Utah Pests News Winter 2009-10

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Are Native Plants Resistant to Pests?

By Dr. Heidi Kratsch and Dr. Larry Rupp, Extension Ornamental Horticulture Specialists in the Department of Plants, Soils, and Climate at USU

Demand for ornamental plant species native to the Intermountain West is expanding rapidly.  People desire plants native to their region for their ability to attract native pollinators and other wildlife to their yard, to conserve water, and for their unique charm and beauty.  Purchasing native plants supports growers who produce these plants for their ornamental value or for use in reclamation and restoration of disturbed rangelands.  Many native plant proponents claim that using native plants decreases plant pest problems.  However, our observations do not bear this out.  In reality, the result is neither an increase nor a decrease in the incidence of pest problems.  Instead, growing native plant species often means a shift in the pest population, which may require an adjustment in approach to landscape and nursery pest management.  We oversee an ornamental native plant production and evaluation program at Utah State University, and work with an ever-changing variety of native plant species.  Here we share some of our experiences in dealing with pest problems during production of native plant species.

In pesticide trials with 4-month-old seedlings, we noticed that disease incidence tended to less be with native plant species, and some native plants appear to be resistant, even when inoculated directly with the disease-causing organism.  This was demonstrated in replicated trials with silver buffaloberry (Shepherdia argentea) inoculated with Phytophthora parasitica, and with Mexican cliffrose (Purshia mexicana) and seaside alder (Alnus maritima) inoculated with Phytophthora cinnamomi, (Phytophthora spp. are the causal agents of root rot).

This disease-resistance also may hold true for mature plants under landscape conditions, but only if plants are placed in the right spot in the landscape and maintained in a way that simulates the conditions to which they are native.  For example, penstemons are native to arid and semi-arid regions in North America.  They are sensitive to heavy, wet soils and will develop root-rot diseases under these conditions.  Growing penstemons in heavy clay soil, or over-watering plants in containers, can encourage development of disease because they are better adapted to faster draining soils lower in organic matter, which are likely to harbor a different variety of microbes.

Relatively small changes in the growing environment (such as the north side of a house versus the south side) may put a native plant in a distinctly non-native setting, where it may suffer from stress and become more susceptible to pests.  We often assume that because of their adaptation to a certain environment and co-evolution with beneficial insects and soil microbes, native plants are free from disease and insect problems.  But native plants have also co-evolved with the pests that are native to their region.  On the other hand, it is possible for an exotic plant to be pest-free when introduced to a new environment if there are no organisms adapted to attacking it.

Figure 1. Terminal growth of nursery-grown canyon maple on July 22 (left) and typical wild maple on June 9 (right). Note difference in terminal bud set and the presence of leaf tatter on the nursery-grown plant.

A nursery by nature is an artificial environment, unlike the environment to which plants are adapted.  Plants are grown under conditions that restrict root growth, either in pots or in the ground situated close to other plants.  A greenhouse is even more artificial because natural enemies that might protect species in their native habitat usually are not present in a greenhouse.  For example, although globemallow (Sphaeralcea) species are rarely affected in the wild, in an enclosed setting they are attractive to aphids, which attack the flower buds.  We have used horticultural oils and pyrethroids to control aphids in our native plant propagation greenhouse, but our best long-term solution to this problem was to get globemallows out of the greenhouse into the open air as soon as the weather permitted.

Nursery environments encourage growth patterns that can be very different from those in a native environment; plants may be exposed to pests or physiological conditions they may otherwise avoid simply due to timing.  In the wild, native canyon maple (Acer grandidentatum) typically sets a terminal bud in early June after 3-5 nodes have formed.  Similar plants grown in the same vicinity, but in an irrigated nursery, will continue to have active terminal buds well into July, and produce much longer shoots with many more nodes as well as secondary shoots from lateral buds.  As a result, the growing points and new leaves of nursery-grown maples may be exposed to conditions that simply do not exist at the time wild-grown plants are at the same physiological stage.  This difference exposes growing points and new leaf tissue of nursery-grown plants to stress in July, while wild plants have long since ceased growth before such stress can occur.  As a result, we commonly see leaf tatter on canyon maple in the nursery, but we rarely see it on maples growing in the wild.  We have noticed that thrips are present at the time of the appearance of leaf tatter in the nursery, and they are likely the cause.  Thrips are not present when wild maples are actively growing earlier in the spring (Figure 1).  On the other hand, we have never noticed ovipositor (the insect part that facilitates egg-laying) damage from cicadas in the nursery, while it is a common occurrence in the wild (Figure 2).

Figure 2.  Ovipositor damage from cicada on canyon maple (left) with subsequent flagging (right).  Flagging is the result of cicadas laying eggs inside small terminal branches; these branches eventually die and break off.

Nursery best management practices require an effective weed control program.  The weeds we find in our nurseries are typical of those found in most production agriculture settings in northern Utah.  However, in situations where we have grown plants under low-volume or drip irrigation, it is possible to see a shift in the species of weeds present.  For example, weeds such as redroot pigweed (Amaranthus retroflexus) may disappear, while others such as Russian thistle (Salsola kali) may begin to invade.  One weed is not worse than the other, but it is important to realize that weed-control strategies may need to change with the shift in weed species.

Finally, native plant species have varying degrees of susceptibility to pesticides, and show phytotoxicity to chemicals that are not a problem for more common horticultural varieties.  Unless cultivated varieties are available, most native plant species have not been tested for potential toxicity to pesticides.  For this reason, we recommend cautious use of pesticides on native plant crops, with an emphasis on “soft” controls such as insecticidal soaps and horticultural oils, and use of weed barriers in planted areas.  Cultural controls can also be effective.  We recently redesigned our outdoor plant holding area with separate irrigation zones for optimal care of plants with different water needs, and we stock planting substrates with various levels of organic matter for optimizing the plant root-zone environment.

Including native plant species is a great way for smaller nurseries to develop a “niche” market, and for all nurseries to diversify their inventory to respond to the increasing demand for regional native plants.  Extra attention to potential changes in pest populations can help growers to protect investments.