Lecture 08: Pestsicides (Linstaedter) Flashcards
Def. Pesticides
Chemical compounds for controlling pest organisms.
It is classified based on the targeted organism to be controlled ending with -icides
E.g. Herbicides, Insecticides, Virucides
„Problem plants of Germany“
- Slender Meadow Foxtail/ Ackerfuchsschwanz ( Damage density: An infestation with 500-1000 fruiting culms per m2 leads to a yield loss of 30-50%)
- Vogelmiere ( Damage density: ≥ circa 25 plants / m2)
- Flughafer (Damage density at 5–10 plants / m2)
Pesticide use in Germany
Herbicides are already used in the last century, but increased rapidly in this century. Fungicides and Insecticides increased dramatically as well.
Amount of pesticides used has increased by > 10% since 1995
How do they work? Modern pesticides
Modern pesticides modify the cell metabolism by binding to specific sites (target sites, mostly Enzyms) and thus inhibit or stop biochemical reactions.
Systemic herbicides are being distributed in the whole plant after uptake; transport in the xylem and/or phloem. -> Man muss nicht mehr gucken, dass alle Pflanzen komplett bedeckt sind sonder nur das Gift auf dem Boden verteilen.
Viewpoint of a chemical company
Modern herbicides are small chemical works of art…. This selectivity is particularly impressive when the harmful and useful plants are closely related botanically..
Better than the old pesticides which were very poisonous for a lot of species and not specific.
BUT unwanted side effects because maybe there are organisms in the future, which are targeted by these pasticieds as well etc
Global increase of weed species with glycoproteins resistance
In 2020 there are up to 40 species which are resistant to glyphosate. There is a cascade and maybe all the pest species will come back… parallel to antibiotics.
We see an exponential increase in glyphosate resistance over time!
Italian raygrass
In Australia, a serious weed of winter crops, resistance again 10 herbicides.
Def. Resistance
Resistance is defined as a change in the sensitivity of a pest population to a pesticide, resulting in the failure of a correct application of the pesticides to control the pest.
- is the result of evolution e.g. mutations and genetics
- spontaneous mutations may lead to resistance: the pesticides is no longer effective or at high doses only
->spraying pesticides provides a strong selective pressure
- Remember: Strong selective pressure occurs when only a few individuals in a population can reproduce, usually because the environment is harsh, and a very specific set of genes is needed to survive to adulthood & reproduce
The development of herbicide resistance
Herbicide resistance is an evolutionary response to weed management practices
The beginnings of chemical control: Spraying with dangerous, unspecific compounds
Early pesticides often caused permanent
damage to soil and groundwater
It appears that modern herbicides are a well- designed, efficient and harmless means to control pest organisms. Are they really?
No, there is the issue of resistance!
How resistance develops:
A decoding of first steps from an evolutionary perspective
Example: herbicide effects on a population of an annual plant („weed“)
1) Before spraying the herbicide:
Only non-resistant genotypes are present in the plant population
2) After herbicide application:
High mortality of non-resistant genotypes (yellow); a mutation has led to one resistant genotype in the weed population, with a much higher fitness (= reproductive success) than the other genotypes
3) The following year:
Resistant genotype multiplies (germination); but non-resistant genotypes also present (germinate from seeds still in the soil seed bank)
How resistance develops:
A decoding of all steps from an evolutionary perspective
Example: insecticide effects on a population of a pest insect species
1) Only non-resistant genotypes are in the insect population before insecticide application
2) A small proportion of the population is able to survive insecticide application (due to a mutation)
3) Resistant individuals reproduce & pass on their resistance to the offspring
4) If an insecticide with the same mode of action is repeatedly used against this population, an even greater proportion will survive
5) Ultimately, the once-effective product no longer controls the resistant population
Resistances evolve rapidly in short-lived organisms (insects, bacteria, annual plants)!
The directional selection of resistance
- Resistances evolve rapidly and are heritable -> ongoing use of a pesticide provides a strong selection pressure
- Evolution of resistance is an example of directional selection: occurs when individuals at one end of the phenotype range have higher fitness than individuals in the middle or the other end of the range
- Spontaneous resistant genotypes spread rapidly in a population, if the use of pesticides is being continued
What are the two other types of selection?
-> Stabilizing and disruptive selection!
Why is it so difficult to detect & fight resistant genotypes?
- Newly evolved resistant individuals are rare & hard to recognise in the field
- Their abundance has to reach a tipping point to be perceived as a problem
-An early recognition requires systematic recording of field data & lab analyses
