Managing resistance-Part III: Insecticides
Plants and insects have co-existed for millions of years. Because many kinds of insects attack plants, plants have evolved many natural insecticides that kill or repel insects. Insects have responded by evolving mechanisms to detoxify or survive the defensive chemicals that plants produce. As a result, many insects are pre-adapted to resistance to insecticides.
For any characteristic, tremendous variability exists in natural populations of any organism. Some people run much faster than others. Pollen makes many of us miserable every spring but leaves others unaffected. These are examples of natural variation within a population. Insect populations are similar with respect to resistance to insecticides-some insects detoxify insecticides (natural or manmade) better than others. If you spray an insect population with an insecticide, you may kill 90 percent of the insects; you may even kill 99 percent of them. It is unlikely, however, that you will kill them all. The insects that survive often have some trait that makes them less susceptible to the insecticide.
Offspring reflect their parents. For example, tall people generally have tall children. If we limit the population to a particular characteristic, the offspring are more likely to carry that trait. Suppose a population existed with only tall people-most of their children would be tall too.
Within a population of insects, susceptibility to insecticides varies because susceptible females mate with resistant males and vice-versa. Consequently, the population remains a mix of resistant and susceptible individuals. However, an insecticide application removes many of the individuals that are vulnerable to the insecticide (that is, it selects for resistance), increasing the proportion of resistant individuals. Therefore, resistant females are more likely to mate with resistant males, and their offspring are more likely to carry the trait of resistance as well. Continued applications of the same insecticide increase the problem until most of the insects in the population are resistant to the insecticide.
Although resistance can occur in many different situations, we know that several factors increase the risk of insecticide resistance: *Regularly treating the insect population with the same insecticide or insecticides within a single chemical class * Short generations, with multiple generations per season (such as aphids, thrips or mites) *An isolated population (susceptible individuals are unlikely to enter the population and "dilute" the resistance of the population).
These conditions are common in greenhouses. Growers use insecticides frequently because economic thresholds are low for greenhouse crops. Plus, greenhouse conditions promote rapid population growth and short generation times. Enormous populations of aphids, thrips or whiteflies can develop quickly. Further, individuals that may be susceptible to the insecticide are excluded from the greenhouse. As the successive insecticide applications eliminate susceptible individuals, the remaining resistant females mate with resistant males, producing a new generation that is even more resistant.
Insecticide resistance is common in populations of Western flower thrips, silverleaf whitefly and some aphids. The resistant insects probably originally developed in greenhouses and then spread to other crops and locations. When you receive plants from greenhouses or nurseries, inspect them carefully for the presence of insects and diseases. Inexpensive plants are no bargain when you have to replace them or spray them repeatedly.
When a new, effective insecticide becomes available, it is common for every company to use the same chemical to control the same pest every time. This behavior encourages the rapid development of resistance. Your local professional society may be able to facilitate cooperation among companies so that everyone rotates among different insecticide classes. This policy would help preserve the efficacy of the pesticide for everyone while also preserving fair competition. Unfortunately, this type of program is difficult to implement in a highly competitive business.
Avoiding insecticide resistance It often is easier to avoid a problem than it is to correct one. Populations of resistant insects can be difficult to control. Therefore, follow these recommendations to prevent or delay insecticide resistance. * Rotate among insecticide classes. Although many different individual insecticides are available, the number of insecticide classes is limited. Diazinon, malathion, acephate (Valent's Orthene) and chlorpyrifos (Dow's Dursban) are all in the same class: organophosphates. Pesticides in the same class tend to have similar chemical structures and kill pests in the same way. Every organophosphate is characterized by an oxygen atom linked to a phosphorous atom, and they all interfere with a key nerve enzyme, acetyl-cholinesterase. If an insect possesses a physiological mechanism to break the oxygen-phosphorus link, or if its acetyl-cholinesterase is changed, it will be resistant to all organophosphates.
