Integrated Pest Management of Sunflower Insects
This is the fourth in a series of articles addressing the Integrated Pest Management (IPM) of insects in sunflower.
Last month, we discussed a specific insect management approach called “biological control” — a tactic which uses predators, parasites and disease to reduce insect pest densities. This current article addresses plant resistance to insects, which is another biologically based method for managing insect numbers and the damage they cause.
The cultivation of plant varieties that resist damage by insects actually dates back thousands of years. When farmers saved seed for the next season’s planting, they inadvertently made plant selections based on a variety of criteria. One criterion was insect-resistant plants wherein (1) the insects did not attack; (2) the insects did poorly on; or (3) the plant itself showed less damage.
Two early examples of purposely developed plant resistance to insects are (1) wheat varieties resistant to the Hessian fly and (2) apple cultivars with resistance to the woolly apple aphid. In a very important development in the late 1800s, resistant U.S. rootstocks were grafted onto French grapes susceptible to a certain underground aphid. That “foreign aid” saved the French wine industry from ruin.
Plants may have physical or chemical traits that repel, kill or enable them to tolerate pests. These traits can be utilized by plant breeders in developing cultivars that possess superior agronomic features and also are resistant to insect pests. The use of resistant plants is both a cost-effective and environmentally sound tool for pest management. It also can be used in conjunction with other strategies to manage pests more effectively in the field. Traditional breeding methods have been used for years to develop insect-resistant plant varieties. Today, genetic engineering technique are used in some crops to supplement traditional breeding and shorten the time frame to develop insect-resistant varieties.
Plant resistance is defined as “plant qualities that are genetically inherited and result in one cultivar being less damaged than a susceptible plant in which those particular qualities are absent.” It is rare for a plant to be totally immune or completely resistant to a particular insect.
Resistance to insects is thus measured in degrees, with the amount of “acceptable” damage depending upon many factors. For high-value crops where the insect attacks the harvestable product (e.g., fruit to be sold fresh), there’s a very low damage tolerance. However, if leaves (rather than fruit) of the plant are being consumed, some feeding damage can often be tolerated before a measurable loss of yield or fruit quality occurs. In that case, the amount of needed resistance can be lower. The relative resistance needed to protect a crop is an important factor in the development and success of a breeding program.
Scientists categorize resistance to insects as being one of three types:
• In “antibiosis,” the biology of the insect is affected, meaning the insect may be destroyed or there might be an impact on its development or reproduction.
• Nonpreference, or “antixenosis,” conveys to the pest that the plant is a bad host (e.g., tastes bad or is unsuitable to lay eggs on). So the insect moves on or selects another plant.
• “Tolerance” means the plant is able to outgrow or overcome the damage caused by the pest when compared to the damage suffered by susceptible varieties. So even though insects are present and feeding, yield remains high.
Of course, there is overlap among the resistance categories. The degree to which a plant exhibits resistance to a particular pest can be affected by environmental conditions and by the size of the insect population.
The major advantage of pest-resistant crops is insect control with lower input costs. With pest-resistant varieties, some or all of the insect management costs are incorporated into the cost of the seed itself.
Among the additional advantages, (1) the effects of plant resistance are limited to the specific pest, (2) the effects accumulate over time (i.e., overall pest populations are reduced, and (3) pest resistance is persistent. There’s also the option of combining other nonchemical management strategies (e.g., biological and cultural controls) with pest resistance to further lower the pest-caused damage. (This is particularly important when crops are attacked by multiple pests.) Finally, plant resistance to insects does not pollute the environment and is nontoxic to humans, natural enemies of the pets, bees and other pollinators, livestock and wildlife.
Research on a number of sunflower pests has shown there is potential for the development of insect-resistant cultivars by using genes from the wild sunflowers (Helianthus spp.). Since sunflower is native to North America, wild sunflower has been subject to attack by many species of insect pests over thousands of years. In response, many wild sunflowers evolved mechanisms to protect themselves from insects and their feeding.
For example, perennial Helianthus tuberosus L. (Jerusalem artichoke) and two other Helianthus species have increased the mortality of banded sunflower moth larvae and also have reduced egg laying by the adults. Certain wild sunflower species also have lower densities of red sunflower seed weevil larvae, while others have been shown to reduce the survival rates of sunflower beetle and stem weevil larvae.
Incorporating pest resistance from these wild sunflower species into commercial lines could be helpful in developing pest-resistant sunflower hybrids. A major obstacle is the fact that many of the wild sunflower species with insect resistance are perennials, which makes the transfer of pest resistance to commercial hybrids difficult.
Insect resistance may be the only practi-cal control method for some insect pests of sunflower. The sunflower midge has been a severe problem in parts of the Red River Valley of North Dakota, Minnesota and Manitoba. Midge are difficult to detect, and insecticide applications are not effective. Even delayed planting may not be effective since the midge has several generations per year. Research has shown that a number of commercially available hybrids do have resistance to the midge. The discovery of a method by which sunflower cultivars can be screened with synthetic chemicals will assist scientists in their search for resistance to this insect.
Research from other parts of the world may help develop resistance to some of our sunflower pests. In Europe, resistant sun-flower varieties are used to reduce damage from larvae of the European sunflower moth. A material called “phytomelanin” is present in seeds of the resistant varieties and greatly reduces larval feeding of this insect. Some U.S. research has been con-ducted to identify lines of sunflower with greater amounts of phytomelanin in their seeds. Transferring this trait to commercial hybrids could have important implications for areas of North America where the sunflower moth is an economic pest.
The screening of sunflower lines for resistance to both the red seed weevil and banded sunflower moth has been conducted in USDA-ARS and North Dakota State University field trials for a number of years — and some gains have been made in identifying germplasm resistant to these two insects.
For example, researchers have discovered four lines that reduce numbers of red seed weevil larvae in sunflower heads by 25 percent. This resistance was based on antixenosis (nonpreference) since adults laid fewer eggs on the heads of these lines compared to susceptible lines.
The ability to detect resistance to insect pests in plants is largely dependent upon field research in which different lines are compared when subjected to attack by natural pest populations. The difficulty with this screening process lies in the inconsistent nature of pest populations from year to year. High pest densities enable the scientist to clearly detect lines offering some type of resistance; low populations, however, make these evaluations difficult. In many cases, there are ways to artificially rear the insects and apply them to the plants; or, there are artificial techniques which can simulate damage to the crop and thus aid in evaluations.
Development of insect resistance can be a very time-consuming process. But its advantages as a pest management strategy make it a tactic worth pursuing.
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