Insect herbivores feeding on plants may be similar to parasites, especially the insects selected for biological control which live in close association with their host. The reduction in host plant fitness due to feeding by the herbivore varies with different insect/plant combinations. Insect attack on meristems, or the growing points of the plant, would be expected to have a greater effect on plant growth than feeding on leaves, but compensatory growth may overcome the effect of meristem damage. Many sap sucking and gall forming insects have little effect on the host plant, but some greatly reduce fitness by acting as an energy sink, diverting nutrients away from growth points in the plant. Flower and fruit feeders appear to greatly reduce 'birth' rate of plants, but this loss of potential progeny may merely substitute for the normal shedding of excess flowers or fruit.

The effect of insect herbivores on the population of their host plant depends not only on the type of damage inflicted by individuals, but also on how many individuals are present (i.e. density relative to the host plant). Factors such as unfavourable abiotic conditions or predators and parasites of the herbivore may prevent the herbivore population increasing to levels that significantly affect the plant host.

Quality of the plant host as food for herbivorous insects

Food quality of host plants can have a major influence on population change in insects. Food quality can be so poor that few larvae survive, and most of the dispersing phase may leave the poor quality food. Herbivores are indirectly vulnerable to effects of water and nutrient availability on their host plant, but effects on the insects are variable. Phloem feeders may be provided with a richer nutrient supply if the host plant is under stress, particularly water stress, whereas leaf feeders may develop more slowly with increased mortality if the host plant is water stressed. Herbivores are particularly sensitive to the nitrogen content of their host.

Herbivores are also influenced by secondary plant substances - metabolic products that act as deterrents or, occasionally, as attractants. Plants that would otherwise be suitable hosts may be rejected as food because of these substances.

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