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Natural habitat is essential to supporting beneficial insect populations, and the loss of such habitat has resulted in a loss of natural pest control services. Farms with natural or seminatural habitat around them tend to have more beneficial insects, less pest pressure, and less damage from pests. In general, farms with smaller field sizes, less pesticide use, and more noncrop habitat have the most beneficial insects.
While beneficial insects can maintain pest populations below economic thresholds, their services often go unnoticed by farmers and pest managers. Yet, when their populations are disrupted by loss of habitat or pesticide use, pest outbreaks may become more common.
A pirate bug nymph feeds on an aphid.
Healthy beneficial insect populations can reduce the need for chemical insecticides. In order to capitalize on beneficial insect suppression of pest outbreaks, farmers must not only monitor pests, but also recognize the presence of beneficial insects. With close monitoring, they can reduce insecticide use and apply more targeted insecticides. This in turn will result in enhanced pest control in future years by allowing beneficial insect populations to build up and thrive.
The majority of beneficial insects in agriculture are native, naturally occurring species. In contrast, many pest species are exotic and were introduced by accident. Since they have left their natural enemies behind in their native range, these pests reproduce unchecked and cause economic losses in crops. Practitioners of classical biocontrol identify and make use of natural enemies that prey on the pest in its native habitat. Intentional release of these nonnative species can provide effective pest control, but the introduction of nonnative biocontrol agents may produce negative impacts on the natural ecosystem.
For example, a tachinid fly, Compsilura concinnata, was released repeatedly in North America from 1906 to 1986 as a biocontrol against several pests, including the introduced European gypsy moth (which defoliates forests throughout the East even today). Now the tachinid fly is implicated in the drastic decline of several nonpest silk moths in New England, including once-common species such as the cecropia moth (Hyalophora cecropia). Similarly, a recent study in Hawaii revealed that 83 percent of parasitoids found inside native moths were nonnative species that had originally been introduced for biological pest control.
Because of these and other unanticipated effects, classical biocontrol is controversial. If farmers and pest managers provide habitat for beneficial insects, there is less need for the release of nonnative insects. Supporting a diverse array of beneficial insects helps to keep native and nonnative pest populations below economically damaging levels. Also, given time, native beneficial insects typically adapt to prey upon new pest species.
Scientists at Cornell University and the Xerces Society estimate the value of native predator and parasitoid insects for crop pest control in the United States to be at least $4.5 billion annually. This is a conservative estimate, based upon limited information. Those same scientists suspect the actual real-world value may ultimately be even higher.
A crop-specific estimation by Doug Landis from Michigan State University and his colleagues found that suppression of soybean aphid by beneficial insects was worth $239 million a year (based on 2008 soybean prices), in Iowa, Michigan, Minnesota, and Wisconsin alone.
A flower fly larva hunts soy-bean aphids, an invasive pest that causes significant damage to soybean crops in the United States.
Conservation biocontrol can provide services to the farm in addition to pest control. For example, the same habitats that support predators and parasitoids also support a diverse array of crop pollinators, such as bumble bees and managed honey bees. These flowers also attract other colorful flower visitors, such as monarch butterflies. All of these, and other insects, in turn provide food for songbirds, game birds, and other wildlife.
Native grass and wildflower plantings for conservation biocontrol can also be incorporated into field buffer systems such as filter strips, grass waterways, roadside embankments, and septic drainage fields. These buffers reduce soil loss and improve water quality by filtering runoff from adjacent fields. They also help absorb and remove excess nutrients in farm systems.
Native shrub, grass, and wildflower plantings can also deprive some crop pests of habitat by reducing their host plants in noncrop areas. For example, in the Pacific Northwest, berry producers are increasingly concerned about the introduced spotted-wing drosophila (Drosophila suzukii), a fly that is a pest of soft fruit. The invasive Himalayan blackberry (Rubus armeniacus), a common weed along farm fencerows in the Northwest, serves as an alternate host for the fly, increasing its populations even when nearby crops are sprayed with insecticide to control it. To combat the fly, some growers are turning their attention to eliminating the Himalayan blackberry and replanting those fencerow areas with native shrubs that lack fruit on which the fly can reproduce.
