Building Habitats for Beneficial Insects
For the most successful and cost effective biological control, the farmscape should be a working ecosystem. Wild natural areas rarely see pest infestations on the same scale that cultivated areas see. This is because plant feeding pests are kept in check by a healthy and diverse natural enemy complex that includes a variety of predators and parasitoids. Building beneficial insect habitats near, or even within, a crop, mimics the diversity of nature. These habitats are a refuge for beneficial insects and encourage a permanent beneficial insect population to settle on your farm and protect your crops. Dedicating as little as one percent of farmland to house beneficial insects will contribute to a functioning, balanced ecosystem.
The seasonal nature of crops prevents beneficial insects from consistently completing their life cycles, as they do in the wild. Once the crop is harvested or the land is plowed insects flee or die, and beneficial releases have to start over again from scratch the next year. Planting areas of habitat cover crops provide refuges for beneficial insects to reproduce in between crop cycles and when there are no pests in the crop to eat. Habitat areas act as field insectaries, on-farm breeding sites for beneficial insects.
Habitat plantings can also protect a crop by acting as trap crops, drawing pests away from the market crop, or by acting as a wall that intercepts pests invading from other areas. High concentrations of pests in the habitat allow the beneficial insect population to expand much faster than if just feeding on the lower levels of pests in the crop itself.
Besides providing a concentrated prey source, a good habitat also provides alternate nutrition in the form of nectar and pollen. The adults of many beneficial insect species subsist only on nectar or live longer and lay more eggs when nectar or pollen is available. For example, adult green lacewings require pollen protein and sugar from nectar and honeydew to produce fertile eggs. The carnivorous larvae of some beneficials mature faster if they have pollen and nectar to eat along with their prey, allowing more larvae to reach adulthood and start reproducing earlier. Habitat plantings also provide water (in the form of dew or gutation droplets, nutrient-laced water that is pushed out of the leaves of grasses such as corn and cereals, to keep predators and parasitoids hydrated.
Habitat can provide structures that help predators. Tall plants provide anchors for the webs of orb weaving spiders. Dense grass crowns provide shelter for diapausing, or estivating insects like ladybugs. Diapause is metabolic slowing in response to low temperatures or short day lengths, similar to hibernation. Conversely, estivation, or “summer sleep” is when insects go dormant during hot and dry weather.
Other benefits of planting habitat are reduced erosion and improved soil. Some habitat plants help improve drainage by loosening hard, compacted soil, others fix nitrogen, and all can be used to increase the organic content and water holding capacity of soil. Covering bare soil with habitat plants decreases radiated heat and dust that help trigger pest flare up at the edges of crops. Habitat areas also invite other beneficial wildlife such as birds and lizards. The high numbers of insects for the birds to eat in the refuge make them less likely to damage ripening fruit.
A healthy farm ecosystem has an unbroken web of life, from soil microbes to plants, to pests that feed on plants, up to predatory insects, lizards and birds that feed on pests. Planting habitat and increasing diversity helps preserve the balance between the parts of these interconnected systems. In a healthy ecosystem, a robust natural enemy complex will develop to protect crops from severe pest infestations.
The first step to building habitat is deciding where to plant it. Questions to ask include: Is the area more suited for a permanent or temporary planting? Is irrigation available to establish the plants, and is regular irrigation necessary in your climate? What direction do prevailing winds blow from? From what direction do pests enter the field? What are the key pests of the crop and how far and fast do they move?
It is helpful to consider the entire timeline of the crop, from land preparation to harvest, and think about how the land will be in use throughout the year. Building an ecosystem takes time, so habitats should be planted ahead of crops if possible. Permanent beds work really well if they fit in the farmscape. There are very few absolute rules for habitat placement, and over time growers usually develop their own arrangement of habitat areas based on observation and experience. Start small and learn what works for your farm.
The easiest spots to see as candidates for habitat plots are empty areas. Try to find area not used or unusable for crop plants, such as slopes too steep to farm, ditches, margins next to fences and roads, even between the tire tracks of dirt roads. In orchards, try planting low growing plants in the lanes between trees. Start looking at bare ground as opportunity to host more beneficial insects. If there's an empty space, there's probably a wildflower that can thrive there. Fill breaks in crop rows, such as holes left by poor germination of market crop seeds, with flowers. Between crop cycles, choose cover crops that can act as temporary habitat, such as alfalfa, or mix annual wildflowers into your regular cover crops. Keep an eye out for irrigated areas where nothing is growing and weedy areas that can be put to work to improve pest control.
