Garden & Greenhouse


Pest Control Using Predatory Insects

Posted August 5th, 2010 by Larry Maki in

Keeping your plants healthy can be a daunting task; the sheer number of fungal, bacterial and viral infections that can grow in or on your plants is staggering. And let’s not forget about the bugs. There are literally hundreds of insect varieties just waiting to suck the life out of your garden plants, use them as nests or as food and all the while propagating madly. It seems at times that the only effective way to eradicate these nuisance bugs is through the use of harsh chemicals which will eventually get into the food that you harvest and eat. Well there is now another way to control the spread of harmful insects and that is through the use of “predatory insects.”

Imagine a green pepper plant overrun by aphids and rather than dusting the plant with a harsh insecticide you unleash an army of beneficial bugs who do battle with and overcome the aphids. These ‘beneficial bugs’ prey upon the bugs that attack your plants; they are the natural enemies of these ‘bad’ bugs and will attack and kill them. It is safe, effective and there are no harsh chemicals involved. You would simply purchase the larval stage of a specific predatory bug or even the adult bug and turn it loose in your garden. Using predatory bugs to eradicate insect pests is not exactly a new idea; many vineyards have been using this technique since the 1920’s but predatory bugs are just now becoming more commercially available.

So you may be asking just what are these predatory organisms and what exactly do they do? Commercially, there seem to be 3 basic categories of predatory bugs: the parasitoids, the nematodes and all the others such as praying mantis and spiders. As a side note, nematodes are actually worms, not insects, but for the purpose of this article I am classifying them with the rest of the predatory bugs.

Let’s look at the parasitoids first. Commercial parasitoids are insects that are literally bred, born and grow up inside a host all  the while using that host as food, eventually killing it. And this is what makes them different from true parasites; they will kill their host while parasites will coexist peacefully and not kill their host. Just think in terms of the movie ‘Aliens’ with Sigourney Weaver, if you have seen it, where an alien creature gestates inside of a human host until it bloodily springs forth killing the human host in the process.

When let loose in your garden, parasitoids can be devastating to aphids, whiteflies, beetles and many varieties of caterpillars. The most popular types of parasitoids are small, stingless wasps measuring about 6mm (1/4 inch). These wasps can be purchased as wasp eggs, wasp adults, parasitized moth eggs and even parasitized aphid mummies to be applied at a rate of 1 to 10 eggs per square foot of garden. Just be sure to purchase the correct species of wasp to attack the infestation you have. For example, if a specific wasp species eats aphids, they will only attack aphids and completely ignore other pests such as whiteflies. In this case you would also need to purchase a wasp species that attacks whiteflies. Once applied, parasitoid wasps will work within 2 to 3 weeks but will need to be reapplied every couple weeks. You can even store them in the refrigerator for a couple days before use.

Although parasitoid wasps have advantages there are also drawbacks: they are expensive, they will leave when their food supply runs out and the majority will never survive the winter, in fact, many will die right after being applied. They are also extremely selective with the host they parasitize so specific wasp species must be matched with specific insect pests.

The second popular type of predatory insect is not an insect at all – it’s a roundworm or nematode. Nematodes are efficient and deadly. They are microscopic worms that enter a host body through natural orifices and once inside, inject bacteria into the host. Within 48 hours this bacteria kills the host through the equivalent of blood poisoning and begins to convert the host tissue into nutrition for the nematodes. Nematodes will eat the dead host and multiply until the host tissue is dissolved then they will start searching for a new host.

These roundworms are perfect for insect pests with a stage of growth that is in the ground. They will spread out and depending on the species, will either stay stationary and attack or move through the soil and attack. They are also versatile hunters. They feed upon a wide range of hosts from weevils, midges and fleas to borers, beetles and moths. Nematodes are shipped overnight or 2nd day in a small clay or other water soluble container and work best when applied at a rate of 3500 nematodes per square foot.

When received they can be stored in the refrigerator for up to 2 months until use in a shallow pan covered by 1/2 inch of water. For application they are mixed with water and sprayed on the garden areas where they are needed. Results will be seen as soon as 3 days from application.

When their host pests are all gone nematodes will move on but will not really ever move very far due to their microscopic size. And they will survive a winter but with a seriously reduced population. There are nematodes available for all climates, those that are adapted to cold climates and those adapted to only warm climates.

Aside from nematodes and parasitic wasps there are several common beneficial bugs that you may find in many gardens at one time or another. They may not be deadly parasites but they will still eat more than their share of garden pests and they can also be obtained commercially.

Many species of mites will attack thrips and spider mites feeding on the egg or the adult pest. “Green lacewings” will eat spider mites, aphids, thrips, whiteflies, scale and mealy bugs and will eat up to 60 aphids an hour. They are voracious predators and once purchased must be applied immediately or they will eat each other. The “insidious flower bug” just loves eating thrips and will actually take a bite out of a person if given the chance. These bites are harmless but can be annoying.

Ladybugs will eat aphids and beetles will eat spider mites and mealy bugs. And let’s not forget praying mantis, spiders, dragonflies… and the list goes on. All of these predatory insects are available for purchase online or at your local garden center. You can buy full grown insects or eggs to hatch these insects. Once you get them in the mail, release them according to the instructions and sit back while your army attacks the invaders…

Are there drawbacks to using predatory insects?

Yes. They are extremely expensive compared to other available options and once released; predatory insects may take weeks to establish themselves maybe long enough for the pests to kill your plant. And when all insect pests have been eradicated, your expensive predatory insects will simply move to a new food source, abandoning your garden. The last drawback to consider: introducing insect species into certain areas of the world where they do not normally live may upset the natural environmental balance.

Larry Maki is an avid, self-taught hydroponics gardener from Connecticut with a passion for alternative types of gardening.

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