1.2 Weeds and pests and their properties

Some of the most common pests are virus, bacteria, fungi, insects, nematode and rodents.

Weeds may be defined as "unwanted plants". This means that no plants are originally defined as weeds. A plant becomes a weed when it no longer is wanted where it grows. A large number of plants may be described as weeds. Plants which are usually regarded as cultivated plants may in some situations be defined as weeds, for example when contaminating seed production. Weeds compete with cultivated plants for water, light and nourishment. The result may be reduced crops of poorer quality. This can happen when inhibited growth of cultivated plants occurs, or when weeds harvested together with the cultivated plants affect the flavour or are toxic. As so many plant species may be defined as weeds, a large number of biological characteristics may be involved. Many common weeds in field crops are annual. They germinate from seed, grow rapidly and set new seeds prior to or at the same time as the cultivated plant. In addition to propagation by seeds these species also have an advanced system of vegetative propagation, such as budding from the root system. Weeds of this kind are highly competitive and are particularly difficult to control.

Plant diseases can also damage cultivated plants. Plant diseases can be caused by virus, bacteria and fungi. However, lack of nutrients and poisoning may produce many of the same symptoms as plant diseases.

Virus propagate in plants and are transmitted from plant to plant. The most common symptoms of a virus disease are mosaic, leaf blotch, chlorosis, yellowing leaves, and necrocytosis of tissue, leaves, tubers and other parts of the plants. Virus may also be transmitted by vegetative propagation, or by nematode, mites, aphids, leafhoppers and certain other insects transmitting virus from plant to plant.

Bacteria cause rot in tubers, fruits, bulbs and other succulent plant organs. Some bacteria grow in the tissue and may cause blight. Bacteria are transmitted by seed, tubers and by vegetative propagation.

Fungi are the cause of most plant diseases. They produce microscopic spores which are transmitted by wind and plant material. Some species survive in the soil and are transmitted via the plants' root systems. Fungi can kill sprouts, cause root rot, rot in tubers, blights and blotches. Some fungi produce mycotoxines in the plant products. Mycotoxines are poisonous to both humans and farms animals.

Pests. All animal species have their function in the ecosystem. An ecosystem is an interactive community of plants, animals, micro-organisms with the environment which they inhabit, which together make up a functional whole. To put it simply: No humans, no pests. Pests on plants can be defined as follows: "A pest is a species whose population density exceeds an acceptable level and which causes financial damage to the crop". In addition, there are pests on stored nutrients and species which directly or indirectly cause serious diseases in both humans and animals. The most common pests belong to the following groups of animals: insects, mites, nematoda, birds and rodents. Insects constitute about 75% of all defined species. An estimate suggests about 4-5 million insect species. About two thirds live in the tropics. About 1000 insect species are considered major pests on cultivated plants, and an estimated 30.000 are minor pests. In tropical and subtropical areas, particularly in areas with only minor seasonal changes in temperature and precipitation, one insect species may have more generations per year than in temperate climates. The insect population may increase rapidly in the case of a continuous access to host plants. Some phytophagous insect species are very omnivorous, but many important insect pests specialise on one plant family and have highly effective mechanisms to find host plants. Traditional tillage systems in the tropics, such as the intercropping of two or more cultivated plants, may interfere with such mechanisms and reduce insect attacks on the cultivated plants. Monocultures, where a single crop is cultivated year after year in large areas, may be highly vulnerable to attacks from specialised insect pests. All pests have many natural enemies. Utility animals may be categorised as predators and parasites. Some predators attack many species, whereas others tend to be more specific. Parasites are specialised. The female ichneumon wasp, for example, lay eggs inside the host animal. The larvae lives inside the host animal and ultimately kills it.