Presentation on theme: "All parts of the plants are subject to pest damage and the pest complex changes during the season as the plants grow. Harvest are reduced in many ways:"— Presentation transcript:
All parts of the plants are subject to pest damage and the pest complex changes during the season as the plants grow. Harvest are reduced in many ways: Loss of roots (root worms) Stems break (stem borers) Leaf area (many species) Reproductive structures Seed weevils Phloem sap (aphids) Pupa of a stem borer Damage by corn rootworm Adults and eggs of bruchids (Larvae do the damage) Leaf miner damage Insect control: Bt crops are an example of the (so far) successful application of biotechnology.
Some insects are generalists others are specialists Whether insects are generalists (e.g. locusts) that eat everything or specialists that thrive on only a few species depends on their ability to overcome the plant’s defenses. Plants have evolved physical barriers (thick Cuticles), chemical barriers (specific defense chemicals) and inducible defenses (e.g. inhibitors of digestive proteases). The proboscis of an aphid species may not be able to penetrate the cuticle and cell wall, or the plant may have toxic chemicals or induce chemicals that prevent growth of the aphid.
Control options for the farmer: avoiding the build- up by cultural control Phenological asynchrony: Safe planting date for winter wheat to escape damage from Hessian fly Crop rotation: when there is no host, the pest does not thrive The other crops may harbor enemies of the pest
Control options for the farmer Planting insect-resistant crop varieties Antixenosis. A physical or chemical property of a plant can make it so unpalatable that it is largely protected from herbivore attack. This type of resistance is often known as nonpreference. It may involve the presence of feeding repellents (or the absence of feeding attractants), or it may involve physical traits such as hairs, waxes, or a thick, tough epidermis that do not provide the pest with a desirable feeding substrate. Alfalfa, for example, has been bred with hairy leaves to deter feeding by the spotted alfalfa aphid.
Control options for the farmer: Biological control agents 1. Narrow host range. Generalized predators may be good natural enemies but they don't kill enough pests when other types of prey are also available. 2. Climatic adaptability. Natural enemies must be able to survive the extremes of temperature and humidity that they will encounter in the new habitat. 3. Synchrony with host (prey) life cycle. The predator or parasite should be present when the pest first emerges or appears. 4. High reproductive potential. Good bio-control agents produce large numbers of offspring. Ideally, a parasite completes more than one generation during each generation of the pest. 5. Efficient search ability. In order to survive, effective natural enemies must be able to locate their host or prey even when it is scarce. In general, better search ability results in lower pest population densities. To improve biological control one can import a new agent, conserve existing agents by changing cultural practices or pesticides or augment agents by periodic release. Through success and failure we Have determined the characteristics of effective agents
Control options for the farmer: insecticides Insecticides have evolved from general poisons to specific poisons! As a result, the total amount of pesticide used has leveled off and insecticide use has actually declined (not acreage, but pounds) Many insecticides are toxic to humans (they affect nerve function),and need to be handled with care. This is not always the case, especially in developing countries.
Bioaccumulation of DDT in the food chain Eggshell thinning was found to be the main reason for the failure of the reproductive success of birds at the top of the food chain Many other chemicals bio-accumulate
The emergence of insecticide resistant insects is the result of continued pesticide use and creates the need for new pesticides.
The goal of insecticides is to kill insects, not to create an ecosystem in which there is an acceptable level of the pest. Pesticide treatment is always followed by resurgence of the pest population. Evolution of pesticide use
Molecular basis of insecticide resistance A.Mutation that makes the target protein insensitive to the pesticide. Pesticide does not bind to the protein. B. Mutation in the promoter of a detoxification enzyme to enhance the expression of the gene. C. Amplification (increase in copy number) of the detoxification enzyme. R S Corn rootworm
With so many options, what is the farmer to do? Use integrated pest management (IPM), which requires monitoring of pests and taking action when required. 1. Cultural practices 2. Genetics 3. Biological control 4. Chemical control Sticky traps are widely used but you need to be able to distinguish pests from non-pests! Farmers contract with special companies for these services. Traps with mating pheromones attract males of flying insects in a species- specific way. Monitoring has to be done throughout the season. Results can vary enor- mously from one year to the next.
