Attracting Beneficial Insects: What the Research Shows

Planting flowers for beneficial insects does help, but less dramatically than garden marketing suggests. A synthesis of field studies found flower strips increased pest control in neighboring crops by 16% on average. The bigger lever for most home gardens is simpler: stop using broad-spectrum sprays that kill the predators and parasitoids already there, then add a long season of small, open flowers.

Who the beneficial insects are

Oregon State University's PNW 550 groups garden beneficials into pollinators, predators, parasitoids, and decomposers. For pest control, the predators and parasitoids matter most:

• Predators such as lady beetles, lacewings, hover fly (syrphid) larvae, ground beetles, and assassin bugs eat pests directly.
• Parasitoids are mostly tiny wasps and flies that lay eggs in or on a host insect, which the developing larva eventually kills.

Many of these insects eat pests only as larvae. The adults of hover flies and parasitoid wasps feed on nectar and pollen. That is the whole logic of planting flowers: feed the adults so they live longer, lay more eggs, and stay near your crops.

What the evidence shows

Flower strips help, modestly

The best summary evidence comes from a 2020 synthesis in Ecology Letters by Albrecht and colleagues, pooling 18 studies on pest control and 17 on pollination across North America, Europe, and New Zealand. Flower strips, but not hedgerows, enhanced pest control in adjacent fields by 16% on average. Effects on pollination and crop yield were more variable. Older, more diverse perennial plantings did better for pollination than young or annual ones.

Those studies were on farms, not backyards. A home garden is small enough that flowers and vegetables are usually close together, which should work in your favor, but there is little direct research at garden scale.

Plant diversity reduces pests, with a cost

A review of 552 experiments by Letourneau et al. (2011) found that diversified cropping systems had more natural enemies, fewer herbivores, and less crop damage than simpler ones. It also found a small but statistically significant drop in crop yield; the authors concluded that pest-suppressive diversification "interfered with production." In a garden, that means devoting some space to flowers rather than squeezing them between every vegetable.

Sweet alyssum in lettuce

The best-known applied example is sweet alyssum in California organic lettuce. UC IPM notes that many organic lettuce growers intercrop with quick-flowering annuals, including sweet alyssum, coriander, and Persian clover, to feed adult syrphid flies whose larvae eat aphids. Research by Brennan (2013) examined replacing 2 to 8% of lettuce transplants with alyssum and found intercropped lettuce heads were slightly smaller, a real trade-off even in a system that works.

UC IPM also flags a catch: do not use sweet alyssum when Bagrada bug numbers are rising, because it is one of that pest's favorite hosts. Insectary plants can feed pests too.

Which plants to use

Most natural enemies are small, with short mouthparts. They do best on flowers with small, shallow, exposed nectar. OSU recommends simple blooms rather than double flowers bred for extra petals, and names three families as especially useful: the carrot family (Apiaceae), the aster family (Asteraceae), and the mint family (Lamiaceae).

Native perennials deserve a place. Fiedler and Landis (2007) at Michigan State compared 43 native perennials with five commonly recommended non-native annuals (sweet alyssum, borage, coriander, dill, and buckwheat). In the first year the annuals attracted more natural enemies. By the second year, many established natives attracted more, and they bloomed across a longer season, from May into October.

The Michigan list is regional. Plants native to your area are a reasonable proxy, but no study tells you exactly which species will perform best in your yard.

The step that matters most: stop killing them

Flowers do not help if the insects they attract are poisoned. UC IPM advises avoiding nonselective, persistent insecticides, making spot treatments rather than blanket sprays, and never applying certain pesticides to flowering plants, including weeds. It warns that systemic insecticides in blooming plants can persist until the next growing season.

OSU's guide is blunter: "Generally, pesticide use does not support beneficial insects in the garden." That applies to organic-approved products too. The Xerces Society rates spinosad as highly toxic to bees and other beneficial insects, and pyrethrins as moderately to highly toxic to beneficials other than bees. See is neem oil safe for bees? for how even milder products fit in.

You also have to tolerate some pests. Predators follow prey. OSU notes that gardeners who want beneficial insects "must tolerate a few pest insects in the garden." A garden sprayed clean of aphids has nothing for lady beetle larvae to eat.

A practical setup

1. Dedicate a strip or border to flowers, rather than scattering a few plants.
2. Choose several species so something is blooming from spring through fall.
3. Mix a few quick annuals for year one with native perennials that take over later.
4. Favor small, open flowers in the carrot, aster, and mint families.
5. Leave some undisturbed ground, mulch, or plant debris for shelter. Ground beetles and many other predators need it.
6. Drop broad-spectrum sprays. Spot-treat only when damage passes what you can accept.

Buying beneficials is not a shortcut. Store-bought convergent lady beetles mostly leave within days; see do store-bought ladybugs work?

Related

Habitat is one layer of No-Spray Pest and Disease Management. It works best alongside exclusion and resistant varieties. For how flowers and vegetables interact in mixed plantings, see companion planting: what has evidence and what doesn't.