Encouraging beneficial insects in the garden

What makes an insect a pest? That may depend on the situation, not just the insect species. Termites in the forest break down wood and return it to the soil, where it becomes available to sustain new plants. This is a beneficial process in the forest. Termites eating wood in a structure, however, are pests. If untreated, the damage could become a safety issue.

The benefits some insects provide vary with species and life stage. Insects might be helpful in one life stage but not another. For instance, butterflies are considered beneficial pollinators, but their larvae (caterpillars) eat plants. By contrast, hover flies are predators while young (in the larval stage), but pollinators as adult flies.

Adult beneficial insects often look completely different from the form they have when young. Learning to identify the different species of beneficial insects and their appearance in various life stages informs pest management decision-making.

Beneficial insects and their roles in the garden

Each type of beneficial insect has distinct food and shelter needs and behaviors. In nature, each insect species fills a niche — a specific role and position that it occupies in the landscape — including its interaction with available resources. The keys to welcoming beneficial insects into the garden are the variety of plant types, flower shapes and blooming times. We call this diversity in the garden. A diverse garden has many ecological niches that different insects can fill.

Insects that help us with pest control by eating or parasitizing pests can greatly reduce populations of pest insects. It’s rare for beneficial insects to completely eliminate a pest population. Instead, beneficial insects help create a balance that usually results in much less damage to plants.

Gardeners hoping to support beneficial insects must tolerate a few pest insects in the garden. Some predatory insect species can survive on pollen and nectar in the absence of pests, but most need a few pests around for continued survival.

In this document, we’ll examine the types of beneficial insects and their helpful behaviors. We’ll also cover what gardeners can do to encourage beneficial insects.

For simplicity, we group beneficial insects into categories: pollinators, predators, parasitoids and decomposers. We list plants that can help you welcome specific insect types, as well as resources that add more detail. Plant a variety of plant and flower types so there are blooms throughout the season. This supports the widest variety of beneficial insects in our garden systems.

Insect life stages

Insects go through different life stages. Young insects molt by shedding their exoskeleton, a hardened outer covering, like skin (think of a shrimp or crab). Most insects molt several times as they mature. Some insects have immature, larval stages that resemble the adult, which is considered incomplete metamorphosis (Illustration 2).

An example of a beneficial insect with incomplete metamorphosis is the minute pirate bug. Overall, minute pirate bugs have a similar shape through each developmental stage, though sometimes with different coloring. During the last juvenile stage, minute pirate bugs have “wing buds,” which are the beginning of wing development. During the final life stage — adulthood — the wings are fully developed. Only adult insects have wings. See the photo gallery for pictures of predatory insects in different life stages.

Insects with larval stages that look quite different from the adult have complete metamorphosis. Complete metamorphosis includes a pupal stage. Butterflies are an example of a beneficial insect that has complete metamorphosis. Insects with complete metamorphosis often have different feeding requirements — even different mouthparts — in the juvenile and adult stages. This is an important consideration when evaluating an insect’s function in your garden. For example, fly larvae often have different mouthparts than adults (Illustration 2).

Insects that have complete metamorphosis often have different diets in the adult and larval stages. Butterflies feed on host plant foliage during the larval (caterpillar) stage. Most caterpillars rely on a specific group of plants as host (food) plants. Female butterflies will seek host plants on which to lay eggs. For example, monarch caterpillars can only survive on milkweed, whereas most swallowtail caterpillars need plants from the carrot family (called Apiaceae).

Once the caterpillars become adult butterflies, food requirements switch to flower nectar. Hover fly larvae are predators of pest insects such as aphids, but feed on pollen and nectar as adults.

Likewise, some insect predators or parasites prefer prey in specific life stages. Yellowjacket wasps seek out caterpillars for feeding their young, but do not eat moths or butterflies in the adult stage. Recognizing the different life stages of both beneficial and pest insects is important — both for supporting beneficial insects and for managing pests in the landscape and garden.

