It sounds like a reasonable question. If we can release predatory mites or parasitoids into a crop and watch them reproduce, why do we need to keep buying them? Why not maintain a population in the glasshouse and produce what we need ourselves?
Sometimes we can do a version of that. Banker plants can support aphid parasitoids, pollen can help some predators establish, and a well-managed crop may sustain beneficial insects for months. I think growers should be doing more to support the insects already present.
But keeping a beneficial insect going in a crop and producing thousands of dependable insects every week are very different jobs.
Commercial production starts with food. A predator needs prey, or a suitable alternative food source. A parasitoid needs hosts. Those prey and hosts must also be produced, kept healthy and supplied at the right stage. For some species, that means managing a whole chain of organisms before you have a single beneficial insect ready to release. Diet and rearing methods remain major areas of research for that reason. (iobc-global.org, sciencedirect.com)
Then there is consistency. A grower ordering beneficials needs them alive, correctly identified, in useful numbers and capable of doing the job when they arrive. Producers have to manage breeding, contamination, storage, packaging and transport while keeping quality consistent from batch to batch. This is serious biological production, with quality-control methods developed specifically for it. (iobc-wprs.org, mrqa.eu)
A crop is less controlled. Temperatures change. Pest numbers change. Sprays may be needed for another problem. A predator may run short of food just when we want its population to grow. An aphid banker plant may produce parasitoids, but it also needs attention and can attract hyperparasitoids that attack the parasitoids we are trying to encourage. Banker systems can work, but they are not simply a pot of plants left in the corner. (academic.oup.com, link.springer.com)
There is a commercial difficulty here as well. Growers are paid to grow a crop. Every square metre, hour of labour and bit of management attention used for insect production has to justify itself against that crop. A specialist producer can spread those costs across many customers. A grower usually needs a reliable supply at particular moments, rather than a permanent insect-rearing operation on site.
I don’t think the answer is that growers must either buy every beneficial or produce everything themselves. The useful middle ground is to buy quality insects when numbers and timing matter, then make the crop a better place for them to survive and reproduce. That might mean earlier introductions, better monitoring, compatible spray choices, banker plants where they suit the crop, or flowering resources around the growing area.
We should keep testing ways for growers to maintain more of their own biological control. But it helps to be honest about what commercial producers provide. We are buying more than insects in a bottle or sachet. We are buying the work needed to have the right living organism, in the right numbers, ready on the week the crop needs it.