Encountering resistance

Insects cause millions of dollars in damage to woody ornamental plants each year. In addition, these pests are often expensive to manage. You must not only consider the cost of replacing dead or severely damaged plants, but also the cost of scouting and monitoring pests and of treating the infestations.

Integrated pest management is the accepted standard approach for dealing with pests. However, a key component of IPM — resistant plants — is greatly underused. Fortunately, that is starting to change.

The resistance movement

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To effectively initiate integrated pest-management programs in landscapes, you must combine several insect-control techniques. If properly instituted, IPM can reduce pests to tolerable levels with fewer control inputs. Your techniques can include pest-quarantine efforts, the use of pest-resistant plants, cultural and physical controls and chemical or biological controls.

Numerous studies have outlined cultural and physical control techniques. Also, chemicals, especially new controls with highly specific targets and application timing, are an essential tool of IPM. However, information on resistant ornamentals is lacking. Therefore, the selection and use of pest-resistant plant materials has not been widely adopted, despite the obvious benefits.

In the past, the criteria for the selection and development of ornamental plants centered on the aesthetic qualities of the plant. These attributes include growth habit, leaf texture, flower color and fall color. Breeders placed little or no emphasis on the resistance or tolerance to various pests. However, as IPM programs have evolved, researchers are beginning to evaluate plants for their aesthetic qualities as well as for their resistance or tolerance to insects and diseases. Some varieties are available now, and more are coming to the marketplace every year.

Ornamental plants exhibit resistance to insect pests through two mechanisms. The first is often referred to as non-preference resistance. The plant has characteristics that impair the insect's ability to use the host plant for egg laying, food or shelter. The characteristics of the host plant can be either chemical (the plant contains a noxious compound that repels the insect) or physical (the plant leaf has long hairs that prevent egg laying or feeding). The second type of resistance is termed antibiosis. With this type of resistance, the insect's metabolic processes are affected as a result of feeding on a resistant plant. Insects feeding on plants with this type of resistance may experience death, reduced growth rates, smaller adults with reduced numbers of eggs, a shortened lifespan or physical deformities.

Recent research

Research recently conducted on pest-resistant or tolerant plants for use in landscapes has expanded the information available. Examples include work on lantana, butterfly bush, hollies, azaleas and ornamental crabapples. The University of Georgia (UGA) recently tested various cultivars and found deciduous azaleas that are resistant to azalea lace bugs, hollies that tolerate spittlebugs and wax scale and butterfly bush that are resistant to two-spotted spider mites.

Under laboratory and field conditions, scientists evaluated 17 species or cultivars of native deciduous azaleas for their resistance to lace bugs. The researchers found high levels of resistance in three azalea species: Rhododendron canescens, R. periclymenoides and R. prunifolium. They also found that the most susceptible azaleas they tested include R. oblongifolium, R. alabamense, R. serrulatum and R. viscosum ‘Buttercup’ and ‘My Mary’.

In other trials, the Georgia researchers screened 137 holly species and cultivars for resistance to adult feeding by the two-lined spittlebug. The research concluded that the species generally resistant to damage include Ilex vomitoria, I. cornuta, I. glabra and I. verticillata. They found that the most highly susceptible hollies were I. cassine and I. opaca, as well as the hybrids from these two parents.

Additional trials evaluated holly resistance to wax scale infestation. The results indicate that Ilex crenata, I. buergeri, I. glabra, I. myrtifolia, I. verticillata and I. vomitoria are the most resistant to wax scale infestation. Ilex aquifolium, I. attenuata, I. cassine, I. ciliospinosa, I. cornuta, I. opaca, I. purpurea, I. rugosa and I. serrata were the most susceptible to wax scale infestation.

Also, 37 species and cultivars of Buddleia (butterfly bush) were examined for resistance to two-spotted spider mites. Buddleia fallowiana ‘Alba’ and B. davidii x B. fallowiana ‘Cornwall Blue’ were found to be highly resistant. The researchers reported that the less pubescent (hairy) Buddleia species such as ‘African Queen’ and ‘Gloster’ were the most highly susceptible.

The University of Tennessee and the University of Kentucky recently researched ornamental crabapple tolerance and resistance to feeding by caterpillars and Japanese beetles. They found that Malus baccata ‘Jackii’, ‘Harvest Gold’ and ‘Jewelberry’ were the least preferred by Japanese beetle adults; ‘Radiant’, ‘Red Splendor’, ‘Royalty’, ‘Liset’ and ‘Ruby Luster’ were preferred the most. At the University of Tennessee, researchers found that ‘Sargent’, ‘Coral Burst’ and ‘Brandywine’ cultivars were least preferred by both leaf rollers and tent caterpillars, while ‘Red Baron’, ‘Winter Gold’ and ‘White Angel’ were preferred the most.

Using resistance as an indicator

Pest-resistant plants provide you with two advantages. First, you can use pest-resistant varieties where specific pests are present and difficult to manage. Second, you can use indicator plants that are highly susceptible to specific pests. These plants may already exist in the landscape, and you can use them as an “early warning system” in scouting programs to detect and monitor pest problems.

Monitor indicator plants on a regular basis when conditions are favorable for development of the pest. By using this strategy, you can detect problems before insects become damaging. Monitor indicator plants to determine developmental stages. They are essential for proper timing of chemical applications. You can use indicator plants to monitor pests that cause damage in other parts of the landscape as well. For instance, Japanese beetles cause extensive damage to turf in immature (grub) stages. If you see numerous adults on ornamental plants, you may need to consider chemical control options in turf.

Research conducted at the University of Georgia on 11 lantana cultivars shows varying degrees of damage from whitefly or lace bugs. While no evidence for complete resistance among lantana cultivars was found in these tests, the researchers did find that larger-leafed cultivars were highly susceptible and exhibited the most significant damage from the pests. By monitoring the most susceptible lantana cultivars (‘Miss Huff’, ‘Confetti’ and ‘Radiation’) on a regular basis, you can detect whitefly and lace bugs early and take control measures before the pest populations explode.

How can you resist?

To put plant resistance to work for you, select and use plant materials that are resistant to, or more tolerant of, specific pest problems in your area. This is the single most important factor in the development your pest-management program. During landscape planning and establishment, you have the opportunity to prevent or limit certain pest problems by selecting the proper plants. Resistant plants will help you maintain an attractive landscape with fewer control inputs.

Numerous books discuss pest-resistant plants. Consult your local state extension office to get information on varieties that will grow well in your area. Also, talk with your plant suppliers; they should be familiar with new and old introductions that are appropriate. As more research defines which plants tolerate pests, this information should become more common and easy to find.

More regulations targeting the use of broad-spectrum, long-residual insecticides are being enacted. More than ever, you need to monitor pest populations and tune into your landscapes. Be aware of how you can use existing plants as indicators for potential pest problems. This will increase your options, including both cultural and chemical controls.

Dr. Beverly Sparks is a professor of entomology at the University of Georgia, (Athens, Ga.).

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