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Building polycultures to manage a damaging new fruit fly species

Spotted wing drosophila (SWD) is a fruit fly species that is newly arrived in the eastern US. It can ruin crops of blueberries, raspberries, cherries, hardy kiwifruit, and other small fruits that ripen in summer and early fall. In Asia where SWD is native, many organisms consume it and help keep it under control. Here, it has few predators that can provide biological control. Farmers and scientists have been hard at work learning how to control SWD. It’s made it much harder to grow organic small fruit, even in biodiverse food forest systems which are usually pretty resilient to insect pests.

Among the most promising SWD management strategies is to plant species like this trumpet creeper that attract hummingbirds, perhaps our best SWD predator. Image CC BY-ND 2.0 Kelly Colgan Azar.

A polyculture is a mix of at least two species of plants. They are designed to minimize competition, maximize cooperation and resource sharing, and provide for ease of management. Often we think about polycultures being of productive plants, to directly provide food for people, but in some cases their role is to make it easier to grow crops in adjacent beds, rows, or fields. If you’d like to learn more about designing polycultures, check out my upcoming webinar July 22, 2023.

SWD on raspberry. Image CC BY-ND 2.0, Hannah Burrack, North Carolina State University,

Some of the approaches being developed by scientists involve harnessing the power of plants and wildlife to help reduce SWD damage. Let’s review some of the categories, and then talk about how they might be combined in polycultures.

Our first category is “dead end” plants. These produce fruits that are very attractive to SWD, which prefers to lay its eggs in these species rather than in your tasty berries. Yet, these fruits kill most or all SWD larvae, meaning that laying eggs on their fruits is a “dead end” for SWD. Relatively few species have been tested. Among the best is our native pokeweed, along with the ornamental shrubs scarlet firethorn and willowleaf cotoneaster. These fruits are not edible for people, but planting them may make growing SWD-vulnerable berries more successful. All three have their challenges: pokeweed is very aggressive and hard to remove once established. Both firethorn and cotoneaster have naturalized in some parts of the US. Hopefully future research will identify additional dead end species, and provide guidance in their use to manage SWD.

Pokeweed, a “dead end” plant, whose fruit SWD loves to lay eggs in, yet the berries kill most of the larvae. Image CC BY 2.0, Under the same moon…

The next category is one to avoid planting: alternate hosts that provide a welcoming home to SWD and allow its populations to grow. These include many edible species recommended for pollinator and beneficial insect plantings such as elderberries, hawthorns, and native cherries and plums. One exception is juneberries (Amelanchier species), which tend to ripen before SWD season.

The third category is species that attract hummingbirds. Recent studies have shown that when feeders are used to attract hummingbirds to berry farms, SWD damage is reduced up to 65%. Why? Because hummingbirds not only drink nectar, they also consume large amounts of insects. Hummingbird feeders require a lot of upkeep and are offer less benefits to the birds than natural attractants. Plants provide an alternative. While many flowers are attractive to hummingbirds, some are spectacularly so. It stands to reason that these are the species you’d want to use in your SWD plantings. Other plants have been noted as important for nesting sites or for nest-building materials.

Cardinal flower is among our best native species to attract hummingbirds. Image CC BY 2.0, Amaury Laporte.

Here’s a short list of such species and some of their functions to assist in designing hypothetical polycultures for SWD management. This is not a comprehensive list, and to my knowledge such plantings have not yet been attempted. However there are many thousands of hummingbird gardens out there, and there are good resources available including Hummingbird Gardens: Turning Your Home into a Hummingbird Haven; Bird-by-Bird Gardening; Gardening for Birds, and of course Edible Forest Gardens (which I co-authored).

Let’s build a few hypothetical polycultures using this species palette and variable goals and site conditions.

Perhaps there’s a wet site near your berry planting, or you need a rain garden to handle some runoff. Woody plants could include alder (nest site) and willow (nectar and nesting material). On the sunny edge, bulrushes and cinnamon ferns could provide nesting material, while swamp milkweed and cardinal flower are excellent nectar sources. In the shade of the willows and alders, jewelweed is an excellent nectar source as well. The alder also provides nitrogen fixation to fertilize the planting.

What if you wanted to build a polyculture around pokeweed, our only native “dead end” species? It’s very aggressive and can grow up to 9′ tall (dying back in winter). Nothing will be able to grow beneath it in summer or fall, though there is a niche available for spring ephemeral species. One such is the hummingbird nectar source bluebell, which dies back by late spring. What can compete with pokeweed in season? Perhaps other highly aggressive species. Black locust, a nectar source, also fixes nitrogen. It’s light shade may not slow pokeweed much, indeed they can sometimes be seen growing together in the wild and in spontaneous urban vegetation. Once it is established, black locust could serve as a living trellis for our outstanding hummingbird nectar (but very aggressive) native vines crossvine and trumpet creeper.

If you’re serious about SWD control, you might consider designing polycultures that contain the most powerful tools in our SWD toolkit. Such a planting could feature the dead-end species cotoneaster and firethorn, both of which will grow in partial shade. They could be planted beneath mimosa trees, an outstanding hummingbird nectar source which also fixes nitrogen (and sometimes invasive). It’s light canopy casts less shade than most trees. Fast growing, mimosa could serve as a trellis for coral honeysuckle vines, an excellent nectar source. Meanwhile below the trees and between the shrubs, shade-tolerant excellent nectar species could include columbine, bee balm, phlox, and woodland pinkroot. Hardy fuchsia (Fuchsia magellanica), an excellent nectar source that grows in part shade, might also be added.

In berry farms or large food gardens, one might even have several islands or strips of SWD management polycultures. That’s because not all of these species will get along with each other – and not all grow in the same conditions. Aggressive species like pokeweed and trumpet creeper offer serious challenges to the designer. Polyculture design addresses not only the functions you desire (like SWD control), but also the ability of plant species to share resources and be compatible with each other.

These particular polycultures are hypothetical, but may be worth experimenting with. I hope they illustrate that polycultures aren’t just for direct food production – they can be built to serve many needs, even a somewhat novel challenge like SWD management. As we learn more, such designs could become more and more targeted.