and therefore continue to aid in pest control. However, we cannot be certain that natural enemies will be within the plant canopy when an insecticide application occurs.

In order to achieve the most effective pest control strategy, growers should apply chemicals that control insect pests but do not harm natural enemies. Systemic insecticides are generally thought to be safer for insects that do not directly ingest the plant material. A worse-case scenario was conducted where natural enemies were trapped in arenas with residue of a systemic or contact insecticide. Reactions to both systemic and contact insecticides were inconsistent between three species implying that no insecticide is inherently “safe” for all natural enemies.