Combating Insecticide Resistance
We have been plagued with resistance problems in the ornamentalindustry for several years. My first exposure to resistance was when theleafminer, Liriomyza trifolii, became resistant to the broad spectrum ofinsecticides used in greenhouse production in the 1970s. This was a new thingto our industry, and we could not figure out why we were getting good resultswith one pesticide in some areas and not in others. First, there were only acouple that could not get the expected control with this insecticide, then afew more, and finally most could not get the expected results. Then it waseverything else; nothing worked anymore. It was like a plague that spreadacross the state. By 1980, leafminer resistance was recognized as a majorproblem nationwide, and monies were channeled into the industry to help solve theproblem. 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 dealwith in the future with various pests.
Since that time, we have had major pest problems because ofthe failure of pesticides to perform as well as they had in the past. Westernflower thrips became a major problem in the early 1980s and silverleafwhiteflies in the late 1980s. These were major control failures, but severalother pests have been problems as well, including mites, aphids, worms,mealybugs and others. Resistance is not as big a puzzle as we once thought; weunderstand what causes it. However, we still do not have a clear picture of whysome pesticides are not as subject to the development of resistance or how todeal with preventing resistance from occurring. There are several theories, andapplying these help in creating a management strategy to try and delay thedevelopment of resistance to specific compounds. An accepted practice is torotate pesticides so pests are not constantly exposed to one specific mode ofaction (the site and method of activity of a pesticide against a pest.)
What Is Resistance?
Resistance occurs when the susceptibility of a particularorganism to a chemical changes such that it takes a larger dosage of thatcompound to cause the same level of activity. This is usually the result ofnatural selection, where a population is exposed to a certain level of atoxicant, and a few individuals survive. Some of the offspring of thesesurviving individuals inherit the ability to survive that dosage of thetoxicant, and this continues to a point that it takes extremely high dosages tobe effective, if they are effective at all. Thus the theory evolved of changingmodes of action: that a new mode of action will be effective against thoseindividuals that survive the previous one. Even if a different pesticide thathas the same mode of action is used, there could be cross-resistance, and itwould not be any more effective than the original pesticide used.Cross-resistance is when resistance develops against one compound, and you canget resistance against other compounds that have the same mode of action, evenbefore the pest was exposed to the other compounds. We are experiencingresistance with many pesticide/pest interactions as well as many othersynthetic chemicals used in other areas, such as the management of disease withantibiotics in humans.
Because of the before-mentioned resistance problems, a concernover resistance developing in whitefly against imidacloprid, or Marathon, hasarisen. The silverleaf whitefly problem surfaced in the late 1980s. The theorywas that this pest, thought to be the sweet potato or cotton whitefly (Bemisiatabaci), was a major problem in field production, and somehow it had changedits host and site selection to greenhouse crops and thrived on ornamental aswell as field crops. The new whitefly was determined to be a different biotypeand was given a new species name, silverleaf ç whitefly (Bemisiaargentifolii). The important thing is that it was resistant to the insecticidescommonly used to control whiteflies, and it was a major pest on greenhouse andfield crops. Several insecticides were tested, including combinations ofinsecticides, with mixed success. This resulted in a tremendous number ofapplications of insecticides used to control whiteflies to get one crop tomarket. In 1996, Marathon was introduced to the greenhouse market withextremely good success against the silverleaf whitefly. As a result, Marathonhas been the most commonly used pesticide against this pest. Any time onecompound is relied upon for the management of a single pest, there is greatconcern over resistance developing. This concern is justified because when youcannot rotate away from one mode of action, the probability of resistancedeveloping is greatly enhanced.
Marathon was introduced as a new class of insecticide, thechloronicotinyls. Since it is a new class of insecticide, one would not expectto have cross-resistance because it has a new mode of action. Silverleafwhiteflies have developed resistance against several insecticides, includingsome pyrethroids, organophosphates and carbamates. Imidacloprid (Marathon) isan acetylcholine receptor agonists/antagonists, which means that it binds to anicotinic acetylcholine receptor, disrupting nerve transmission. This is thesame receptor that organophosphates and carbamates are active against, but themode of action is different. Carbamates and OPs are acetylcholine esteraseinhibitors, which cause the inhibition of the enzyme acetylcholinesterase,interrupting the transmission of nerve impulses. These classes are nervepoisons, which impact the nerve impulses, but imidacloprid has a different siteof activity.
