IPM Strategy 6. Using Targeted Pesticide Applications

 

Pesticides, better described as crop protection agents because of their diversity, are one of the key elements in a modern, integrated pest management (IPM) approach to managing crop pests. They are particularly important as a means of preventing the breakdown of genetic resistance due to pests that develop their own ways around that resistance. First of all, it is important to understand that farmers have no economic or practical incentive to over use pesticides. These materials can be costly as can be the time, fuel and equipment wear involved in applying them. Against that background, a number of other principles guide a farmer in terms of what specific pesticides to use, when to use them and how to deliver them. I’ll touch briefly on some of those principles below:

One of the central principles of IPM is monitoring the pest population to see when it is, in fact, important to make a pesticide application. It is not economically ideal to go to the expense of a spray for certain levels of pests and their damage, such as while the natural enemies of the pest have the best chance to keep it in check. This issue varies greatly depending on the type of pest involved. For some pests, the threshold for action must be low. This is particularly true for certain insects that vector (spread) damaging viruses or bacterial pests. In that case, the insect may not be causing significant damage through its own feeding activity, but the disease it is spreading can quickly become devastating. Growers of each crop in each region have to be particularly tuned into these issues to spend time and/or money on monitoring for the worst pest threats in their situation

  • Protecting Beneficial Organisms

One of the six elements of IPM is fostering beneficial organisms. In many cases, these organisms are biologically similar enough to the targeted pests that they too may be killed by a pesticide. Fortunately, not all pesticides are alike in this regard and a farmer can often choose a particular crop protection product that is effective on the pest but relatively safe for beneficial organisms. These options are preferable whenever possible, and a good deal of guidance is available on this subject from State Cooperative Extension agencies and from private pest control advisors.

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Lacewing image from Wikimedia
  • Protecting Pollinators

Different crops are pollinated in different ways (e.g. by the wind or by birds), but in many cases crops are pollinated by an insect like a honeybee, a bumble bee, various other wild bees or other insects. During the period when these crops are blooming, a spray for an insect pest can hurt the pollinators. Different pesticides have different profiles in this regard. Some are quite safe for pollinators and others are safe as long as growers apply them at a time of day when the pollinators are not working.

Farmers must make careful choices during the bloom of their crops and need to be aware of when bees are being employed to pollinate a neighbor’s crop. In recent years, some general problems with bee populations have arisen and pesticides have been widely blamed as the culprit. The truth is that this is a complex issue that often has more to do with pests of the bees themselves, like the Varroa mite and the virus diseases that it spreads among bee colonies. Through making thoughtful choices with pesticide options and by good neighbor-to-neighbor communications, farmers can and are doing their part to protect pollinators.

  • Targeted Delivery

The IPM ideal is not only to apply the right pesticide at the right time but also to apply it as specifically as possible to where it is needed. Depending on the crop and pest in question, growers can achieve the latter goal with considerable precision. Some pesticides can be injurious to fish or other aquatic organisms. In those cases, growers must usually follow formal limits on how close their application can be to bodies of water. For weeds growing in an orchard or a vineyard, it is possible to spray downwards at the ground with virtually no danger of the herbicide contacting the susceptible, leafy part of the crop. In some situations, it is possible to attract the first wave of a given pest to physical traps or to a trap crop which is not intended for harvest but planted along the edge of the field. Once the pests arrive at the trap crop, farmers can spray them there without need to spray the crop itself. This only works in certain situations but is another example of targeted delivery.

For annual crops, the time during seed germination and initial growth is critical within the crop cycle. When the plants are small, feeding by an insect or infection by a fungus can much more easily kill the seedling or severely stunt it in ways that the plant cannot fully overcome throughout the growing season. In recent decades, scientists have developed technologies to treat/coat seeds with protective fungicides and insecticides which are present specifically around the germinating seed and/or picked up through the plant roots to provide protection during this most vulnerable stage. When crop protection products are applied this way, the total quantity used per acre is a tiny fraction of what it would be with a drench or spray application. The levels of these pesticides declines as the season progresses so that by the time the crop is blooming and long before the crop is harvested, the pesticide has been broken down to harmless components.

One more example of targeted delivery is the use of modern machine vision systems to selectively spray only weeds while passing over a field.

