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.
Considerations for Gardeners
Habitat features like beetle banks, brush piles, tunnel nests, and insect hotels can all be scaled down for use in garden settings. Recently cultivated garden beds offer little shelter for ground-dwelling beneficial arthropods taking cover from inhospitable weather, predators that would otherwise dwell under leaf litter and mulch, and those beneficial insects that live part of their life beneath the soil. To support these diverse beneficial insects, gardeners can plant small beetle banks between vegetable beds, or even “beetle bumps,” small mounds planted with perennial grasses, within vegetable plots.
There are other steps you can take to bolster natural pest control by ground-dwelling arthropods. Using leaf mulch or straw in garden beds can offer shelter to predators such as ground beetles, sheet-weaving spiders, and wolf spiders. Consider providing some nesting opportunities for the predatory wasps that hunt common garden pests such as tomato hornworms and armyworms. You can create small wooden nest blocks with only a few openings or make small stem bundles and hang them throughout your yard for predatory wasps. Consider leaving naturally occurring bare patches of ground for nonaggressive ground-nesting solitary wasps that also hunt garden pests. Finally, insect hotels, creative stacks made from myriad materials, can be ideal for gardeners who like to experiment but are pressed for space.
How to Attract Social Wasps
In some cases, it may be worth encouraging social wasps to nest near your crop fields. Paper wasps (Polistes spp.) are important predators of caterpillars. Researchers in North Carolina found that enhancing wasp nesting habitat near tobacco fields reduced the damage caused by tobacco and tomato hornworms, as paper wasps feed the caterpillars to their larvae.
Paper wasps build nests from chewed plant fibers, creating open cells where brood are reared. The entire nest structure is usually suspended from a stalk anchored in a sheltered area, such as under the eave of a barn. New wasp nests are initiated by a mated queen in the spring; the colony grows throughout the summer and eventually dies, except for a new mated queen, in the fall. By midsummer, paper wasp nests may contain between five and 50 individuals.
How to Create Shelters for Social Wasps
To create new nesting opportunities for social wasps, some growers construct shelters consisting of wooden boxes that are open on the bottom; old birdhouses can be reused for this purpose. To make your own, cut five 4-by-4 inch (10-by-10 cm) lengths of 1-inch (2.5 cm) board. (Larger dimensions can also be used.) Assemble the pieces to construct a box that is open on the bottom. Attach the nest shelter to a post or tree at least 3 to 4 feet (1 to 1.2 m) off the ground along fencerows or field borders. Wasp shelters should be located away from places with lots of human or animal activity where the nests might be a hazard. Although you can observe the nests if you approach them carefully and slowly, disturbances such as vibrations caused by mowing nearby will disturb them. If disturbed, social wasps will defend their nest by stinging.
Create Nests for Solitary Wasps
Wooden Nest Blocks
You can construct wooden nest blocks by drilling holes in various kinds of wood. Blocks of 4-by-4-inch (10 by 10 cm) or 4-by-6-inch (10 by 5 cm) preservative-free lumber are most commonly used, but rougher blocks of wood can also be used. You can drill holes in a stump, an existing snag, or a fence post. Pithy sticks measuring 3/4 by 3/4 by 6 inches (2 by 2 by 15 cm) may also be drilled and then bundled together.
On one side of your block, drill a series of holes separated from each other and from the edges by 1/2 to 3/4 inch (1.3 to 2 cm). Holes should be between 1/8-inch (3 mm) and 1/2-inch (1.3 cm) in diameter. Drill holes as deeply as you can into the block. Holes should be a minimum of 3 inches (8 cm) deep, ideally around 5 to 6 inches (13 to 15 cm) deep, but should not go all the way through the block.
Cavity-nesting wasps do not like to nest in tunnels open at both ends, which are harder to defend from intruders, so don’t drill the block all the way through. Use sharp drill bits, such as brad point bits, to make the smoothest, straightest holes, to increase the likelihood of wasps using the tunnel.
Sections of bamboo or reeds, such as the common reed (Phragmites australis), can also be used as nests. Use plants with hollow stems that are divided into sections by nodes, since these nodes will serve as the closed end of the tunnel nest. You can buy bamboo poles or stakes from garden centers at minimal cost, or collect dead reed stems from wetlands or marshes. You can also use cupplant stems (Silphium perfoliatum) or teasel (Dipsacus fullonum).
Cut bamboo and reeds with hand clippers, pruning shears, or a fine-toothed saw. Cut reeds to lengths with one node each, so that each length has one open end only. Use pieces with thick walls and with lengths that exceed 3 inches (8 cm), but discard pieces that have openings larger than 1/2 inch (1.3 cm) and smaller than 1/8 inch (3 mm) in diameter, as they are too large or too small to be used by most predatory wasps. Bundle the reeds together, with the closed ends of the stems at one end of the bundle, and secure tightly with string or wire.
Another option is to pack the node ends of the stems into a jar or tin can, so that the open ends face outward. Place the bundles with the stems parallel to the ground. If the bundles contain irregular lengths and diameters, they can shelter various species of wasps, which often have specific preferences for nest cavity dimensions.
Hang your nest block with the entrance holes in a horizontal position facing east or southeast, so that morning sunlight reaches the entrance holes early, but isn’t too direct during the hottest portion of the day, which can be too harsh for developing brood. Provide protection from rain either by attaching a small overhanging roof to the nest, or by hanging the nest in a protected location such as next to a building.
Nest blocks or bundles should be placed at least 3 feet (1 m) off the ground, and in a visible location, such as on a fence post, or near a large tree, in order to help wasps locate nests in the landscape. Nests may be more rapidly colonized when they are near permanent strips of habitat, such as hedgerows or field borders.
Tunnel nests do need attention over time to avoid the buildup of parasites and diseases that can affect wasp populations. There are several approaches to maintaining nest blocks. Paper liners can be used only with wooden nest blocks and are the most intensive management approach. First, line the tunnels of wooden nest blocks with removable paper straws, which can be purchased or made using waxed paper curled around a dowel rod. At the end of the growing season, gently pull out the straws containing the wasps and place the straws in a cool place, such as a refrigerator or garage. Then place new straws inside the block. Place old straws in an emergence chamber, a dark container such as a plastic bucket with a lid that has a single 3/8-inch (9 mm) hole at the bottom of the side of the container. In the spring, place the emergence container next to the empty block. As wasps emerge from their nests, they will crawl toward and then exit the hole in the emergence container and find the clean nest nearby. Leave the old straws in the emergence box for the season.
If you don’t use paper liners, it is important to phase out and replace nesting blocks or bundles regularly. Every two or three years, place entire bundles or blocks in emergence chambers. Place the emergence chambers next to new nests, and the wasps will colonize the new, clean nests rather than returning to the old nests.
The least intensive option is to use multiple smaller bundles of reeds or small blocks with fewer tunnels. Spread the blocks out across your property, which mimics natural conditions of limited nest sources separated spatially. Smaller nests also decompose more rapidly. Add small nests to the landscape periodically, and allow the old nests to deteriorate naturally.
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Excerpted from Farming with Native Beneficial Insects © The Xerces Society, photography by © Tess Grasswitz. Used with permission from Storey Publishing. You can buy this book from our store: Farming with Native Beneficial Insects.