The host range of an insect is the group of plant species on which larval development occurs in nature. In most phytophagous insects the larval food is determined by the ovipositing adult female, not by the larva. Hence, the process of host selection by ovipositing females is used by most biological control workers as the most important indicator of host range.

Oviposition tests determine the behavioural host range of an insect, that is, the host range that occurs as a result of insect behaviour. Larval development tests determine the physiological host range, that is, the range of plants upon which larvae are physiologically capable of developing.

Host-testing cages being inspected.

To test this host selection, insects are given access to a range of plant species including the target weed. The adults are later removed and the number of eggs laid on each plant is counted.

Oviposition tests are not possible for many insects which do not express natural host selection behaviour under cage conditions. This may be due to sensitisation, where some attribute of the host, e.g. volatiles, has excited and increased the responsiveness of the insect resulting in it ovipositing on non-hosts or even on cage walls. Sometimes it is possible to evoke natural behaviour by using a larger cage, adding a natural substrate or otherwise making the conditions more natural. If these are unsuccessful, other types of tests are required.

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