A lot of the permaculture literature praises parasitic and predatory wasps in the garden, where they keep insect populations balanced. If a certain "pest" insect becomes too abundant in your wasp-friendly garden, the chances are good that predators will find that abundance, feast away, and bring the prey numbers back down to an unproblematic level. Bees, Wasps, and Ants by Eric Grissell goes further into the amazing lives of all the Hymenoptera, paying special attention to parasitic and predatory wasps. The book gives lots of gory details, and even the non-gory bits are generally more than one strictly needs to know, but it's fasinating if you're into that sort of thing. It's re-inspired me to take a closer look at flowers and who's around.
In my Portland garden, I saw increasing numbers of individuals, and entire new species of bees and wasps every year. Permaculture sources recommend scattering beneficial insectary plants throughout your garden, especially plants in the carrot (Apiaciae), sunflower (Asteraceae), and mint (Lamiaceae) families. Our success in attracting so many insects to our old garden probably lies in our planting 50+ species of perennial flowering plants, ensuring a yard full of blooms from early spring to late fall. (Also, we didn't "clean up" the garden over the winter, instead allowing dead stalks and ground litter to provide overwintering shelter.) Our reliance on perennial vegetables effortly provided food to our insect friends, an important benefit alongside the near elimination of our need to tend the garden and of soil disturbance. I'm concerned that as people maximize food production within cities, they'll convert ornamental plantings to annual vegetables and starve out the insects. Besides the loss of "pest" control provided by wasps, cities could lose honey production as bees lose their foraging grounds. And of course, these insects have every right to live and exist for their own sakes, regardless of any benefit to us. For this and many other reasons, I strongly advocate perennial vegetables, shrubs, and trees as the backbone of any food system.
Sadly, Hawaii has been something of a shock. Apart from a diversity of ants (no species of which occurred in Hawai'i before European arrival), the Hymenoptera we've seen to date are Sonoran Carpenter Bees (lots), honey bees (rare), a stinging paper wasp (a few times - generally accompanied by shrieks), yellow jackets (once, at the Volcanoes National Park), and maybe I saw a small green bee on a flower once. This paucity in biodiversity, unfortunately typical of my experience here thus far, could result from the GMO Papaya Fields of Death in the neighborhood, the general oddness and artificial simplicity of this island ecosystem heavily reshaped by humans, and/or the lack of flowers around here. I'm hoping the latter, as we could rectify this with plantings.
We don't have many flowers here period, and of those present, most are deep, suited to birds, large bees, and large wasps, but not so much to smaller bees and wasps, especially the parasitoids. Grisell reiterates the importance of the sunflower, carrot, and mint family flowers, then writes:
"Bees and wasps come in a huge array of different shapes and sizes. Some have long tongues, others have short ones. Thus, the more shapes of flowers found in a garden, the more and different kinds of hymenopterans will be attracted. Even within a single group such as bumble bees, there are long-tongued species that can reach directly into the nectar spur of a tubular flower such as a columbine, whereas short-tongued species will simply bite a hole at the base of the tube to reach the nectar. Most bees apparently prefer bilaterally symmetrical flowers, by which they orient themselves and then enter the flower's throat. Shallow, dish-shaped flowers, such as apple or blackberry, which are radially symmetrical, are called open-access flowers because many different sorts of insects, including Hymenoptera, have access to the nectar."
Grissell explains the role of color and scent in attracting pollinators, then concludes:
"Creating a garden that enhances the likelihood of attracting adult bees, predatory wasps, and parasitic wasps merely requires that it be a garden of diversity. To achieve diversity, a garden must have many different kinds of plants including native and cultivated species; flowers of different sizes, petal count, shapes, colors, and seasonality; and integration of plants into a network that attracts so much biological diversity that attacks on its structure are absorbed with scarcely a notice by the gardener."
We've noticed, on this land and on other farms and homesteads, a near complete disregard for tree and orchard understories, where most of this diversity can take place. We hope to find many suitable understory plants and arrange them into functional polycultures, but for now we mostly have just theoretical ideas.
We did notice a pretty little insect on the pretty little flowers of Ageratum conyzoides, in abundance around here as a weed. I thought at first that it may be a small bee, but after a good look we suspect a fly of some sort. (UPDATE: thanks to BugGuide.net, I've identified this a male syrphid fly, Toxomerus genus, probably T. marginatus) Still, I appreciate the prompting of Bees, Wasps, and Ants to look at our smaller neighbors, of whatever orders!