Beneficial insect habitat strips can be used to form a protective wall. Surround fields with a mix of flowering hedges or tall growing flowers and grasses. This type of planting does for the crop what our skin does for our immune system – it is the first line of defense preventing invading organisms from getting in. The predators and parasitoids will attack invading pests, and follow any that make it through the wall into the crop. If you can't surround the whole field, at least plant a border on the upwind side of the crop. This will act as a windbreak and catch pests hitching a ride on prevailing winds towards your crop.
Perennial hedgerows should be large enough to host beneficials for the crop. Start with 1% of the crop area. Choose areas with moderately good soil, where irrigation is easily available for at least part of the year (for example, when cropland is fallow). For ease of maintenance, avoid areas with nut grass or Bermuda grass – these persistent weeds will demand intensive weeding and crowd and steal water from habitat plants. Areas without these grasses should only require a couple hours of weeding per season. Dusty areas should also be avoided, as dust may interfere with beneficials and reduce the hedgerow's effectiveness as a habitat. Hedgerows take a year or two to get established and provide welcoming habitat, so consider planting a strip of annuals alongside the hedges for the first year.
A habitat border will slow pest invasions, but as you move into the field, away from the border, its influence is less predictable. A lot depends on your key pests and the beneficials that control them. Some insects are weak fliers or only fly a couple hundred feet, others only walk, leaving the center of the crop undefended. To keep the whole field protected, plan for regular pest break strips and islands of habitat interspersed throughout the crop. For pest break strips, interplant a row of alfalfa and other flowering plants between crop rows every 350 to 700 feet or so. Pest breaks work best aligned perpendicular to prevailing winds. Plant up to 20 habitat islands per acre – hills of corn mixed with flowers works well for these. Lacewing adults, Trichogramma and Orius like to spend the day on an island, then fly out into the crop at dusk to lay eggs. Reducing your crop area slightly will be rewarded with reduced pest control and labor costs – you're paying for the services of beneficials instead with room and board.
In areas where the birds welcomed by the habitat may become pests themselves, leave a dirt road or bare swath of land between the habitat and the crop.
Corn is an easy to grow plant that can host a wide variety of predators, parasitoids, and the pests they feed on. It is especially attractive to green lacewing and Trichogramma and draws many pests away from the market crop, especially tomato fruit worm and cotton bollworm. A wind pollinated plant, it produces large amounts of pollen to support predatory mites and other pollen eating beneficials until there is a pest infestation for them to attack. It hydrates and feeds nectar-feeders with nutrient-rich gutation droplets along the edges of its leaves. Corn plantings are also a cool refuge for beneficials on hot days.
Beneficials found in corn plantings include spiders, lacewing, minute pirate bugs (Orius), various lady beetles, predatory mites, and a variety of different parasitoids including parasitic wasps and Tachinid flies.
Plant a mix of different varieties that mature at different times to have a constant source of pollen. Intersperse throughout border plantings, or plant 20-30 per island or "corn hill." Supplement corn hills with other grasses, such as sorghum and Sudan grass, as well as wildflowers. Plant smaller amounts of corn, if corn ear worm (Heliocoverpa sp.) is present and is a pest on neighboring crops.
Alfalfa is another plant that attracts many pests and natural enemies. Field reports in Kern County, California, show that alfalfa supports the primary natural enemies of vegetable and field crops in the area. Alfalfa is also more attractive than many market crops to some pests. For example, lygus bugs prefer it over cotton, as long as there is succulent new growth to munch on. Stinkbugs are especially attracted to alfalfa or beans when there are seeds in the milk stage present.
Succession planting (and later, alternate strip cutting) every two weeks will keep some plants in the attractive milk stage and make pests less likely to migrate when the plants mature. Plant with Daikon radish, low growing mustards, and beans for a lygus trap crop. Mixing in some umbels attracts beneficial insects, especially Tachinid and hover flies. Make hay and remove alfalfa stems before seeds enter milk stage if alfalfa seed chalcid is a problem.
What to Plant
When choosing habitat plants to include in the mix, pay attention to flower timing, shape and color. Research the beneficial insects you're trying to attract – what are the nutritional requirements of each life stage? Do they eat nectar and pollen, or just other insects? What other insects host them that aren't pests of the crop you are growing? For example, you can plant grasses such as rye or barley around a vegetable crop to attract grass aphids as an alternate host for aphid parasites. What type of plant do the beneficials you want to attract tend to lay eggs or pupate on? For best results, plant a variety of types of refuges. Varying the habitat plantings from area to area and including a diverse mixture of plants in each planting will promote insect biodiversity and make it more likely that a wide variety of beneficials will call your farm home.
