Biological Pest Control Made Simple

A Tomato Hornworm with wasp eggs. A wasp has injected her eggs into this hornworm. When the eggs hatch into larvae, the caterpillar will be eaten.

Biological control of undesirable garden pests is the desirable way to go  Biological control is a garden management tool that encourages natural processes by bringing one living organism into the garden to munch on another. Besides, having chemicals around to control garden pests is always a nuisance.  Their preparation for use and application must be done while protecting skin, eyes, and breathing passages.  And storing chemicals is a hassle, too.  Chemical pest control may also kill or discourage beneficial insects and other friendly wildlife.

In truth, biological control is going on all around us without any human assistance.  Native birds have a ravenous appetite for insect pests, as do naturally occurring ladybug and green lacewing larvae.  The grain, vegetable, and fruit crops on which our diet depends could not grow without the nearly invisible presence of hordes of minute parasitic wasps that do the heavy lifting where worldwide insect pest control is concerned.
You can bring birds into your garden with birdbaths and birdhouses and by planting perennials with tubular flowers, nearly all of which seem to attract hummingbirds. Beneficial insects are attracted to yarrows (Achillea species), sweet alyssum, penstemon, herbs (thyme, lemon balm, spearmint, English lavender, feverfew, coriander), daisy family members (zinnia, cosmos, chrysanthemum), and, especially, alfalfa.
Still, the presence of birds and visits by beneficial insects through prudent plant selection may not be sufficient to overcome insect pest issues.  In such cases, there are insectaries such as Rincon-Vitova ( in Ventura that nurture and mass produce dozens of predatory insect species for control of common insect pests and mites.  They will ship these beneficial predators right to your door.
Garden pests do not include only insects, but snails and slugs, raccoons, squirrels, and deer.  Yet if the only pests that we had to worry about belonged to the animal kingdom, garden maintenance would be a lot simpler than it is.  One way or another, animal pests can nearly always be controlled or discouraged.
However, when it comes to pestiferous plants — weeds, that is — it seems that the only way to control certain particularly pernicious types is through chemical application.  Yet this, too, may be changing due to increased aversion to chemical use and to expanding research into biological weed control.
Biological pest control is not new.  Nearly 2,000 years ago, the Chinese brought nests of yellow citrus ants into mandarin orchards to control undesirable caterpillars and beetle borers.  And today the Chinese use other kinds of ants to deter insect pests in rice paddies and sugarcane fields.
An Australian story of biological control of an unwanted plant is similarly illuminating.  By 1925, the build-up of prickly pear cactus (Opuntia ficus-nitida) had become treacherous down under. Hundreds of square miles of prime grazing land had been consumed and made inaccessible by the thorny cactus.  In a desperate attempt to slow its spread, a lepidopterous prickly pear cactus pest from South America was shipped to Australia.  Soon, its larvae began to feast on the succulent pads of the cactus.  Today, only 1% of the original prickly pear cactus remains.
I was inspired to investigate biological weed control upon receiving an email from Sharon Reeve, who gardens organically in La Mesa near San Diego.  She has been struggling to contain tropical dayflower (Commelina benghalensis) without success.  Worldwide, and especially in the tropics, tropical dayflower is a highly destructive weed, having a severe impact on crop production.
Two years ago, a researcher noticed that a close relative of tropical dayflower (known as spreading dayflower) had foliage which was blighted with a leaf spot fungus.  The infested leaves were taken into the laboratory where the fungus was extracted and cultured until a sufficient quantity was produced for spraying on the spreading dayflower.  Through use of the fungal spray, significant containment of the dayflower was achieved.
Although this product is not yet available for commercial use, and it is not known whether it would be effective on other dayflowers, it is reasonable to assume that such products are part of our future.  Pathogenic bacteria, viruses, and microscopic worms (known as nematodes) that feed on leaves, roots, or seeds have also been introduced as biological control agents for certain weeds.
Like every other plant that grows by leaps and bounds, tropical dayflower has positive qualities, aside from its bright sky blue flowers and lush leaves, too.  Many African tribes rely on it for treatment of burns and cold symptoms.  It is also edible and, although not especially tasty, is relied upon in tropical countries as an emergency food source during periods of famine.
One recourse to controlling tropical dayflower or, for that matter, any other weed, is to plant something that grows quickly and deprives it of light.  Squash, for example, is reasonably fast growing and has large leaves that would discourage any low growing plant from development.  I would dig up as much of your dayflower as possible and then plant squash.  Once the squash had leafed out, the dayflower would probably be discouraged from growing and, as a bonus, you would have a nice addition to your vegetable platter. And, of course, a consistent application of mulch, keeping a three or four inch layer covering the dayflower plot at all times, would also depress its growth.
Tip of the Week:  The following insect biological control tip comes from Craig Endler, who gardens in Santa Clarita.
“Sooner or later your garden pots will break, but rather than toss them into the trash, recycle your broken pots into a beneficial critter shelter. I made pieces of my broken pots into lizard community condos.  I layered the broken pieces in a remote part of my backyard.  There is a large open middle space in the shelter with many openings and exits throughout.  This gives the lizards a cool protective shelter in the summer, and a warm place to live during the winter. The lizards reward me with eating a lot of unwanted bugs in my garden, and they are entertaining to watch as well.”
As for attracting birds, who are the very best insect control agents, Endler has come up with a creative way of bringing them into his garden.
“I’ve noticed over the years that birds seem to like the coco fibers from my hanging baskets, pulling them out to use in their nest building.
I had one hanging basket where the plant was long dead and birds would not only check out the coco fibers, but would look along the top of the dirt for nest materials. I later added some dried grass from my yard, small twigs, and anything that I thought would make good nest material, and set them all on top of the dirt.
If the birds like what they see, they will use it to build their nests — hopefully, in a birdhouse in my yard.”

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