Is there resistance to imidacloprid (Marathon)? I am notaware of any confirmed resistance to Marathon in the field in the UnitedStates. I do feel that there are cases of difference in susceptibility toimidacloprid in populations of whiteflies in the United States. This is notresistance, but it could be an indication of populations where resistance coulddevelop if not properly managed. Is Marathon immune to the development ofresistance? I doubt it. If the compound is not properly managed, thenresistance will occur. ç We have had a difference in susceptibility ofwhiteflies to Marathon ever since it came on the market. We have observedproblems controlling the greenhouse whitefly (Trialeurodes vaporariorum) incooler parts of Georgia ever since Marathon came on the market. I feel that tocontrol greenhouse whiteflies with Marathon, it must be used before thewhitefly population has a chance to become established. The silverleaf whiteflyis more susceptible to Marathon, and we get very good control of this pest inour area. However, there are populations within the silverleaf species that aremore susceptible than others.
In some areas, imidacloprid is used for many different cropsto manage whiteflies and other pests. In these areas, whiteflies couldpotentially be exposed to imidacloprid continuously throughout the year, so wehave to be especially careful to monitor whitefly populations for reduction insusceptibility to imidacloprid. These will probably be the first areas whereresistance can develop and areas where it is especially important to rotate theinsecticides used to manage whitefly populations. The ideal situation would beto control the amount of one chemical used in all of the crops in a region andmaintain a limited exposure to that particular chemical.
There are some countries that have programs recommending theuse of a particular chemical only during a certain time. One of these programsis called the ?windows? method, where pesticide X can only be usedbetween certain dates or a window of application. This could only beaccomplished with cooperation among neighbors and would surely delay thedevelopment of resistance to insecticides. Chemical companies are trying to dothe same thing by limiting the number of applications on a particular crop orseason. The problem with this approach is that it does not control what aneighbor is doing.
What can we do to avoid the development of resistance toMarathon? One key thing that each person can do is to rotate modes of actionsuch that a pest is not exposed to a particular one continuously. There arethree formulations of Marathon currently on the market for greenhouseproduction. It is important that these be considered as one chemical and not beused repeatedly, especially if a grower feels that they have not gotten theresults that they expect from Marathon. If you are not satisfied with controlobtained with Marathon, it is important not to come back with a repeatapplication with any of the three formulations. ç Marathon II, appliedas a foliar spray, can be used in repeat applications ? if nomedia-applied Marathon has been used on that crop. This could increase thechance 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 goodworking condition and good coverage on the plants treated is achieved. If goodcoverage is not obtained, there could be pests not reached within the canopythat later come in contact with the reduced dosage of the chemical residue andstart developing immunity to the chemical. Marathon does have systemic activitywhen applied to the root zone, which is an added advantage to this compoundbecause it can be applied to the potting medium and taken up by the plant togive control of pests throughout the canopy. Timing is still important becausethe age of the leaves and root system development will influence the uptake ofMarathon. Marathon is taken up by the roots and translocated to the activelygrowing foliage. This is not a problem on newly established plants with a goodroot system, but on older plants the chemical will not be translocated to theold leaves as efficiently. The result could be control failure on these olderleaves, 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, butyou would not get the systemic activity typical of application to the rootzone. Systemics are by nature water-soluble, and excess watering could also bea problem, especially for a few days after chemical application. The roots musthave a chance to take the chemical up into the vascular system before it isleached out of the pot. The type of medium in the potting mix can also affectthe speed of uptake.
There are many things that can affect the development ofresistance of any pesticide, and many things must be considered before blaminga control failure on resistance. In most situations it is something else thatis the cause. The most common cause is in the application and handling of thepesticide. If you suspect control failure as the result of the application ofany chemical, it is a good idea to consider all things that could havecontributed to the failure. When you repeat an application to get bettercontrol, do not immediately return with the same chemical and do not increasethe dosage above the labeled rate ? select an alternative. Rotate modesof action whenever possible, and follow the programs recommended by yourextension service.