Combating Insecticide Resistance

We have been plagued with resistance problems in the ornamental industry for several years. My first exposure to resistance was when the leafminer, Liriomyza trifolii, became resistant to the broad spectrum of insecticides used in greenhouse production in the 1970s. This was a new thing to our industry, and we could not figure out why we were getting good results with one pesticide in some areas and not in others. First, there were only a couple that could not get the expected control with this insecticide, then a few more, and finally most could not get the expected results. Then it was everything else; nothing worked anymore. It was like a plague that spread across the state. By 1980, leafminer resistance was recognized as a major problem nationwide, and monies were channeled into the industry to help solve the problem. We worked with various insecticides and finally worked out solutions. It became very obvious that this was a problem we were going to have to deal with in the future with various pests.

Since that time, we have had major pest problems because of the failure of pesticides to perform as well as they had in the past. Western flower thrips became a major problem in the early 1980s and silverleaf whiteflies in the late 1980s. These were major control failures, but several other pests have been problems as well, including mites, aphids, worms, mealybugs and others. Resistance is not as big a puzzle as we once thought; we understand what causes it. However, we still do not have a clear picture of why some pesticides are not as subject to the development of resistance or how to deal with preventing resistance from occurring. There are several theories, and applying these help in creating a management strategy to try and delay the development of resistance to specific compounds. An accepted practice is to rotate pesticides so pests are not constantly exposed to one specific mode of action (the site and method of activity of a pesticide against a pest.)

What Is Resistance?

Resistance occurs when the susceptibility of a particular organism to a chemical changes such that it takes a larger dosage of that compound to cause the same level of activity. This is usually the result of natural selection, where a population is exposed to a certain level of a toxicant, and a few individuals survive. Some of the offspring of these surviving individuals inherit the ability to survive that dosage of the toxicant, and this continues to a point that it takes extremely high dosages to be effective, if they are effective at all. Thus the theory evolved of changing modes of action: that a new mode of action will be effective against those individuals that survive the previous one. Even if a different pesticide that has the same mode of action is used, there could be cross-resistance, and it would not be any more effective than the original pesticide used. Cross-resistance is when resistance develops against one compound, and you can get resistance against other compounds that have the same mode of action, even before the pest was exposed to the other compounds. We are experiencing resistance with many pesticide/pest interactions as well as many other synthetic chemicals used in other areas, such as the management of disease with antibiotics in humans.

Current Resistance

Because of the before-mentioned resistance problems, a concern over resistance developing in whitefly against imidacloprid, or Marathon, has arisen. The silverleaf whitefly problem surfaced in the late 1980s. The theory was that this pest, thought to be the sweet potato or cotton whitefly (Bemisia tabaci), was a major problem in field production, and somehow it had changed its host and site selection to greenhouse crops and thrived on ornamental as well as field crops. The new whitefly was determined to be a different biotype and was given a new species name, silverleaf Á whitefly (Bemisia argentifolii). The important thing is that it was resistant to the insecticides commonly used to control whiteflies, and it was a major pest on greenhouse and field crops. Several insecticides were tested, including combinations of insecticides, with mixed success. This resulted in a tremendous number of applications of insecticides used to control whiteflies to get one crop to market. In 1996, Marathon was introduced to the greenhouse market with extremely good success against the silverleaf whitefly. As a result, Marathon has been the most commonly used pesticide against this pest. Any time one compound is relied upon for the management of a single pest, there is great concern over resistance developing. This concern is justified because when you cannot rotate away from one mode of action, the probability of resistance developing is greatly enhanced.

Marathon was introduced as a new class of insecticide, the chloronicotinyls. Since it is a new class of insecticide, one would not expect to have cross-resistance because it has a new mode of action. Silverleaf whiteflies have developed resistance against several insecticides, including some pyrethroids, organophosphates and carbamates. Imidacloprid (Marathon) is an acetylcholine receptor agonists/antagonists, which means that it binds to a nicotinic acetylcholine receptor, disrupting nerve transmission. This is the same receptor that organophosphates and carbamates are active against, but the mode of action is different. Carbamates and OPs are acetylcholine esterase inhibitors, which cause the inhibition of the enzyme acetylcholinesterase, interrupting the transmission of nerve impulses. These classes are nerve poisons, which impact the nerve impulses, but imidacloprid has a different site of activity.

