Module 4. Interventions
The United Nations Food and Agriculture Organization (FAO) recommend Integrated Pest Management for pest control, with focus on three main activities: prevention, observation and intervention. These activities are explained in the following lessons.
Lesson 4.1: Preventions
Prevention is a key element in an Integrated Pest Management strategy, however a very difficult concept to achieve by farmers in general; wheat growers are not the exception.
In wheat, diseases such Gaeumannomyces graminis, variety Tritici, which disseminates by contact, can easily be avoided with crop rotation. Therefore, farmers should avoid planting new wheat in an area which was affected in the past by this disease. This will limit or prevent pests managing the crop, allowing the increase in natural enemies, decreasing the hiding places of different pests and / or decreasing the food for pests.
To break the pest cycle some tools to use are: crop rotation, resistant varieties, good sanitation, removal of pests and hosts, and elimination of crop residues, stubble and seeds. There are also spatial, sequential and control methods of the planting or seeding material.
Spatial methods: as, for example, the use of multiple cropping patterns, plant spacing, intercropping, row crops, use of a trap crop or intercropped with others, habitat management.
Sequential methods: as, for example, crop rotation, multiple cropping, crops that are intermixed or under others.
Control of planting / sowing material: as, for example, host plant resistance, use of disease-free seeds and plants, genetic diversity of the crop, fertilization and proper irrigation, etc.
Lesson 4.2: Observation
The goal of this aspect of crop protection is to determine what action to take and when to take it.
Crop inspection in wheat in a weekly basis and in regular intervals is key to a successful pest control. It is important to determine crop growth patterns, as well as identify weeds that do not compete with the crop or host insect pests or diseases. Based on soil analysis, the farmer should make decisions on the use of fertilizers and apply the recommended herbicide after assessing weeds present in the crop. Also, the application of insecticides and fungicides should always be depending on the pest damage levels.
Natural enemies must be also evaluated, since their presence may allow minimization of the use of pesticides. The number of pests present must be counted and, compared to known thresholds and the number of natural enemies present in the crop, a decision may be reached as to the appropriate actions.
Decision support systems:
In order to help farmers make decisions regarding the incidence of pests in their fields, research is conducted to determine at what point certain actions should be performed. For example, when the population of a harmful insect reaches a certain level on cultivated plants, treatment with an insecticide may be recommended. Such a recommendation would be made according to the growth stage of the crop and the presence of beneficial insects.
It is also possible that public organizations agents or others have forecasting programs to give advice to farmers about when to undertake pest control.
To allow effective control of certain pests, it may be necessary to take extensive control measures, especially when it comes to highly mobile pests. In these cases, all farmers in a given locality must take appropriate action. Typically, such coordinated action would be organized by public organizations. For example, in wheat, this action should be taken when chrysomelidae or white worms are present or when outbreaks of new pests occur.
Lesson 4.3: Types of interventions
Soil management comprises a series of agronomic practices such as crop rotations, residue management of the previous crop, tillage, fertilization, liming, irrigation, and avoiding erosion and compaction, to create good physical and chemical conditions for crop growth and development, and to obtain the maximum yield at harvest.
4.3.1 Chemical interventions
Insecticide, fungicide and herbicide applications are important components of a Wheat Integrated Pest Management Program. Their use should be supervised by a professional who periodically evaluates the efficacy of these products. Most of the crop protection products are effective; however, some pests and microorganisms can develop resistance or tolerance. This can occur when pests and diseases are long-term exposed or treated with the same mode of action of the crop protection product, or just because environmental factors negatively impact the correct application of crop protection products.
Wheat growers should keep in mind that an effective use of crop protection products is very much in line with the costs. When environmental conditions are favorable to pests and diseases, and they are not controlled rapidly and adequately through Integrated Pest Management, which includes the use of crop protection products, consequences for farmers are usually very negative.
Identifying the causal agent, selecting the appropriate crop protection product, adequate calibration of sprayers, use of correct dosage and safety measures when handling crop protection products, are some of the good practices recommended by experts in training manuals on Integrated Pest Management and the responsible use of pesticides. Government officials, as responsible for authorizing the product sale and use in each country, should continue to be involved in this type of training.
