Importation and augmentation biological controls both focus on releasing natural enemies into the landscape. Importation involves the release of nonnative natural enemies in order to reduce populations of invasive pests accidentally introduced into the United States. As gardeners, we are not going to perform these importation biological control releases ourselves, but we benefit from successful programs that reduce damage to garden and landscape plants. We can perform our own augmentation biological control releases, where naturally occurring predators or parasitoids can be purchased and introduced into the garden to combat pest infestations.
Home Biological Controls
When thinking about applying biological control practices in a home landscape, however, conservation biological control is the place to start. Conservation biological control involves building up existing natural enemy communities by creating a suitable habitat that provides all the resources they need to thrive. The application of these conservation practices can mitigate the need to take further action, whether that is the release of biological control agents or the use of chemicals to control insect pests.
Also consider other nonchemical control methods that can be used with biological control to manage pests. This can be as simple as removing the pests by hand. For small gardens, dropping pests into a bucket of soapy water works well for immobile or slow-moving pests such as pest eggs and many larvae. For other very mobile and damaging pests, such as striped and spotted cucumber beetles (Chrysomelidae: Acalymma vittatum and Diabrotica undecimpunctata), try covering the plants with row covers to prevent the beetles from accessing their food source.
These covers can be placed over the crop and may be used with or without hoops (when used without hoops, they are called floating row covers). For crops that require bee pollination, however, you will need to remove the cover when the plant is flowering. This gives the plant a pest-free window prior to flowering, with biological control coming into play after the row cover is removed.
Importation Biological Control
Importation biological control targets species that have been accidentally introduced into the United States. These pests are referred to as invasive, meaning a non-native species that causes significant harm to our environment. Examples of such pests include the emerald ash borer (Buprestidae: Agrilus planipennis), which has decimated our native ash trees, and the red imported fire ant (Formicidae: Solenopsis invicta), which have displaced native ants and, when encountered in home landscapes, will sting people and pets. These invasive pests arrive in many ways, including on infested nursery stock or in the surrounding soil, on imported fruits and vegetables, or, as is the case with wood-boring insects, within pallets of untreated lumber or furniture. And, like many invasive plants, some have been intentionally released.
Aquatic invaders have also been known to hitch a ride within the ballast water of ships. The vast majority of these invaders are detected when the contaminated shipments arrive and are inspected at a United States port of entry. Unfortunately, however, the small number of invasive species that make it past this process can cause considerable damage to the balance within existing ecosystems.
When an invasive species establishes itself in the United States, government agencies and university researchers attempt to eradicate it when populations are small and this type of intensive effort can be successful. But if eradication fails and the species becomes abundant over a large area, goals shift to managing the population at as low of a level as possible. Importation biological control is one method that can be used to reduce the abundance of an invader. Typically, invasive organisms are not nearly as abundant and damaging in their native range due to suppression by natural enemies. When they are released into a new area, however, where their natural enemies are not around to keep them under control, they can cause devastating damage.
How It Works
In importation biological control, researchers travel to the home range of the invader and search for natural enemies of the invasive pest. Any natural enemies found during these explorations are brought to the United States, and colonies are established within a quarantine facility. In this facility, years of research will be conducted to evaluate the collected natural enemies for possible release. This is an intensive process because it is critical to determine if the natural enemy will attack other native species that may or may not be closely related to the invasive pest.
Only natural enemies found to be host specific– meaning that they only consume the target invasive pest–will be considered for release. For this reason, the natural enemies of arthropod pests considered for release as part of an importation biological control program are nearly always parasitoids. While predators may be effective as well, they are far more likely to be generalists that will attack a broad range of arthropods, including native species. If, after intensive testing, a biological control agent is approved by the U.S. Department of Agriculture for release, the natural enemy will be reared in quarantine to obtain enough individuals to release several small populations. Follow-up studies are then done to determine if these populations establish and are able to effectively control the target pest.
The release of biological control agents was not always evaluated as carefully and extensively as it is today. These protocols were developed to prevent the introduction of non-native natural enemies that consume native species in addition to the target invasive pest, as has happened following some previous releases. These negative effects, called non-target impacts, can also include competition between native species and released natural enemies for food or other resources. Due to stricter regulations, the number of importation biological control releases has decreased over time, but the proportion of species released that have a significant impact on their target invasive pest has remained relatively consistent.
Augmentation Biological Control
This is the type of biological control that is most familiar to gardeners. Augmentation biological control involves the periodic release of natural enemies that already occur naturally in your region by individuals, including gardeners and farmers. The idea behind this pest management strategy is that in some cases, existing natural enemy communities are not sufficient to control garden pests; perhaps they have been negatively affected by practices such as tillage or pesticide application, or simply have not yet colonized a garden in sufficient number. With augmentation, the goal is to supplement the existing population with enough natural enemies to enhance pest control.
