Biological control: how to attract the right insects to your garden

Ladybug larva eating aphid.
Credit: Shutterstock
Ladybug larva eating aphid
Biological control: how to attract the right insects to your garden

Until the turn of the third millennium, my only interest was in garden pests. To my mind, they had to be eliminated as quickly as possible before they did too much damage. What sparked my interest in the usefulness of biological-control agents (predators, parasitoids) was noticing the presence of ladybug larvae on aphid-infested purple toadflax seedlings. And in fact I decided not to intervene, and see if the ladybug larvae would manage to control the aphid population. My patience was rewarded: after a few days, the aphid colony had practically disappeared. As a bonus I noticed that syrphid fly larvae had joined the feast.

Twelve years later, reading the Jessica Walliser book Attracting Beneficial Bugs to Your Garden, I learned that the American horticulturist had experienced more or less the same dilemma that I did. In her case it was aphids on a tulip tree decimated by ladybug adults and larvae. And as a result of that experience, Walliser came to the same conclusion as me: better to invite beneficial insects into the garden than try to eliminate pests with huge doses of pesticides.

Biological control through conservation

To increase the number and diversity of the biological-control agents that are already present in the environment, those agents have to be provided with what they need to live and prevail, namely:

  • A broad diversity of plants that will furnish the pollen and nectar that a number of them feed on when no prey is available;
  • Habitats where they can camouflage themselves, reproduce and hibernate, such as clumps of grass or perennials, or piles of stone or wood.

Inundative biological control or through acclimatization

Of course, it’s possible to introduce biological-control agents produced in the laboratory. We’re talking about biological control “through acclimatization” when control agents end up remaining in the environment. It’s more a question of “inundative” biological control when regular releases of agents are needed to control pests.

Acclimatization does not always yield good results, and in most cases it takes a number of years for control agents to settle in once and for all. Inundative control methods produce good results in greenhouses. But they’re much more difficult to implement outdoors, since control agents are quick to migrate to other food sources when the prey population declines. Plus, the space has to be inoculated on a regular basis to maintain a balance between control agents and pests.

Biological control through conservation therefore remains the most interesting approach for people who don’t necessarily have the required knowhow or the time to devote to the two other techniques of biological control.

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