  • Good Neighbor Standards

The control of some pests doesn’t just depend on the outcome in a given field but rather on the balance with the IPM programs in neighboring fields. If the local populations of some pests gets too high, IPM principles like action thresholds and preserving beneficial organisms become too difficult to employ. With too much pest pressure, control strategies like mating disruption will no longer work.

To prevent an abundance of pest pressure, some growing regions have laws in place such that if a grower is allowing the population of particularly damaging pests to grow too much, then the county or state authorities have the ability to step in and control the pest at the owner’s expense. Fortunately, these incidents are quite rare within strong crop sectors. In some particularly difficult pest examples, even an untended backyard tree can compromise the commercial production within a certain radius. Good neighbors of all types need to be aware of these sorts of issues.

  • Managing the Risk of Pest Resistance Development

Part of good pest management with pesticides is preserving the usefulness of the tools available, particularly the most beneficial ones in terms of safety and low risk to non-target species. Pests tend to go through many life cycles during each single season of crop growth, so their potential to evolve pesticide resistance is significant.

Starting in the 1980s, guidance has been available to farmers about how to mix and match modes of action in their choice of pesticides. Most modern pesticides work through rather specific effects on a target enzyme in the pest, which is how they can be effective as a pesticide and yet essentially non-toxic to other organisms, including humans. The downside of that kind of specificity is that a single mutation in the pest can render it resistant. That is why farmers are advised, and sometimes even required, to use pesticides with different modes of action either in combinations or in consecutive treatments, a strategy called resistance management. These practices prevent, or at least significantly delay, the development of pest resistance to the most desirable pesticide options.

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IPM Strategy 5. Fostering Beneficial Organisms

Even pests have pests, and often growers can encourage those natural enemies sufficiently to keep crop pest populations at tolerable levels. For example, the cottony cushion scale was once a big problem in the California citrus industry, but a natural predator called the videlia beetle greatly reduced the problem once it was introduced into the state.

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The cottony cushion scale pest of citrus (Photo by Lucarelli via CreativeCommons.org)
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The vedalia beetle which was introduced to California to feed on cottony cushion scale (Photo by Katja Schulz via CreativeCommons.org)

The grape leafhopper can be a damaging pest, but when growers plant wild blackberry vines near their vineyards, they encourage the build-up of a certain kind of parasitic wasp which attacks the blackberry leafhopper species in addition to the grape leafhopper. This can keep the grape leafhopper numbers sufficiently low, reducing the need for pesticidal control. Some insect predators and some parasites of insects are raised commercially for release on farms. Growers can also control some diseases and nematodes by applying biocontrol agents such as bacteria, nematodes or fungi which act as hyperparasites.

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IPM Strategy 4. Disrupting the Pest’s Life Cycle

Perhaps the most common way that growers control pests in annual crops is through the use of crop rotation. For instance, growers typically rotate corn with soybeans throughout much of the Midwestern U.S. This change of crops sets back the population of certain pests. For potatoes, it is often necessary to have several seasons of other crops planted between each potato crop, otherwise pests build up to damaging levels.

Some pests spend different parts of their lifecycle on different hosts. A damaging and even deadly bacterial disease of grapes is spread by an insect called the glassy-winged sharpshooter. A state-wide program in California has been in place for the last 15 years to minimize the damage caused by this pest to the highly valued wine grape industry. One element of this program is based on the fact that the insect spends the winter feeding in citrus orchards while the grapes are dormant. By disrupting the pest’s life cycle stage on citrus, the state greatly diminishes the risk to grapes.

California uses a similar off-season program to control the sugar-beet leafhopper which is a vector of the curly top virus disease of peppers and tomatoes. In the winter, the hoppers survive on weeds in rangeland and unused cropland and then return to the tomato and pepper crops in the summer. Through a state program, California monitors for off-season populations and treats non-crop areas as needed to protect the next season’s vegetables.

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Tomato infected with curly top virus (Image from California Department of Food and Ag)

 

 

 

 

 

 

 

 

Another way that growers can control insect pests through life-cycle disruption is an approach called mating disruption.

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Diagram from Washington State University Extension, originally published by J. Brunner and Alan Knight in 1993

Growers place synthetic versions of the insect’s mating hormones throughout a field or orchard so that the males can’t detect the gradient of the hormone that guides them to females. This sort of program works best with relatively low populations of the insects, so it is best paired with a targeted insecticide program.

