Higher Biology Unit 3 3.3- Crop Protection

Weeds, pests and disease Weeds, pests and diseases are all issues for farmers growing crop plants as they reduce productivity This can be a huge issue where crops are grown as monocultures as there is less genetic variation and resistance to disease and pests may be reduced

Monoculture A monoculture is the name given to single crop species grown over a large area

Weeds A weed is a plant that grows somewhere it is not wanted Weeds will compete for the crop plant for resources such as water, lights, and nutrients from the soil Weeds may also release chemical inhibitors into the soil which affect crop plant growth, and can act as hosts for pests and disease

Annual Weeds Annual weeds grow quickly, have a short life cycle and produce many seeds which are viable for long periods of time Examples include: Groundsel

Perennial weeds Perennial plants live for many years, becoming dormant in winter and growing again in spring Some perennial weeds have competitive adaptations to ensure survival such as storage organs for food and vegetative (asexual) reproduction Dandelions

Pests Pests of crop plants tend to be invertebrates such as insects, nematode worms and molluscs

Effects of pests Plant pests eat the leaves of the crop plant which reduces their ability to photosynthesise This reduces yield and vigour of the crop

Plant Diseases Plant diseases are often carried by invertebrates These diseases include bacteria, viruses and fungi

Control of weeds, pests and diseases Different methods can be used to protect crops from harm These include cultural means, chemical means and biological control

Cultural Means Ploughing a field can result in weeds being buried deep enough into the soil that they die and decompose

Weeding can be used to remove weeds and prevent competition This should be carried out early in the life of the crop plant The perimeter of the field should also be weeded to remove hosts for pathogens

Crop rotation means that pathogens are unable to build up as they not able to utilise the new crop The use of leguminous plants in crop rotation allows for more nutrients to be kept in the soil

Chemical control Chemicals can be applied to crops to protect them from weeds and pests Herbicides are chemicals used to kill weeds Insecticides are used to kill insect pests Fungicides are used to kill fungal parasites

Selective Herbicides Selective herbicides mimic hormones in broad leafed plants causing their metabolism to speed up. This results in them using all of their resources more quickly and they will die. Narrow leafed plants are not affected.

Systemic Herbicides Systemic herbicides are absorbed by the plant and enter its circulatory system. This then has a lethal effect to the roots and leaves of the plant.

Contact Herbicides Contact herbicides kill all green plant tissue they come into contact with. The roots can survive and the plant can grow back.

Contact Pesticides Contact pesticides work in 2 ways: They kill the invertebrates they come into contact with directly Or They leave a residue on the plants which will kill the pest at a later time

Systemic pesticides Systemic pesticides are absorbed by the plant They will kill any pests that try to eat the plant when they ingest plant material

Contact Fungicides Contact fungicides are applied to plants directly They will be absorbed by fungal spores as they start to germinate causing them to die

Systemic Fungicides Systemic fungicides are absorbed by the crop plant and are not washed away by rain They will only kill fungi affecting the plant

Protective applications If there is a known crop fungus outbreak fungicides can be applied as a preventative measure This can be a more effective method than trying attempting to treat already infected plants

Biological Control Pest species can be managed using biological control This is a method where a predator of the pest species is introduced to manage and reduce its numbers An example would be introducing ladybirds to feed on aphids

Crop Protection Issues The use of chemicals can be useful to protect crops but there are associated issues Chemicals may be non-specific and kill plants other than those intended They may be toxic to animals They may persist in the environment and accumulate in the food chain Their use can result in resistant populations forming Biological control methods may upset the food web and cause an ecosystem to collapse