Surveys must be carried out over the whole region where the target plant is believed to be native; they should proceed along transects traversing altitude, temperature, rainfall and other ecological gradients. 

It is hypothesised that the greatest number of natural enemies will occur near the centre of diversification or evolution of the weed genus or subgenus. Some of these are likely to have evolved with the weed and have a host range specific to the weed, or to the weed and a few species within the same genus. If the centre of diversification is known, intensive surveys must be made in that region.

Surveys should be designed to take account of season, habitat, soil or water type and follow a predetermined plan. Many natural enemies are seasonal in abundance, even in the tropics, and it is desirable to conduct preliminary surveys at a time when the weed and natural enemies are abundant.

If a population of the weed can be found in the native range which is genetically identical to the population in the target country, then particular attention should be given to this population. If matching populations cannot be found then control agents must be sought from dissimilar populations.

Occasionally, biological control agents are selected from closely related species. Although the chances of success may be reduced, there are some very successful programs where agents have been collected from one plant species and used against another closely related species.

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