Biological control is the use of predators, parasitoids, pathogens, etc. to control pests.

Predators: A "Predator" is an organism that captures and eats another. Predators can be either indiscriminate, feeding on a wide variety of prey, or specialists, feeding on one species only. Some are predaceous only as larvae and others like, lacewings, lady beetle, ground beetles, praying mantis, etc. are predaceous as nymphs and adults. The most common predators are: Beetles (lady beetles, rove beetles, ground beetles, etc.), lacewings, syrphid fly larvae, Mantids, spiders and mites.

Parasitoids: Parasitoids are organisms that have their young develop on or within another host organism. They can be the same size as their hosts and their development always kills the host. Normally, a female parasitoid will deposit her eggs into or near the body of a host egg, larvae or insect. The larvae hatch and feed internally or externally on the host’s tissues and body fluids, consuming it slowly; the host may remain alive during the early stages, but will die later as the parasitoid pupates inside or outside of the host’s body. Finally, the adult parasitoid will emerge from the dead host or a cocoon nearby. Wasps are the most common parasitoids.

Unfortunately, "Biological Control" of pests is not an option for orchid growers. Firstly, I do not know of any orchid growers that are prepared to purchase predators for a couple of mealy bugs and secondly, the fungicides and insecticides used daily by orchid growers, will kill any natural predators that accidently entered the greenhouse for a feed.

Some useful predators:

Ladybirds: (Various species). Ladybrids are aphid predators. Adult Ladybirds as well as larvae feed on a wide range of aphids, mites, moth eggs and other small larvae. Adult Ladybirds are small, round to obal and have disticnt colourings.

The Convergent Lady Beetle (Hippodamia convergens): The convergent lady beetle is one of the best known predators. Aphids form their primary target. The adult beetle has orange wing covers, with 5-6 small black spots on each side. The larvae and adult the convergent lady beetle feed primarily on aphids. Where aphids are not available, the lady beetles may feed on scale insects or other small soft-bodied insect larvae, etc.

The Mealybug ladybird (Cryptolaemus montrouzeri): The Mealybug Ladybird is a predator of mealy bugs and scales, endemic to East Australia. It is small, round with orange head and black wing covers. It has no spots. A female can lay up to 500 eggs. Both, larvae and adults feed on mealy bugs and scale insects. It has been used to control them for many years.

The Green Lacewing (Mallada signata), and The Brown Lacewing (Micromus tasmaniae). Both are indiscriminate predators of a wide range of of soft-bodied insects but prefer aphids, scales, white flies, moth eggs and other small larvae. The adult lacewings are up to 1.5cm in size and have a delicate bodies. The eggs are laid singly or in small groups on top of fine, silken stalks for protection. Lacewing larvae are also cannibalistic if prey populations are low.

Praying Mantis nymphs and adults, are indiscriminate predators that feed on a wide variety of insects and other Mantis. Adult females lay up to two hundred eggs. The eggs hatch early summer and the tiny nymphs straight away search for prey. Mantids are very territorial and most nymphs die young as a result of starvation, predation, or cannibalism. They are not effective predators of aphids and mites.

Pirate bug (Orius armatus): The pirate bug is also an indiscriminate predator. It prefers thrips (larvae or adult) but also feeds on aphids, spider mites, and butterfy eggs. Unlike other insects, orius has 7 developmental stages. Females, lays eggs in the plant tissue of the stem and on the underside of the leaves. In all its developmental stages the pirate bug is able to hunt and kill its prey by sucking its body fluids. Adult insects can fly and search for new prey.

Predatory mites (Phytoseiulus persimilis): The predator mites are red and spider like in appearance and feed predominantly on two spotter spider mites. Can be easily distinguished from the "two spotted spider mites" by their long legs, lack of spots and rapid movements. They prefer warm and humid environments. They are fast-moving, pear-shaped predators with short life cycles (1-3 weeks) and high reproductive capacities. Voracious feeder, nymphs feed on spider mite eggs, larvae, and nymphs, adults feed on all developmental stages of spider mites. Tolerant to some insecticides and most fungicides.

Some Parasitoid wasps:

Aphidius (Aphidius colemani): Aphidius is a wasp that targets aphids. As all the parasitoid wasps do, the female wasp will lay its eggs into the body of the aphid. The eggs hatch inside and the larvae take advantage of the aphids, feed and develop into a fully formed wasp, killing the host in the process.

Aphytis (Aphytis melinus; Aphytis linganensis): Tiny yellow wasps that target scales.

Metaphycus Sp. Parasitoid wasp that target scales.

Pteromalid wasps (Family Pteromalidae): Large family of wasps (over 300 known species) that use many types of insects as hosts. Hosts include: larvae of moths, beetles, wasps, scale insects and mealybugs.

Encyrtid wasps (Family Encyrtidae): Family with over 3000 species in the world. A very important chalcidoid family for biological control. Hosts include: soft and armored scales, mealybugs, moths and larvae and eggs of other insects. Leptomastix dactylopii and Metaphycus helvolus are commercially (to control scales).