Habitat plantings should have a mix of different flowers that bloom at different times of year so that there is always something flowering. This is to provide a constant nectar and pollen source throughout the year. Besides looking at when during the year a plant blooms, be aware of what time of day the flower is open – flowers that only open at night won't feed insects that are active when the sun is out.
Another consideration is what kind of flowers the plants have – is the nectar or pollen easy to get to or is the flower designed for very specific pollinators? Can you see distinct petals (open structure) or are they fused into a tube (closed structure)? Some flowers are shaped to only allow pollinators with a particular head shape have access to its nectar or pollen. Flowers where the petals form a long tube are generally evolved for bird and moth pollination. The best plants for beneficial insect habitat purposes have clusters of small open flowers arranged in dense heads or horizontal planes (mats). This arrangement lets pollen or nectar feeding insects find the most food while expending the least amount of energy because they don't have to travel far to find the next flower. The carrot and dill family, Apiaceae or Umbelliferae, fulfills this requirement well. Its members have flowers arranged in "umbels," or inside-out umbrella shaped flower clusters. Other plants with closely arranged flowers are sweet alyssum, elderberry, and plants in the daisy family (Asteraceae or Compositae), such as yarrow. The large, showy flowers of many members in this family, such as sunflowers, are actually clusters of hundreds of tiny "disc" or "ray" flowers – the "petal" we see on the edge of sunflowers and daisies is actually an individual ray flower.
Some plants, such as passionflower and many varieties of peach, also produce nectar in areas other than flowers, in organs called extrafloral nectaries. Annuals with extafloral nectaries include corn, cowpeas (black-eyed peas), common and hairy vetch, snap beans, bell beans and sunflower.
Wind pollinated flowers, such as grasses, produce large amounts of pollen that benefit some beneficials, but their usually drab, inconspicuous flowers provide no nectar. White flowers seem to get the most insect visits, but diverse colors have more human eye appeal.
The surface of a plant's leaves also affect how well it will work as a habitat plant. Choose plants with smooth leaves. The tiny hairs, or trichomes, that cover plants such as tomatoes make it hard for many predators to move around the plant and hunt for prey. What feels soft and fuzzy to us can be as inviting as a wall of pikes from an insect's perspective. Be sure to check the bottom of the leaf, where most of the insect action occurs.
A beneficial property of some plants' leaf surfaces are the presence of micro habitats or domatia that can shelter tiny insects or predator mites. Experiments have shown that leaves with domatia usually have higher populations of predatory mites and lower populations of herbivorous (pest) mites. In a few cases, plant-eating mites have been found to take shelter in domatia, but this usually has a positive effect on the predator mite population living outside the domatia. These hide outs are frequently pits, pockets or tufts of trichomes alongside the main veins of a leaf.
Other factors to weigh when looking for plants for your habitat are how well they work in the area you plan on planting. For a permanent habitat, plant perennials or reseeding annuals. For temporary plantings, such as pest break strips interplanted in a row crop, choose annuals that won't make themselves a nuisance next year. If planning on using a hedgerow as a habitat border, plant annuals to support beneficials until the hedges are established. By the second year, the hedgerow should be able to house beneficials without the help of annuals. Depending on climate and availability of irrigation, you may need to find drought tolerant wildflowers. Plant height, ability to survive trampling (or driving over), and attractiveness to humans are other considerations. (Beneficials might love a weedy looking habitat planting, but your neighbors might not!)
For an extra bonus, some plants can do double duty and help improve soil or deter soil pests while housing beneficials above ground. Plant legumes, such as clover, to attract beneficials while they fix nitrogen and fertilize the other plants in the mix. Mallow or malva, generally seen as a weed, has deep tap roots that help loosen compacted soil. It also is a host of Vanessa or painted lady butterfly. Some plants, such as marigold (Tagetes spp., French dwarf varieties especially), mustard, fodder radish, and cahaba vetch, help suppress plant parasitic nematodes. Some plants, such as sorghum, sunflower and wormwood, can suppress seed germination or seedling growth in an interaction known as allelopathy and can be used for selective weed control. Other plants however, such as amaranth and lambsquarter (chenopod), have the negative effect of suppressing mycorrhizal fungi; fungi that form beneficial associations with plant roots.
Habitat plants can have important economic uses such as cut flowers, culinary herbs, medicinal herbs or decorative dry flowers. Corn ears can be harvested by farm workers and serve as motivation to care for the island plantings. A number of seedsmen have formulated habitat seed mixes that use these considerations to fit into a wide range of niches on farms.
