By the end of this lesson
- Explain the reasons for pest control.
- Evaluate the advantages and disadvantages of pesticides.
- Evaluate the advantages and disadvantages of biological control.
BIG QUESTION
Is it better to spray aphids or release ladybirds?
Both methods reduce pest damage, but they differ in speed, specificity, persistence and risk to other organisms.
Why control pests
pestAn organism that damages crops or reduces yield, such as an insect, weed or fungus. are organisms that reduce crop yield or quality. Insects eat leaves or suck phloem sap; fungi cause disease; weeds compete for light, water and minerals.
Controlling pests means more of the energy fixed by the crop ends up in the harvested product, so yield and quality rise.
Pesticides
pesticideA chemical used to kill pests, such as insects, weeds or fungi. are chemicals that kill pests: insecticides, fungicides and herbicides. They act quickly and are easy to apply over large areas.
Disadvantages: they can kill non-target organisms such as pollinators; some persist in the environment and can bioaccumulateTo build up in an organism's tissues over time and increase in concentration along a food chain, as some pesticides do. along food chains; repeated use selects for pesticide-resistantDescribes a pest that is not killed by a pesticide, because it has an allele for resistance that natural selection has made more common. pests; and residues may remain on food. They must be reapplied.
Biological control
biological controlUsing a natural predator, parasite or pathogen of a pest to reduce the pest population. uses a natural predator, parasite or pathogen of the pest, such as ladybirds eating aphids or parasitic wasps attacking whitefly in glasshouses.
Advantages: it is usually specific to the pest, leaves no chemical residues, and the control organism may reproduce so repeated treatment is less often needed. Disadvantages: it is slower, does not eliminate the pest completely, and an introduced species may become a pest itself or harm native species.
Pesticides versus biological control
Neither method is best in every situation.
| Feature | Pesticides | Biological control |
|---|---|---|
| Speed | Fast. | Slow to take effect. |
| Specificity | Often kills non-target species. | Usually targets one pest. |
| Residues | May persist and bioaccumulate. | No chemical residues. |
| Resistance | Pests may evolve resistance. | Resistance is less likely. |
| Risk | Toxic to other organisms. | Control species may become invasive. |
| Effectiveness | Can kill most pests. | Reduces but rarely eliminates pests. |
Language that earns precision
- pest
- An organism that reduces the yield or quality of a crop.
- pesticide
- A chemical used to kill pests.
- biological control
- Using a natural enemy of a pest to reduce its population.
- bioaccumulation
- Build-up of a substance in organisms, increasing in concentration along a food chain.
Common misconceptions
Claim: Biological control eradicates the pest.
Correction: It reduces the population; if the pest disappeared the predator would decline too.
Claim: Pesticides make pests resistant.
Correction: Pesticides select resistant individuals that already exist in the population.
Claim: Biological control has no risks.
Correction: Introduced species can harm non-target organisms or become invasive.
Match the answer to the command word
Give advantages and disadvantages of both methods, then a justified conclusion for the stated situation.
Use natural-selection language: variation, survival of resistant pests, reproduction.
Recall, apply, then justify
Give two disadvantages of using insecticides on crops.
Hint
Other organisms and resistance.
Suggested marking points
- They may kill useful / non-target insects such as pollinators.
- Pests may become resistant / chemicals may persist or bioaccumulate.
Explain why biological control is often used in glasshouses.
Hint
Enclosed space and residues.
Suggested marking points
- The enclosed space keeps the control organism near the crop.
- It targets the pest specifically.
- No chemical residues are left on food crops.
Explain how repeated use of the same insecticide can lead to a population of resistant aphids.
Planning hint
Natural selection sequence.
What a strong design includes
- There is variation in the aphid population.
- A few aphids have alleles giving resistance (from mutation).
- The insecticide kills susceptible aphids.
- Resistant aphids survive and reproduce.
- They pass on the resistance alleles.
- Over generations, resistant aphids become a larger proportion of the population.