Example: IPM of the sweet potato whitefly (Bemisia tabaci) in Cuba (infects many plants and spreads tomato yellow leaf curl virus) Steps in the IPM program: 1. Plant only early tolerant, or resistant varieties 2. Healthy or disinfected cuttings 3. Sex pheromones to disrupt mating 4. Beauveria bassiana (entomophagous fungus) 5. Colonization by predatory ants 6. Irrigation management (no soil cracking) 7. Early harvest 8. Crop rotation 9. Destroy crop residues and volunteer plants 10. Nationwide monitoring program (traps) Beauveria bassiana Program developed by the Centro Internacional de la Papa in Peru. Whiteflies on the leaves
sporulation induces synthesis of crystals pore-forming domain receptor binding domains crystals are made up of pore-forming toxins Bacillus thuringiensis is a ubiquitous soil bacterium that produces proteins that kill insects and nematodes. The proteins are called “Cry”, because they occur in crystals
Physiological mechanism of Cry toxin action
Lepidoptera Coleoptera Diptera Cry Toxin Specificity Crickmore et al Nematoda
Bacillus thuringiensis (Bt) spores can be formulated as dusts or sprays and have been used for years as “natural” insecticides. About 100 different Cry proteins have been identified and all have some specificity. Spores of Bt are dusted on vegetables by home gardeners and organic producers or sprayed (1000 spores per sq meter) to kill larvae of lepidoptera (e.g. corn ear worm)
Genetically engineered Bt cotton and Bt corn have been a commercial success Advantages of transgenic approach: Reduction in insecticide sprays (labor and chemical costs). Increased activity of natural enemies. Biological control can be used on secondary pests. Limitations of transgenic approach You still need to control the “secondary” pests Cost of transgenics Development of resistance because of persistent exposure.
A side benefit of Bt corn: reduction in mycotoxins Infection of corn (and other seeds) by Fusarium is more common when corn is damaged by insects. Fusarium produces fumonisin, a potent mycotoxin. The FDA “Guidance for Industry” for fumonisin levels of 2 to 4 µg/g in human food and animal feeds, but higher levels are normally found in corn produced in some parts of the country or some years (5 to 10 g/g). The histogram below compares fumonisin levels in control (green) and Bt (blue) corn. Bt 176 does not make Bt protein and is a control.
Example of the economic benefit of Bt cotton to small farmers in South Africa.
The total worldwide cost of insect control is $ 8 B. About $ 2.6 B could be substituted by Bt crops.
Resistance to Bt sprays has already emerged for some insects. So, management of Bt crops is needed to avoid the emergence of Bt resistant pests. Two farmers stand among rows of non-Bt corn, which has suffered insect damage. To either side are rows of Bt corn, for which insect damage is greatly reduced. Carrie Daniel, Novartis Seeds “Management” means that a certain acreage must be set aside for the non-GM crop so that the insects will thrive there. This will reduce the selection pressure and the occasional mutant that evolves will find a non-mutant mate. This greatly delays the emergence of resistance.
Does Bt corn endanger the monarch butterfly as alleged by many “Green” organizations? Pollen shed by the Bt corn falls on milkweed that grows in and around corn fields. Monarch larvae feed exclusively on milkweed leaves. Cornell university researchers showed that high levels of pollen on the leaves can kill the larvae. But are such high levels a frequent occurrence in nature?