Types of Releases
There are two types of augmentative releases: inundation and inoculation. Gardeners may conduct inundation releases when they have a significant pest problem and are releasing a large number of natural enemies to suppress the pest outbreak. This approach garners fast results by quickly overwhelming the pest population, but this large number of natural enemies is likely to die off or leave the area after exhausting all sources of food. So inundation can be effective when pest populations are very high, but if the pest population continues to grow or reestablishes later, additional releases may be necessary.
In an inoculation release, a small number of natural enemies are released with the idea that these individuals will establish within the garden and reproduce. Some control will be provided by the released individuals, but the real benefits will come from subsequent generations. To establish a natural enemy population that will survive and flourish in a home garden, it is critical to consider how you manage your garden habitat to support these arthropods. See Conservation Biological Control on page 23 for tips on how to make your garden more inviting to beneficial insects.
When examining your options for augmentative biological control releases, first identify your target pest. If you are not sure what type of insect you are dealing with, take a sample of the damaged plant material and the pest itself to a nearby university extension office or reputable local garden store for identification. Some of the most common insects controlled through augmentative releases include aphids (Aphididae), scales (Coccoidea), mealybugs (Pseudococcidae), spider mites (Tetranychidae), thrips (Thripidae), and caterpillars (Lepidoptera).
Next, determine if purchasing and releasing natural enemies is worth the investment. To do this, it is important to monitor your plants frequently. If you notice that an insect pest is beginning to multiply, look for evidence of natural enemies at work. Do you see predators foraging on the infested plant? For pests that do not move far, such as aphids (Aphididae) or caterpillars (Lepidoptera), do you see fewer pests present over a few-day period? Or perhaps there is evidence of parasitism, such as some aphids within a patch turning to brown or black aphid mummies? If so, releasing natural enemies may not be necessary.
If you do not see evidence of naturally occurring biological control, you can consider augmenting the population by purchasing natural enemies. Typically, gardeners purchase natural enemies from a garden center, home improvement store, or from a biological control supply company found online. A wide range of species are commercially available, including parasitoids and predators.
Parasitoids for Augmentation Releases
If releasing a parasitoid, it is critical to purchase the correct species that will attack your pest. Most parasitoids are very selective about the hosts they will attack. A reputable biological control company will clearly state the parasitoid’s host range, lifecycle information, recommended release rates, and instructions for the release, with tips on how to evaluate the effectiveness of the natural enemies.
Before purchasing parasitoids, check the extended weather forecast in your area to make sure rain or extreme heat is not expected. Most companies advise against holding onto your shipments for more than a couple of days, so you want to be sure conditions will be favorable for release before purchasing the insects. Parasitoids are often shipped in the pupal stage, but some may have already emerged as adults. For best results, make sure to read and follow all the provided release instructions carefully. Your pupae may arrive on cards that are specifically designed to be attached to plants, or they may be sent in a bottle mixed with a carrier such as woodchips, seed husks, or vermiculite.
Predators for Augmentation Releases
Many generalist predators, including the spined soldier bug (Pentatomidae: Podisus maculiventris), minute pirate bugs (Anthocoridae: Orius), green lacewings (Chrysopidae), lady beetles (Coccinellidae), hover flies (Syrphidae), and predatory mites (Acari), are available commercially. Predators may be shipped as eggs, nymphs or larvae, pupae, or adults. Depending on the natural enemy, one or more life stages will be available. When deciding what life stage to order, consider whether both adults and immatures (nymphs or larvae) feed on prey. If only the immature stage is a meat eater and you release adults, you will have a delay before immatures from the next generation begin feeding.
This isn’t necessarily a bad thing if you are trying to build up a population, but it is something to consider if the pest infestation is significant and plant damage or loss is already occurring. Also, adults and immatures might differ in their ability to disperse. Immature arthropods lack functional wings, so they are more likely to stay and feed where you release them, whereas adults may or may not be winged.
Predators may be sold as eggs on cards or as immatures, pupae, or adults in containers. Sometimes adults might be shipped with a sugar-water solution or other food source, or, like parasitoid pupae, they may be sent in carrier such as vermiculite, seed husks, or woodchips. Make sure to read and follow the directions for release carefully to know how and when to introduce your predators to the garden.
Conservation Biological Control: Designing a Natural Enemy-Friendly Landscape
Gardening practices such as planting, mulching, weeding, pesticide application, and fruit and vegetable harvesting can disturb natural enemy communities and affect the amount of biological control they can provide in a home landscape. Conservation biological control focuses on minimizing the impacts of these practices by adding the resources natural enemies need to thrive locally.
