Mitigate Pesticide Resistance
October 1, 2013 Several questions come to the mind of crop protection professionals as they ready to transverse a given field and scout for potential threats to a crop. Whether it is a field of sweet corn, a block of crisp leafy greens, rows of seedlings just emerging or transplants flowering, the history of previous observations and actions are both considerations that should be made before setting foot in the field.
Knowing what the situation was during the most recent scouting excursion and the consequent decision made is essential to gauging how effective the action taken was and influences the next step to take. Once boots are on the ground and in the field, careful inspection of the crop is another critical component of the decision making process. Only thorough scouting methods like turning over leaves, inspecting blossoms with a hand lens, digging up roots, checking traps, soil moisture and the like can the severity of threats to the crop be determined. A pesticide application may not be needed and therefore can be conserved for future use when actually warranted.
Sometimes, however, a spray is justified and proper identification of pests, diseases and abiotic disorders becomes of the upmost importance to crop protection professionals so they can make the best management decision. Crop consultants and managers rely on scouts to confidently and accurately report what is found standing, crawling, creeping, hopping, flying or trying to hide within the crop. These decision makers depend on scouts to know whether what they find is friend or foe as well as the developmental stage of the crop and potential pest. Frequency of occurrence and spatial distribution within the field are also needed to access the threat to the crop. Consultants and managers can begin to determine the best action to take with such information in-hand.
How immediate the threat of damage to the crop may be plays a significant role in the consideration of a pesticide application or whether another means of management should be implemented. An accurate, complete and timely scouting report allows the examination of alternative measures and appropriate pesticides. Therefore critical inspection of the crop is essential so unnecessary sprays are avoided. This is especially true since there are cases where abnormal looking plants are the result of abiotic factors such as a nutrient deficiency, improper soil moisture, high winds, temperature extremes or herbicide damage. Other times symptoms of a sickly plant may be very similar in appearance but the result different biotic causal agents.
One case in point includes symptoms of distorted new growth commonly seen in peppers. New growers are delving into pepper production as the popularity of fresh farmers and green markets increases. Broad mite damage caused by Polyphagotarsonemus latus is commonly misdiagnosed as virus symptoms or herbicide damage.
Broad mites feed using piercing-sucking mouthparts, which inject a toxin causing leaf petioles to elongate and leaves to become twisted, hardened and shrunken. Their feeding can also result in vegetative and flower buds abscission. The new foliage should be examined thoroughly with a hand lense when such symptoms are discovered. Early detection and treatment will mitigate any impending damage and minimize the use of unnecessary pesticide applications due to an inaccurate identification.
Refer to EDIS publications EENY-183 and ENY-658 for more information on broad mites and thrips management respectively.
Properly Identifying Pest Problems
We also can consider the case of a wilted plant as another example of like symptoms having different causal agents. A pesticide application could be a complete waste depending on whether the wilt is the result of a biotic or abiotic factor. Plants can develop wilt symptoms as the result of any one of a variety of factors. Wilting could be the result of inadequate or possibly excessive soil moisture. It also could be the result of nematode damage to the root system or a pathogen preventing water flow through the plants. Yet another possibility to consider is an insect boring into and feeding upon the interior plant tissue. Wilted plants may also have been damaged mechanically during weeding.
Misapplication of herbicides also can cause plants to wilt. With the number of potential causal agents resulting in wilt symptoms, it takes thoughtful inspection and experience to accurately diagnosis the true cause. A pesticide application may or may not be warranted depending on what is causing the damage. Therefore if there is any doubt as to the identification of a pest, scouts are strongly encouraged to visit their county extension office and utilized the extensive network of cooperative extension agents and specialists available through the partnership between UF/IFAS and local county government.
An accurate and complete scouting report helps narrow down an initially broad scope of management choices to a limited few or perhaps just a single feasible option in a given situation. Professional crop managers recognize the importance of proper identification and an accurate diagnosis of an aliment as the first steps in the management decision making process. The result of misidentification or misdiagnosis is ultimately the same whether caring for vegetables, turf, fruit, ornamentals, lawns or landscapes. Faltering here can lead to wasted capital, lost time, increased crop damage and unnecessary chemical inputs into the environment. Knowing what a particular critter is or the causal agent of a particular plant disease allows the decision makers to determine if mitigation is warranted and if so, helps with the selection of the best management options to take.
More so than ever before, it is of the upmost importance to utilize as many pest management tactics as are practical in an integrated way so pesticide use is limited to an as needed basis. If pesticides are to remain effective options for future use, crop consultants and managers must give consideration to which are the most effective against the pest as well as which product(s) were previously used, when the application was made and how efficacious it was. The consequence of neglecting these considerations has proven to be detrimental by leading to the development of a resistant pest population to which a pesticide is no longer effective.
How Pesticide Resistance Develops
Pesticide resistance involves inherited genetic physiological and/or behavioral adaptations that confer a selective advantage in the presence of a pesticide and lead to control failures of a once susceptible pest population despite proper application. Pesticide tolerance is on the other hand, when physiological and/or behavioral adaptations lead to some toxicity baseline but is not the result genetics (Buss et. al. ENY-842). Resistance to insecticides was documented as far back as 1914 (Fishel PI-83). Fungicide resistance was then detected in 1960 and can take as few as two years to develop (Fishel and Dewdney PI94). Herbicide-resistant plants took longer to develop due to the large soil seed bank limiting the number of plants being exposed to an herbicide at any one time.
With the advent and adoption in recent years of genetically modified crops designed to withstand repeated exposure to the same pesticide, it was not long before herbicide resistant weeds were detected in fields across the nation when planted successively with RoundUp Ready corn.
