Plant
Flowers to Encourage Beneficial Insects
Many natural enemy adults – particularly tiny
wasps and flies – visit flowering plants to obtain
nectar and pollen. By providing nectar and pollen, flowers can attract and keep the
natural enemies of many pests in the home landscape, enhancing biological
control. Some of the many beneficial insects that visit flowers include lady
beetles, green
lacewings, syrphid
flies, tachinid
flies, sphecid wasps and various parasitic wasps.
Predatory bugs (such as nabid bugs, minute
pirate bugs or twospotted stink bugs) can also be found on flowers, but they're
generally there to feed on thrips or other prey on
the blossoms rather than feeding on the flowers themselves. Whether or not planting a
specific flower will actually help reduce the abundance of a problematic garden
pest depends on how effectively the flower brings in the more effective natural
enemies.
Flowers provide
pollen and nectar as a sort of bribe to induce insects to transfer pollen from
one flower to the next and cause pollination. However, the size and shape of a flower limit the kinds of insects that can access its pollen and nectar.
Many of the natural enemies that can benefit most from floral resources are very
tiny wasps, which will get lost in a big flower. The best flowers for these
small natural enemies are also small. Many of these small flowers come in big
packages that we'd call a single flower, but are actually lots of tiny
individual flowers grouped together. Take a look at a Queen Anne's lace
"flower" – it's a large inflorescence (an umbel, technically
speaking). But to a little syrphid fly, less than ¼ inch long, it's a
smorgasboard of hundreds of tiny nectar-producing flowers. Each individual
flower is shallow (not tubular), and has exposed nectaries (although you
probably can't see that!) so it's easy for the fly to gets its tiny mouthparts
to the good stuff. These types of flowers are frequently included in lists of
flowers to plant for beneficials, or are in commercially available "insectary
seed blends."
Most of the flowers with these characteristics
are concentrated in a few plant families. The Umbelliferae, or carrot family,
contains many such species which are frequently included on lists of good plants
for insects. Queen Anne's lace, wild parsnip, fennel, dill, coriander, caraway
and others are quite attractive to beneficials. All have an inflorescence with
lots of tiny, accessible flowers. In an organic market garden in Massachusetts,
flowering fennel attracted 48 species of ichneumonid wasps and 8 species of
predatory wasps. In the Compositae, or aster family, are cosmos,
coneflowers, coreopsis, blanketflower and many others. Several types of
sunflowers have extrafloral nectaries (nectar-producing glands on stems or
foliage) that attract and feed many types of natural enemies. Lettuce or chicory
can be left to go to seed for their flowers. 
The
Brassicaceae, or mustard family includes many common
crops such as radish, mustard, arugula, broccoli, and bok choi that can be left to
flower after harvest. Sweet alyssum and candy-tufts are common garden flower that provide useful flowers.
Many legumes that are planted as cover crops, such as white or
yellow sweet clover, other clovers, hairy vetch, big flower vetch, alfalfa, and
fava beans, are good for natural enemies, not only for their flowers, but as
good habitat and a source of alternate
prey. Fava beans, like sunflowers, have extrafloral nectaries that
attract many natural enemies. Plants in the family Polygonaceae, such as domestic and wild buckwheats and
common knotweed (Polygonum aviculare) are also reported as providing
easily-accessible nectar. A number of plant species
have been shown to encourage natural enemies (see table), but this list is
certainly far from complete. Notice that many of these plants are considered
weeds!
So what exactly makes a good flower –
for a bug? A study at Rutgers University used two wasp
parasitoids of Colorado potato beetle
to explore how flower architecture and nectar scent affect parasitoid attraction
and retention. Using artificial flowers, they found that flower
nectar greatly increased the time the wasps stayed on the flower, compared
with a sugar control. 
And the more concealed the nectar was, the sooner the parasitoids left
the flowers. In a garden planted with flowers of varying nectar accessibility,
the researchers used a video camera to follow foraging activity of the two
wasps. The wasps remained longer on those flowers with the most accessible
nectar. Interestingly, they found that on
umbelliferous flowers, nectar accessibility was determined not only by both flower
morphology, but also by the strength of the wasp. Pediobius foveolatus, the
stronger of the two wasps in this study, was better able to spread petals
and stamens than Edovum puttleri, and therefore stayed much
longer on flowers. It seems that both flower architecture and
insect morphology are important in determining what's a good bug flower.
Equally important as flower size and shape is
when pollen and nectar are produced by the flowers. Many natural enemy species are
around as adults only for short periods during the growing season, so to be
useful, pollen and
nectar must be available when the adults are active. This is most easily
achieved by planting a mixture of plants that have relatively long blooming
periods that overlap in time. Perennial plants often have shorter blooming
periods than annuals, so particular attention should be given to plant diversity
and blooming times in perennial borders designed as habitat for natural enemies.
In the home garden,
sequential plantings of dill, coriander, and caraway can be made to provide a
continuous source of their valuable flowers throughout the season.
Does planting flowers really increase the number
of natural enemies in your yard, and consequently decrease pest problems? Most
scientific studies have looked at just the impact of flowers on the longevity and fecundity of
many natural enemies, rather than decreases in pest populations. In the few
studies that addressed this topic specifically, planting flowers did make a
difference. For example, parasitism of bagworms
was about 40% higher on shrubs that were surrounded by flowering plants. And in
one year, plantings of perennial flowers improved parasitism of the summer generation of obliquebanded
leafrollers on apple (mainly by the tachinid fly Actia interrupta) but didn't
in another year.
While the
idea of prescribing flowers to manage pests has a long way to go, getting home gardeners in
the habit of planting more flowers may make it easier to get them to plant the
flowers that could help them with pest problems in the future.
– Susan Mahr, University
of Wisconsin - Madison
Additional Information:
Return to Horticulture
Information Page