Therefore, alternating between diazinon, malathion and chlorpyrifos, for example, is not a good way to manage resistance because they are all organophosphates. A good rotation for scale insects might be between an organophosphate, a pyrethroid and imidacloprid (Bayer's Merit). If several generations of the pest occur each season, change insecticide classes between generations. Keep good pesticide records and, if possible, make a single person responsible for all pesticide records to keep them consistent and accurate. * Practice integrated pest management (IPM). IPM combines several different tactics, including pesticides, to control pests. The particular IPM techniques you use will depend on the pest(s) you need to control. * Choose healthy plants. Healthy plants are the foundation of pest management. Healthy plants can better withstand pest attacks, and stressed plants are often more attractive to pests. Thus, good pest management depends on good cultural practices: proper irrigation, good fertility, drainage, etc.
Species selection also is vital. Choose plants that will thrive in the location where you'll plant them. Choose plant varieties resistant to key pests in your area. When market constraints allow, avoid plants that consistently have substantial pest problems in your area. Educate your customers about proper plant selection.
Inspect plants before you buy them. Avoid unhealthy plants or plants infested with insects, and establish a formal policy for inspecting plants before you accept them. * Monitor for pests. Monitoring is a critical component of IPM. Inspect plants regularly to identify pest problems early. It is often difficult or impossible to control pests if they become large or numerous. Visual inspection often allows you to detect infestations while they're still small. Monitoring traps also are available for many insect pests. * Practice sanitation. Sanitation helps you avoid pest problems. Therefore, control weeds in and around the planted areas. In addition to being a direct problem, weeds also harbor insect pests and plant diseases. Remove dead, diseased or insect-infested plants whenever possible. * Use mechanical controls. You may be able to exclude pests with screens or other barriers in some situations. You can remove small numbers of insects by hand and prune out heavily infested or diseased plant parts. * Consider biologicals. Biological controls are an important element of IPM. Most professionals recognize that ladybeetles and spiders are beneficial insects that eat aphids and other small pests, but other insects help control pest populations as well. Learn to recognize those that occur in your area. Nectar and pollen of flowers will attract many beneficials. Use pesticides sparingly to preserve beneficial insects and choose selective pesticides (for example, Bacillus thuringiensis) when possible. You may need to educate some clients about the difference between "good" and "bad" insects. * Employ insecticides sparingly. Insecticides are part of good IPM. However, you should use them only when they are necessary. Regular monitoring allows you to discern when you do and do not need pesticides. Avoid regular applications of the same insecticide for the same pest over large areas and use spot applications whenever possible.
Recognizing and controlling resistance Pest-control operators sometimes blame resistance for poor pesticide performance. However, if your pesticide application does not work, do not automatically assume the pests are resistant. First, carefully review your procedures. * Did you identify the pest correctly? Your university extension service can help you with identification. * Did you apply the correct pesticide? Did you use any needed adjuvants? * Did you apply the pesticide at the right time and at the proper rate? Review the calculations you used to determine the pesticide rate and to calibrate application equipment. Did the application method assure good coverage? Thorough coverage is essential to control many of the most troublesome pests, including mites, scales and aphids. * Is the pesticide out-of-date? The shelf life of most chemical pesticides is about 2 years. However, storage conditions can greatly increase or decrease the shelf life of a product. Do not allow pesticides to become too hot or to freeze. Storage conditions are critical for pesticides based on living organisms, such as Bacillus thuringiensis. Labels usually describe proper storage conditions.
If you determine that a pest is resistant to an insecticide, you may be able to control it with a higher application rate, an increased number of insecticide applications (do not exceed the labeled rate or number of applications), another insecticide or some non-chemical technique. In difficult cases, you may have to replace some or all of the plants with a less susceptible species or variety.
Insects' ability to develop resistance to pesticides is a natural consequence of the co-evolution of plants and insects. Any major selective pressure (including insecticides) on an insect population will cause a shift toward resistance in the population. It is imperative to avoid or delay insecticide resistance as long as possible because resistance, once it occurs, may be impossible to reverse or eliminate. To manage resistance, follow two primary strategies: practice good IPM and, when pesticides are necessary, rotate among different chemical classes.
Dr. Paul Guillebeau is the IPM/pesticide coordinator at the University of Georgia-Athens.
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