Beneficial insects in the garden can be loosely housed in four groups: pollinators, predators, parasitoids and decomposers. Each group serves a different role in the garden. They also tend to occupy very different spaces in the garden.

Becoming familiar with these insect groups and their roles in the garden is worth the effort. It’s possible to deduce from insect behaviors and physical features the beneficial category to which they belong.

Pollinators

Insect pollinators include many more species of insects than just honey bees. Native bees are also important pollinators, in addition to many species of beetles, butterflies, moths and flies. These insects provide ecosystem services by spreading the pollen among different plants of the same species, helping them complete their life cycles by producing seed.

However, pollination is not a goal of the insects. Pollination occurs as they move around the flowers, seeking nectar or pollen as food sources. Most insects that spread pollen when feeding often have hairs that hold onto and then transfer pollen among plants.

Pollination syndrome

Different pollinators are attracted to different types of plants. A wide variety of plant shapes, colors and fragrances will attract a wide variety of pollinators. For example, bees are most attracted to blue, yellow and white flowers, while butterflies prefer brightly colored flowers with nectar (Table 1).

Insect ‘stratification’

In addition to preferring a specific type of flower, each type of beneficial insect may occupy a specific niche in both time and space in the garden. In other words, a niche for each insect type can be provided through careful plant selection. For instance, some native bees may be present in the garden only early or late in the growing season, while other species may be found from spring through fall. Long-tongued bees can feed from bell- and trumpet-shaped flowers, while short-tongued bees need flat flowers like daisies. Butterflies are active during the day, while most moths access flowers at night.

There is a beneficial insect for every flower type, time of day and part of the growing season — not all are present in the garden at the same time. The concept of different animals and plants in layers — both in time and in physical space — is sometimes referred to as stratification. Insects and flowers occur in various layers of the landscape, at different times of the day and at different times of the year. For more details on supporting pollinators, including plant selections and planting plans, see Enhancing urban and suburban landscapes to protect pollinators, EM 9289.

Ground beetles

Ground beetles include multiple species of predatory beetles that actively hunt insect prey on or near the ground. They can vary in size and color; however, adults of the most common species are usually black with long legs and parallel grooves in the outer wings that resemble stripes.

Ground beetles generally have visible jaws (mandibles) that they use for grasping their prey.

Ground beetles can be seen scurrying along the ground if disturbed. They prefer to remain hidden during the day and search for food at night.

Both adults and larvae are predators and feed on other insects or even slugs. Juvenile insects undergo complete metamorphosis and have little resemblance to adults.

Lacewings

The lacewing is another insect that undergoes complete metamorphosis — lacewing larvae have no resemblance to adults. Larvae are generally green or brown, with soft, alligator-shaped bodies. Adults may be green, brown or golden.

Lacewings have large, clear wings with many veins that are held in an upright position, tented over the body. Female adult lacewings need nectar as part of their diet to lay eggs. Nectar-producing flowers may encourage these beneficial insects in the garden.

Lacewings are associated with many vegetable crops, especially those commonly infested by aphids.

Lady beetles

Lady beetles, or ladybugs as they are commonly known, are generalist predators, meaning they eat many kinds of prey. Both the adults and larvae are predatory.

Lady beetle larvae look quite different than adults: Larvae are soft-bodied and elongated, looking like mini-alligators (see Photo 10). The colors of the adults and larvae are usually red to orange and black, but adults of some species may be beige or yellow.

Lady beetles are most effective as aphid predators, especially when aphid populations are high. Depending on the species, an individual immature or adult female ladybug may eat several hundred aphids per day. Lady beetles will feed on insect eggs and other small insects when aphids are not present. They also need pollen in their diet.

Mantids

Young and adult forms of this large predator are easy to recognize. Mantids demonstrate a preference for large insects and don’t distinguish between pests and other beneficial insects.

While easy to identify, mantids are less likely to prey on common home garden pests like aphids, mites, thrips and whitefly. They prefer larger prey.

