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Seattle University


Beneficial Pests

Written by Janice Murphy
November 5, 2012

Early this past summer, I was out in my garden, contemplating the damage done to my struggling cabbage plants by the pesky green larva that hatches from the eggs of the Cabbage White Butterfly (Pieris rapae.) As an organic gardener, I had a few options, but for many of these, the timing was wrong. Hand picking the cabbage worm was the best option, it seemed, and I was losing the battle. 

As I considered this dilemma, I happened to see a wasp flying in and amongst the plants, dipping in and out, disappearing from sight for several seconds at a time. It was a Paper Wasp, (Polistes aurifer), the wicked-looking kind with the long, dangling legs; the one that looks (to the uninformed) most likely to come after and sting you. Since I know that the Paper Wasp is one of the least aggressive of our stinging insects, I watched her antics close in, delighting in her sinister beauty. It took me a few minutes to realize that she was hunting; her prey was the cabbage worm, which she carried back to her larval young. And there it was: the balance of nature, being acted out right in front of me. 

This was a pretty straightforward example of predator and prey. But there are lots of other not-so-easy-to-see interactions going on in our gardens all the time. The definition of "beneficial" extends beyond pest-eating insects. Those that pollinate, those that parasitize, those that decompose organic matter, all these organisms are part of a rich, complex web of interdependence that, for the organic gardener, is the cornerstone of success in the garden. The great entomologist and ecologist of the late twentieth century Carl Huffaker once remarked, "If you destroy all the beneficials, you inherit their work." 

Conservation Biological Control, as it's formally called, is an important principle that we talk a lot about in the Grounds Department. How can we foster these relationships, find ways to provide habitat for the beneficials to complete their life cycles? How do our choices in plant selection and garden management affect the natural allies that help keep our gardens healthy and vigorous? Our efforts include choosing plants that provide nectar and pollen sources for adults over a long season, using plants for larval food and "banker plants" that attract insect pests, providing a water source, and making shelter and undisturbed soils a priority in designated areas.

It also takes knowing when to do nothing. In the fall, it's very tempting to "clean up" the garden and you will often find recommendations to do just that in gardening how-to books: rake and remove debris is often the prescription for fall activities. But consider that the egg cases, the pupae, and the burrowing adults of many of these beneficials need a place of refuge through the winter. Leaving fallen leaves in place and adding a layer of compost or wood chip mulch to aid in their decomposition, keep them from blowing away and make an attractive appearance is a strategy that we use wherever possible in the SU gardens.

So consider adding to, rather than removing from, your garden this year, leaving seed heads and plant stalks in place, and you'll be on your way to creating a garden that supports biological diversity in the natural world. 

My cabbage plants outgrew their pest problem, provided a food source for one of the great garden predators and fed us too!  

For more information about garden beneficials and a list of habitat plants, check the Grounds website at click on the Grounds tab.

Janice Murphy serves as SU's integrated pest management coordinator and is an occasional contributor to The Commons.