A New Mode of Resistance Management By Jim Bethke

With insecticide numbers shrinking and population resistance growing, resistance management is more important than ever. A new method of rotation could be the solution we’ve all been waiting for.

The management of insecticide resistance is a somewhatcontroversial subject, and not because we are ignorant of the problem. In fact,scientists have a good handle on the mechanisms of pesticide resistancedevelopment. It is controversial because scientists do not have a good handleon the solutions to insecticide resistance. At several recent conferences,there were varying opinions on the subject, with many extension scientistsbegging for answers they could take to growers. The growers, like my uncle insuburban Seattle, just want the pests dead and gone — although my unclewants them to suffer and, with their last dying breath, tell their bug buddiesnever to return.

Professional opinions on the best way to manage insecticideresistance range from those that suggest we need to stop using pesticidesaltogether to those that recommend tank-mixing the most efficacious products.Of course, these two examples are at the extremes and neither is likely to bean appropriate solution, especially in the world of ornamentals where nothingis simple. Few studies have actually tested the theories of insecticide resistance in practical applications. So in most cases, people can only make educated guesses at which strategies should work best for each product and each pest.

Through the 1990s, more detailed investigations of themechanisms involved in pesticide resistance have shown that insects have aninnate ability to develop resistance to most pesticide chemical classes, andsome insects have the ability to develop resistance to many classessimultaneously. Everyone is aware of the classic example of cross-resistancebetween carbamates and organophosphates. However, with the development of newinsecticide chemistries, new forms of cross-resistance (resistance to more thanone pesticide with the same or similar mode of action) and multiple-resistance(resistance to two or more pesticides with different modes of action) have beenexhibiting themselves, such as multiple-resistance to both a chitin synthesisinhibitor and a carbamate or an imidacloprid resistant whitefly that is moretolerant to bifenthrin. Many of the highly resistant insects we see today are aresult of overuse or off-label use of pesticides.

So why is insecticide resistance management important,especially since it seems so difficult? Primarily because the number ofpesticides registered is dwindling, and the cost and time required to developand register new ones is high. In addition, resistance management is importantto manufacturers so that their products have a long life in the industry.Therefore, they are imposing user restrictions on the label or recommending aspecific IPM program on the label to delay or reduce the chance of resistancebuildup.

Classifying Chemicals

Understanding insecticide resistance management requiressome knowledge of pesticide chemistry and mode of action. The actual chemicalstructure, or molecular structure, of the active ingredient in the pesticidedetermines the chemical class. Unfortunately, the chemical class is not on thepesticide label, and in many cases it is not on the MSDS. Researchers have beentelling growers to rotate by chemical class for a long time, but untilrecently, there’s not been an easy way to identify a pesticide’schemical class. This information is available from many sources, includingOlympic Horticultural Products’ Chemical Class Chart, various Web sitesand the GPN Resource and Buyer’s Guide. The EPA has been listening andnow has an extensive chemical class list for pesticides used in the industry(see appendices, http://www.epa.gov/opppmsd1/PR_Notices/).

The mode of action is the mechanism by which the pesticidekills the pest. The reason for suggesting rotation based on chemical classes isthat the chemical class usually denotes the mechanism of action of thepesticide, i.e., the way the chemical works (See Table 1, page 34) It isimportant to note, however, that it is possible for different chemical classes tohave the same or similar modes of action, such as with organophosphates andcarbamates. Back-to-back rotation of these chemical classes would not berecommended. For a more technical description of insecticide mode of action,see the following Web site: http://ipmworld.umn.edu/chapters/bloomq.htm.

Figure 1, above, is a simple representation of the nervoussystem in animals. Nerve cells are connected by synapses. The nerve impulsemoves along the axon (sodium NA+ and chlorine CL¯ channels) until itreaches the synapse and Acetylcholine (ACh) is produced to carry the impulse tothe next nerve cell. Once the Acetylcholine has done its job, it is metabolizedby Acetylcholine esterase (AChE) to empty the synapse for the next nerveimpulse. You will note that the majority of the modes of action listed in Table1, page 34, affect the nervous system in some way.

Mechanisms of Resistance

Resistant pest populations that can no longer be controlledby a pesticide usually develop over several generations. Resistance developsfastest in insects that have a high rate of reproduction and are under heavypesticide pressure. The more you spray, the more you increase the proportion ofresistant individuals in the population. This fact has been demonstrated manytimes. Insecticide resistance is genetically based, and a resistant populationmay have developed one or more of the following resistance mechanisms:

• They may have a different behavior.

• They may have changed the outer cuticular layer sothe pesticide can’t penetrate.

• They may produce an increased amount of pesticideinsensitive ACh.

• They may produce an increased amount of pesticideinsensitive AChE.

• They may have increased the number of types andquantities of detoxifying enzymes.

The insect cuticle and blood are full of many differenttypes of enzymes called mixed function oxidases (MFOs) and cytochrome p450s.The latest research shows that some resistant insects have the ability toproduce increasing amounts of and different types of detoxifying enzymes. In addition to the metabolic enzymes, resistant insects have the ability to producepesticide-tolerant or increasing quantities of ACh and AChE, so much so thatthe pesticide is rendered ineffective. Unfortunately, some of the highlyresistant insects (leafminer, western flower thrips, etc.) use several of thesemechanisms at the same time. This is known as multiple resistance.