Minute pirate bugs

Among the smallest of the “true bugs,” minute pirate bugs can barely be seen without magnification. These insects specialize in feeding on small insects like thrips and insect eggs. They also feed on spider mites.

Like big-eyed bugs, minute pirate bugs can switch to plant material when pests and eggs are not present. This characteristic makes these insects more susceptible to systemic insecticides. Avoid or delay using systemic pesticides when these predators are present in the garden system.

Predatory mites

Predatory mites can be as small as 1/50 inch, and are teardrop- or pear-shaped with three to four pairs of legs, depending upon the life stage. They can vary in color from cream to green to red and are often smaller and faster-moving than the other mites and insects they feed upon.

All four life stages of mites feed on small arthropods, including fungus gnat larvae, as well as thrips and mites, such as spider, bulb and rust mites.

Predatory mites can be found naturally in farms and gardens and can also be purchased for use in humid spaces such as greenhouses.

Sphecid and vespid wasps

These insect groups include hornets, paper wasps and yellowjackets. While often considered pests due to the painful stings and aggressive nature of some species, wasps are also important predators in the garden. Adults are known for attacking caterpillars and taking them back to the nest to feed larvae; however, some species will attack any insect available, including those that we consider beneficial. Larvae feed on insects provided by the adult female or parasitize other insects, depending on the species. See Vegetarians, parasitoids and predators: Lesser-known wasps of Oregon, EM 9426.

Noninsect predators in the garden

In addition to insects and predatory mites, a variety of other insect-feeders may be found in the home garden. These include bats, birds, spiders and centipedes.

While in-depth discussion of these creatures is outside the scope of this publication, it is worth noting that many of the same practices used for encouraging insects in the garden apply to noninsect predators. Diverse plantings and appropriate habitat can encourage their presence in the garden. Providing appropriate habitat may be as simple as leaving leaf litter at ground level among plants instead of maintaining an overly tidy garden space.

Spiders and predatory mites may be susceptible to some pesticides. Whenever possible, avoid broad-spectrum pesticides that kill a wide range of species.

Parasitoid flies

Bombyliidae

Commonly known as “bee flies,” bombyliidae flies are bee mimics (Table 4). The adults use their proboscis (sucking mouthparts) to feed on nectar and are often seen hovering near flowers. Bee flies can be distinguished from hover flies by their hairy bodies and long proboscises.

The larvae of bee flies parasitize the ground-dwelling larval stage of various species of wasps, beetles and flies (Photo 8).

Tachinid flies

The Tachinids are a large family of flies that includes numerous parasites, with approximately 8,200 species worldwide. They vary in size and appearance; however, many are large (up to 2/3 inch) and look similar to house flies. Tachinid flies lay oval, whitish eggs on their prey. These eggs can often be seen on the surface of their hosts, which include caterpillars, sawflies, beetles or true bugs such as stink bugs.

Most parasitoid species attack only a single host species with which they have evolved closely. For example, an aphid parasitoid will not parasitize a caterpillar. Generally, most native insects have at least one parasitoid or predator that helps keep their populations in check.

Unfortunately, many invasive pests, which originate from other countries or regions, may not have parasites or predators present in the invaded area. This lack of natural enemies is a major reason why invasive insects are so problematic: The predators and parasites meant to help reduce pest populations are not part of the equation.

Invasive insects are typically not problematic pests in their native regions, where parasitoids and predatory insects exist. Outside of their native regions, some insects build up huge populations that severely impact crops and landscapes.

Encouraging beneficial insects

Plant selection

Garden spaces well-suited for beneficial insects have a diversity of flower types, shapes and colors. Diverse flowers provide pollen and nectar for a broader range of insects.

Bee species are adapted to flowers of specific sizes and shapes. Bees with long tongues utilize deeper-throated flowers such as penstemons, while smaller bees with shorter tongues visit easily accessible, smaller blooms such as aster and goldenrod. Small insects, such as parasitoid wasps, need small flowers to forage upon.

