Plant diversity and benficial insects
Last night I attended a talk on plant diversity and its influence on beneficial insects, presented by University of Maryland's Dr. Paula Shrewsbury at our monthly Master Gardeners meeting. Going into it, I figured I already knew the gist of the message: gardens with a greater diversity of plant types will provide habitat for a wide variety of insects, and so, in this bug-eat-bug world, there will be enough "good bugs" to take care of the "bad bugs," naturally, without chemical intervention. That was the essence of the presentation, but there were several points that were new and particularly interesting to me:
1.) Structural complexity is as important as plant species diversity. Translating the bio-speak, that means it's good to have a mix of distinctive layers of vegetation in your landscape, if possible: overstory trees, understory trees, shrubs, herbaceous plants, and ground covers, and then a variety of different plants at each level.
2.) Fertilized plants attract more insects. In Shrewsbury's research, insect populations (a bad kind) were twice as high on fertilized plants versus the non-fertilized. I had learned previously that when fertilized plants send out a lot of succulent new growth, they're especially attractive to pests such as aphids. The take-away: don't fertilize your plants if they don't really need it. Personally, I think compost goes a long way to keeping things healthy.
3.) Insects referred to as "natural enemies," the good guys, are beneficial only at certain stages in their life cycle. So, for example, the syrphid flies that I knew to be beneficial only feed upon prey while they're in their larval stage. Adult syrphids don't eat other bugs, they eat nectar and pollen. The take-away: Plant flowers that provide a variety of nectar and pollen sources, so your garden will support natural enemy insects in all of their life stages.
What to plant in order to attract beneficial insects?
Choose plants with long-season blooms and varying architectures. This includes big flowers like daisies or coneflowers, and small lacy flowers such as sweet alyssum, caraway, dill, and parsley. Herbs are great in general, as are cover crops like buckwheat and clover. Members of the mint family, such as catnip and hyssop, are good choices too.
In my own garden, I have catnip, herbs, and yarrow that are good for the insects. And I usually let a few veggies go to flower too, such as my mustards. I'd like to gradually add more native plants, and I definitely want a more "structurally complex" yard, especially the front, which is still mostly plain old turfgrass and ho-hum azaleas.
For more on natural enemy insects, what they look like, what they eat, etc., check out Michigan State University's excellent website: http://ipm.msu.edu/natural-enemies.htm.