Similarly, studies in the Central Valley of California clearly indicate that nonnative weedy plant species found along farm and road margins harbor more crop pests than do plantings of native species. Therefore, any efforts to replace Eurasian weeds with native habitats for beneficial insects can be doubly useful by both increasing predators and parasitoids and removing potential crop-pest habitat.
The value of encouraging beneficial insects in your yard or garden extends beyond natural pest control. Garden habitat for beneficial insects can be attractive, can reduce the need for pesticides, and can support charismatic wildlife like pollinators and birds. Plants that attract beneficial insects can also play multiple functional roles in your garden, such as helping to soak up excess rainwater or to return nutrients to the soil.
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Habitat for beneficial insects can support a variety of valuable wildlife, including pollinators such as this long-horned bee (Melissodes spp.).
Habitat enhancements made on farms certified as organic receive additional benefits. The USDA National Organic Program mandates that organic farmers maintain or improve their on-farm natural resources, including soil, water, wetlands, woodlands, and wildlife. Over the last several years, advocates for wildlife and farm sustainability have worked with the National Organic Standards Board to develop mechanisms for organic farm certifiers to document that organic farms are maintaining this standard.
Beneficial insect habitat can help organic farmers fulfill the biodiversity conservation requirements of NOP certification.
Since the summer of 2012, organic farm certifiers have been required to check if farms are helping to secure on-farm biodiversity. Organic farmers who demonstrate that they are working to maintain, protect, or enhance habitat for beneficial insects on their land clearly are meeting these criteria.
Native wildflowers like those on this farm do not typically support pest insects.
Conservation biocontrol can help farmers fulfill the biodiversity requirements for organic certification described in the National Organic Program (NOP) Rule. For example, the NOP definition of organic farming includes practices that:
“Foster cycling of resources, promote ecological balance, and conserve biodiversity.” (§205.2)
“Maintain or improve the natural resources — the physical, hydrological, and biological features, including soil, water, wetlands, woodlands, and wildlife — of the operation.” (§205.200 and §205.2)
For eastern Washington’s vineyards in the 1990s, controlling grape pests meant using pesticides, and lots of them. At that time grape growers applied more than seven pounds of active ingredient of pesticide per acre per season, and the vineyards were alive with spray application machinery. Worse, most of the pesticides were broad-spectrum, as likely to kill the good insects and mites as the bad ones. The landscape was virtually devoid of life except the grapevines.
With the new century came a new awareness and desire by grape growers to reduce their dependence on pesticides and to produce fruit and wine containing as few chemical residues as possible. Washington State University scientists researched and developed biologically based Integrated Pest Management systems, and progressive growers put them into practice. Persistent, broad-spectrum pesticides were replaced by fewer applications of short-lived, narrow-spectrum “soft” chemicals identified as safe to key predators and parasitoids. Almost immediately, things began to change. Outbreaks of spider mites and mealybugs, common in the 1990s, became less frequent as natural enemies of these pests colonized vineyards.
Research on conservation biocontrol of grape pests intensified in multiple states with identification of key predators and parasitoids followed by a progressive understanding of their biology and ecology. For example, researchers found that the key natural enemies of grape leafhoppers, minute parasitic wasps in the genus Anagrus, overwinter within other leafhoppers living on rosaceous plants. Plantings of roses at the ends of grape rows or in specific areas within vineyards provided overwintering habitats for Anagrus hosts and hastened the migration of wasps into vines in the spring, improving biocontrol of leafhoppers.
Beyond just Washington, the integration of beneficial insect habitat into vineyards is an increasingly common practice throughout the U.S. wine industry.
