Plants have adapted over millions of years to defend themselves against wildlife and insect predation. Many have thick woody stems, thorns and toxins. Others use pheromones when their host insects are too densely proportioned on the host plants.
For instance, if you had 10 host plants in a small area and the female does not have an adequate number of host plants to lay her eggs on, this would be problematic for the host plant as it cannot sustain a large number of heavy feeders and needs to protect itself from extreme damage. When this happens, the plant releases a pheromone, which in turn attracts necessary predatory insects such as parasitic wasps, flies, hornets, etc.
These insects are called in and they do a quick clean up of a majority of these insects that are damaging the plants. Most of your predators will be using the soft body of the insects to lay their eggs and continue amending their predatory population as the other adults will eventually die off. This is a natural cycle that is very necessary. Without these insects, the hosts of the plant could eventually do too much damage to the host plant, causing disease and death to the plant. This would not be beneficial to the host insects in the future as the plants would disappear in large portions if this continued on a regular basis. In larger natural areas you will find that most host insects are laid in less dense patterns as there is a much more abundant food source available.
If you are planning on planting a host plant area, please remember that volume is necessary to compensate for the females egg laying needs. Native grasses and wildflowers should also be incorporated into that area as a barrier of protection from other predation as well.
Good questions to ask yourself:
How many eggs does the female lay that uses this specific host plant?
Are they heavy feeders with long larval cycles?
How much does the average larvae eat of it's host plant in it's life cycle?
Once you can find the answer to these questions, you will have a better understanding of how many plants you'll need in your natural area to do a proper job of hosting the insects.
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