Establishing Insectary Plantings
Insectary plantings are a great way to support a diverse and abundant natural enemy population within a home landscape. Insectary plantings are groupings of annual and/or perennial plants that sustain and enhance populations of beneficial arthropods–both natural enemies and pollinators–by providing resources such as pollen and nectar, alternative prey, winter cover, and habitat for nesting and retreats.
Nearly all gardeners know that bees visit flowers to feed on and collect protein-rich pollen and sugar-rich nectar, but few recognize the importance of these resources for predators and parasitoids. Many groups of natural enemies feed on pollen and nectar, including many beetles (Coleoptera), true bugs (Hemiptera), lacewings (Neuroptera), predatory wasps (Hymenoptera), parasitoids (Diptera and Hymenoptera), and even spiders (Araneae). Access to pollen and nectar increases their longevity and egg production, improving the ability of a natural enemy community to provide biological control. For some, access to these resources is essential– parasitoid wasps, for example, cannot survive and reproduce without access to flowering plants.
Not only do natural enemies benefit from the foods plants naturally produce, but also from the herbivores found feeding on them. Many times, the herbivores that feed on flowering garden plants are not the same pests that can build up in large numbers within fruit and vegetable gardens. When gardeners provide a diversity of flowering plants, natural enemies are able to reproduce and multiply within a home landscape by feeding on these arthropods and are then present to begin feeding on pests that may arrive.
When designing an insectary habitat in a home landscape, the goal is to provide both the food and shelter resources natural enemies need to survive year-round. Natural enemies require floral resources throughout the growing season, so selecting several different flowering plants to create a consistent supply of blooms across the season is key.
It’s especially important to look for early spring and late fall blooming species because it is at these times of the year that resources tend to dwindle in home landscapes. Also, since many natural enemies do not have the specialized mouthparts that allow some bees and butterflies to gather nectar and pollen from deep flowers, make sure to include some with open, shallow flowers for easier access to nectar and pollen and/or extrafloral nectaries. Extrafloral nectaries are glands found on leaf surfaces and margins, petioles, leaf and flower bracts, and sepals that provide natural enemies with sugar, water, and amino acids.
Insectary habitats also can provide areas for natural enemies to seek refuge from the sun, hide from predators, and find cover in which to spend the winter. By offering plants that vary in height and growth form, and by allowing the dead vegetation to remain throughout winter, you increase your chances of attracting biological control insects to your garden.
So, what plants create the most effective insectary? Many studies have focused on the value of annual plants for conservation biological control because they are often prolific pollen and nectar producers, generally have low seed or plant cost, and can be incorporated directly within fruit and vegetable crop beds each spring. Several annuals have been identified as highly attractive to beneficial arthropods, including borage (Borago officinalis), buckwheat (Fagopyrum esculentum), coriander or cilantro (Coriandrum sativum), dill (Anethum graveolens), fava bean (Vicia faba), phacelia (Phacelia tanacetifolia), and sweet alyssum (Lobularia maritima).
Annual plants work well in containers, as mobile insectaries that can be moved throughout a home landscape. Annuals also can be planted in rows between vegetable and fruit crops or as borders surrounding food gardens. Sweet alyssum is a great choice to add nectar and pollen resources directly within vegetable gardens because it is relatively low growing and blooms throughout the growing season.
The presence of extrafloral nectaries–glands found on leaf surfaces and margins, petioles, leaf and flower bracts, and sepals that provide nourishment to natural enemies–is another plant feature to consider when designing your insectary. Plants with these nectaries produce nectar not only in flowers, but also on other plant parts. Many species including ants (Formicidae), lady beetles (Coccinellidae), predatory and parasitoid wasps (Hymenoptera), and spiders (Araneae) are known to feed from extrafloral nectaries. More than 100 plant families contain species that have extrafloral nectaries, including many found in home landscapes.
You may already be growing some of these crop and ornamental plants now. The addition of others can help to improve the abundance and diversity of natural enemies in your garden; be sure to select species that are suitable for your location and are not invasive.
Many gardeners are interested in incorporating native flora into their home landscape, and these perennials are excellent additions to an insectary planting. Perennials offer pollen and nectar resources, alternative prey, and cover for natural enemies to overwinter if they are not cut back in the fall. Native plants are also adapted to the conditions present in your region and often require less watering and fertilizer than non-natives. As an added side benefit, many of these plants have become rare or even endangered, so incorporating them into your home landscape improves native biodiversity in your area.
When selecting your plant mix, it is a good idea to talk with a local native plant producer to learn more about the native flora for your region and to determine the species that will do best. These plantings can include native trees, shrubs, and herbaceous plants.
