Beetle banks are a simple way to attract and maintain a population of beneficial insects near your crops.
Using native beneficial insects for pest and weed control serves to reduce or eliminate the need for chemical pesticides while improving the biodiversity of your farm or garden. With the inspiration and instructions in Farming with Native Beneficial Insects (Storey Publishing, 2014) from The Xerces Society, you can learn to identify beneficial insects and implement a host of projects designed to improve habitat for them.
You can purchase this book from the MOTHER EARTH NEWS store: Farming with Native Beneficial Insects.
Providing shelter for beneficial insects can enhance their ability to survive overwintering, and offers them protected areas for egg-laying and pupation. Common types of shelter include the crowns and understories of tall bunch grasses, thick piles of dead brush, decomposing logs, stumps and snags, and similar features.
Beetle banks are a habitat enhancement intended to provide shelter for a specific group of beneficial insects: predatory ground beetles. Beetle banks consist of long, elevated earthen berms planted with perennial bunch grasses; occasionally the bunch grasses are interplanted with native wildflowers. These banks provide undisturbed winter cover for ground beetles adjacent to cultivated fields, and are intended to promote rapid movement by beetles back into crop fields when warm weather returns the following year.
The beetle bank concept originated in Great Britain to provide habitat for beetles and other beneficial insects that had declined due to the loss of hedgerows and other habitat adjacent to cropland. British farmers have used beetle banks successfully to control grain crop pests like aphids and wheat blossom midges; in some cases they have eliminated the need for pesticides altogether. Additional research suggests that various ground beetle species supported by beetle banks may feed extensively on weed seed, and can play an important role in suppressing crop weeds. Despite these promising results, beetle banks remain largely untested in the United States, especially outside the Pacific Northwest.
Researchers in Britain wanted to find out whether beetle banks could help reduce cereal aphids in winter wheat. To answer this question, they constructed barriers at various distances from the beetle banks that blocked beetle movement into wheat fields. They then measured the abundance of aphids and aphid predators on either side of the barriers. The researchers found that aphid populations were larger in wheat when barriers blocked predators such as ground beetles, rove beetles, and linyphiid spiders. The impact of the predation by beneficial insects and spiders on aphids declined as distance from the beetle banks increased.
You can construct a beetle bank by plowing two reverse furrows side by side to create an embankment roughly 2–6 feet (60 cm to 1.8 m) wide and at least 1 foot (30 cm) high. Seed the bank with native bunch grasses or plant it with grass plugs. In western states, blue wild rye (Elymus glaucus), California oatgrass (Danthonia californica), slender wheatgrass (Elymus trachycaulus), and Roemer’s fescue (Festuca idahoensis roemerii) have been successfully used in beetle bank construction. In other regions native bunch grasses like switchgrass (Panicum virgatum), big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparium), Indian grass (Sorghastrum nutans), Junegrass (Koeleria macrantha), Canada wild rye (Elymus canadensis), and prairie dropseed (Sporobolus heterolepis) are good potential candidates.
Beetle banks are often positioned in the center of crop fields and may extend almost to the field edges, leaving enough room on each end for equipment to pass. In this way, you can continue to cultivate the entire field around the beetle bank. If your crop field is large, you may need multiple beetle banks positioned at regular intervals to account for the dispersal distance of ground beetles and other beneficial insects. Current guidelines in Britain recommend at least one beetle bank for every 20 hectares (roughly one beetle bank for every 50 acres).
If you want to construct a beetle bank, you should anticipate having to mow it routinely during the first year after planting to suppress annual weeds that may shade and compete with the newly established bunch grasses. In future years you may have to control woody plants and spot-spray or spot-pull weeds. Over time, as the grasses mature, they should be fairly effective at preventing weed encroachment. As with all types of beneficial insect habitat, it is important to protect beetle banks from insecticide spraying.
Because the beetle bank concept is still very new in North America, there is still very little research on questions such as how big the banks should be. This makes it difficult to provide precise guidance on how best to construct a new beetle bank. Most farmers who are experimenting with beetle banks use one of two common construction approaches: opposite plowing to create a berm, or using a bed shaper.
To create a berm using opposite plowing, employ a single blade plow, driving carefully in a straight line across the field, pushing the soil to one side in a narrow hill. Lift the plow blade before you reach the end of the field to maintain a vehicle path between the end of the bank and the beginning of the field edge (in the future you can plant crops in this headland area), allowing the entire field to continue to be farmed as a single unit. After creating this initial hill of soil, turn and plow in the opposite direction, pushing soil up against the first hill you created. The result will be a single long mound about 2 feet (60 cm) across.
If you would like a wider bank, repeat the process on both sides, pushing more soil from the sides into the mound. Depending on your soil, the plowing will leave rough clumps that you’ll need to smooth before you plant. To do this, drag a lightweight harrow over the hill.
Using a Bed Shaper
A much simpler approach to creating a beetle bank is to use a single-row bed shaper. The disadvantage of this approach is that the width of your beetle bank will be limited to the width of the bed shaper. Despite this, most bed shapers produce a smooth, packed surface, eliminating the need for follow-up harrowing.
Beetle banks can be established either from seed or with transplants, but they should be planted relatively soon after you’ve created the mound, to stabilize it. On narrow banks of at least 2 feet (60 cm) wide, you should be able to transplant grass plugs in a staggered pattern around 18 inches (46 cm) apart. Wider banks may require multiple rows of transplants, staggered in a checkerboard pattern to ensure adequate coverage.
Grass seed can also be broadcast by hand over the mound, but expect some of it to show up in the adjacent crop fields. A typical seeding rate for native grass of 5 pounds (2.3 kg) per 1,000 square feet (93 square m) should provide good cover. For even greater beneficial insect value, you might also consider adding a few easy-to-grow wildflowers, as plugs or seed, to the beetle bank. In general, however, your vegetation should consist of at least 75 percent native bunch grasses.
Spring is the best time to establish a new beetle bank; if you begin in the fall, winter precipitation might wash away seed and loose soil. Many native grasses do not require cold stratification to germinate from seed, so spring planting, with a little irrigation as needed, will usually be enough to establish a good cover.
Cropland weeds will be a problem in most beetle banks during the first year, and sometimes into the second year as well. You should plan to mow the bank if possible once or twice during the first few years, when annual weeds begin to take over. This occasional mowing won’t hurt your perennial native grasses, and it can actually help them compete with weeds by encouraging them to send out tillers and expand their crowns.
After several years, the grasses will increase in size and crowd out most weeds. As this happens, you should reduce, and ideally stop, any further mowing since it could harm ground-nesting birds and will definitely reduce winter cover for beneficial insects. In the long term, if shrubby weeds begin to take hold in your beetle bank, you can control them by spot spraying, or pulling them out.
Excerpted from Farming with Native Beneficial Insects © The Xerces Society, photography by © Andrew Dunham, Gwendolyn Ellen and Laura Westwood and illustrations by © Marjorie C. Leggitt. Used with permission from Storey Publishing. Buy this book from our store: Farming with Native Beneficial Insects.