Research on biocontrol of spider mites showed that improving the vineyard habitat by reducing the number of sulfur sprays used for disease control strengthened the diversity and abundance of predatory mites and suppressed spider mite populations. Other studies showed native bushes and trees acted as harbors for predatory mites, encouraging growers to set aside areas within vineyards for native plants.
Experiments with the use of nonnative flowering annuals as ground covers indicated that these ground covers improved biocontrol by attracting and retaining natural enemies of vineyard pests. Although it is difficult to sustain these nonnative plants in the dry environment of eastern Washington, the experiments stimulated contemporary research on native shrubs and bushes that could be used as beneficial ground covers.
Further improvements to vineyard habitats are under way, with native habitat restoration programs designed not only for predators and parasitoids but also for pollinators. The Washington grape industry is an excellent example of what can be achieved for agriculture, the local community, and the environment by farming for beneficial insects.
— Dr. David G. James
Washington State University Department of Entomology
Finally, conservation biocontrol offers the additional benefit of beautifying landscapes. Having functional mass plantings of native wildflowers or flowering shrub hedgerows can light up a landscape. These habitats can provide recreational opportunities for farm families, such as high-quality habitat for hunting, bird watching, and learning about insects and wildlife. Attractive plantings along roadsides or market stands also support agritourism business models, especially if they include interpretive signs or other educational materials, and offer a way to connect with farm customers who are interested in sustainable agriculture issues.
Recent studies show that pest control by naturally occurring predators and parasitoids on some organic farms may be as effective as pest control with pesticides on some conventional farms.
The flowers of milkweed (Asclepias spp.) can produce a rich supply of nectar. We wanted to find out the extent to which planting insecticide-free milkweed habitats in agricultural farmscapes could conserve bees and other insect pollinators, as well as enhance parasitism of insect pests.
In peanut-cotton farmscapes in Georgia, stink bugs, including the southern green stink bug (Nezara viridula) and the green stink bug (Chinavia hilaris), develop in peanut fields and then move on to feed on fruit in an adjacent cotton crop. We wanted to know whether strategic placement, in time and space, of a milkweed habitat between crops could lead to successfully increasing biocontrol of stink bugs in these agricultural settings.
In our experiments, we aimed to: (1) document feeding of stink bug parasites on milkweed nectar in the field; and (2) determine the impact of strategic placement of milkweed nectar provision on parasitism rates of stink bugs in cotton.
In this two-year study, we established plots with 25 milkweed plants per plot, and control plots without milkweed, along the edges of neighboring peanut and cotton fields. Weekly, throughout the growing season, we observed and recorded insects visiting flowering milkweed plants. We sampled cotton for stink bugs each week during the five-week period these pests colonized the crop.
Over both years of the study, parasites and predators that attack the stink bugs (including both eggs and adults) readily fed on milkweed nectar in these insecticide-free habitats, as did insect pollinators, including honey bees, bumble bees, carpenter bees, leafcutter bees, and hover flies. For the first year of the study, parasitism of southern green stink bug adults by the stink bug parasitoid fly Trichopoda pennipes was close to five times higher in cotton plots with nearby milkweed habitat than in the control plots. In the second year of the study, combined parasitism of southern green stink bug, green stink bug, and leaf-footed bug (Leptoglossus phyllopus) adults by this parasite was at least three times greater in plots with nearby milkweed habitat than in control plots.
In only one year, there was indication that this management strategy alone can help maintain stink bugs below the economic damage threshold for this crop. Complementary management strategies, however, such as use of selective insecticides and trap cropping, may also need to be incorporated to suppress pests below economic thresholds throughout the growing season.
In conclusion, provision of a milkweed insectary habitat between peanut and cotton fields aided beneficial insect conservation and increased the rate of adult stink bug parasitism in cotton.
Milkweed plants are very attractive to a wide range of beneficial insects, including solitary predatory wasps.
— Dr. Glynn Tillman, Crop Protection & Management Research Laboratory, U.S. Department of Agriculture Agricultural Research Service