Is there resistance to imidacloprid (Marathon)? I am not aware of any confirmed resistance to Marathon in the field in the United States. I do feel that there are cases of difference in susceptibility to imidacloprid in populations of whiteflies in the United States. This is not resistance, but it could be an indication of populations where resistance could develop if not properly managed. Is Marathon immune to the development of resistance? I doubt it. If the compound is not properly managed, then resistance will occur. Á We have had a difference in susceptibility of whiteflies to Marathon ever since it came on the market. We have observed problems controlling the greenhouse whitefly (Trialeurodes vaporariorum) in cooler parts of Georgia ever since Marathon came on the market. I feel that to control greenhouse whiteflies with Marathon, it must be used before the whitefly population has a chance to become established. The silverleaf whitefly is more susceptible to Marathon, and we get very good control of this pest in our area. However, there are populations within the silverleaf species that are more susceptible than others.

In some areas, imidacloprid is used for many different crops to manage whiteflies and other pests. In these areas, whiteflies could potentially be exposed to imidacloprid continuously throughout the year, so we have to be especially careful to monitor whitefly populations for reduction in susceptibility to imidacloprid. These will probably be the first areas where resistance can develop and areas where it is especially important to rotate the insecticides used to manage whitefly populations. The ideal situation would be to control the amount of one chemical used in all of the crops in a region and maintain a limited exposure to that particular chemical.

There are some countries that have programs recommending the use of a particular chemical only during a certain time. One of these programs is called the ?windows? method, where pesticide X can only be used between certain dates or a window of application. This could only be accomplished with cooperation among neighbors and would surely delay the development of resistance to insecticides. Chemical companies are trying to do the same thing by limiting the number of applications on a particular crop or season. The problem with this approach is that it does not control what a neighbor is doing.

Avoiding Resistance

What can we do to avoid the development of resistance to Marathon? One key thing that each person can do is to rotate modes of action such that a pest is not exposed to a particular one continuously. There are three formulations of Marathon currently on the market for greenhouse production. It is important that these be considered as one chemical and not be used repeatedly, especially if a grower feels that they have not gotten the results that they expect from Marathon. If you are not satisfied with control obtained with Marathon, it is important not to come back with a repeat application with any of the three formulations. Á Marathon II, applied as a foliar spray, can be used in repeat applications ? if no media-applied Marathon has been used on that crop. This could increase the chance of a low-susceptibility population developing resistance to Marathon. Media-applied Marathon can be used once every 16 weeks.

Good application technique is key to resistance management. Make sure the proper recommended dosage is used, application equipment is in good working condition and good coverage on the plants treated is achieved. If good coverage is not obtained, there could be pests not reached within the canopy that later come in contact with the reduced dosage of the chemical residue and start developing immunity to the chemical. Marathon does have systemic activity when applied to the root zone, which is an added advantage to this compound because it can be applied to the potting medium and taken up by the plant to give control of pests throughout the canopy. Timing is still important because the age of the leaves and root system development will influence the uptake of Marathon. Marathon is taken up by the roots and translocated to the actively growing foliage. This is not a problem on newly established plants with a good root system, but on older plants the chemical will not be translocated to the old leaves as efficiently. The result could be control failure on these older leaves, which is not a sign of resistance but the lack of chemical in these leaves. A foliar spray can also be taken up in the leaves to move within the leaf, but you would not get the systemic activity typical of application to the root zone. Systemics are by nature water-soluble, and excess watering could also be a problem, especially for a few days after chemical application. The roots must have a chance to take the chemical up into the vascular system before it is leached out of the pot. The type of medium in the potting mix can also affect the speed of uptake.

There are many things that can affect the development of resistance of any pesticide, and many things must be considered before blaming a control failure on resistance. In most situations it is something else that is the cause. The most common cause is in the application and handling of the pesticide. If you suspect control failure as the result of the application of any chemical, it is a good idea to consider all things that could have contributed to the failure. When you repeat an application to get better control, do not immediately return with the same chemical and do not increase the dosage above the labeled rate ? select an alternative. Rotate modes of action whenever possible, and follow the programs recommended by your extension service.