Plants that bloom at different times are necessary so beneficial insects will have pollen and nectar sources throughout the growing season. Insects such as mason bees and hover flies are prominent early in the season, while insects such as the long-horned bee are commonly seen later, in summer through fall. Bumble bees are active all season and are one of the earliest pollinators seen in spring and among the latest in fall.

Having multiple flower shapes and sizes in bloom from spring to fall will provide food to all these insects.
In addition, provide host plants for all stages of beneficial insect development. Many insects, especially those that undergo complete metamorphosis, have different feeding requirements for immature and adult life stages. Provide food sources for multiple life stages if possible. Consider growing host plants for caterpillars to feed on, as well as flowers that give nectar for adult butterflies.

When choosing plants for the garden or patio, consider which plants will provide nectar and pollen. Plants bred for ornamental purposes may not be ideal food sources. For example, breeders may be interested in increasing petal number to enlarge the bloom size, which adds a “pop of color” for consumers. However, this can reduce the amount or accessibility of the pollen and nectar. Single-flowered varieties are best for insects.

Common plant families and the insects they attract

Purchasing beneficial insects

Some people purchase beneficial insects for pest control. This tactic is most likely to succeed in a greenhouse or high tunnel. Success is influenced by greenhouse temperature, degree of infestation, life cycle of the pest, and the appropriate ratio of beneficial insects for the job. Research is needed to match the situation with the best choice of beneficial insect.

Insect predators and parasites can be purchased either online or at a local garden center. Insects released into an open landscape or home garden often scatter rather than stay where needed to control pests. Lady beetles are commonly available in garden centers. These are collected in the wild in winter, then refrigerated. Due to their biology, these lady beetles typically disperse when released from their packaging.

Commercial insects may require specific temperature, humidity and prey conditions to become established. Without these appropriate conditions, it’s challenging and expensive to establish beneficial insect populations via release. Purchased insects may help the home gardener in the event of a severe outbreak, but long-term establishment of beneficial insects is likely best achieved by providing an inviting habitat.

In home gardens and landscapes, encourage predators and parasites by providing diverse resources. Beneficial insects need habitat, food sources for all life stages and water in an area without harmful pesticides. When these needs are met, beneficial insects are likely to be present. Use the strategies described in “Habitat.”

Pesticide use

Generally, pesticide use does not support beneficial insects in the garden. Even organic pesticides can harm beneficial insects when not used properly. A few holes in leaves are expected; do not automatically reach for a pesticide. Instead, check if the damage is affecting a harvestable part of the plant.

Plants can tolerate feeding damage in some situations. For example, beetles feeding on potato leaves early in the season will prevent proper tuber development. Later in the season, a few beetles feeding on leaves are less of a problem because most tubers have already developed.

Inspect plants regularly to identify and estimate the abundance of pests. Often, predators and parasitoids will establish themselves and manage pest populations. Beneficial insects may not be present at the beginning of a pest outbreak. Be patient and allow time for these insects to arrive, reproduce and reduce pests. Going straight to a pesticide disrupts this natural process. Regular scouting helps determine the number of pests relative to beneficial insects.

Unfortunately, invasive insects originally from other parts of the world may not be well managed by naturally occurring enemies. In this situation, physical insect removal or a change in practices may be helpful. Pesticides are usually a last resort. Contact your local Extension office for insect identification or assistance with developing a least-toxic management strategy.

Beneficial insects are susceptible to pesticides, just like pests. Consider beneficial insects when deciding which pesticide to use. If a pesticide is needed, choose the least-toxic, yet effective pesticide for your situation. Avoid spraying blooming plants — pollinators can be affected when they visit flowers.

For detailed information about protecting beneficial insects when pesticides are needed, see How to reduce bee poisoning from pesticides, PNW 591.

