Insects survive and breed best when conditions are most favourable. A high degree of variation in quality of sites or suitability of sites for agent population growth is common. Ecoclimatic considerations may also influence site selection. Remember also that, unlike humans, insects prefer to eat food that is good for them or in some way aids their survival.

To maximise the chances of achieving successful establishment, agents are best liberated at sites where plants are known to possess the preferred quality attributes, or, if the plant quality preferences of the agent are unknown, at sites where plants are likely to possess high diversity in plant quality allowing the agents to select the appropriate resource. Given the unpredictability of establishment, release sites should be spread across a range of factors such as soil types, altitude, climate, etc.

It is theoretically possible to manipulate plant quality to favour insect attack but so far little work has been done on this. Plant growth regulators, including auxin-class herbicides, are of possible use here. For example, Oka and Pimental (1976) presented several references which reported increases in pest problems on herbicide-treated crops, probably due to consequential increased nitrogen levels in the plants. Improving quality by fertilising the weed with urea is another example and this was used in a program with Cyrtobagous salviniae on Salvinia molesta reported by Room and Thomas (1985). Where information is known on the agent's preferences, this should be included in release instructions provided to project collaborators.

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