Crop Water Information on Wheat (FAO, 2015)
Fungicides are essential to wheat crop management. Wheat plantations are easily attack by diseases, as diseases attack all the plant structures, including roots, leaves, stems and spikes, affecting the photosynthesis process and thus yields. Some diseases are accidently introduced to the crop by farmers themselves whenever they use contaminated seed; this emphasis the need for high quality seed only, which has been treated with pesticides and free of pests and diseases.
In wheat, the carbons Ustilago tritici , Tilletia caries y T. foetida are usually transmitted via seed. Seed treatments are then an effective way to prevent fungi, as the treatment will attack and kill any spores or mycelium of the fungi pathogens present in the seed. The seed treatment will also protect the plant from other potential pathogens that live in the soil.
Systemic fungicides such as triazole have historically controlled internal infections caused by carbons in cereals. Developed in 1960; many of them are linked to carbamate of methyl-benzimidazol and other components, such as benomilo, carbendazima, tiabendazol and metiltiofanato. These products are very effective against the large majority of ascomycete fungi and imperfect fungi. The components of triazole such as tiradimefon, diclobutrazol are highly utilized in wheat against a variety of diseases. Antibiotics such as blasticina and kasugamisina, usually act systematically and are also used as fungicides and bactericide in wheat. Some of these products, such as ciclobenximida and estreptomicina are usually used in crops with a high return, as they are rather costly.
Another major foliar issue that farmers face is rust. Rust can be caused by fungi parasites such as Uredinales, Basidioycetes. The fungus develops in the growing tissue, namely foliage and spikes, affecting the metabolism in susceptible varieties and creating thousands of yellow-orange or brownish spots or spores. The most known rusts are: Stem rust: Puccinia graminis; Red rust: P. triticina and Stripe rust: P. striformis. In wheat plantations, when the foliage remains wet for long periods (more than three hours), strong attacks of rust are likely to happen. Other factors that intervene in rust attacks are temperature (around 20 °C) and upwind conditions.
The chemical control of rust in wheat should always be preventive in susceptible varieties; the use of a mixture of active ingredients including triazole and strobilurins is very common. Triazole such as fluconazole, itraconazole, voriconazole and posaconazole are usually used as they have a direct impact on the foliar area. This chemical control can avoid diseases such as chlorosis, tissue senescence and estrobilurinas; all derived from the beta acid methoxyacrylic and developed by various fungi species.
Other diseases in wheat such as, septoriosis foliar, caused by Septoria tritici and that mainly attacks during fall, are usually controlled by farmers with a mixture of fungicides, such as: tebuconazole + triadimenol; epoxiconazole + carbendazima; or epoxiconazoles + krexoxim methyl.
Another example is Oidios, caused by the ascomicete fungi Erysiphe graminis, and commonly known as Blumeria graminis. It directly attacks the spikes, covering them with a mass of fungi spores similar to a white powder. Some wheat farmers continue to use fenbuconazole for fungi control as it is also very effective for rust control.
However, as mentioned before, the effectiveness of fungicides in controlling wheat diseases is limited. Erysiphe graminis and fusariosis’caused by Fusarium culmorum, F. pseudograminearum and Gaeumannomyces graminis, variety Tritici known as foot disease, are very difficult to treat. For these diseases, it is highly recommended crops rotation and using certified seed only.
In terms of seed treatment and disinfection, it is very important to use efficient spraying mechanisms to ensure an even application of the treatment on all seeds (to avoid untreated or over treated seeds); this will guarantee high levels of germination and an effective disease control.
Each country offers a number of seed disinfectants developed based on triazole; concentrated solutions are preferred over solids and their use should always be advised by a professional in agriculture. A common used term in seed treatment is “pelleted”, which refers to the technic of mixing simultaneously a fungicide and a binder to provide a thick protective coat to the seed.
Fungicide Resistance Action Committee
Insecticides in wheat were first introduced in 1960 and 1970 mainly for control of sucking insects such as aphides and white worms, Hylamorpha, Phytoloema and Athila; and for sporadic attacks of the mosquito Tipula spp.
Currently, wheat growers use fewer insecticides, as pest control is mainly targeting aphides, including Rhopalosiphum padi, Metopolophium dirhodum, Sitobion anevenae, Schizaphis graninum and Diauraphis noxia. Aphides are usually controlled with biological controllers, and pesticides are applied only when the number of aphides exceeds the economic threshold level, or when more than 25 individuals are present in the flag leaf during flowering or grain filling. In areas where aphides frequently exceed the economic threshold level, pesticides applications are recommended in early fall. An application of Imidacloprid on the seed and soil will have the ideal prolonged residual effect on sucking insects, as once applied, it moves into the roots, stems, leaves and spikes.