Pesticides have different active ingredients or combinations of ingredients with varying formulations. Some products are less toxic and safer for beneficial insects than others. Insecticides can be broad-spectrum, meaning they kill a wide range of insects, or narrow-spectrum, meaning they are specific to particular kinds of insects. When possible, choose a narrow-spectrum product selected for the target pest. You need a proper insect identification to use these products, since they need to be matched to the specific pest. Your local Extension office can help you with pest identification.

Read insecticide labels to see which pests the product treats. Those with large lists of pests killed or statements such as “complete insect killer” indicate a broad-spectrum product. Some plant-derived products, such as neem, are broad-spectrum but are broken down quickly by the environment. These products are safer for beneficial insects because they are active for a shorter period.

Systemic insecticides, such as imidacloprid and other neonicotinoids, persist within a plant. The plant absorbs these products into its tissues. Depending on the formulation, the pesticide can remain active for a few weeks to a full year. An insect feeding on any part of a plant treated with systemic insecticides will be negatively affected. The insecticide can also be present in a plant’s nectar and pollen, where it can kill feeding beneficial insects. Avoid these products when gardening for beneficial insects. Whenever using pesticides, remember to follow the label exactly; the label is the law.

Putting it all together

Growing a garden that supports beneficial insects is not only rewarding but also beautifies your yard. The variety of plant types will provide habitat for insects, as well as other wildlife. Encouraging beneficial insects can improve pollination in the vegetable garden and home orchard, increasing food harvest.

In summary, when gardening for beneficial insects, we recommend the following practices:

• Plant a diversity of flowers. This includes plants with different bloom times, flower colors, sizes and shapes.
• Consider planting native plants and using plants with “simple blooms” rather than those bred for high petal count.
• Allow your garden space to be a little “messy,” creating habitat for beneficial insects.
• Be familiar with the identities and characteristics of common beneficial insects.
• Accept some insect feeding damage.
• Allow predatory and parasitic insects time to manage pests before reaching for pesticides.
• Reduce pesticide use and use the least toxic, yet effective, product for your pest.

Resources

The content of this document is intended to provide actionable ideas and information for use in the home garden and landscape. In recent years, appreciation for the value of pollinators and other beneficial insects has greatly expanded. This has resulted in a multitude of resources for gardeners seeking to encourage beneficial insects. Some of these additional resources are listed below.

Pollinators

• Mader, E., Smith, M., Vaughan, M., and The Xerces Society. 2011. Attracting Native Pollinators: Protecting North America's Bees and Butterflies: The Xerces Society Guide. Storey Publishing.
• Creating pollinator habitat, Colorado State University Extension.
• Enhancing urban and suburban landscapes to protect pollinators, EM 9289.
• Native plant picks for bees: 10 species you can grow to support wild bees in Oregon, EM 9363.
• Native Plants for Pollinators and Beneficial Insects: Maritime Northwest Region, The Xerces Society.
• Oregon Bee Project. Numerous references and publications are featured in the Resources section.
• Mader, E., Fowler, J., Vento, J., Hopwood J. The Xerces Society. 2016. 100 Plants to Feed the Bees: Provide a Healthy Habitat to Help Pollinators Thrive: The Xerces Society Guide. Storey Publishing.
• Pollinator Conservation Resources, Pacific Northwest Region, The Xerces Society.
• Shrubs and trees for bees: Attract a diversity of bees with plants native to Oregon, EM 9391.
• Wilson, J.S., and Carril, O.M. 2016. The Bees in Your Backyard: A Guide to North America’s Bees. Princeton University Press.
• Pyle, R.M. 2002. The Butterflies of Cascadia: A Field Guide to all the Species of Washinton, Oregon and Surrounding Territories. Seattle Audubon Society.

Beneficial insects

• Attracting Beneficial Insects, Clemson Cooperative Extension.
• Beneficial Insects of Utah, Utah State University Extension.
• How to reduce bee poisoning from pesticides, PNW 591
• Flint, M.L., and S.H. Dreistadt. 1998. Natural Enemies Handbook: The Illustrated Guide to Biological Pest Control. UC ANR Publication 3386, Oakland, California.