Are Native Plants Resistant to Pests?
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.
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 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.