The Online Gardener's Handbook 2010
Chapter 2: Integrated Pest Management
Natural Control Methods

Table of Contents

1. Biological Control Methods
2. Promoting Beneficial Insects
3. Augmentation
   
4. Learn More

Biological Control Methods

Biological control uses a pest's natural enemies to help suppress populations. The natural enemies, collectively known as "beneficials", may be predatory insects, parasites, pathogens or nematodes. Predatory insects kill and consume multiple other (usually smaller) insects and include ground beetles, lady beetles, praying mantids and certain mites (Table 3). Parasitic insects or parastioids lay their eggs in or on the bodies of another insect, and the parasitoids young feed on the host insect, typically killing it (Table 4). Most parasitic insects are tiny wasps or flies. Pathogens are microorganisms which infect and kill insects and mites. Insects are naturally susceptible to a wide variety of bacteria, fungi, protozoa and viruses. Finally, nematodes are tiny worms similar to those that attack plants. However instead of plants, they invade insects through natural openings and feed on them from the inside out, killing them.

Many predators and parasitoids are closely related to the pest species and will look very similar. You would be wise, therefore, to get to know the more common beneficial species in the garden. Pictures and information on beneficial insects can be found on the OMAFRA website. To determine whether an insect is the problem or part of the solution, look for feeding injury on the plant. Watch the insect to see if it is feeding on the plant or on another insect. Look at its movements; fast-moving insects are generally predators as they need to catch their prey.

There are two main ways gardeners use biological control - by taking various actions to promote and conserve naturally occurring populations of beneficial organisms, and by augmenting these populations through the addition of commercially available natural enemies.

Promoting Beneficial Insects

The best way to encourage biological control in your garden is to promote natural enemy populations by providing them with favourable habitat. For many insect parasites, the adults will survive longer if they have a source of nectar to feed on, while some predators must feed on pollen before they can reproduce. Many natural enemies also do better when sources of shelter are provided. To promote populations of these insects in your garden, plant a variety of flowering plants which provide pollen and nectar for adult beneficial insects. When selecting plants, ensure you have selected species that will not become weedy or invasive in your garden. Native plant species may be a good choice for this purpose. Maintaining a diverse habitat in and around the garden will provide beneficial insects with shelter. There are several tactics that you can use to do this, but there are risks. Mulch between rows of plants provides a natural refuge for predators such as ground beetles, but may also encourage slugs and sowbugs.

You must also be careful in your use of pesticides in the garden, as some beneficial insects are killed by smaller doses of insecticides than are required to kill harmful insects. You must also take care to avoid pesticide drift to neighbouring plants which may harbour beneficial insects.

Augmentation

Numerous species of natural enemies are sold for release in crops and gardens as biological control agents. While release of these biological controls has been very successful in greenhouses, many have limited value in home gardens. Some, such as preying mantids, are generalist predators which feed on so many species that they will have limited impact on populations of the particular pest you want to control. Many introduced natural enemies will not remain in your garden for long periods of time.

Native predators such as ladybird beetles can be introduced, but if you buy beetles (Hippodamia convergens) imported from the U.S. you will realize little benefit. These beetles are collected while wintering in the mountains of California. Upon release here, they fly away before settling down to feed again. Few if any will remain in your garden to provide the desired control. As well, the ladybird beetle can only help control outbreaks of non mobile insects such as aphids, and is only effective if introduced before the pest population is well established. On the other hand, there are beneficial parasitoids and predators that occur naturally in Ontario, are suited for home use, and can be bought from suppliers. Routine introductions are necessary if they are to be successful.

A number of products based on insect-parasitic nematodes are available for sale in Ontario. These can be effective in helping to control populations of certain soil pests. Because these products are based on living organisms, close attention to storage and application instructions on the labels is important to their effective use.

A variety of pathogens of pest insects and plants are available commercially, however they are formulated and regulated as insecticides, and will not be discussed in this handbook.

If you do choose to introduce natural enemies, stick with commercially available products from reputable suppliers. Never introduce a beneficial insect you or someone else have collected from other areas and transported to Ontario. Introducing foreign organisms is against the law, and could lead to serious ecological problems in the long run.

Table 3: Common Predators and Their Prey

Predator	Prey
Lady beetles and larvae	aphids, mites, thrips and other small insects and insect eggs
Ground beetles	caterpillars and small soft bodied insects
Syrphid flies	aphids and small caterpillars
Lacewing larvae	aphids, insect eggs, thrips and other small insects and larvae
Assassin bugs	aphids, insect eggs, leafhoppers and other small insects and caterpillars
Other true bugs, including predatory stink bugs, pirate bugs, big-eyed bugs and damsel bugs	mites, thrips, aphids and other small insects and larvae, insect eggs and caterpillors
Dragon flies and damsel flies	mosquitoes and other flying insects
Praying mantis	general feeders on a wide variety of insects
Frogs, snakes and mice	a variety of insects

 

Table 4: Common Parasitoids and Their Hosts

Parasitoid	Host
Ichneumonid wasps	moth, butterfly, beetle and fly larvae and pupae
Braconid wasps	moth, beetle, and fly larvae and various insect pupae and adults

 

Table 5: Commercially Available Natural Enemies

Target Pest	Natural Enemy
	Type	Species
Mites	Predatory Mites	Phytoseiulus persimilis Galandromus occidentalis Amblyseius fallacis Mesoseiulus longipes Neoseiulus californicus
	Predatory Flies	Feltiella acarisuga
	Predatory ladybeetles	Stethorus punctillum
Whiteflies	Parasitic wasps	Eretomocerus mundus Eretomocerus eremicus Encarsia formosa
	Predatory Ladybeetles	Delphastus catalinae
Aphids	Predatory Ladybeetles	Hippodamia convergens Adalia bipunctata
	Predatory Lacewings	Chrysoperla carnea Chrysoperla rufilabris
	Praying mantis (predatory)	Tenodera aridifolia
	Predatory fly larvae	Aphidoletes aphidomyza
	Parasitic wasps	Aphidius colemani Aphidius matricariae Aphelinus abdominalis
Thrips	Predatory mites	Amblyseius swirskii Amblyseius cucumeris Amblyseius degenerans Orius insidiosus
Caterpillars	Predatory Soldier bugs	Podisus maculiventris
	Parasitic wasps	Trichogramma pretisoum Trichogramma minutum Trichogramma ostriniae
	Parasitic nematodes	Steinernema carpocapsae
Various soil beetle larvae	Parasitic nematodes	Heterorhabditis bacteriophora Heterorhabditis megidis Steinernema kraussei
Leafminers	Parasitic wasps	Dacnusa sibirica Diglyphus isea
Mealybugs	Parasitic wasps	Leptomastix dactylopii
	Predatory ladybeetles	Cryptolaemus montrouzieri
Scale insects	Parasitic wasps	Aphytis melinus Metaphycus spp.
	Predatory ladybeetles	Lindorus lophanthae
Slugs	Parasitic nematodes	Phasmarhabditis hermaphrodita

Note that many, though not all, of these natural enemies are targeted more for greenhouse situations and may not achieve successful control in outdoor landscapes. Not all natural enemies are equally effective.

Learn More

• Online Gardener's Handbook - Links to More Information