Aphid control is often achieved with parasitoids and entomopathogens fungi, as it will be further explained in lesson 4.3.2. Applications of wide-spectrum insecticides should be considered carefully, as they also eliminate natural enemies and beneficial insects such as fly-bees Syrphidea, larvae of Eriopsis connexa and the endemic parasitoids Aphidius and Praon.
Insecticide Resistance Action Committee
4.3.2 Biological interventions
Biological control and the use of live organisms such as parasites, parasitoids, predators and pest micro-controllers, have proven an effective Integrated Pest Management strategy for aphides control. However, there are other methods equally effective that are based on the pest biology, including the behavior of the host plant, attractants, release of sterile males and resistant crops. Likewise, vegetal natural products (e.g. nicotine and Neem oil) and other live organisms are also considered biological controllers.
Further information on biological control here Booklet on Biological control (Thomas S. Bellows, 1999)
Biological control in wheat mainly refers to organisms such as parasites and parasitoids, pathogen microorganisms and predators. Predators are known for having a free live and consume a certain type of preys. Biological control also includes other groups such as pathogen controllers’ microorganisms and insects that attack weeds.
It is important to acknowledge that biological control agents are great allies in the management of phytosanitary problems or in Integrated Pest Management, but with them we cannot to completely eradicate or eliminate a pest, as they are specific and this may limit their use. They are essential organisms in the sustainability of agro-ecosystems, which is evident when natural enemies are scarce or are eliminated, resulting in an imbalance in the dynamics or epidemiology of pests, increasing pest populations, which may be solved doing releases. In some cases pest resistance and resurgence also occurs, mainly entomopathogens.
Common control agents
Parasites: organism that lives at the expense of another and from which derives its livelihood; usually pass through a free state which is very short and is linked to the spread of the pest which is called the host. In some cases they can be lethal, but when they do not kill the pest, they successfully reduce its reproduction, especially when they parasite the eggs.
Parasitoids: there is a large group of lethal parasites with such a very special biology that are often called parasitoids, which are mostly insects belonging to the Hymenoptera and the orders Diptera. They are very important group in introduced biological control since they comprise more than 300,000 spp. and virtually all insect pests are attacked by one or more parasitoids.
Parasitoid females deposit their eggs near the host; the larvae feed internally or externally on the host, immobilizing and eventually killing the host. The pupa is formed and developed internally from where the adults emerge; they usually mate immediately and then disperse. In most cases the larvae kill their host and perhaps for this reason they are called parasitoids, but some wasps or flies acting as parasitic insects may be found. In wheat, it is common to find Aphidius colemani and A. testaceipes (Hym. Branconidae); these are two species of parasitoids that attack aphid populations mummifying the adults and nymphs.
Predators: are organisms that live freely, each of which will consume a large number of preys during their life cycle either as larvae or as adults and are less specific than parasites, parasitoids and pathogens. Besides insect they include some vertebrates, birds, reptiles, spiders among many others. The most effective predators are invertebrates, such as insects that multiply rapidly, such as ants, wasps, dragonflies, ladybugs, lacewings and bedbugs. In wheat plantations, there are predators such as coccinellid (Hippodamia convergens, Coleomegilla maculate, Eriopsis connexa, Cycloneda sanginea and Rodolia cardinals) in addition to the lacewing predators from Cosmopolitan Chrysoperta which control not only aphides but larvae of coleopterons, lepidopteron and mites.
Pathogens: are microorganisms such as viruses, mycoplasma, bacteria, fungi and protozoa, which in contrast with parasite nematodes and parasitoids, are treated as a separate group of biological control agents. Pathogen infections generally occur when a particle is consumed, but the fungal ones can occur by direct penetration to the body of the host. They multiply rapidly within the host causing a disease which in some cases is fatal to the host. The particles or spores can be released even when the host is alive but generally this occurs at the death of the same, when millions of pathogen particles are released. For control and effectiveness it is ideal to have an epidemic when pest density is high. In wheat there are pathogens such as Entomophtora and Nomuraea.
Other biological control agents may be groups of insects or microorganisms with very specific behavior such as hyperparasites or antagonists or competitors for space. Also included here may be insects that control weeds by feeding on them or those microorganisms that are weed pathogens.
