The Coastal Gardener: Life-and-death garden struggles

Not long ago, gardeners seemed to spend as much time controlling pests as they did any other garden activity. Malathion, diazinon, Dursban and chlordane were about as popular as marigolds, dianthus, dahlias and chrysanthemum.

It’s nice to see that gardeners have evolved a much more laissez-faire approach to the six-legged inhabitants that share our gardens. Maybe gardeners today are just too busy, or maybe they’re just not paying as close of attention as they used to, but there seems to be a lot less spraying, dusting, baiting and generalized killing going on in gardens nowadays.

But mostly I think that gardeners are now realizing that in a healthy, well-managed garden, some pests are best controlled by letting things be, by letting species interactions run their course. After all, this is the basis of biological pest control.

Biological control agents are often referred to as natural enemies or simply as beneficials. Every day, in every garden, there is a continual interaction between pests and their predators, parasites and pathogens. Most of this interaction we, as gardeners, don’t participate in and may not even be aware of.

So why is there significantly less use of pesticides today than there was 10 or 20 years ago? Certainly there are no fewer pests.

Let’s look at the example of one of our most abundant garden pests, the aphid. Five possible things can happen to an aphid in a garden, four of which are bad for the aphid but good for the gardener.

First, they could become mummies. Aphid mummies are the little crispy, golden, puffy-looking, empty aphid shells that we sometimes see stuck on leaves and stems. These golden husks are not the molted skeletons of growing aphids, but are the parasitized remnants of unlucky ones. Insect parasites make a living at their host’s expense, coexisting with the host, at least for awhile. After all, killing the host would be counterproductive and leave the parasite with no place to live and pupate. This is why parasitic insects, especially many small flies and wasps, are good garden allies.

Elsewhere, a different aphid is having another type of bad experience. Ladybugs seem to be constantly hungry and soft, so slow-moving, dim-witted aphids are a favorite breakfast, lunch and dinner. An adult or larval ladybug can eat hundreds of aphids in a day. Other predators, like lacewings, soldier beetles, syrphid flies and several true bugs are in the garden, too, searching for a tasty aphid meal.

A third aphid on another part of the plant is happily slurping down a plant-sap smoothie, unaware that things are about to go horribly wrong. Predaceous flies are also hungry, and an adult aphidoletes has come to feed on the aphid’s sweet honeydew. This doesn’t harm the aphid, but in a couple of days when the fly’s newly laid eggs hatch, the larvae will devour the aphid, and several of its next of kin. Like parasitic wasps, some predators let their children do the dirty work.

Unfortunately, aphid No. 4 is about to bite the dust, too. Most insects, especially colonial species, are susceptible to simple pathogens and a colony of aphids is vulnerable to infection by several fungi. Just one microscopic spore, contacting the aphid, usually spells doom. The spore quickly germinates and takes over, turning the aphid into a fuzzy mess. While other insects, especially caterpillars, are prone to deadly viruses and bacteria while chewing leaves, aphid colonies are continually plagued by fungal outbreaks.

The fifth aphid managed to avoid the parasite, the hungry ladybug, the predaceous fly and the fungal disease. This one managed to survive, perhaps providing a minor annoyance to the gardener, but also ensuring the continuation of the parasites, predators and beneficial pathogens that forever keep pests in balance.

Next time you head into the garden to take a peak at the roses or the lemon tree with a hand lens. You’ll soon see the various life stages of both pests and beneficials, and you’ll see lots of evidence of biological control that is always at play in a healthy garden.

There are some easy ways to help conserve your garden allies and enhance their activities. The most important thing you can do is not use broad-spectrum or residual insecticides, as these can kill the beneficials along with pests and throw the entire equation out of balance, because pests multiply and return much more quickly than do beneficials. If you need to use sprays, spot-treating heavily infested areas or using a narrow-spectrum insecticide that kills only the target pest and not the good guys is the best way to go.

Making your garden an inviting place for beneficials is a good step toward solving your pest problems naturally, so that you’ll have more time to spend on the marigolds, dianthus, dahlias and chrysanthemum.