In certain cases, it is possible to release large numbers of male insects that have been raised to be sterile. These males out-compete the wild ones to mate with the females, which then generate few offspring within the population. This strategy has often been applied to control mosquitoes that vector (or spread) human diseases or to control introduced exotic pests like the Mexican fruit fly.

Historically, sterile male release programs have been based on the use of gamma irradiation to sterilize the insects. These males may be compromised in other ways, so programs must use large numbers of sterile males to effectively compete with the wild males. Scientists have developed a technology that genetically modifies males to need a specific, supplied nutrient in their diet to survive. Growers or others involved in insect control programs can release such males to mate, but the males will not live long and their offspring will not survive because they inherit the requirement for the nutrient.

Field-testing of this technology has demonstrated that it can be effective to lower the populations of the mosquitoes that spread human diseases like dengue fever, Zika virus or chikungunya virus. This approach could also be helpful for controlling some of the newly introduced exotic pests that now threaten various crops around the world.

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IPM Strategy 3. Modifying the Climate

Perhaps the most familiar example of pest management by climate management is the use of refrigeration for foods like fresh fruits and vegetables. Most consumers have seen their favorite produce items mold and decay, but careful maintenance of the cold chain from harvest to consumer greatly reduces such food losses. Within a given growing area, certain methods can change the effective crop environment by modifying the microclimate in which the plants grow.

One aspect of the microclimate has to do with the nature of reflected light. In the late 1970s, scientists discovered that growers could use reflective mulches on vegetable beds to confuse aphids that spread damaging viruses to squash plants. As one article from 1979 states, “the mulches reflect the sun’s ultraviolet rays, which the aphids ‘see’ instead of the blue-green light of the plants. In effect, they receive a signal to ‘keep flying’ instead of landing.” Tarps are also widely used to block light getting to weeds so that they are not able to grow and compete with the crop.

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Vineyard following “leaf removal” to modify the canopy microclimate (Photo by Andy Allen, Univ. Missouri)

Wine grape growers often use trellising methods and remove lower leaves to change the microclimate where the grape clusters are developing. By lowering the humidity, it is possible to reduce damage by fungal pathogens such as Botrytis bunch rot.

A variety of growing systems, called protected culture, range from a simple rain shield to a passive greenhouse to a high-tech greenhouse with complete climate control. These measures provide relief from certain diseases that would otherwise be fostered by rain. They also provide other advantages, such as an extended growing season.

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Hoop houses for raspberry production in the Salinas Valley of California (photo by Steve Savage)

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IPM Strategy 2. Employing the Plant’s Own Genetic Defenses

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Wheat infected with stem rust (Image from USDA Agricultural Research Service)

Plants have evolved various ways to resist infection or attack by pests, and pests often evolve ways to get around a plant’s resistance. This back and forth evolutionary battle generally means that a given crop species includes a range of resistant and susceptible types. In the 1960s, plant breeders found wheat plants with resistance to a particularly damaging disease called wheat stem rust, and that advancement was a major contributor to the success of the Green Revolution. In 1999, a strain of the fungus in Uganda overcame the plant’s resistance and began to spread throughout the world’s wheat crops. Because some institutions maintain extensive seed banks of diverse genetic types, it was possible to find a new source of resistance. The global network of wheat breeders are now cooperating internationally to move that resistance trait into the myriad types of wheat grown around the world.

 

Genetically-based pest resistance is highly desirable for farmers, but it is best to integrate that strategy with other methods of pest control, otherwise the pest is likely to evolve around the host defense. IPM systems are critical to preserving genetic resistance for as long as possible.

 

Another genetic pest control strategy involves joining two different plant types. From ancient times, people learned that if they found a particularly desirable example of a fruit or nut, it was possible to grow more of the same trees or vines by grafting a branch or bud of the desirable species on to the roots/trunk of a different kind of the same species or of a related species. For instance, when an insect that attacks grapes was introduced to Europe from North America in the 1870s, the only way to save the crop was to graft the venerated European grape varieties on to rootstocks of North American grape species which were resistant to the pest. This sort of two-variety or two-species grafted plant is a pest resistance strategy that has been widely used in perennial crops.

