The more insects available of a particular agent species, the more freedom we have to choose how many and which insects (e.g. age, sex) to release and how many sites to release them at. There are no hard rules here; these aspects are open to argument and mostly depend on the agent's biology and especially on what is possible and practicable. However, given that few sites may be fully secure from disturbance, it follows that the more release sites available the better.

Regarding insect numbers for release, examples exist where an agent has become established following release of surprisingly few insects. Nevertheless it is generally assumed that releases of large numbers should mean that the population is starting out further along the 'lag phase' of its theoretical sigmoidal population growth curve.

Sometimes limited resources make it impracticable to liberate large numbers of insects, for example where weed plants infested with immature stages of the agent are to be planted into existing weed infestations. If numbers of adults per shipment for release must be limited, remember that gravid females are more valuable for release than males or unmated females. However shipping mixed-sex adults has an advantage in that mating may occur during transit.

On the question of releasing one or several agent species against a target weed, there has been much discussion regarding the advantages or disadvantages of establishing a complex of agents. Arguments against releasing a complex have been put by workers dealing mainly with biocontrol of insect pests, and Huffaker (1978) saw greater benefits in using a complex of agents against weeds than arthropod pests. I think most weed biocontrol workers support the argument of Hassell (1978) that additional agent species will either coexist with the first agent or replace it and whatever the outcome, the equilibrium density of the host will decrease. The campaign against prickly pear in Australia, where 51 species were introduced although only 5 were effective (Wilson 1960), strongly supports the pro-complex theory.

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