It is important to understand the basic biology of the pestsin question. How long does it take for the pest to complete one generation? Withaphids, it’s very short, so rotation should occur in approximately twoweeks. However, whiteflies take about 25 days to develop into an adult. Theirgeneration time is much longer.

Resistance Factors

Before we can discuss pesticide resistance, we shouldinvestigate the other potential causes for control failure. Control failure maynot be a result of resistance. It may be due to some other factor in the pestmanagement scheme.

Possible reasons for control failure include :

• failure to initiate other parts of an overallintegrated pest management program;

• failure to intensively monitor for pest species;

• misidentification of pest species;

• wrong choice of pesticide;

• incorrect rates/off-label use/use of an old ordegraded pesticide;

• poor choice of tank mixes, adjuvants, pH, waterquality, etc.;

• misuse of equipment, inadequate agitation, impropercalibration, inadequate maintenance, etc.;

• inadequate coverage, improper placement of thepesticide; and/or

• very high pest populations, tolerant or resistantpopulations.

Managing pests should begin with the basics of integrated control. For instance, all efforts should be made to exclude or inhibit thedevelopment of pest populations using cultural or environmental controls.Biological control should be considered where possible, and an intensivemonitoring or scouting program should be in place so that one can treat or spottreat when necessary. Another good reason to intensively monitor for pests isthat a very heavy pest infestation is very difficult, if not impossible, tocontrol before damage occurs. Ten aphids on a terminal today can become 250 ina week. If spider mite webbing is visible, the population is probably enormous.Very high pest populations should be avoided at all costs. Concentrating onintegrated methods of control will reduce the reliance on chemical control. When chemical control is necessary, proper application of the chemicals is the next best method of avoiding or delaying pesticide resistance.

Some of the most critical components of successful pesticideapplications are coverage, timing and placement. Not spending enough timecovering dense foliage will, in all likelihood, allow some pests to survive.Sub-lethal doses due to inadequate coverage may increase the chance ofpesticide resistance. The timing of pesticide application must coincide with astage of the pest that is vulnerable to the application. For instance,whiteflies are most susceptible when they are in the early nymphal stages. Theyare most tolerant when they are in the redeye stage. Therefore, monitoring toestablish the pest stage will help determine which pesticide to use and when touse it. In addition, whitefly nymphs are located on the lower surface of theleaves. So, the application should be directed to the undersides of the leavesfor effective coverage. Knowing where to direct the pesticide is just asimportant as proper coverage.

Using the New Mode

Rotation of pesticides with different modes of action is avery important concept. Research has shown that pests can develop high levelsof resistance, like western flower thrips, which can detoxify more than onechemical or mode of action per generation. Quite simply, this means thattreating insect populations with tank mixes of insecticides that have severalmodes of action can result in a pest population with multiple resistancemechanisms and produce what seems to be a superbug. Therefore, it is importantto use the pesticide according to the label and to rotate by both chemicalclass and mode of action. Rotation should occur every one or two pestgenerations so that the new chemical will kill surviving individuals tolerantto the previous pesticide.

So what is the best approach to manage pests withoutenhancing pesticide resistance? We cannot in good conscience suggest thateveryone just stop using pesticides. We also cannot suggest enhancing theproblem by adding a greater amount of pesticide or a greater number ofpesticides to the tank. Minimizing chemical control by incorporating other pestmanagement tactics is a more sensible solution. I have observed that successfulgrowers monitor their crop very closely and treat hot spots. On the other hand,growers who are treating on a scheduled basis and tank mixing more than one modeof action at one time are usually less successful. If you want to examine arecent example of how things can go wrong, see “The Return of theLeafminer,” in the June 2001 issue of GPN. The products that successfulgrowers use are highly effective because the pests they treat are sensitive tothe pesticides.

Tank mixing insecticides with different modes of action isrisky and, in my opinion, should be avoided. It exposes the pest population toseveral modes of action at the same time, enhancing the potential forresistance development. There are a few successful tank mixes that arewell-known such as the mix of an OP with a pyrethroid (e.g., acephate +permethrin), or avermectin with insecticidal soap. In my opinion, it would berisky to rely on the OP-pyrethroid mix religiously without rotation. However,resistance to soaps and oils is unlikely, so the inclusion of soaps and oils toa rotation or a tank mix is an option to keep open.

Many have asked me about tank mixing to kill both the adult(e.g., a pyrethroid) and immature (e.g., an IGR) at the same time. Again, thisis where I believe some growers have gotten into trouble with resistant pests.All stages of the pest are being exposed to two modes of action at the sametime. It is possible, with an effective monitoring program, to know which stagethe population is in and treat only that stage. It is also possible to treatthe untreated stage with another pesticide at a different time, therebydelaying the exposure of both stages to both modes of action.

If a resistance problem occurs, then it’s time to takea step back and review the overall integrated pest management program. Thetendency is to seek a new chemical solution rather than looking at the entireprogram. There is evidence that reversion can occur. That is, if the use ofpesticides against a resistant insect is curbed, in time, that chemical can beused again. This needs more study in ornamentals.

Jim Bethke

Jim Bethke is a research assistant for the Department of Entomology, University of California, Riverside, Calif. He can be reached by phone at (909) 787-4733 or E-mail at bethke@citrus.ucr.edu.