Biological Control and Natural Enemies of Invertebrates (University of California, 2014)
Population dynamics of biological controllers
The success of biological control depends essentially on our knowledge of population dynamics of beneficial insects, the pest, the pathogens and their hosts. The population of beneficial insects, pathogens, pests or the plants varies with time, often showing fluctuations within certain limits. Then, the target of biological control, as well as all measures aimed at managing pests is to reduce and maintain these fluctuating populations below the economic threshold level.
The size of a particular pest population over time is a balance between factors that increase it: reproduction and immigration and those that make it decline: death and emigration. Natural enemies as well as insecticides are among the causes of death, but pests also die due to abiotic agents and natural aging. To only measure the abundance of natural enemy or the percentage of parasitism, without measuring the size of the population in which the natural enemies act, often is useless information. Mortality can be dependent and independent of the population density.
One way to conserve the natural enemies on the systems where pathogens are sprayed or beneficial insects are released is to do it at the proper application timing. Avoid partial releases or unnecessary applications; make complete sprays and releases; establish the rate, the application equipment, the selective pesticides such as growth regulators, microbial pesticides, formulations and take into account the environmental conditions.
When applying synthetic pesticides in general, natural enemies are more susceptible to pesticides than pests, because being smaller, they require less active ingredient, remain exposed on the plant surface, and have less enzymes that enable them to detoxify pesticides. It could be argued that killing the natural enemy is not so bad if the pesticide use helps with the direct death of the pest.
However in pest management priority is given to minimizing the use of pesticides, then the applications that simply replace the action of natural enemies are not desirable or conversely, if the contribution of a natural enemy is so small there is no reason to preserve them limiting the use of pesticides. In many cases the use of both methods in the context of pest management is both desirable and necessary, but this must be the result of research.
4.3.3 Cultural interventions
Cultural control is a wide range of pest management options and techniques to improve crop production, which deliberately alter the normal agricultural production systems. They are management measures, traditional preventive or interventionist. They act by making the plant unacceptable to the pest, adjusting the establishment of the plant in space or season, or making it dangerous for the pest by the population numbers of natural enemies. The techniques or management options are: destruction of breeding or wintering shelter, such as the aphids affecting wheat in temperate areas and overwinter in the egg stage in some woody hosts, likewise some white grubs that remain feeding on the corn roots after harvest. Crop rotation interrupts the normal development of the insect’s life cycle forcing them to stay in other hosts; it is a very practical chrysomelidae management mechanism in the corn/soybean rotation. Tillage allows turning the soil to expose the larvae and thus control the borers.
Management of planting dates or of harvests may allow plants to escape the damaging infestations, but also early crops may be less affected than delayed planted crops. Timely harvests do not allow insects to fulfill their lifecycle. Trap crops may in some cases exacerbate the problems if the behaviors of the pests are not well known. Hygiene, cleanliness is perhaps one of the aspects that should be most improve in wheat; it is very important to reduce sources of inoculum mainly of microorganisms. The management of water or nutrients can suffocate insects or improve plant health because some pests and microorganisms grow when there is poor crop growth and others when there is succulent development. Flooding is used for control of wireworm in wheat. Fertilization; undernourished plants are more affected by aphids.