For high value crops such as tomatoes, peppers and cucumbers, growers are increasingly dividing the task of genetic improvement between the roots and top of the crop. Growers use rootstocks to develop resistance to soil-borne pests and overall plant vigor. Using the above ground part of the plant, growers focus on breeding for yield, quality and resistance to the pests that attack the leaves and fruit. The two plant types are then grafted together for use in commercial production. Sometimes, you can find such grafted vegetables for sale in garden centers.

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Tomato top grafted onto a different tomato rootstock (Image by Goldlocki via Wikimedia Commons)
Genetic engineering provides a means of using genetic pest resistance in situations where ordinary breeding for such a trait is either impossible or far too slow. For instance, a gene for resistance to a bacterial disease of peppers has been moved to tomatoes, making them resistant to that same bacterium. Potatoes are quite difficult to breed, but by transferring a gene from wild potatoes from the Andes, scientists have moved disease resistance into modern, commercial-type potatoes. Potatoes with this improvement have recently been approved for planting in North America. Crops can also be engineered to be genetically resistant to virusesa strategy that saved the Hawaiian papaya industry.

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Test plot in Uganda of potatoes genetically engineered by 2blades Foundation to resist late blight using a gene from wild potatoes

Genetic resistance through biotechnology could provide solutions to important pest issues in many crops, but opponents of the technology have largely blocked the implementation of this approach. In many cases, the final result of the gene transfer would be the same as what would have happened through a much longer traditional breeding effort. In other cases, biotechnology offers a solution to a threat that may not be successfully addressed by any other approach.

 

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IPM Strategy 1. Avoiding the Pest

 

Not all pests occur in all places. Pests, like insects and diseases, have co-evolved with the plant species that they attack, often in the geography where the crop was first domesticated. Sometimes by moving the crop to a new location, farmers can avoid the pest. Eventually the pest tends to catch up and, with the intensity of modern travel, pest redistribution is inevitable.

The most successful examples of avoiding pests involve growing crops like fruits and vegetables in Mediterranean climates (e.g. California, Italy, Spain, Northern Africa…) where there is little or no rain during the growing season. Without moisture on the leaves of their crops, farmers can avoid many fungal diseases. This of course requires irrigation.

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Google Maps view of the isolated, agricultural region in the San Luis Valley of Colorado

Farmers also have some other, successful, long-term means of avoiding a pest. The San Luis Valley in Colorado is surrounded by high mountains and has cold winters. Insects that spread viruses to potatoes cannot survive the winter there, and the mountains prevent their arrival during the growing season. For that reason, and because it is important for farmers in all areas to plant virus-free potato seed tubers, the potato seed crop industry thrives in the valley. Sometimes the planting date of a crop can be modified to get a crop through a critical stage for pest damage either before or after the time when the pest arrives in a given region. Unfortunately, this strategy becomes more difficult to employ in a time of changing climate.

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Tomatoes grown without soil and physically protected from pests.  Image from Goldlocki via Wikimedia Commons
In some high-tech greenhouses and vertical farming systems, it is possible to completely exclude pests using physical barriers and by using hydroponic growing systems to avoid exposure to the pests that reside in soil.

Although this approach is too capital-intensive for most crops, it is very effective for high value vegetable and fruit crops and is a rapidly growing segment of agriculture around the world.

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The Many Ways Farmers Control Pests

 

Whether a farmer is growing in an organic or conventional system, his or her crop needs to be protected from damage from plant pests (insects, fungi, bacteria, viruses, nematodes, weeds…). To fail to minimize pest damage leads to inefficient use of scarce resources like prime farm land, water, or inputs. The quality and safety of the final products can also be compromised.

While materials we think of as “pesticides” play an important role, modern agricultural pest management depends on a combination of several tools and strategies which, when used together, offer a more resilient, economic, and effective means of crop protection. Though some of these practices have been part of traditional farming, many are more recent innovations. The explicit design of these multi-strategy programs began in the 1970s, and the approach is now widely adopted as integrated pest management (IPM). The optimal IPM program varies widely by crop and geography; this post will describe some examples that highlight the various components.

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The approaches used to implement IPM programs generally fall into six categories:

  1. Avoiding the pest
  2. Employing the plant’s own genetic defenses
  3. Modifying the climate
  4. Disrupting the pest’s life cycle
  5. Fostering beneficial organisms
  6. Using targeted pesticide applications

A brief introduction to each of the six approaches follows with additional links to the more detailed presentations. Each post will link back to the list above.

