The biological control simply aims to use naturally occuring enemies to help reduce the invasive plant’s impact on agriculture and environment. Some herbivores and pathogens can be effective biocontrol agents for weeds. Not all weeds are suitable for biological control. The selection of appropriate target weeds is a serious consideration. It can take up to twenty scientist years to reach a successful conclusion. Environmental considerations may restrict future biological control practice because of increased concerns about possible damage to non target native or even agricultural plants. However, biological control has been successfully used against a wide variety of weed types, from a broad taxonomic spectrum, from annual herbs to trees. The diversity of weeds that have been controlled biologically Is a clear indication of a great utility of the method, even it is not possible to predict the outcome of particular projects. Developing a biological control project requires a substantial investment. Considerably host-specifity testing is done prior to the release of biological control agents to ensure they will not pose a threat to non target species such as native and agricultural plants In Europe no classical biological control agent has been released against an invasive weed, but many studies are carried out and future expectations lead in this direction.
Host plant quality directly affects potential and achieved herbivore fecundity and reproductive strategies. Insect herbivores strongly regulate their nutrient intake including allochemicals. The nitrogenous nutrients are a limiting factor for herbivore survival, growth and fecundity. Herbivore insects usually exhibit enhanced performance and outbreak dynamics on stressed plants do to increased changes in plant physiology. Closely related. coocuring and generalist feeding herbivores eat protein and carbohydrate in different amounts and ratios even if they eat the same plant taxa. Plants in rban landscapes are subject to stress factors (pollutants, automobile exhaust, higher temperature) may reduce the abundance of natural enemies, parasitoids and predators. Many species of insects possess the capacity to learn to guide an active search for food and hosts. Both plants and arthropod herbivores generate select forces that lead to the evolution of plant defence against herbivores and pathogens.
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