Stephen Powles notes that there now are evolved glyphosate-resistant populations of such economically damaging weed species as common rag weed (Ambrosia artemissifolia), giant ragweed (Ambrosia trifida), several Amaranthus spp. including A. palmeri, A. rudis, A. tuberculatus and various Conyza and Lolium spp. in the U.S. Herbicide resistant weeds are not however limited to North America. In Argentina and Brazil where areas of transgenic glyphosate-resistant crops are grown successively, glyphosate-resistance has evolved in populations of Sorghum halepense and Euphorbia heterophylla (Pest Management Science Special Issue: Glyphosate-Resistant Weeds and Crops Volume 64, Issue 4, pages 360–365, April 2008).
Prior to pesticide exposure there are susceptible and resistant individuals within any pest population. The later typically makes up a very small percentage of the whole until the population is exposed to a pesticide. The resistant individuals survive and remain following the pesticide application to swap their DNA among themselves thereby increasing the likelihood of resistance development. When a new population of like pests arrive, their DNA dilutes the resistance carrying genes of the previous pest population and lessen the likelihood of transferring resistance to the progeny. Potential for pesticide resistance still remains and this is especially true of repeated exposure of pests to the same pesticide or mode of action (MOA).
It is often not enough to simply rotate between two different pesticide products as part of a resistance management plan to avoid development of resistance. This is because pesticide products may have the same active ingredient or have the same mode of action despite having different trade names. The Insecticide Resistance Action Committee (IRAC) was formed to communicate and educate agricultural producers and crop protection professionals across the globe by providing resistance management information. Groups have been created to do the same for fungicides (FRAC) and herbicides (HRAC). These Resistance Action Committees classify pesticides on the basis of their mode of action which is the specific physiological activity of a toxin resulting in the death of a pest.
As seen in Table 1 of EDIS publication PI-83 “IRAC’s Insecticide Mode of Action Classification,” each column lists the number (or letter in the case of herbicides) of the group code for each pesticide class, the group name or general description of that group of pesticides, the common name of pesticides that belong to each group and examples of trade names of pesticides for each common name listed. Care should be given to rotate between using products with different modes of action by choosing products with a different group code than was previously used. These types of tables are available for several crops through EDIS and similar information can be found in the Vegetable Production Handbook for Florida.
Pesticide Resistance Mitigation Tactics
Pesticide resistance is a serious and growing problem with a worldwide estimate of more than 600 species of pests having developed some level of pesticide resistance. Crop protection specialists must do their due diligence to delay the onset of pesticide resistance to a particular pesticide or “family” of pesticides if they are to remain viable management tools for any great length of time. When resistance develops, products can no longer be effectively utilized and pest management options are reduced.
Additional consequences of a failed pesticide application may include yield losses or crop quality reduction. The tendency of some is to go out at a higher rate or shorten the spray interval when a pesticide fails. Acting this way without determining the true cause of the failure leads to increased exposure of toxins to the environment and higher control costs as more product is used. Control costs will climb even higher to contend with a resistant pest population since more expensive products will likely be required. Rotating different modes of action is one method of alleviating these consequences but resistance management can also be achieved with other tactics as well.
Another tactic that may be employed is the practice of mixing two pesticides of dissimilar modes of action that target the same pest. This practice exposes the pest to more than one toxicant simultaneously with the idea that the few resistant individuals to one MOA will be susceptible to the other. Pesticide applicators frequently tank-mix their own mixtures but pre-mix products have become more and more available in recent years from several manufacturers.
It is always advisable to consider and implement pest management tactics in an integrated way so pesticide use is minimized thereby delaying resistance development as long as possible. Scouting, accurate pest identification and resistance management are all part of the broader concept of Integrated Pest Management (IPM) whereby multiple, cost-effective efforts to decrease pest populations to tolerable levels with minimal environmental impact are made by utilizing plant, pest and environmental information. The main principle of IPM is using the best scientific data to develop a site specific management plan.
In addition to scouting, pest identification and making a management decisions based on threat assessments, an IPM plan also could include exclusion efforts such as using barriers, a clean seed source or rouging diseased plants in an attempt to delay pest activity in a field or protected structure. Cultural methods via environmental manipulation may also be employed in an IPM plan which could include using resistant crop varieties, adjusting growing conditions, laying down reflective mulch and enhancing surrounding vegetation to make it more attractive to beneficials. IPM also includes sanitation efforts like disking under harvested crops, scouting crops from youngest to the most mature plantings and eliminating debris. As part of an IPM, these types of efforts can delay the development of pesticide resistance by minimizing the need for pesticide applications.
It is important to realize that pesticide resistance is not the sole cause of pesticide failure. Other causes of pesticide failure may include misidentification of a pest resulting in incorrect pesticide selection, incorrect pesticide dosage, poor timing of the application, the pesticide not reaching the target pest, and there could have been unfavorable environmental conditions or poor pesticide conditions (F. Fishel PI-163).
As a crop protection professional, it is critical to keep sustainability in mind and be good stewards of the tools we have available to manage pest populations. We should carefully consider if a pesticide application is justified, know which MOA was used previously and when as well as be mindful of the impact on beneficials such as pollinators, predators and parasitoids before a product is selected for use. Professionals stay informed and are lifelong learners keen to avoid making the same mistake twice.
Remember above all else, the pesticide label is the law and must be followed. For the sake our industry and your bottom-line, we should remember that all cases of pesticide resistance are driven by selection through the repeated use of one or similar chemistries and we therefore need to make an effort to recall the Resistance Action Committee group designation and mode of action as well as we do product names.
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