4.3.4 Genetic interventions
Wheat breeding seeks to improve the development characteristic, and its response to photoperiod and temperature. This is because the duration of each phase is determined by the interaction of genetic and environmental factors which are also responsible for yields. The sources of genetic resistance against diseases start by selecting parents with effective resistance in each region. Until now the main work has been aimed at seeking resistance to rusts given the damage they cause. Being a monogenic heredity, it is possible to clearly identify whether the resistance was incorporated. This type of resistance is called breed specific or vertical resistance, which is based on the theory gene to gene by Harold Henry Flor (1955), that states that a plant will not be attacked if it has a gene that protects it from the virulence gene of the pathogen, otherwise it will be susceptible. Another alternative is attributed to higher qualitative resistance genes, characterized by plants that carry these complete genes and present an initial resistance that a short time later become susceptible. Some geneticists have found that in qualitative resistance there are also lesser genes where severity is expressed in a gradual manner. In the future it is expected to include parents with specific resistance genes to certain races of rusts which usually cause artificial epidemics following the introduction of inoculum from other regions and varieties
Wheat diseases are mainly caused by fungi and to a lesser extent by bacteria and viruses (Rajaram and van Ginkel, 1996; McIntosh, 1998). With the discovery of the genetic basis of resistance by Biffen (1905), the physiological specialization of rust pathogens by Stakman and Levine (1962) and the gene interaction gene-by-gene by Flor (1955), utilizing the hypersensitivity (race-specific) type of resistance started and has dominated in wheat breeding ever since. This approach seems to be very attractive from the standpoint of the cleanliness of crops and because it is easy to incorporate into improved germplasm. The phenomenon of erosion of these genes, or combinations thereof, prompted scientists to search for alternative approaches to resistance management. The multilinear approach promoted by Jensen (1952) and Borlaug (1953) arose from the frustrations associated with the frequent failures of specific genes for each race. Van der Plank (1963) was the first epidemiologist to clearly define the theoretical basis of the concepts of resistance. In the late 1960s and 1970s, there was a resurgence of the concept of general resistance (race-nonspecific) and its application in crop improvement (Caldwell, 1968). This approach is widely used for stem rust resistance in wheat by Borlaug (1972), leaf rust resistance by Caldwell (1968) and yellow rust resistance by Johnson (1988). The large-scale application of this concept in improving leaf rust resistance, commonly known as the slow rusting, has dominated in improving CIMMYT bread wheat for over 25 years. During this period, numerous terms have been used in the literature to describe the various features of resistance. A simple term is often not enough, since resistance can be described in terms of its epidemiological and genetic characteristics. During this period, numerous terms have been used in the literature to describe the various features of resistance. A simple term is often not enough, since resistance can be described in terms of its epidemiological and genetic characteristics
Achieving lasting resistance
The wheat genotypes of CIMMYT germplasm are grown in a large area and are exposed to a variety of pathogens in conditions that favor the development of disease; CIMMYT’s strategy has been to use germplasm sources as diverse as possible to resist rust. Gene flow to and from the bread wheat breeding program is continuous, and scientists are in constant contact with their colleagues in national programs to ensure this exchange. The lines showing stable yield across locations are useful for the understanding of the genetic basis of resistance (FAO, 2002).
Genetic diversity and durability are the two most important characteristics of resistance for the global wheat breeding program at CIMMYT. Genetic diversity serves as insurance against the vulnerability of pathogens. Genetic analysis to understand the genetic basis of this resistance could help the targeted transfer of resistance, as well as the search for additional genes that might contribute to new combinations of genes for durable resistance to the three wheat rusts.
The stem rust resistance gene Sr2 and other unknown lesser genes are the hope of the crop. The Yaqui 50 genotype, launched in Mexico in 1950, and other Sr2 carrier wheats have been released since then and have stabilized the stem rust problem in Mexico. In recent studies, changes of new stem rust races have not been observed in Mexico. The Sonalika genotype launched in 1960 in the Indian subcontinent and then planted in millions of hectares has also remained resistant. Knott (1988) has shown that appropriate levels of multigenic resistance to stop rust can be selected by the accumulation of about five minor genes. In his studies, the genes were different from Sr2. It is likely that similar genes are present in CIMMYT germplasm, but more research is needed to document their existence.
Genes for durable resistance to leaf rust
The South American Frontana genotype is considered one of the best sources of durable leaf rust resistance (Roelfs, 1988). The Rockefeller-Mexican program first used the variety in the 1950s. Subsequent derivatives, such as Penjamo 62, Torim 73, Kalyan/Bluebird, etc., showed characteristics of horizontal resistance, possibly derived from Frontana. Genetic analysis of Frontana and several CIMMYT wheats possessing excellent partial resistance to leaf rust around the world have indicated that such resistance in adult plants is based on the additive interaction of Lr34 and two or three additional minor genes (Singh and Rajaram, 1992). In Mexico, the severity of leaf rust in most cultivars may be related to the number of minor genes. When susceptible genotypes exhibit 100% leaf rust severity, cultivars with Lr34 manifest just 40 percent of the disease; cultivars with Lr34 and additional one or two genes show under 10 to 15 percent of disease; and cultivars with Lr34 and two or three additional genes show 1-5 percent. Leaf rust could increase to unacceptable levels in cultivars carrying only Lr34 or Lr34 and one or two additional genes. However, cultivars with Lr34 and two or three additional genes show a stable response in the environments tested so far, with final scores of leaf rust lower than 10 percent.