  1. Avoiding the pest

Not all pests occur in all places either because they have not spread there or because they cannot flourish in the climate of a given region. Both of these limitations have been historically important factors to consider when deciding what crops to grow where, and these pest limitations continue to be important considerations for farmers. Long-term, this strategy is limited by climate change and by the extensive movement of people and goods around the world

   2. Employing the plant’s own genetic defenses

Plants fight back against pests by evolving a variety of defensive strategies controlled by genetic traits. Built-in genetic resistance is an attractive form of pest control for farmers, but it is a resource that requires considerable effort to employ and stewardship to maintain as an effective part of an IPM program. For some crops, farmers can maintain a seed bank of genetic variation and draw upon it to keep ahead of the pest’s inevitable tendency to evolve around plant defenses.

When genetic resistance is available, it is generally wise to complement it with other IPM elements, such as pesticides, to avoid losing the valuable traits. For many crops, conventional methods of breeding are too slow and/or complex to easily employ genetic solutions. Traditional and advanced grafting approaches offer a dual plant genetics approach that has been quite useful in many systems. Advancements in biotechnology allow farmers to use same-species resistance genes in hard-to-breed crops as well as novel genetic approaches that have shown considerable benefit in the few cases where they have been allowed to-date.

   3. Modifying the climate

In some cases, farmers can shift the microclimate in which the plant is grown enough to reduce the threat of certain pests. Various degrees of protected culture have been widely used to shield crops from rain and/or to shift the temperature regime to extend the growing season at either end. The nature of the plant canopy can sometimes be managed to reduce humidity, increase light or otherwise create a microenvironment that is suppressive to certain pests.

   4. Disrupting the pest’s life cycle

Several strategies for pest control center on making it more difficult for the pests to reproduce. These range from crop rotation to insect pheromones to removal of damaged or infested plant parts. Other approaches involve the release of male insects which are sterile so that the females with which they mate do not produce any offspring.

   5. Fostering beneficial organisms 

Even pests have pests, and often there are ways that farmers can encourage these natural enemies to help keep pest populations low enough to obviate the need for other control measures. Sometimes, it is possible to actively produce and add the bio-control organisms to the system.

   6. Using targeted pesticide applications

Farmers can use a wide range of crop protection agents as part of an IPM system. In a great many cases, these agents are low hazard options in terms of environmental, beneficial, or human impact, but the use of all such agents is highly regulated on a national and state level. These crop protection agents are often important for preserving the utility of other IPM approaches, particularly genetic resistance. Farmers have many economic and practical incentives to only use these materials on an as-needed basis.

Pest control in agriculture is a multi-dimensional effort, and pesticides are just one of the important tools that farmers employ. Some of these tools have been in use for a long time and some are new. With climate change, the control of pests will become even more difficult. As the global population grows and standards of living increase, it will be even more important for farmers to avoid the sort of losses and food waste that pest cause. Fortunately, the toolbox available to fight pests is diverse and constantly improving.

The Challenges of Pest Control: Farming in a frost-free, urban environment (San Diego County)

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As many San Diego homeowners know, pests like the whiteflies on this hybiscus are one of the downsides of our beautiful, mild climate

When most people think of San Diego, California, they think of the beach or Legoland or other tourist attractions.  It turns out that in addition to being a major urban and tourist center, San Diego is a major agricultural player.

In this 30 minute podcast ( https://www.youtube.com/watch?v=_VHBlUT2aeQ ) I speak with Eric Larsen, the Executive Director of the San Diego Farm Bureau.  We talk about all sorts of interesting features of our farm economy (I also live in this county).  There are several points that come up concerning pest management and crop protection:

  • The fact that San Diego has very mild winters actually makes farming (and gardening) more difficult because the pest populations are not set back the way they are in harsher climates

 

  • A great many of the pests that SD farmers battle are introduced species, and unfortunately in many cases that introduction happened because urbanites and suburbanites bring in prohibited plant materials that carry the exotic pests.  They don’t do this on purpose, but there are reasons that this is an illegal practice

 

  • All farmers have to deal with pests – this includes conventional and organic farmers here in San Diego as in any region.  Some of our greenhouse producers are effectively able to prevent pests from ever getting to their crops, but only with careful management and exclusion strategies

This is the first podcast in a series that I will be working on for the CropLife Foundation