Benefits of Insectary Plants

Americans spend millions of dollars annually on insecticides to protect ornamental plants from insect damage. These pesticides are an economic burden and a source of environmental contamination. They also pose a risk to human health and non-target organisms. With funding from a CSREES National Research Initiative grant, researchers at the University of Illinois have been studying the role of insectary plants (i.e., those attractive to insects) in minimizing insect pest damage to Scotch pine trees and to wintergreen groundcover. The research grant, which was funded from 2001 through 2004, may eventually lead to environmentally benign and cost effective methods of protecting ornamental landscapes from pest insect damage.

The research consisted of replicates of a mini-landscape. A Scotch pine (Pinus sylvestris) or a grouping of wintergreens (Euonymous fortunei ‘Colorata’) were set in an area of turf and hardwood mulch and infested with armored scale insects (Pine needle scale - Chionaspis pinifoliae and Euonymous scale - Enaspis euonymi). Four insectary plants were used: white clover (Trifolium repens), goldenrod (Solidago canadensis ‘Golden Baby’), cushion spurge (Euphorbia epithymoides syn. E. polychroma), and threadleaf coreopsis (Coreopsis verticillata ‘Moonbeam’). These flowering insectary plants were installed around the tree or groundcover at either a high or low density level. One-third of the plots were left bare as a control. The numbers of beneficial insects (predators and parasitoids) were then counted periodically, and the number of killed or parasitized scale insects determined.

Beneficial insects were as much as ten times more abundant in the insectary plantings as in the control plots. Mortality of the pest scale insects (caused by natural enemies) was up to double the amount in the insectary plantings as in the bare plots. The researchers found the diversity of plants explained the increased levels of beneficial insects and these numbers remained higher, even when researchers artificially removed all of the flowers during one portion of the experiment – showing the increase in beneficiary insects was not due to an increase in pollen and nectar rewards.

The use of appropriate insectary plants has enormous potential to reduce pest populations in the urban landscape. This technique combined with proper spatial positioning of the ornamentals, diverse plantings, and the avoidance of susceptible ornamental plants can prevent the need for insecticidal use. Less dependence on pesticides will lead to a cleaner, healthier environment and less harm to beneficial insects and other wildlife species.

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