Prepare a seed bed by scratching the surface to get aggregate particle size of 1/2 to 1 inch. Poor soils will benefit from amendments of organic matter (i.e. humus, compost, ground bark, aged sawdust or wood shavings). This step is optional but will promote drought tolerance and a healthy, long lasting stand of plants.
To reduce weed problems later, water area before planting habitat, then wait for at least two weeks to germinate weed seeds. Kill weeds with shallow cultivation, a contact weed killer (a less toxic option, such as plant based product, Phydura, is preferred), flaming, mowing, or by hand weeding. Tilling weeds in can bring new weed seeds to the surface. Rake lightly before planting seeds to break up soil.
Drill in or broadcast seed and lightly cover. Sand or sawdust may be used to control seed flow rate. Don't plant too densely, or the slower growing plants will be crowded out by the faster plants and the overall diversity will be lower. To ensure even distribution of plant species when planting mixes, sow 1/4 of the seeds going in one direction, and sow the rest in three different applications going in different directions. Because seed mixtures are made up of seeds of different sizes and shapes, the seeds of some plants will settle to the bottom of the seeder, and some will rise to the top. Planting them all in one pass may lead to one end of the strip having more of one species of plant and the other end being dominated by another plant. Light mulching (1/8 inch) after planting can help seedlings retain moisture.
The best time to plant depends on local climate. In areas with cold winters plant in the spring, after the danger of frost has passed. In California's mild Mediterranean climate, plant right before or during the winter rainy season if possible, so that the rains help the seeds establish. If planting in spring, water to establish. Seedlings will need to be irrigated for about three weeks. The planting may also need to be watered several times in the summer.
Depending on climate and what plants were chosen, the habitat patch may need to be irrigated. In desert or other dry areas, hedgerows will need frequent watering for the first two months. Once established, watering can be cut back to every 2–4 weeks, depending on weather. Irrigation can also extend the blooming period. Observe the health of your habitat and adjust watering schedules. If irrigation isn't easily available, make a note of which plants do and don't do well in your climate and adjust the mix of plants when reseeding next year.
Once flowers start blooming or a pest population is observed, habitat areas can be colonized by releasing insectary grown beneficials before pests are seen in the market crop. Small releases early in the season can prevent or reduce flare-ups later in the season and lower the overall amount of insectary-grown bugs that need to be released that year.
Avoid spraying pesticides or other harsh chemicals on habitat areas. Even soft pesticides or botanicals should be avoided – a high pest population in this area may be desirable to help the beneficial insect population grow. If pest densities get to the point where pests are migrating into the crop, try using physical controls such as vacuuming. You can use a two-stage screen (16 mesh, or window screen sized mesh, and 90 mesh) while vacuuming to separate larger pests (like lygus) from smaller beneficial insects (like lacewing and Trichogramma). With a little patience, the large, easy to identify beneficials (i.e. lady beetles) can be separated from the pests and released, too. Alternatively, run chickens through the strip to bring down insect numbers. Keep them where they're needed using a "chicken tractor" or movable electric fencing.
Weeding, if necessary, is easier earlier in the growing season. If you are unfamiliar with how the plants in your habitat mix look before they flower, try planting a few pinches of seeds in an area you know to be free of weeds, such as a flower pot with weed seed free potting soil, to familiarize yourself with your habitat plants.
To prolong flowering period, use a string trimmer to deadhead old flowers. Be sure to let some flowers set seed if you want the stand to reseed itself for next year. For annuals interplanted within crops, removing the spent flowers before they set seed can help prevent habitat flowers from becoming weeds, if that is a worry.
Cut as needed to reduce the height of plantings for ease of field operations. Prune shrubs and trees to improve vigor and maintain diversity. Non-hedge habitat areas, especially ones that include interplantings of alfalfa, should be alternately strip cut. Pests are more attracted to habitat plants when they are putting out lush new growth. Once alfalfa or other plants set seed, pests become less interested and are more likely to migrate to other areas. Regular mowing "resets the clock," ensuring there is always tender new growth to lure pests away from the market crop. Only cut or mow half of a habitat strip at once. Mow the other half about two weeks later or when the alfalfa sets seed. This preserves a section of unmown habitat to keep predators and pests in the habitat areas and stops pests from migrating into crops. Alternate strip cutting also concentrates pests and beneficials into the uncut habitat area. This suddenly increased population density makes it easier for beneficials to find hosts or prey, making their population grow even faster. Beneficial populations are 4 to 20 times higher in alternate strip cut areas than in solid cut strips. For permanent habitat areas, let plants reseed themselves by letting flowers set seed before mowing.