There are excellent resources online that offer lists of plants known to be attractive to natural enemies and pollinators. Michigan State University researchers have developed a listing of recommended native plants to attract both bees and natural enemies that is suitable for use in the Great Lakes region (nativeplants.msu.edu), and The Xerces Society for Invertebrate Conservation has developed regional plant lists covering the United States to support pollinator populations (xerces.org). These plants were selected with bees in mind, but many will be highly attractive to natural enemies as well.
Lawns as a Habitat
Looking for other ways to create beneficial insect habitat? Lawns take up the greatest area of most residential landscapes, but typically the goal is to keep these weed free. Making peace with weeds saves on herbicide use and provides additional resources for beneficial insects. But if you are not ready to surrender in the battle against dandelions, there are also “no mow” grass seed mixes available that contain annual and perennial flowering plants such as clover, yarrow, sweet alyssum, thyme, poppies, and daisies.
Shelter from the Elements
Another way to attract natural enemies to your yard is to incorporate garden features with protected cracks and crevices in which beneficial insects can take shelter. For example, natural enemies will nestle under rocks and in woodpiles. Adding or maintaining these elements provides a habitat for predators and parasitoids during the growing season and protected space to spend the winter.
You can also add nesting boxes and retreats to a garden to provide protected spaces for natural enemies to nest and/or overwinter. Many are actually designed for bees, but they can also be colonized by solitary wasps. The bee nesting boxes available online or from garden centers are often designed to attract mason bees (Megachilidae: Osmia) and have 16 mm holes. To attract a greater diversity of bees and wasps, you can embark on a relatively easy DIY project. Take a 4 x 4 inch (10 x 10 cm) or 4 x 6 inch (10 x 15 cm) block of wood and drill holes of varying diameter into it (2.5 to 10 mm), with a depth of 3 to 5 inches (8 to 13 cm) for holes smaller than 6 mm, and 43⁄4 to 51⁄2 inches (12 to 14 cm) for larger holes (recommended by the Xerces Society for Invertebrate Conservation).
You can paint the outside of the house, add a roof, and mount the block to a post, fence, or shed where it will be secure and ideally catch the morning sun. Another option is to collect hollow stems from plants in your home landscape and place them together inside a structure. Paper straws are commercially available that will be readily colonized as well. You can leave these stems or straws outside in their structure during the winter, but you are likely to increase survivorship if you place them in a plastic container with a mesh vent and store them in a refrigerator or unheated building. In the early spring, place the plastic container in the garden and remove the mesh to allow emerging bees and wasps to leave.
Other types of overwintering retreats are also commercially available, including some that are marketed specifically for natural enemies, such as lady beetle (Coccinellidae) and lacewing (Chrysopidae) shelters. They are typically open boxes with small slits for the natural enemies to enter. The idea is that these shelters provide protection from the elements and potential predators of the natural enemies during the growing season as well as a protected place to overwinter. I personally have not tried any of these to see if arthropods will colonize them, but DIY instructions are readily available online, so it might be a fun rainy day project to make one and give it a try.
Adding mulch to the garden is a great conservation biological control strategy to enhance ground-dwelling predators such as wolf spiders (Lycosidae) and ground beetles (Carabidae). Spreading mulches, such as straw, bark, or woodchips, between plants adds moisture and offers natural enemies sun protection, enabling them to effectively move throughout the garden without drying out. Mulches may also support alternative prey for predators. For example, soil-dwelling mites can be enhanced with the addition of mulch. These mites are not garden pests and can serve as alternative prey for predatory mites, which can build up in mulched gardens and suppress spider mite populations.
Keep in mind, however, that mulches are not uniformly beneficial for arthropods. Some soil-nesting bees prefer bare soil, so leaving some spaces without mulch might encourage local nesting by these species within your garden.
In addition, if you are looking for a break from weeding, the presence of weeds can provide natural enemies with shelter, alternative prey, and in some cases, pollen and nectar resources. Other plants can also be established within gardens as living mulch. Plants such as hairy vetch (Vicia villosa), rye grass (Lolium), or alfalfa (Medicago sativa) can be seeded into a garden space in the fall. In the spring, strips can be tilled into this mulch, and crops may be established into the strips. The strips of mulch remaining between garden rows provide shelter and alternative prey for natural enemies.
Natural Enemy Food Sprays
Commercially available food sprays can be applied directly to plant foliage to attract and retain natural enemies. These products, which contain sugar and sometimes have added protein resources, can be an easy and effective way to provide food for natural enemies when prey populations are low. You can also make your own sugar solution by heating one pound (450 g) of sugar mixed with one gallon (3.8 L)of water and stirring until all the sugar is dissolved. The solution can then be cooled and applied with a hand-held pump sprayer.
Reprinted with permission from Good Garden Bugs by Mary M. Gardiner and published by Quarry Books, 2015.