A Risk Assessment... Acute toxic effects of pollen Probability of larvae being exposed to toxic levels in and around corn fields Hazard = Exposure = Risk = “This two year study suggests that the impact of Bt corn pollen from current commercial hybrids on Monarch Butterfly populations is negligible.” From Sears et al., 2001 X
Pests have short life cycles and their populations can build up during the season and over the years. Populations fluctuate depending on conditions (food, enemies, weather) Pest control options: (1) prevent the buildup, (2) decrease the level or (3) delay the buildup beyond the point where it damages the crop. Typical buildup and decline of a pest population during the growing season
When is a herbivore a pest? Economic injury level Pest populations change over time and not every population will reach the economic injury level (EIL). The farmer has to balance the cost of control with the loss of revenue. When cosmetic appearance is important (fruit, sweet corn, canned/frozen vegetables) even a little damage (blemishes, presence of insect larvae) may cause economic loss. Pest control has to be initiated before that level is reached at the economic threshold (ET).
A different approach uses inhibitors of digestive enzymes Peas on the left have been transformed with the gene from bean that encodes an inhibitor of digestive -amylase. This inhibitory protein is normally present in bean seeds and prevents the seeds from being eaten by the larvae of certain species of bruchids, when the gene is transferred to peas and expressed in the pea seeds, the seeds are now resistant to those species of bruchids whose digestive amylase is inhibited by this inhibitor (amylases are needed to digest starch).
Pyrethrum is a non-synthetic insecticide (botanical) Pyrethrum is extracted from the flowers of the chrysanthemum grown in Kenya and Ecuador. It is one of the oldest and safest insecticides available. The ground, dried flowers were used in the early 19th century as the original louse powder to control body lice in the Napoleonic Wars. Pyrethrum acts on insects with phenomenal speed causing immediate paralysis, thus its popularity in fast knockdown household aerosols. However, unless it is formulated with one of the synergists, most of the paralyzed insects recover to once again become pests. Pyrethrum is a mixture of four compounds: pyrethrins I and II and cinerins I and II.
What happens to all those insecticides? Only a minor proportion falls on the plant
A study from Cornell University showed that pollen from Bt corn, when dusted on milkweed leaves killed the monarch larvae. But how realistic are the conditions?
Plants, herbivores and their enemies all evolve together (co- evolution). Plants evolve defenses, but herbivores evolve to overcome them. Predators evolve to live off the herbivores, but the herbivores evolve defenses. Spined soldier bug attacking larva of Mexican bean beetle. All the interactions between herbivores and their predators and diseases are still poorly understood Azadirachtin, an allelochemical from the Neem tree that is an anti-feedant and can be used as a spray. Plants contain tens of thousands of chemicals, most of which have not been identified let alone studied. Their role is in plant-plant or plant- herbivore interaction is poorly understood.
Many insecticides affect nerve function Three major classes of insecticides: DDT and other organochlorines) are now banned because of bio-accumulation and effects on mammals. Mode of action of DDT was not clearly established but it interferes with nerve function (not used in the US). Organophosphates discovered by research on nerve gasses. Inhibit acetyl-choline esterase an enzyme essential for nerve function. Inhibition causes accu- mulation of acetyl-choline at the nerve synapses resulting in muscle twitching. Pyrethrum is a naturally occurring insecticide, but it is unstable after isolation. Chemists have made a series of synthetic pyrethrins. They also block nerve function (keep Na+ channels in the open position). ipmworld.umn.edu/chapters/ware.htm
Insects and insect control Pests attack all parts of the plants; some are generalists, others specialists. Plants defend themselves in several ways, but co-evolution means that someone always gets eaten. The goal of pest control should be to have a stable manageable pest population that causes no economic hardship. The farmer has many pest control options: breeding, cultural methods, chemicals (pesticides, natural or synthetic), biological control. Selection pressure results in the emergence of pesticide-resistant pests Integrated pest management (IPM) is the best pest control method Bacillus thuringiensis (Bt) produces a protein toxin used by farmers to kill lepidoptera Bt-crops are genetically engineered with the Bt gene encoding the Cry toxin protein. Bt crops are highly successful biotech crops.